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3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer. 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management - PowerPoint PPT Presentation
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Transport Layer 3-1
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-2
Transport services and protocols provide logical
communication between app processes running on different hosts
transport protocols run in end systems send side breaks app
messages into segments passes to network layer
rcv side reassembles segments into messages passes to app layer
more than one transport protocol available to apps Internet TCP and UDP
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
logical end-end transport
Transport Layer 3-3
Internet transport-layer protocols reliable in-order
delivery (TCP) congestion control flow control connection setup
unreliable unordered delivery UDP no-frills extension of
ldquobest-effortrdquo IP
services not available delay guarantees bandwidth guarantees
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
logical end-end transport
Transport Layer 3-4
Transport vs network layer
network layer logical communication between hosts
transport layer logical communication between processes relies on enhances
network layer services
What is the minimum that the Transport layer must do
Household analogy12 kids sending letters
to 12 kids processes = kids app messages =
letters in envelopes hosts = houses transport protocol =
Ann and Bill network-layer protocol
= postal service
Transport Layer 3-5
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-6
Multiplexingdemultiplexing
application
transport
network
link
physical
P1 application
transport
network
link
physical
application
transport
network
link
physical
P2P3 P4P1
host 1 host 2 host 3
= process= socket
delivering received segmentsto correct socket
Demultiplexing at rcv hostgathering data from multiplesockets enveloping data with header (later used for demultiplexing)
Multiplexing at send host
Transport Layer 3-7
How demultiplexing works host receives IP datagrams
each datagram has source IP address destination IP address
each datagram carries 1 transport-layer segment
each segment has source destination port number
host uses IP addresses amp port numbers to direct segment to appropriate socket
source port dest port
32 bits
applicationdata
(message)
other header fields
TCPUDP segment format
Transport Layer 3-8
Connectionless demultiplexing Create sockets with port
numbersDatagramSocket mySocket1 = new
DatagramSocket(12534)
DatagramSocket mySocket2 = new DatagramSocket(12535)
UDP socket identified by two-tuple
(dest IP address dest port number)
When host receives UDP segment checks destination port
number in segment directs UDP segment to
socket with that port number
IP datagrams with different source IP addresses andor source port numbers directed to same socket
Transport Layer 3-9
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
SP 9157
DP 6428
SP 6428
DP 5775
SP 5775
DP 6428
SP provides ldquoreturn addressrdquo
Transport Layer 3-10
Connection-oriented demux
TCP socket identified by 4-tuple source IP address source port number dest IP address dest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets each socket identified
by its own 4-tuple
Web servers have different sockets for each connecting client non-persistent HTTP will
have different socket for each request
Transport Layer 3-11
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
client IP A
P1P2P4
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P5 P6 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-13
Review
Transport layer Logical connection between two PORTS
Transport layer lives on endpoints ldquoAnnerdquo and ldquoBillrdquo Minimum job multiplexing and
demultiplexing Can also provide reliability ordering etchellip
Sockets Multiplexing demultiplexing
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-2
Transport services and protocols provide logical
communication between app processes running on different hosts
transport protocols run in end systems send side breaks app
messages into segments passes to network layer
rcv side reassembles segments into messages passes to app layer
more than one transport protocol available to apps Internet TCP and UDP
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
logical end-end transport
Transport Layer 3-3
Internet transport-layer protocols reliable in-order
delivery (TCP) congestion control flow control connection setup
unreliable unordered delivery UDP no-frills extension of
ldquobest-effortrdquo IP
services not available delay guarantees bandwidth guarantees
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
logical end-end transport
Transport Layer 3-4
Transport vs network layer
network layer logical communication between hosts
transport layer logical communication between processes relies on enhances
network layer services
What is the minimum that the Transport layer must do
Household analogy12 kids sending letters
to 12 kids processes = kids app messages =
letters in envelopes hosts = houses transport protocol =
Ann and Bill network-layer protocol
= postal service
Transport Layer 3-5
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-6
Multiplexingdemultiplexing
application
transport
network
link
physical
P1 application
transport
network
link
physical
application
transport
network
link
physical
P2P3 P4P1
host 1 host 2 host 3
= process= socket
delivering received segmentsto correct socket
Demultiplexing at rcv hostgathering data from multiplesockets enveloping data with header (later used for demultiplexing)
Multiplexing at send host
Transport Layer 3-7
How demultiplexing works host receives IP datagrams
each datagram has source IP address destination IP address
each datagram carries 1 transport-layer segment
each segment has source destination port number
host uses IP addresses amp port numbers to direct segment to appropriate socket
source port dest port
32 bits
applicationdata
(message)
other header fields
TCPUDP segment format
Transport Layer 3-8
Connectionless demultiplexing Create sockets with port
numbersDatagramSocket mySocket1 = new
DatagramSocket(12534)
DatagramSocket mySocket2 = new DatagramSocket(12535)
UDP socket identified by two-tuple
(dest IP address dest port number)
When host receives UDP segment checks destination port
number in segment directs UDP segment to
socket with that port number
IP datagrams with different source IP addresses andor source port numbers directed to same socket
Transport Layer 3-9
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
SP 9157
DP 6428
SP 6428
DP 5775
SP 5775
DP 6428
SP provides ldquoreturn addressrdquo
Transport Layer 3-10
Connection-oriented demux
TCP socket identified by 4-tuple source IP address source port number dest IP address dest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets each socket identified
by its own 4-tuple
Web servers have different sockets for each connecting client non-persistent HTTP will
have different socket for each request
Transport Layer 3-11
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
client IP A
P1P2P4
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P5 P6 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-13
Review
Transport layer Logical connection between two PORTS
Transport layer lives on endpoints ldquoAnnerdquo and ldquoBillrdquo Minimum job multiplexing and
