Information Network 1 Transport layer: TCP - iplab.naist.jp · " Congestion control Next lecture P Q (P, Q) ... Unique identification of TCP connection " (source IP, source port,

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Information Network 1 Transport layer: TCP

Youki Kadobayashi NAIST�

Copyright(C)2015 Youki Kadobayashi. All rights reserved.

Transport layer: a birds-eye view�

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H HR�R�R� R�

IP�IP� IP� IP�IP�

Transport�

Routers don’t maintain per-host state�

Hosts maintain state for each transport-layer endpoint�

Application Presentation

Session Transport Network

Data Link Physical

Application Presentation

Session Transport Network

Data Link Physical

Copyright(C)2015 Youki Kadobayashi. All rights reserved. 3

Functions provided by the transport layer

!  Communication between processes "  Identify process "  Identify inter-process channel

!  Interface for upper layer

" Connection-oriented (virtual circuit) " Connectionless (datagram)

!  Competition and arbitration of network resource

" Flow control " Congestion control

Next lecture

P Q

(P, Q)�


Transport protocols in the Internet protocol suite�

!  TCP (RFC793) "  Transmission Control Protocol

" Connection-oriented " Multiple functions

!  for reliability

!  SCTP (RFC4960)

!  UDP (RFC768) "  User Datagram Protocol

" Connectionless "  IP & Process identification�

!  Packets can be lost

!  DCCP (RFC4340)

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For self study

Advanced topic


Identifying process and connection in TCP

!  Unique identification of process " (IP, port)

!  Unique identification of TCP connection "  (source IP, source port, destination IP, destination port)

1040

80

22

2137

(203.178.136.36, 22)

163.221.52.100 203.178.136.36

(163.221.52.100, 1040)

connection

connection

process


TCP service model (1)

!  Connection-oriented !  Virtual Circuit

" Looks like a circuit, but virtual: no physical wires between specific endpoints

!  Adapts to speed " Adapts to speed of intermediate networks as well as the

speed of endpoints


Establishing a TCP connection

!  3-way handshake !  SYN, SYN-ACK, ACK !  Ensure full-duplex communication

16bit source port 16bit destination port

32bit sequence number

32bit acknowledgment number

4bit hlen 16bit window size

16bit TCP checksum 16bit urgent pointer

reserved flags

URG ACK PSH RST SYN FIN

SYN

SYN-ACK

ACK


Establishing TCP connection: a concrete example with Wireshark !  # tshark -i en0 -n -f 'port 80' !  tcpdump: listening on de0 !  Capturing on en0 !  0.164720 10.0.1.148 -> 163.221.8.221 TCP 63428 > 80 [SYN] Seq=0

Win=65535 Len=0 MSS=1460 WS=16 TSval=1135377929 TSecr=0 SACK_PERM=1

!  0.195154 163.221.8.221 -> 10.0.1.148 TCP 80 > 63428 [SYN, ACK] Seq=0 Ack=1 Win=50137 Len=0 TSval=190559467 TSecr=1135377929 MSS=1460 WS=8 SACK_PERM=1

!  0.195322 10.0.1.148 -> 163.221.8.221 TCP 63428 > 80 [ACK] Seq=1 Ack=1 Win=131760 Len=0 TSval=1135377958 TSecr=190559467

Reply with Sequence number + 1 as an ACK implies acknowledgment�


Meanings of Wireshark output

!  time source IP -> dest IP TCP source port > dest port [ flags ] Seq=n Ack=n …

!  0.195154 163.221.8.221 -> 10.0.1.148 TCP 80 > 63428 [SYN, ACK] Seq=0 Ack=1 Win=50137 Len=0 TSval=190559467 TSecr=1135377929 MSS=1460 WS=8 SACK_PERM=1


Virtual Circuit(2):TCP connection release

FIN

Ack of FIN

FIN

Ack of FIN

close

close

5.749366 163.221.8.221 -> 10.0.1.148 TCP 80 > 63428 [FIN, ACK] Seq=659 Ack=2449 Win=401096 Len=0 TSval=190560021 TSecr=1135378482 5.749464 10.0.1.148 -> 163.221.8.221 TCP 63428 > 80 [ACK] Seq=2449 Ack=660 Win=131104 Len=0 TSval=1135383482 TSecr=190560021 5.749650 10.0.1.148 -> 163.221.8.221 TCP 63428 > 80 [FIN, ACK] Seq=2449 Ack=660 Win=131104 Len=0 TSval=1135383482 TSecr=190560021 5.765279 163.221.8.221 -> 10.0.1.148 TCP 80 > 63428 [ACK] Seq=660 Ack=2450 Win=401096 Len=0 TSval=190560024 TSecr=1135383482


