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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)

TCP Connection Termination 1 TCP Message (Segment) Format

The previous section described how two devices using the Transmission Control Protocol establish a TCP connection, as well as how that connection is managed and eventually terminated. While connections are a key part of how TCP works, they are really a means to the ultimate end of the protocol: sending data. Employing the TCP sliding window mechanism, a special segment format and several features, TCP devices are able to package and send data over the connection, enabling applications to communicate.

In this section, I describe the actual mechanism by which TCP messages are formatted and data is transferred between devices. I begin with a look at the important TCP segment format, which describes the fields in each TCP message and how they are used. I provide a description of the method used to calculate the checksum in TCP (as well as UDP) messages, and the reason why a special “pseudo header” is used. I discuss the Maximum Segment Size (MSS) parameter and its significance. I then talk about exactly how the sliding window mechanism is used to transfer and acknowledge data. I conclude with a description of two special data transfer features: the “push” feature for immediate data transfer, and the “urgent” feature for priority data transfer.

TCP Connection Termination 1 TCP Message (Segment) Format