demultiplexing Can also provide reliability ordering etchellip
Sockets Multiplexing demultiplexing
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-3
Internet transport-layer protocols reliable in-order
delivery (TCP) congestion control flow control connection setup
unreliable unordered delivery UDP no-frills extension of
ldquobest-effortrdquo IP
services not available delay guarantees bandwidth guarantees
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysicalnetwork
data linkphysical
logical end-end transport
Transport Layer 3-4
Transport vs network layer
network layer logical communication between hosts
transport layer logical communication between processes relies on enhances
network layer services
What is the minimum that the Transport layer must do
Household analogy12 kids sending letters
to 12 kids processes = kids app messages =
letters in envelopes hosts = houses transport protocol =
Ann and Bill network-layer protocol
= postal service
Transport Layer 3-5
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-6
Multiplexingdemultiplexing
application
transport
network
link
physical
P1 application
transport
network
link
physical
application
transport
network
link
physical
P2P3 P4P1
host 1 host 2 host 3
= process= socket
delivering received segmentsto correct socket
Demultiplexing at rcv hostgathering data from multiplesockets enveloping data with header (later used for demultiplexing)
Multiplexing at send host
Transport Layer 3-7
How demultiplexing works host receives IP datagrams
each datagram has source IP address destination IP address
each datagram carries 1 transport-layer segment
each segment has source destination port number
host uses IP addresses amp port numbers to direct segment to appropriate socket
source port dest port
32 bits
applicationdata
(message)
other header fields
TCPUDP segment format
Transport Layer 3-8
Connectionless demultiplexing Create sockets with port
numbersDatagramSocket mySocket1 = new
DatagramSocket(12534)
DatagramSocket mySocket2 = new DatagramSocket(12535)
UDP socket identified by two-tuple
(dest IP address dest port number)
When host receives UDP segment checks destination port
number in segment directs UDP segment to
socket with that port number
IP datagrams with different source IP addresses andor source port numbers directed to same socket
Transport Layer 3-9
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
SP 9157
DP 6428
SP 6428
DP 5775
SP 5775
DP 6428
SP provides ldquoreturn addressrdquo
Transport Layer 3-10
Connection-oriented demux
TCP socket identified by 4-tuple source IP address source port number dest IP address dest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets each socket identified
by its own 4-tuple
Web servers have different sockets for each connecting client non-persistent HTTP will
have different socket for each request
Transport Layer 3-11
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
client IP A
P1P2P4
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P5 P6 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-13
Review
Transport layer Logical connection between two PORTS
Transport layer lives on endpoints ldquoAnnerdquo and ldquoBillrdquo Minimum job multiplexing and
demultiplexing Can also provide reliability ordering etchellip
Sockets Multiplexing demultiplexing
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-4
Transport vs network layer
network layer logical communication between hosts
transport layer logical communication between processes relies on enhances
network layer services
What is the minimum that the Transport layer must do
Household analogy12 kids sending letters
to 12 kids processes = kids app messages =
letters in envelopes hosts = houses transport protocol =
Ann and Bill network-layer protocol
= postal service
Transport Layer 3-5
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-6
Multiplexingdemultiplexing
application
transport
network
link
physical
P1 application
transport
network
link
physical
application
transport
network
link
physical
P2P3 P4P1
host 1 host 2 host 3
= process= socket
delivering received segmentsto correct socket
Demultiplexing at rcv hostgathering data from multiplesockets enveloping data with header (later used for demultiplexing)
Multiplexing at send host
Transport Layer 3-7
How demultiplexing works host receives IP datagrams
each datagram has source IP address destination IP address
each datagram carries 1 transport-layer segment
each segment has source destination port number
host uses IP addresses amp port numbers to direct segment to appropriate socket
source port dest port
32 bits
applicationdata
(message)
other header fields
TCPUDP segment format
Transport Layer 3-8
Connectionless demultiplexing Create sockets with port
numbersDatagramSocket mySocket1 = new
DatagramSocket(12534)
DatagramSocket mySocket2 = new DatagramSocket(12535)
UDP socket identified by two-tuple
(dest IP address dest port number)
When host receives UDP segment checks destination port
number in segment directs UDP segment to
socket with that port number
IP datagrams with different source IP addresses andor source port numbers directed to same socket
Transport Layer 3-9
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
SP 9157
DP 6428
SP 6428
DP 5775
SP 5775
DP 6428
SP provides ldquoreturn addressrdquo
Transport Layer 3-10
Connection-oriented demux
TCP socket identified by 4-tuple source IP address source port number dest IP address dest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets each socket identified
by its own 4-tuple
Web servers have different sockets for each connecting client non-persistent HTTP will
have different socket for each request
Transport Layer 3-11
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
client IP A
P1P2P4
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P5 P6 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-13
Review
Transport layer Logical connection between two PORTS
Transport layer lives on endpoints ldquoAnnerdquo and ldquoBillrdquo Minimum job multiplexing and
demultiplexing Can also provide reliability ordering etchellip
Sockets Multiplexing demultiplexing
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-5
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-6
Multiplexingdemultiplexing
application
transport
network
link
physical
P1 application
transport
network
link
physical
application
transport
network
link
physical
P2P3 P4P1
host 1 host 2 host 3
= process= socket
delivering received segmentsto correct socket
Demultiplexing at rcv hostgathering data from multiplesockets enveloping data with header (later used for demultiplexing)
Multiplexing at send host
Transport Layer 3-7
How demultiplexing works host receives IP datagrams
each datagram has source IP address destination IP address
each datagram carries 1 transport-layer segment
each segment has source destination port number
host uses IP addresses amp port numbers to direct segment to appropriate socket
source port dest port
32 bits
applicationdata
(message)
other header fields
TCPUDP segment format
Transport Layer 3-8
Connectionless demultiplexing Create sockets with port
numbersDatagramSocket mySocket1 = new
DatagramSocket(12534)
DatagramSocket mySocket2 = new DatagramSocket(12535)
UDP socket identified by two-tuple
(dest IP address dest port number)
When host receives UDP segment checks destination port
number in segment directs UDP segment to
socket with that port number
IP datagrams with different source IP addresses andor source port numbers directed to same socket
