TCP: Transmission Control Protocol

• Overview• Connection set-up and termination• Interactive• Bulk transfer• Timers• Improvements

TCP: Overview• Connection oriented, byte stream service• Full or half duplex service• Reliability (ARQ)

– Sliding window with variable sized window– Stream is sent in segments (IP datagrams)– SN for bytes– Receiver buffer reorders bytes– Checksum on header and data– Discards duplicate data– Flow control

TCP: Overview

IP Header TCP Header TCP Data

TCP segment

Source port # Destination port #Sequence #

Acknowledgement #HL reserv flags Window size

Checksum Urgent pointerOptions if any

65535-20-20=65495

TCP: Flags• URG: The urgent pointer is used• ACK: The acknowledgement number is valid• PSH: The receiver should pass this data to the

application as soon as possible• RST: Reset the connection• SYN: Synchronize sequence numbers to initiate

a connection. • FIN: The sender is finished sending data

TCP: Set-Up Syn=1 Ack = 0

A:SYN, MSS, SN=ISN

B: SYN, MSS, SN=ISNSyn1 ack 1

B: ACK

A: ACK

A B

Fullduplex

TCP: Termination

A:FIN

B: ack of FIN

B: FIN

A: ack of FIN

A B

Bothsidesclose

TCP windows

1 2 3 4 5 6 7 8 9 10 11

sent andacked

sent butnot acked

can sendasap

min{offered window from receiver,cwnd}

usablewindow

Actually,

Transmission window: cwnd

1

cwnd1

ack1

2 23 ack2

ack34 4

567

Doublingevery RT!

TCP Timeout and Retransmission

Each packet has a retransmission timer• It is initialized by the retransmission time out

(RTO) value• When the timer expires, a time out occurs and

the data is retransmitted• If a retransmission fails then the time-out doubles

i.e., exponential backoff.

It’s important to find a good RTO value

TCP• Slow start: cwnd =1

cwnd exponentially increasing• Congestion avoidance: cwnd reaches ssthresh,

cwnd linearly increasing

TCP: Tahoe and Reno

Tahoe: slow start + congestion avoidanceReno: fast retransmit + fast recovery