Slide Set 13: TCP

In this set....• TCP Connection Termination• TCP State Transition Diagram• Flow Control• How does TCP control its

sliding window ?

Connection Termination• Note that after

the server receives the FIN_WAIT_1, it may still have messages -- thus, connection not yet closed.

DATA/ACK

Active Close

FIN_WAIT1

FIN_WAIT2

CLOSE_WAIT

LAST_ACK

CLOSED

FIN, Seq Num = M

ACK= M+1

FIN, Seq Num = N

ACK N+1

TCP State Transitions

CLOSED

LISTEN

SYN_RCVD SYN_SENT

ESTABLISHED

CLOSE_WAIT

LAST_ACKCLOSING

TIME_WAIT

FIN_WAIT_2

FIN_WAIT_1

Passive open Close

Send/SYNSYN/SYN + ACK

SYN + ACK/ACK

SYN/SYN + ACK

ACK

Close/FIN

FIN/ACKClose/FIN

FIN/ACKACK + FIN/ACK

Timeout after twosegment lifetimesFIN/ACK

ACK

ACK

ACK

Close/FIN

Close

CLOSED

Active open/SYN

• Note: Retransmissions and Data Packet /ACK exchanges are not represented in the state transition diagram.• Final Wait time needed to ensure that the ACK is not lost.

• Simultaneous Connection Inceptions/ Terminations possible !

An Simpler View of the Client Side

CLOSED

TIME_WAIT

FIN_WAIT2

FIN_WAIT1

ESTABLISHED

SYN_SENT

SYN (Send)

Rcv. SYN+ACK, Send ACK

Send FINRcv. ACK, Send Nothing

Rcv. FIN, Send ACK

120 secs

Simpler Server Model

CLOSED

LAST ACK

CLOSE_WAIT

ESTABLISHED

SYN_RCVD

LISTEN

Passive OPEN, Create Listen socket

Rcv. SYN, Send SYN+ACK

RCV ACKRcv. FIN, Send ACK

Send FIN

Rcv. ACK, Send nothing

More about Termination

• Applications on both sides have to “independently” close their half of the connection.

• If one side does it, this means that this side has no data to send but it is willing to receive.

• In the TIME_WAIT state, a client waits for 2 X MSL (typically). During this time the socket cannot be reused.– If ACK is lost, a new FIN may be forthcoming and

this second FIN may be delayed.– Thus, if a new connection uses the same

connection i.e., the same port numbers, this FIN would initiate termination of later connection !

Sequence Numbers and ACKs

• How does one set Sequence numbers ?– Implicitly a number in every byte in the stream.– If we have 500000 bytes and each segment =

MSS and = 1000 bytes, SN of 1st segment = 0, SN of second segment = 1000 and so on.

• Note that ACK number is the number that the receiving host puts in -- indicates the “next” byte that it is expecting.

• ACKs are cumulative -- ACK up to all the bytes that are received.

Flow Control• Flow control ensures that the

sender does not send at a rate that causes the receiver buffer to overflow.

• Note that flow control is “end-to-end”.

Buffers at End Hosts• Sending buffer

– Maintains data sent but not ACKed

– Data written by application but not sent.

• Receive buffer– Data that arrives out of order– Data that is in correct order but

not yet read by application.

Sender Side View• For now, let us forget SN

wrap around.• Three pointers are

maintained, LastByteAcked, LastByteSent, LastByteWritten.

• LastByteAcked ≤ LastByteSent

• LastByteSent ≤ LastByteWritten

Sending application

LastByteWrittenTCP

LastByteSentLastByteAcked(a)

Receiver Side View• Three pointers

maintained again.• LastByteRead <

NextByteExpected• NextByteExpected

≤ LastByteRcvd + 1

Receiving application

LastByteReadTCP

LastByteRcvdNextByteExpected

How is Flow Control done?

• Receiver “advertises” a window size to the sender based on the buffer size allocated for the connection.– Remember the “Advertised Window” field in

the TCP header ?• Sender cannot have more than “Advertised

Window” bytes of unacknowledged data.• Remember -- buffers are of finite size - i.e.,

there is a MaxRcvBuffer and MaxSendBuffer.

Setting the Advertised Window

• On the TCP receive side, clearly, LastByteRcvd -LastByteRead ≤ MaxRcvBuffer

• Thus, it advertises the space left in the buffer i.e., Advertised Window =

MaxRcvBuffer - (LastByteRcvd -LastByteRead)

• As more data arrives i.e., more received bytes than read bytes, LastByteRcvd increases and hence, Advertised Window reduces.