Transport Layer 3-9
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
SP 9157
DP 6428
SP 6428
DP 5775
SP 5775
DP 6428
SP provides ldquoreturn addressrdquo
Transport Layer 3-10
Connection-oriented demux
TCP socket identified by 4-tuple source IP address source port number dest IP address dest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets each socket identified
by its own 4-tuple
Web servers have different sockets for each connecting client non-persistent HTTP will
have different socket for each request
Transport Layer 3-11
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
client IP A
P1P2P4
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P5 P6 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-13
Review
Transport layer Logical connection between two PORTS
Transport layer lives on endpoints ldquoAnnerdquo and ldquoBillrdquo Minimum job multiplexing and
demultiplexing Can also provide reliability ordering etchellip
Sockets Multiplexing demultiplexing
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-6
Multiplexingdemultiplexing
application
transport
network
link
physical
P1 application
transport
network
link
physical
application
transport
network
link
physical
P2P3 P4P1
host 1 host 2 host 3
= process= socket
delivering received segmentsto correct socket
Demultiplexing at rcv hostgathering data from multiplesockets enveloping data with header (later used for demultiplexing)
Multiplexing at send host
Transport Layer 3-7
How demultiplexing works host receives IP datagrams
each datagram has source IP address destination IP address
each datagram carries 1 transport-layer segment
each segment has source destination port number
host uses IP addresses amp port numbers to direct segment to appropriate socket
source port dest port
32 bits
applicationdata
(message)
other header fields
TCPUDP segment format
Transport Layer 3-8
Connectionless demultiplexing Create sockets with port
numbersDatagramSocket mySocket1 = new
DatagramSocket(12534)
DatagramSocket mySocket2 = new DatagramSocket(12535)
UDP socket identified by two-tuple
(dest IP address dest port number)
When host receives UDP segment checks destination port
number in segment directs UDP segment to
socket with that port number
IP datagrams with different source IP addresses andor source port numbers directed to same socket
Transport Layer 3-9
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
SP 9157
DP 6428
SP 6428
DP 5775
SP 5775
DP 6428
SP provides ldquoreturn addressrdquo
Transport Layer 3-10
Connection-oriented demux
TCP socket identified by 4-tuple source IP address source port number dest IP address dest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets each socket identified
by its own 4-tuple
Web servers have different sockets for each connecting client non-persistent HTTP will
have different socket for each request
Transport Layer 3-11
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
client IP A
P1P2P4
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P5 P6 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-13
Review
Transport layer Logical connection between two PORTS
Transport layer lives on endpoints ldquoAnnerdquo and ldquoBillrdquo Minimum job multiplexing and
demultiplexing Can also provide reliability ordering etchellip
Sockets Multiplexing demultiplexing
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-7
How demultiplexing works host receives IP datagrams
each datagram has source IP address destination IP address
each datagram carries 1 transport-layer segment
each segment has source destination port number
host uses IP addresses amp port numbers to direct segment to appropriate socket
source port dest port
32 bits
applicationdata
(message)
other header fields
TCPUDP segment format
Transport Layer 3-8
Connectionless demultiplexing Create sockets with port
numbersDatagramSocket mySocket1 = new
DatagramSocket(12534)
DatagramSocket mySocket2 = new DatagramSocket(12535)
UDP socket identified by two-tuple
(dest IP address dest port number)
When host receives UDP segment checks destination port
number in segment directs UDP segment to
socket with that port number
IP datagrams with different source IP addresses andor source port numbers directed to same socket
Transport Layer 3-9
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
SP 9157
DP 6428
SP 6428
DP 5775
SP 5775
DP 6428
SP provides ldquoreturn addressrdquo
Transport Layer 3-10
Connection-oriented demux
TCP socket identified by 4-tuple source IP address source port number dest IP address dest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets each socket identified
by its own 4-tuple
Web servers have different sockets for each connecting client non-persistent HTTP will
have different socket for each request
Transport Layer 3-11
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
client IP A
P1P2P4
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P5 P6 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-13
Review
Transport layer Logical connection between two PORTS
Transport layer lives on endpoints ldquoAnnerdquo and ldquoBillrdquo Minimum job multiplexing and
demultiplexing Can also provide reliability ordering etchellip
Sockets Multiplexing demultiplexing
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-8
Connectionless demultiplexing Create sockets with port
numbersDatagramSocket mySocket1 = new
DatagramSocket(12534)
DatagramSocket mySocket2 = new DatagramSocket(12535)
UDP socket identified by two-tuple
(dest IP address dest port number)
When host receives UDP segment checks destination port
number in segment directs UDP segment to
socket with that port number
IP datagrams with different source IP addresses andor source port numbers directed to same socket
Transport Layer 3-9
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
SP 9157
DP 6428
SP 6428
DP 5775
SP 5775
DP 6428
SP provides ldquoreturn addressrdquo
Transport Layer 3-10
Connection-oriented demux
TCP socket identified by 4-tuple source IP address source port number dest IP address dest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets each socket identified
by its own 4-tuple
Web servers have different sockets for each connecting client non-persistent HTTP will
have different socket for each request
Transport Layer 3-11
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
client IP A
P1P2P4
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P5 P6 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-13
Review
Transport layer Logical connection between two PORTS
Transport layer lives on endpoints ldquoAnnerdquo and ldquoBillrdquo Minimum job multiplexing and
demultiplexing Can also provide reliability ordering etchellip
Sockets Multiplexing demultiplexing
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-9
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
SP 9157
DP 6428
SP 6428
DP 5775
SP 5775
DP 6428
SP provides ldquoreturn addressrdquo
Transport Layer 3-10
Connection-oriented demux
TCP socket identified by 4-tuple source IP address source port number dest IP address dest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets each socket identified
by its own 4-tuple
Web servers have different sockets for each connecting client non-persistent HTTP will
have different socket for each request
Transport Layer 3-11
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
client IP A
P1P2P4
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P5 P6 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-13
