TCP Initiation and Header field functions

PSH FLAG - Telnet application 

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 Header Field Functions

The price we pay for this flexibility is that the TCP header is large: 20 bytes for regular segments and more for those carrying options. This is one of the reasons why some protocols prefer to use UDP if they don't need TCP's features. The TCP header fields are used for the following general purposes:

• Process Addressing: The processes on the source and destination devices are identified using port numbers.
  
• Sliding Window System Implementation: Sequence numbers, acknowledgment numbers and window size fields implement the TCP sliding window system.
  
• Control Bits and Fields: Special bits that implement various control functions, and fields that carry pointers and other data needed for them.
  
• Carrying Data: The Data field carries the actual bytes of data being sent between devices.
  
• Miscellaneous Functions: A checksum for data protection and options for connection setup.