Sender Side Response• At the sender side, the TCP sender should ensure

that: LastByteSent - LastByteAcked ≤ Advertised Window.Thus, we define what is called the “Effective Window”

which limits the amount of data that TCP can send :Effective Window =

Advertised Window - (LastByteSent - LastByteAcked)• Note here that ACKing does not imply that the

process has read the data!• In order to prevent the overflow of the Send Side

buffer: LastByteWritten - LastByteAcked ≤ MaxSendBuffer

– If application tries to write more, TCP blocks.

Persistency• What does one do when Advertised

Window = 0 ?• The sender will persist by sending 1

segment.• Note that this segment may not be

accepted by the receiver initially.• But at some point, it would trigger a

response that may contain a new Advertised window.

Sequence Number Wraparound

• TCP Sequence Number is 32 bits long.• Advertised Window is 16 bits. Since 232

>> 2 X 216, it is almost impossible for the same sequence number to exist twice -- wrap around unlikely.

• In addition, MSL = 120 seconds to make sure that there is no wrap-around.

• Time-stamps may also be used.

How long should the time-out be ?

• Remember, TCP has to ensure reliability.

• So bytes need to be resent if there is no “timely” acknowledgement.

• How long should the sender wait ?• It should be adaptive -- fluctuation in

load on the network.– If too short, false time-outs– If too long, then poor rate of sending.

• Depends on round trip time estimation

RTT Estimation• Simple mechanism could be:

– Send packet, record time T1– When ACK is returned, record time = T2.– T2 -T1 = Estimated RTT.

• To avoid fluctuations, estimated RTT is a weighted average of previous time and current sample

Estimated RTT = (1-) Estimated RTT + SampleRTT• In the original specification = 0.125• The Time out is set to 2 * RTT.

A problemSender Receiver

Original transmission

ACK

Retransmission

Sender ReceiverOriginal transmission

ACKRetransmission

(a) (b)

• When there are retransmissions, it is unclear if the ACK is for the original transmission or for a retransmission.

• How do we overcome this ?

The Karn Patridge Algorithm• Take SampleRTT measurements only for

segments that have been sent once !• This eliminates the possibility that

wrong RTT estimates are factored into the estimation.

• Another change -- Each time TCP retransmits, it sets the next timeout to 2 X Last timeout --> This is called the Exponential Back-off (primarily for avoiding congestion).

Jacobson Karels Algorithm

• The main problem with the Karn/Patridge scheme is that it does not take into account the variation between RTT samples.

• New method proposed -- the Jacobson Karels Algorithm.

• Estimated RTT = Estimated RTT + X Difference– Difference = Sample RTT - Estimated RTT

• Deviation = Deviation + (|Difference| - deviation)

• Timeout = Estimated RTT + deviation.• The values of and are computed based on

experience -- Typically = 1 and = 4.

Silly Window Syndrome• Suppose a MSS worth of data is collected and

advertised window is MSS/2.• What should the sender do ? -- transmit half

full segments or wait to send a full MSS when window opens ?

• Early implementations were aggressive -- transmit MSS/2.

• Aggressively doing this, would consistently result in small segment sizes -- called the Silly Window Syndrome.

Issues ..• We cannot eliminate the possibility of

small segments being sent.• However, we can introduce methods to

coalesce small chunks.– Delaying ACKs -- receiver does not send

ACKs as soon as it receives segments.• How long to delay ? Not very clear.

– Ultimate solution falls to the sender -- when should I transmit ?

Nagle’s Algorithm• If sender waits too long --> bad for interactive

connections.• If it does not wait long enough -- silly window

syndrome.• How to solve ?• Timer -- clock based

– If both available data and Window ≥ MSS, send full segment.

– Else, if there is unACKed data in flight, buffer new data until ACK returns.

– Else, send new data now.• Note -- Socket interface allows some applications

to turn off Nagle’s algorithm by setting the TCP-NODELAY option.

TCP Throughput• If a connection sends W segments

of MSS size (in bytes) in RTT seconds, then, the throughput is defined as : W *MSS / RTT bytes/second.

• If there is a link of capacity R, if there are K connections, what we want is for each TCP connection to have a throughput = R/K.

Throughput (cont)• If a TCP session goes through n links

and if link j has a rate Rj and is shared by Kj connections, ideally the throughput = Rj/Kj.

• Thus, a connection’s end-to-end rate is r = min (R1/K1, R2/K2, .. Rj/Kj... Rn/Kn).

• In reality not so simple, some connections may be unable to use their share -- so the share may be higher.

Where are we ?• We have covered Chapter 5 --

Sections 5.1 and 5.2.• Whatever I left out from

Section 5.2 is for self-study.

Where are we headed ?• We will look at Congestion

Control with TCP next time.– Chapter 6 -- Sections 6.3 and 6.4.