The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transmission Control Protocol (TCP)                      9  TCP Basic Operation: Connection Establishment, Management and Termination

TCP Connection Establishment Process: The "Three-Way Handshake" 1 2 3 TCP Connection Management and Problem Handling, the Connection Reset Function, and TCP "Keepalives"

TCP Parameter Exchange

In addition to initial sequence numbers, SYN messages also are designed to convey important parameters about how the connection should operate. TCP includes a flexible scheme for carrying these parameters, in the form of a variable-length Options field in the TCP segment format that can be expanded to carry multiple parameters. Only a single parameter is defined in TCP 793 to be exchanged during connection setup: Maximum Segment Size (MSS). The significance of this parameter is explained in the TCP data transfer section.

Each device sends the other the MSS that it wants to use for the connection, if it wishes to use a non-default value. When receiving the SYN, the server records the MSS value that the client sent, and will never send a segment larger than that value to the client. The client does the same for the server. The client and server MSS values are independent, so a connection can be established where the client can receive larger segments than the server or vice-versa.

Later RFCs have defined additional parameters that may be exchanged during connection setup. Some of these include:

• Window Scale Factor: Allows a pair of devices to specify larger window sizes than would normally be possible given the 16-bit size of the TCP Window field.
  
  
• Selective Acknowledgment Permitted: Allows a pair of devices to use the optional selective acknowledgment feature to allow only certain lost segments to be retransmitted.
  
  
• Alternate Checksum Method: Lets devices specify an alternative method of performing checksums than the standard TCP mechanism.

TCP Connection Establishment Process: The "Three-Way Handshake" 1 2 3 TCP Connection Management and Problem Handling, the Connection Reset Function, and TCP "Keepalives"