TCP Three-way Handshake

TCP Connection Management

Out-of-Order and Duplication(复制) Problem

Old segment from previous connections to come

Use long sequence number (32-bits)

Establish randomly selected initial sequence number (ISN)

Accept sequence numbers from a small window(To allow the network to clear old segments from the network)

Enforces a time-out period at end of connection, called maximum segment lifetime (MSL), usually 2 minutes but round-trip delay dependent

TCP Header – Seq and Ack

Sequence Number

Byte count

First byte in segment

32 bits long

0 <= SN <= 2^32-1

Initial sequence number (ISN) selected during connection setup (SYN flag bit is 1);

Acknowledgement Number

SN of next byte expected by receiver

Acknowledges that all prior bytes in stream have been received correctly

Valid if ACK flag is set

TCP Header – Control bits

Control

6 bits

URG: urgent pointer flag

Urgent message end = SN + urgent pointer

ACK: ACK packet flag

PSH: override TCP buffering

RST: reset connection

Upon receipt of RST, connection is terminated and application layer notified(被通知)

SYN: establish connection

FIN: close connection

TCP Connection Management

“Three-way Handshake”

ISN’s protect against segments from prior connections



If host always uses the same ISN

TCP Connection Closing

“Graceful Close”

TCP Tree-way handshake

Establish connection

Transfer data

Close the connection