Review
Transport layer Logical connection between two PORTS
Transport layer lives on endpoints ldquoAnnerdquo and ldquoBillrdquo Minimum job multiplexing and
demultiplexing Can also provide reliability ordering etchellip
Sockets Multiplexing demultiplexing
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-10
Connection-oriented demux
TCP socket identified by 4-tuple source IP address source port number dest IP address dest port number
recv host uses all four values to direct segment to appropriate socket
Server host may support many simultaneous TCP sockets each socket identified
by its own 4-tuple
Web servers have different sockets for each connecting client non-persistent HTTP will
have different socket for each request
Transport Layer 3-11
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
client IP A
P1P2P4
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P5 P6 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-13
Review
Transport layer Logical connection between two PORTS
Transport layer lives on endpoints ldquoAnnerdquo and ldquoBillrdquo Minimum job multiplexing and
demultiplexing Can also provide reliability ordering etchellip
Sockets Multiplexing demultiplexing
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-11
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
client IP A
P1P2P4
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P5 P6 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-13
Review
Transport layer Logical connection between two PORTS
Transport layer lives on endpoints ldquoAnnerdquo and ldquoBillrdquo Minimum job multiplexing and
demultiplexing Can also provide reliability ordering etchellip
Sockets Multiplexing demultiplexing
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-12
Connection-oriented demux (cont)
ClientIPB
P1
client IP A
P1P2P4
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P5 P6 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-13
Review
Transport layer Logical connection between two PORTS
Transport layer lives on endpoints ldquoAnnerdquo and ldquoBillrdquo Minimum job multiplexing and
demultiplexing Can also provide reliability ordering etchellip
Sockets Multiplexing demultiplexing
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-13
Review
Transport layer Logical connection between two PORTS
Transport layer lives on endpoints ldquoAnnerdquo and ldquoBillrdquo Minimum job multiplexing and
demultiplexing Can also provide reliability ordering etchellip
Sockets Multiplexing demultiplexing
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-14
TCP Socket like a phone
connectionSocket =welcomeSocketaccept()
welcomeSocket = ServerSocket(port)
clientSocket = Socket(hostport)
clientSocketread()
Server (running on hostid) Client
clientSocketwrite(data)connectionSocketread()
connectionSocketwrite(data)
TCP connection setup
New Thread()
connectionSocket =welcomeSocketaccept()
clientSocket = Socket(addressport)
clientSocketread()
clientSocketwrite(data)
connectionSocketread()connectionSocketwrite(data)
TCP connection setup
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-15
UDP Socket like a mailbox
serverSocket = DatagramSocket(port)
clientSocket = DatagramSocket()
hpd = clientSocketread()
Server (running on hostid)Client
clientSocketwrite(host port data)hpd = serverSocketread()
connectionSocketwrite(host port data)
hpd = serverSocketread()
connectionSocketwrite(host port data) hpd = clientSocketread()
clientSocketwrite(host port data)
clientSocket = DatagramSocket()
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-16
Notice
TCP sockets Server port required to create listening socket Server address and port needed by client for
connection setup Nodes can talk freely after that
UDP sockets Server port required to create listening socket Every message requires dest addressport All reads provide source addressport
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-17
Connection-oriented demux Threaded Web Server
ClientIPB
P1
client IP A
P1P2
serverIP C
SP 9157
DP 80
SP 9157
DP 80
P4 P3
D-IPCS-IP A
D-IPC
S-IP B
SP 5775
DP 80
D-IPCS-IP B
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-18
Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428)
ClientIPB
P2
client IP A
P1P1P3
serverIP C
SP 6428
DP 9157
S-IP C
SP 9157
DP 6428
S-IP A
SP 6428
DP 5775
S-IP C
SP 5775
DP 6428
S-IP B
SP provides ldquoreturn addressrdquo
D-IP C
D-IP A D-IP B
D-IP C
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-19
Puzzle sockets
How many simultaneous TCP sockets can a server have
How many simultaneous UDP sockets
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-20
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-21
UDP User Datagram Protocol [RFC 768]
ldquono frillsrdquo ldquobare bonesrdquo Internet transport protocol
ldquobest effortrdquo service UDP segments may be lost delivered out of order
to app connectionless
no handshaking between UDP sender receiver
each UDP segment handled independently of others
Why is there a UDP no connection
establishment (which can add delay)
simple no connection state at sender receiver
small segment header no congestion control
UDP can blast away as fast as desired
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-22
UDP checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-23
Internet Checksum Example Note
When adding numbers a carryout from the most significant bit needs to be added to the result
Example add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-24
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-25
Reliable data transfer getting startedWersquoll incrementally develop sender receiver
sides of reliable data transfer protocol (rdt) consider only unidirectional data transfer
but control info will flow on both directions
use finite state machines (FSM) to specify sender receiver
state1
state2
event causing state transitionactions taken on state transition
state when in this ldquostaterdquo next state
uniquely determined by next event
eventactions
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-26
Reliable data transfer (rdt) interface
sendside
receiveside
rdt_send() called from above (eg by app) Passed data to deliver to receiver upper layer
udt_send() called by rdtto transfer packet over unreliable channel to
receiver
rdt_rcv() called when packet arrives on rcv-side of channel
deliver_data() called by rdt to deliver data to
upper
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-27
Rdt10 reliable transfer over a reliable channel
underlying channel perfectly reliable no bit errors no loss of packets
separate FSMs for sender receiver sender sends data into underlying channel receiver read data from underlying channel
Wait for call from above packet = make_pkt(data)
udt_send(packet)
rdt_send(data)
extract (packetdata)deliver_data(data)
Wait for call from
below
rdt_rcv(packet)
sender receiver
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-28
Rdt20 channel with bit errors
underlying channel may flip bits in packet checksum to detect bit errors
the question how to recover from errors acknowledgements (ACKs) receiver explicitly tells
sender that pkt received OK negative acknowledgements (NAKs) receiver
explicitly tells sender that pkt had errors sender retransmits pkt on receipt of NAK
new mechanisms in rdt20 (beyond rdt10) error detection receiver feedback control msgs (ACKNAK) rcvr-
gtsender
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-29
