Internet Core Protocols: The Definitive Guide

The TCP Header

TCP segments consist of header and body parts, just like IP datagrams. The body part contains whatever data was provided by the application that generated it, while the header contains the fields that tell the destination TCP software what to do with the data.

A TCP segment is made up of at least ten fields. Unlike the other core protocols, some TCP segments do not contain data. In addition, there are a variety of supplementary header fields that may show up as “options” in the header. The total size of the segment will vary according to the size of the data and any options that may be in use.

Table 7.2 lists all of the mandatory fields in a TCP header, along with their size (in bits) and some usage notes. For more detailed descriptions of these fields, refer to the individual sections throughout this chapter.

Table 7.2. The Fields in a TCP Segment

Field	Bits	Usage Notes
Source Port	16	Identifies the 16-bit port number in use by the application that is sending the data.
Destination Port	16	Identifies the 16-bit target port number of the application that is to receive this data.
Sequence Identifier	32	Each byte of data sent across a virtual circuit is assigned a somewhat unique number. The Sequence Identifier field is used to identify the number associated with the first byte of data in this segment.
Acknowledgment Identifier	32	Each byte of data sent across a virtual circuit is assigned a somewhat unique number. The Acknowledgment Identifier field is used to identify the next byte ...