Transmission Control Protocol
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Each TCP/IP application typically chooses to use either TCP or UDP based on the application’s
requirements. For example, TCP provides error recovery, but to do so, it consumes
more bandwidth and uses more processing cycles. UDP does not perform error recovery,
but it takes less bandwidth and uses fewer processing cycles. Regardless of which of these
two TCP/IP transport layer protocols the application chooses to use, you should understand
the basics of how each of these transport layer protocols works.
TCP, as defined in Request For Comments (RFC) 793, accomplishes the functions listed in
Table 5-2 through mechanisms at the endpoint computers. TCP relies on IP for end-to-end
delivery of the data, including routing issues. In other words, TCP performs only part of the
functions necessary to deliver the data between applications. Also, the role that it plays is
directed toward providing services for the applications that sit at the endpoint computers.
Regardless of whether two computers are on the same Ethernet, or are separated by the
entire Internet, TCP performs its functions the same way.
Figure 5-1 shows the fields in the TCP header. Although you don’t need to memorize the
names of the fields or their locations, the rest of this section refers to several of the fields,
so the entire header is included here for reference.
Figure 5-1 TCP Header Fields
The message created by TCP that begins with the TCP header, followed by any application
data, is called a TCP segment. Alternatively, the more generic term Layer 4 PDU, or
L4PDU, can also be used.