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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)

UDP Common Applications and Server Port Assignments 1 TCP Overview, Functions and Characteristics

In my description of the Internet Protocol, I call it the “workhorse” of the TCP/IP protocol suite. IP is, in fact, the foundation upon which the other protocols of the suite are built. Well, if IP is the “workhorse”, then the “worker” that rides on that horse would have to be the TCP/IP Transmission Control Protocol (TCP). Like horse and rider, TCP and IP form a team that work together to make it possible for applications to easily run over an internetwork.

TCP and IP share the marquee in the name of the suite, and are very important complements to each other. IP concerns itself with classic network-layer tasks such as addressing, datagram packaging and routing, which provide basic internetworking capabilities. TCP provides to applications a method of easily making use of IP, while filling in the capabilities that IP lacks. It allows TCP/IP devices to establish and manage connections and send data reliably, and takes care of handling all the potential “gotchas” that can occur during transmission so each application doesn't need to worry about such matters. To applications, TCP could thus be considered almost like a nice user interface to the fairly rudimentary capabilities of IP.

This section provides a comprehensive description of the concepts, characteristics and functions of the Transmission Control Protocol (TCP). TCP is a rather complex protocol that includes a number of sophisticated functions to ensure that applications function in the potentially difficult environment of a large internetwork. It's also, as I said above, a very important part of the TCP/IP protocol suite. For this reason, the section is rather large, and has been divided into five subsections.

The first subsection provides an overview of TCP, describing its history, what it does and how it works. The second paints some important background information that is necessary to understanding how TCP operates. This is done by explaining key concepts such as streams and segments, sliding windows and TCP ports and connections. The third subsection describes the process used by TCP to establish, maintain and terminate sessions. The fourth describes TCP messages, and how they are formatted and transferred. Finally, the last subsection shows how TCP provides reliability and other important transport layer functions to applications, such as flow control, retransmission of lost data and congestion avoidance.

UDP Common Applications and Server Port Assignments 1 TCP Overview, Functions and Characteristics