Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 192
Transmission Control Protocol (TCP)

TCP is also a transport-layer protocol. Unlike the UDP protocol, the purpose of the transport-layer software TCP is to allow data to be reliably exchanged with another station on the network. It, too, provides the demultiplexes of port numbers to identify an application in the host, but also provides reliable transport of data, including many different options that may or may not be sent by the originating station. A communications facility needs to be able to reliably transfer data between two points. Imagine setting up a communications system that only allowed for unreliable data transfer—the post office transfers most of its mail in this manner. When you mail a letter, you have no idea if it really reached its destination unless you make the effort to check. Should make you a little nervous on that critical data. This is further exemplified by a packet switch network in which the same communication channel is used by multiple entities all vying for the same path, and each header contains its own directional information.

TCP/IP hosts originally were connected via telephone lines (commonly known as serial lines). This mode of communication was not the same in the early 1970s as it is today. The lines were noisy and were not conditioned to handle high-speed data. Therefore, the TCP protocol has strict error-detection algorithms built in to ensure the integrity of the data. The following paragraphs explain the TCP protocol and show how its strictness in its structure ensures the integrity of the data.