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Table Of Contents  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)

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TCP Connection Termination
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TCP Message (Segment) Format
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TCP Message Formatting and Data Transfer

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.

Background Information: This section assumes that you are already familiar with TCP concepts such as sequence numbers, segments, and the basics of the TCP sliding window mechanism. If you are not, I’d strongly recommend reading the TCP fundamentals section before proceeding here.


Quick navigation to subsections and regular topics in this section



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Version 3.0 - Version Date: September 20, 2005

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