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);

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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
  

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If host always uses the same ISN

TCP Connection Closing

“Graceful Close”

TCP Tree-way handshake

• Establish connection
• Transfer data
• Close the connection