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 Message Formatting and Data Transfer

TCP Message Formatting and Data Transfer 1 2 3 4 TCP Checksum Calculation and the TCP "Pseudo Header"

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.

TCP Message Formatting and Data Transfer 1 2 3 4 TCP Checksum Calculation and the TCP "Pseudo Header"