rdt20 FSM specification
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
belowsender
receiverrdt_send(data)
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-30
rdt20 operation with no errors
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-31
rdt20 error scenario
Wait for call from above
snkpkt = make_pkt(data checksum)udt_send(sndpkt)
extract(rcvpktdata)deliver_data(data)udt_send(ACK)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
rdt_rcv(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp isNAK(rcvpkt)
udt_send(NAK)
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Wait for ACK or
NAK
Wait for call from
below
rdt_send(data)
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-32
rdt20 has a fatal flaw
What happens if ACKNAK corrupted
sender doesnrsquot know what happened at receiver
canrsquot just retransmit possible duplicate
Handling duplicates sender retransmits current
pkt if ACKNAK garbled sender adds sequence
number to each pkt receiver discards (doesnrsquot
deliver up) duplicate pkt
Sender sends one packet then waits for receiver response
stop and wait
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-33
rdt21 sender handles garbled ACKNAKs
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
Wait for ACK or NAK 0 udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isNAK(rcvpkt) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt)
Wait for call 1 from
above
Wait for ACK or NAK 1
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-34
rdt21 receiver handles garbled ACKNAKs
Wait for 0 from below
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq0(rcvpkt)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
Wait for 1 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq0(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp not corrupt(rcvpkt) ampamp has_seq1(rcvpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt)
sndpkt = make_pkt(ACK chksum)udt_send(sndpkt)
sndpkt = make_pkt(NAK chksum)udt_send(sndpkt)
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-35
rdt21 discussion
Sender seq added to pkt two seq rsquos (01)
will suffice Why must check if
received ACKNAK corrupted
twice as many states state must
ldquorememberrdquo whether ldquocurrentrdquo pkt has 0 or 1 seq
Receiver must check if
received packet is duplicate state indicates
whether 0 or 1 is expected pkt seq
note receiver can not know if its last ACKNAK received OK at sender
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-36
rdt22 a NAK-free protocol
same functionality as rdt21 using ACKs only instead of NAK receiver sends ACK for last pkt
received OK receiver must explicitly include seq of pkt being
ACKed
duplicate ACK at sender results in same action as NAK retransmit current pkt
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-37
rdt22 sender receiver fragments
Wait for call 0 from
above
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)
rdt_send(data)
udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) || isACK(rcvpkt1) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
Wait for ACK
0
sender FSMfragment
Wait for 0 from below
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp has_seq1(rcvpkt)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(ACK1 chksum)udt_send(sndpkt)
rdt_rcv(rcvpkt) ampamp (corrupt(rcvpkt) || has_seq1(rcvpkt))
udt_send(sndpkt)
receiver FSMfragment
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-38
rdt30 channels with errors and loss
New assumption underlying channel can also lose packets (data or ACKs) checksum seq
ACKs retransmissions will be of help but not enough
Approach sender waits ldquoreasonablerdquo amount of time for ACK
retransmits if no ACK received in this time
if pkt (or ACK) just delayed (not lost) retransmission will be
duplicate but use of seq rsquos already handles this
receiver must specify seq of pkt being ACKed
requires countdown timer
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-39
rdt30 sender
sndpkt = make_pkt(0 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
Wait for
ACK0
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt1) )
Wait for call 1 from
above
sndpkt = make_pkt(1 data checksum)udt_send(sndpkt)start_timer
rdt_send(data)
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt0)
rdt_rcv(rcvpkt) ampamp ( corrupt(rcvpkt) ||isACK(rcvpkt0) )
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt) ampamp isACK(rcvpkt1)
stop_timerstop_timer
udt_send(sndpkt)start_timer
timeout
udt_send(sndpkt)start_timer
timeout
rdt_rcv(rcvpkt)
Wait for call 0from
above
Wait for
ACK1
rdt_rcv(rcvpkt)
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-40
rdt30 in action
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-41
rdt30 in action
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-42
Performance of rdt30
rdt30 works but performance stinks example 1 Gbps link 15 ms e-e prop delay 1KB packet
Ttransmit
= 8kbpkt109 bsec
= 8 microsec
U sender utilization ndash fraction of time sender busy sending
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
L (packet length in bits)R (transmission rate bps)
=
1KB pkt every 30 msec -gt 33kBsec thruput over 1 Gbps link network protocol limits use of physical resources
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-43
Review
rdt 10 just send it rdt 20 bit errors acks and nacks rdt 21 corrupted acksnacks seqNos rdt 22 acks + seqNo no need for
nacks rdt 30 packet loss timers
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-44
rdt30 stop-and-wait operation
first packet bit transmitted t = 0
sender receiver
RTT
last packet bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
U sender
= 008
30008 = 000027
microseconds
L R
RTT + L R =
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-45
Pipelining increased utilization
first packet bit transmitted t = 0
sender receiver
RTT
last bit transmitted t = L R
first packet bit arriveslast packet bit arrives send ACK
ACK arrives send next packet t = RTT + L R
last bit of 2nd packet arrives send ACKlast bit of 3rd packet arrives send ACK
U sender
= 024
30008 = 00008
microseconds
3 L R
RTT + L R =
Increase utilizationby a factor of 3
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-46
Pipelined protocols
Pipelining sender allows multiple ldquoin-flightrdquo yet-to-be-acknowledged pkts range of sequence numbers must be increased buffering at sender andor receiver
Two generic forms of pipelined protocols go-Back-N selective repeat
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-47
Go-Back-NSender ACK(n) ACKs all pkts up to including seq n - ldquocumulative ACKrdquo
may receive duplicate ACKs (see receiver) timer for each in-flight pkt timeout(n) retransmit pkt n and all higher seq pkts in window
k-bit seq in pkt header ldquowindowrdquo of up to N consecutive unackrsquoed pkts allowed
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-48
GBN sender extended FSM
Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])hellipudt_send(sndpkt[nextseqnum-1])
timeout
rdt_send(data)
if (nextseqnum lt base+N) sndpkt[nextseqnum] = make_pkt(nextseqnumdatachksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ else refuse_data(data)
base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer
rdt_rcv(rcvpkt) ampamp notcorrupt(rcvpkt)
base=1nextseqnum=1
rdt_rcv(rcvpkt) ampamp corrupt(rcvpkt)
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-49
GBN receiver extended FSM
ACK-only always send ACK for correctly-received pkt with highest in-order seq may generate duplicate ACKs need only remember expectedseqnum