TCP connection reset

!  RST " Abortive release " Nonexistent port (e.g., dead process)

telnet to sh.naist.jp, port 80 will result in connection reset: !  0.000000 10.0.1.148 -> 163.221.10.10 TCP 63436 > 80 [SYN] Seq=0 Win=65535

Len=0 MSS=1460 WS=16 TSval=1137289324 TSecr=0 SACK_PERM=1 !  0.018841 163.221.10.10 -> 10.0.1.148 TCP 80 > 63436 [RST, ACK] Seq=1

Ack=1 Win=0 Len=0

If you kill “Dropbox”: !  55.992214 10.0.1.148 -> 108.160.163.51 TCP 62991 > 80 [RST] Seq=310 Win=0

Len=0


Virtual Circuit Summary: TCP state transition diagram�

State transition: trigger / response�


$ netstat!Active Internet connections!Proto Recv-Q Send-Q Local Address Foreign Address (state)!tcp4 0 0 45.1.20.101.57456 74.125.235.138.http SYN_SENT!tcp4 0 0 45.1.20.101.57455 ey-in-f101.1e100.http ESTABLISHED!tcp4 0 0 45.1.20.101.57454 74.125.235.148.http ESTABLISHED!�

Troubleshooting TCP with state machine�

!  netstat�

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SYN sent, awaiting SYN+ACK response

Many other open-source tools are available: lsof, trpt, tcptraceroute, tcptrace, tcpflow, etc. Try some of these tools in the provided VM image.�


Hands on: observe connection setup/release

In the provided virtual machine, 1.  Observe connection setup/release;

" e.g., by using browser within VM 2.  Observe connection reset by terminating program;

" e.g., by skill firefox 3.  Observe connection reset by connecting to

nonexistent port on the working machine. " e.g., by telnet sh.naist.jp 80

!  Try many tools: wireshark, tshark, tshark -V


Buffered transfer: adapts to varying speed of endpoints, as well as intermediate networks

TCP connection

send buffer

recv buffer

send buffer

recv buffer

Process

OS kernel

block/unblock Read() Write()

Process

Read() Write()

OS kernel


TCP service model (2)

!  Byte-stream service " Upper layer can’t see boundaries between packets " no boundary: structuring (framing) is needed at upper layer

!  Full duplex "  independent two streams in single connection

!  Reliable " masks packet reordering, duplication, discard and bit error

O L L E H O L L E H TCP being viewed as byte-stream service OK OK


How does TCP implement reliable stream service?

!  ACK: Acknowledgment " Active acknowledgment

!  Explicitly acknowledge the receipt of packet " Duplicate ACK

!  Implicitly communicate the packet loss information

!  Timeout and Retransmission " Whenever sender doesn’t receive ACK after fixed time,

a “Timeout” event is triggered # Retransmission: assuming that transmission has failed

!  Exponential back-off: 3, 6, 12, …


ACK: Acknowledgment

Packets in transit

Sent but unacknowledged Sent and acknowledged

User data arrives

Sender

Receiver

Nara Institute of Science and Technology

Nara Insti

10

16


Piggybacking: Exploiting full-duplex channel

Packets in transit


User data arrives

Sender Receiver

Receiver Sender


Nara Insti

Graduate School of Information Science

Graduate S

User data arrives

Note: rough sketch �



Duplicate ACK: implicit communication of packet loss

Packets in transit


User data arrives

Sender

Receiver


Nara Institute o

Packet loss

10

16

16


From byte-stream to packets: TCP header

IP Header

TCP Header TCP data

TCP segment

16bit source port 16bit destination port

32bit sequence number

32bit acknowledgment number

4bit hlen 16bit window size

16bit TCP checksum 16bit urgent pointer

(options)

(TCP data)

reserved flags

20 octets

Chop appropriate length from byte-stream buffer & add TCP header�


Nagle algorithm

!  Q. If you added 20byte+20byte size header to 1byte data, isn’t that overhead big?

