TCP Three-way Handshake

2024-10-04 20:23:24

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

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