out-of-order pkt discard (donrsquot buffer) -gt no receiver buffering Re-ACK pkt with highest in-order seq
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt) ampamp notcurrupt(rcvpkt) ampamp hasseqnum(rcvpktexpectedseqnum)
extract(rcvpktdata)deliver_data(data)sndpkt = make_pkt(expectedseqnumACKchksum)udt_send(sndpkt)expectedseqnum++
expectedseqnum=1sndpkt = make_pkt(expectedseqnumACKchksum)
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-50
GBN inaction
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-51
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts as needed for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq rsquos again limits seq s of sent unACKed pkts
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-52
Selective repeat sender receiver windows
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-53
Selective repeat
data from above if next available seq in
window send pkt
timeout(n) resend pkt n restart timer
ACK(n) in [sendbasesendbase+N]
Stop timer mark pkt n as received if n smallest unACKed pkt
advance window base to next unACKed seq
senderpkt n in [rcvbase rcvbase+N-
1]
send ACK(n) out-of-order buffer in-order deliver (also
deliver buffered in-order pkts) advance window to next not-yet-received pkt
pkt n in [rcvbase-Nrcvbase-1]
ACK(n)
otherwise ignore
receiver
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-54
Selective repeat in action
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-55
Selective repeat dilemma
Example seq rsquos 0 1 2 3 window size=3
receiver sees no difference in two scenarios
incorrectly passes duplicate data as new in (a)
Q what relationship between seq size and window size
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-56
Review
Mechanisms for Reliable Transport Packet Corruption AcksNacks Ack corruption Sequence s Loss Timeouts
Pipelining Go-Back-N cumulative acks no rxr
buffering Selective Repeat individual acks rxr
bufferingbull Must be careful that rxrWindow lt= max seq no
2
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-57
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-58
TCP segment structure
source port dest port
32 bits
applicationdata
(variable length)
sequence number
acknowledgement numberReceive window
Urg data pnterchecksum
FSRPAUheadlen
notused
Options (variable length)
URG urgent data (generally not used)
ACK ACK valid
PSH push data now(generally not used)
RST SYN FINconnection estab(setup teardown
commands)
bytes rcvr willingto accept
countingby bytes of data(not segments)
Internetchecksum
(as in UDP)
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-59
TCP seq rsquos and ACKsSeq rsquos
byte stream ldquonumberrdquo of first byte in segmentrsquos data
ACKs seq of next byte
expected from other side
cumulative ACKQ how receiver handles
out-of-order segments A TCP spec doesnrsquot
say - up to implementor
Host A Host B
Seq=42 ACK=79 data = lsquoCrsquo
Seq=79 ACK=43 data = lsquoCrsquo
Seq=43 ACK=80
Usertypes
lsquoCrsquo
host ACKsreceipt
of echoedlsquoCrsquo
host ACKsreceipt of
lsquoCrsquo echoesback lsquoCrsquo
timesimple telnet scenario
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-60
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-61
TCP sender eventsdata rcvd from app Create segment with
seq seq is byte-stream
number of first data byte in segment
start timer if not already running (think of timer as for oldest unacked segment)
expiration interval TimeOutInterval
timeout retransmit segment
that caused timeout restart timer Ack rcvd If acknowledges
previously unacked segments update what is known
to be acked start timer if there are
outstanding segments
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-62
TCP retransmission scenarios
Host A
Seq=100 20 bytes data
ACK=100
timepremature timeout
Host B
Seq=92 8 bytes data
ACK=120
Seq=92 8 bytes data
Seq=
92
tim
eout
ACK=120
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
lost ACK scenario
Host B
X
Seq=92 8 bytes data
ACK=100
time
Seq=
92
tim
eout
SendBase= 100
SendBase= 120
SendBase= 120
Sendbase= 100
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-63
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
ACK=100
loss
tim
eout
Cumulative ACK scenario
Host B
X
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-64
TCP Round Trip Time and TimeoutQ how to set TCP
timeout value longer than RTT
but RTT varies too short premature
timeout unnecessary
retransmissions too long slow
reaction to segment loss
Q how to estimate RTT SampleRTT measured time
from segment transmission until ACK receipt ignore retransmissions
SampleRTT will vary want estimated RTT ldquosmootherrdquo average several recent
measurements not just current SampleRTT
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-65
TCP Round Trip Time and TimeoutEstimatedRTT = (1- )EstimatedRTT + SampleRTT
Exponential weighted moving average influence of past sample decreases exponentially fast typical value = 0125
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-66
Example RTT estimationRTT gaiacsumassedu to fantasiaeurecomfr
100
150
200
250
300
350
1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106
time (seconnds)
RTT
(mill
isec
onds
)
SampleRTT Estimated RTT
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-67
TCP Round Trip Time and TimeoutSetting the timeout EstimtedRTT plus ldquosafety marginrdquo
large variation in EstimatedRTT -gt larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT
TimeoutInterval = EstimatedRTT + 4DevRTT
DevRTT = (1-)DevRTT + |SampleRTT-EstimatedRTT|
(typically = 025)
Then set timeout interval
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-68
TCP ACK generation [RFC 1122 RFC 2581]
Event at Receiver
Arrival of in-order segment withexpected seq All data up toexpected seq already ACKed
Arrival of in-order segment withexpected seq One other segment has ACK pending
Arrival of out-of-order segmenthigher-than-expect seq Gap detected
Arrival of segment that partially or completely fills gap
TCP Receiver action
Delayed ACK Wait up to 500msfor next segment If no next segmentsend ACK
Immediately send single cumulative ACK ACKing both in-order segments
Buffer packetImmediately send duplicate ACK indicating seq of next expected byte
Immediate send ACK provided thatsegment starts at lower end of gap
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-69
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Close Transmission
Host B
Seq=100 20 bytes data
ACK=120
time
SendBase= 120
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-70
TCP retransmission scenarios (more)
Host A
Seq=92 8 bytes data
tim
eout
Duplicate acks
Host B
Seq=100 20 bytes data
ACK=100
time
SendBase= 120
Seq=120 16 bytes dataSeq=136 20 bytes data
lossX
ACK=100
ACK=100
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-71
Fast Retransmit
Time-out period often relatively long long delay before
resending lost packet
Detect lost segments via duplicate ACKs Sender often sends
many segments back-to-back
If segment is lost there will likely be many duplicate ACKs
If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend
segment before timer expires
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-72
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-73
TCP Flow Control
receive side of TCP connection has a receive buffer
speed-matching service matching the send rate to the receiving apprsquos drain rate app process may be
slow at reading from buffer