!  Nagle algorithm (RFC896) " There is only one small segment which is unacknowledged

in the network. "  In case of short RTT:

$ acceptable overhead, as LAN bandwidth is abundant $ send packets with small buffering

"  In case of long RTT $ reduce overhead, due to WAN bandwidth constraints


Hands on: observe packet loss & retransmission

Within provided virtual machine, 1.  Observe duplicate ACK;

" Emulate lossy network with: " # tc qdisc change dev eth0 root netem loss 10%

2.  Observe the effect of Nagle algorithm; " Emulate satellite network with: " # tc qdisc add dev eth0 root netem delay 500ms


Questions?


Summary thus far�

!  Transport layer !  The transport protocol on the internet – TCP !  TCP: service model, and features

!  Efficiency: ACK, piggybacking, Nagle algorithm !  TCP connection establishment and release !  Diagnosis: tools + state machine knowledge


Transport protocol challenges�

!  With a number of unknown parameters: " Number of active communications " Bottleneck bandwidth " Error rate

!  how can we accommodate as much communications as possible, without collapsing network itself?�

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1� 1�

n� k�

…�

…�

Key ideas: probing, estimation, self-policing, macro-level stability�


Flow control and congestion control: Competition and Arbitration of Network Resource�

!  Flow control " Absorb difference of transmission rate " Recovery from sequence error " Recovery from duplication, packets drop and bit

error !  Congestion control

" Sharing the bandwidth while suppressing the congestion

" Fair sharing of bandwidth�


Characteristics of TCP Flow Control�

!  End to end " No global assignment of resource " Estimate available bandwidth at individual hosts " Routers do not explicitly allocate resource

!  Implicit signaling through packet drops

!  Scalable " Host-based autonomous method has better scalability,

because it doesn’t need state management inside network. !  Autonomous $ Less state management $ Scalable


TCP: key characteristics�

!  Very simple algorithm " Macroscopic self-stabilization

!  TCP doesn't assume the presence of greedy nodes. " Elimination of global control system " Reject the idea of intermediate policing system

!  Modest performance across almost all data-links " As opposed to optimal performance in specific condition

!  Stepwise improvements over 30 years�


Flow Control on TCP�

!  Control available bandwidth " Sliding window

!  Using sequence number, not packet number " Window size

!  Control transmission interval of packets " ACK clocking

!  Miscellaneous " Error detection with TCP checksum " Packet drop detection with duplicate ACK, timeout


Sliding window

Packets in flight


User data arrives

Sender

Receiver


Nara Insti

10

16

Window size Sequence number


Congestion Control on TCP�

!  Fair-share model: fair distribution of bandwidth End to end

!  Increase and decrease of window size " additive increase " multiplicative decrease

"  AIMD is known to be self-stabilizing (R. Jain, et al., “Analysis of the increase and decrease algorithms for congestion avoidance in computer networks”, 1989)

!  Switch increasing strategy of window size !  Detect congestion through packet drops


Increasing Window size�

!  TCP switches increasing strategy of congestion window (cwnd) by slow start threshold (ssthresh)

(Outline of the algorithm) !  On receiving an ACK:

if (cwnd < ssthresh) { /* slow start: exponential increase */ cwnd += 1; } else { /* congestion avoidance: additive increase */ cwnd += 1 / cwnd; }


Increasing Window size�

!  Slow start " exponential increase

!  Congestion avoidance " additive increase

!  Effectiveness: see V. Jacobson, “Congestion Avoidance and Control”, SIGCOMM’88.�

Source: TCP/IP Illustrated, Vol.1�


Decreasing Window size�

(Outline of algorithm ) !  On detecting packet drop:

ssthresh = cwnd / 2; if (timeout) { cwnd = 1; }



Questions?


Packet drop

!  Detection with 2 methods: " Duplicate ACK " Retransmission Time Out (RTO)

!  How do I judge the time out? → RTO ~ RTT Estimator


RTO Calculation

!  Err = M – A A <- A + gErr D <- D + h(|Err| - D) RTO = A + 4D

" A: smoothed RTT " D: smoothed mean deviation " g: gain for the average (1/8) " h: gain for the deviation (1/4)



RTO calculation�

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Source: A Quick Tour Around TCP, http://web.eecs.utk.edu/~dunigan/tcptour/


Summary�

!  Transport layer !  The transport protocol on the internet – TCP !  TCP: service model, and features

!  Efficiency: ACK, piggybacking, Nagle algorithm !  TCP connection establishment and release !  Flow control and congestion control in TCP

Documents

Information Network 1 Transport layer: TCP - iplab.naist.jp · " Congestion control Next lecture P Q (P, Q) ... Unique identification of TCP connection " (source IP, source port,