sender wonrsquot overflow
receiverrsquos buffer bytransmitting too
much too fast
flow control
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-74
TCP Flow control how it works
(Suppose TCP receiver discards out-of-order segments)
spare room in buffer= RcvWindow
= RcvBuffer-[LastByteRcvd - LastByteRead]
Rcvr advertises spare room by including value of RcvWindow in segments
Sender limits unACKed data to RcvWindow guarantees receive
buffer doesnrsquot overflow
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-75
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-76
TCP Connection Management
Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments
initialize TCP variables seq s buffers flow control info
(eg RcvWindow) client connection initiator Socket clientSocket = new
Socket(hostnameport
number) server contacted by client Socket connectionSocket =
welcomeSocketaccept()
Three way handshake
Step 1 client host sends TCP SYN segment to server specifies initial seq no data
Step 2 server host receives SYN replies with SYNACK segment
server allocates buffers specifies server initial
seq Step 3 client receives SYNACK
replies with ACK segment which may contain data
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-77
TCP Connection Management (cont)
Closing a connection
client closes socket clientSocketclose()
Step 1 client end system sends TCP FIN control segment to server
Step 2 server receives FIN replies with ACK Closes connection sends FIN
client
FIN
server
ACK
ACK
FIN
close
close
closed
tim
ed w
ait
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-78
TCP Connection Management (cont)
Step 3 client receives FIN replies with ACK
Enters ldquotimed waitrdquo - will respond with ACK to received FINs
Step 4 server receives ACK Connection closed
Note with small modification can handle simultaneous FINs
client
FIN
server
ACK
ACK
FIN
closing
closing
closed
tim
ed w
ait
closed
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-79
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-80
Principles of Congestion Control
Congestion informally ldquotoo many sources sending too
much data too fast for network to handlerdquo different from flow control manifestations
lost packets (buffer overflow at routers) long delays (queueing in router buffers)
a top-10 problem
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-81
Causescosts of congestion scenario 1
two senders two receivers
one router infinite buffers
no retransmission
large delays when congested
maximum achievable throughput
unlimited shared output link buffers
Host Ain original data
Host B
out
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-82
Causescosts of congestion scenario 2
one router finite buffers sender retransmission of lost packet
finite shared output link buffers
Host A in original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-83
Causescosts of congestion scenario 2
ldquocostsrdquo of congestion with finite buffers Loss more transmissions for given data less throughput Delay redundant retransmissions = loss 2
R2
R2in
ou
t
b
R2
R2in
ou
t
a
R2
R2in
ou
t
c
R4
R3
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-84
Causescosts of congestion scenario 3 four senders multihop paths timeoutretransmit
in
Q what happens as and increase
in
finite shared output link buffers
Host Ain original data
Host B
out
in original data plus retransmitted data
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-85
Causescosts of congestion scenario 3
Another ldquocostrdquo of congestion when packet dropped any ldquoupstream transmission capacity
used for that packet was wasted
Host A
Host B
o
u
t
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-86
Approaches towards congestion control
End-end congestion control
no explicit feedback from network
congestion inferred from end-system observed loss delay
approach taken by TCP
Network-assisted congestion control
routers provide feedback to end systems single bit indicating
congestion (SNA DECbit TCPIP ECN ATM)
explicit rate sender should send at
Two broad approaches towards congestion control
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-87
Case study ATM ABR congestion control
ABR available bit rate
ldquoelastic servicerdquo if senderrsquos path
ldquounderloadedrdquo sender should use
available bandwidth if senderrsquos path
congested sender throttled to
minimum guaranteed rate
RM (resource management) cells
sent by sender interspersed with data cells
bits in RM cell set by switches (ldquonetwork-assistedrdquo) NI bit no increase in rate
(mild congestion) CI bit congestion
indication RM cells returned to sender
by receiver with bits intact
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-88
Case study ATM ABR congestion control
two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell senderrsquo send rate thus maximum supportable rate on
path
EFCI bit in data cells set to 1 in congested switch if data cell preceding RM cell has EFCI set sender sets CI
bit in returned RM cell
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-89
Chapter 3 outline
31 Transport-layer services
32 Multiplexing and demultiplexing
33 Connectionless transport UDP
34 Principles of reliable data transfer
35 Connection-oriented transport TCP segment structure reliable data transfer flow control connection
management
36 Principles of congestion control
37 TCP congestion control
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-90
TCP Congestion Control details
sender limits transmission LastByteSent-LastByteAcked
min( CongWin rcvWin ) Roughly
CongWin is dynamic function of perceived network congestion
How does sender perceive congestion
loss event = timeout or 3 duplicate acks
TCP sender reduces rate (CongWin) after loss event
three mechanisms AIMD slow start conservative after
timeout events
rate = CongWin
RTT Bytessec
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-91
TCP congestion control additive increase multiplicative decrease
8 Kbytes
16 Kbytes
24 Kbytes
time
congestionwindow
if no loss increase congestion window until loss occurs additive increase increase CongWin by 1 MSS every RTT until loss
detectedbull CongWin += 1CongWin
multiplicative decrease cut CongWin in half after loss
timecong
estio
n w
indo
w s
ize
Saw toothbehavior probing
for bandwidth
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-92
TCP Slow Start
When connection begins CongWin = 1 MSS Example MSS = 500
bytes amp RTT = 200 msec
initial rate = 25 kbps
available bandwidth may be gtgt MSSRTT desirable to quickly
ramp up to respectable rate
When connection begins increase rate exponentially fast until first loss event
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-93
TCP Slow Start (more)
When connection begins increase rate exponentially until first loss event double CongWin every
RTT congWin += 1
Summary initial rate is slow but ramps up exponentially fast
Host A
one segment
RTT
Host B
time
two segments
four segments
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-94
Combining the two
Implementation Define Threshold=CongWin At loss event Threshold is set to
12 of CongWin just before loss event
Switch the exponential increase switch to linear when CongWin gets to 12 of its value before timeout
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-95
Conservative on Timeout
After 3 dup ACKs CongWin is cut in half window then grows
linearly But after timeout
event CongWin instead set
to 1 MSS window then grows
exponentially to a threshold then
grows linearly
3 dup ACKs indicates
network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario
Philosophy
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-96
Summary TCP Congestion Control
When CongWin is below Threshold sender in slow-start phase window grows exponentially
When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly
When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold
When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-97
TCP sender congestion control
State Event TCP Sender Action Commentary
Slow Start (SS)
ACK receipt for previously unacked data
CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo
Resulting in a doubling of CongWin every RTT
CongestionAvoidance (CA)
ACK receipt for previously unacked data
CongWin = CongWin+MSS (MSSCongWin)
Additive increase resulting in increase of CongWin by 1 MSS every RTT
SS or CA Loss event detected by triple duplicate ACK
Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo
Fast recovery implementing multiplicative decrease CongWin will not drop below 1 MSS
SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo
Enter slow start
SS or CA Duplicate ACK
Increment duplicate ACK count for segment being acked
CongWin and Threshold not changed
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-98
TCP throughput
Whatrsquos the average throughout of TCP as a function of window size and RTT Ignore slow start
Let W be the window size when loss occurs
When window is W throughput is WRTT Just after loss window drops to W2
throughput to W2RTT Average throughout 75 WRTT
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-99
TCP Futures TCP over ldquolong fat pipesrdquo
Example 1500 byte segments 100ms RTT want 10 Gbps throughput
Requires window size W = 83333 in-flight segments
Throughput in terms of loss rate
L = 210-10 Wow New versions of TCP for high-speed needed
LRTT
MSS221
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-100
Fairness goal if K TCP sessions share same bottleneck link of bandwidth R each should have average rate of RK
TCP connection 1
bottleneckrouter
capacity R
TCP connection 2
TCP Fairness
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-101
Why is TCP fair
Two competing sessions Additive increase gives slope of 1 as throughout increases multiplicative decrease decreases throughput proportionally
R
R
equal bandwidth share
Connection 1 throughputConnect
ion 2
th
roughput
congestion avoidance additive increaseloss decrease window by factor of 2
congestion avoidance additive increaseloss decrease window by factor of 2
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-102
Fairness (more)
Fairness and UDP Multimedia apps
often do not use TCP do not want rate
throttled by congestion control
Instead use UDP pump audiovideo at
constant rate tolerate packet loss
Research area TCP friendly
Fairness and parallel TCP connections
nothing prevents app from opening parallel connections between 2 hosts
Web browsers do this Example link of rate R
supporting 9 cnctions new app asks for 1 TCP
gets rate R10 new app asks for 11 TCPs
gets R2
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-103
Delay modeling
Q How long does it take to receive an object from a Web server after sending a request
Ignoring congestion delay is influenced by
TCP connection establishment
data transmission delay slow start
Notation assumptions Assume one link between
client and server of rate R S MSS (bits) O object size (bits) no retransmissions (no loss no
corruption)
Window size First assume fixed congestion
window W segments Then dynamic window
modeling slow start
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-104
Fixed congestion window (1)
First caseWSR gt RTT + SR ACK for
first segment in window returns before windowrsquos worth of data sent
delay = 2RTT + OR
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-105
Fixed congestion window (2)
Second case WSR lt RTT + SR wait
for ACK after sending windowrsquos worth of data sent
K is number of windows that cover the object
For fixed W K=O(WS)
delay = 2RTT + OR+ (K-1)[SR + RTT - WSR]
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-106
TCP Delay Modeling Slow Start (1)
Now suppose window grows according to slow start
Will show that the delay for one object is
R
S
R
SRTTP
R
ORTTLatency P )12(2
where P is the number of times TCP idles at server
1min KQP
- where Q is the number of times the server idles if the object were of infinite size
- and K is the number of windows that cover the object
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-107
TCP Delay Modeling Slow Start (2)
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Examplebull OS = 15 segmentsbull K = 4 windowsbull Q = 2bull P = minK-1Q = 2
Server idles P=2 times
Delay componentsbull 2 RTT for connection estab and requestbull OR to transmit objectbull time server idles due to slow start
Server idles P = minK-1Q times
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-108
TCP Delay Modeling (3)
R
S
R
SRTTPRTT
R
O
R
SRTT
R
SRTT
R
O
idleTimeRTTR
O
P
kP
k
P
pp
)12(][2
]2[2
2delay
1
1
1
th window after the timeidle 2 1 kR
SRTT
R
S k
ementacknowledg receivesserver until
segment send tostartsserver whenfrom time RTTR
S
window kth the transmit totime2 1
R
Sk
RTT
initia te TCPconnection
requestobject
first w indow= S R
second w indow= 2S R
third w indow= 4S R
fourth w indow= 8S R
com pletetransm issionobject
delivered
tim e atc lient
tim e atserver
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-109
TCP Delay Modeling (4)
)1(log
)1(logmin
12min
222min
222min
2
2
110
110
S
OS
Okk
S
Ok
SOk
OSSSkK
k
k
k
Calculation of Q number of idles for infinite-size objectis similar (see HW)
Recall K = number of windows that cover object
How do we calculate K during slow start
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-110
HTTP Modeling Assume Web page consists of
1 base HTML page (of size O bits) M images (each of size O bits)
Non-persistent HTTP M+1 TCP connections in series Response time = (M+1)OR + (M+1)2RTT + sum of idle
times Persistent HTTP
2 RTT to request and receive base HTML file 1 RTT to request and receive M images Response time = (M+1)OR + 3RTT + sum of idle times
Non-persistent HTTP with X parallel connections Suppose MX integer 1 TCP connection for base file MX sets of parallel connections for images Response time = (M+1)OR + (MX + 1)2RTT + sum of
idle times
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-111
02468
101214161820
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)RTT = 100 msec O = 5 Kbytes M=10 and X=5
For low bandwidth connection amp response time dominated by transmission time
Persistent connections only give minor improvement over parallel connections
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-112
0
10
20
30
40
50
60
70
28Kbps
100Kbps
1Mbps
10Mbps
non-persistent
persistent
parallel non-persistent
HTTP Response time (in seconds)
RTT =1 sec O = 5 Kbytes M=10 and X=5
For larger RTT response time dominated by TCP establishment amp slow start delays Persistent connections now give important improvement particularly in high delaybandwidth networks
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
Transport Layer 3-113
Chapter 3 Summary principles behind transport
layer services multiplexing
demultiplexing reliable data transfer flow control congestion control
instantiation and implementation in the Internet UDP TCP
Next leaving the network
ldquoedgerdquo (application transport layers)
into the network ldquocorerdquo
