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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

TCP/IP Transport Layer Protocols 1 TCP and UDP Overview and Role In TCP/IP

TCP/IP is the most important internetworking protocol suite in the world; it is the basis for the Internet, and the “language” spoken by the vast majority of the world's networked computers. TCP/IP includes a large set of protocols that operate at the network layer and above. The suite as a whole is anchored at layer three by the Internet Protocol (IP), which many people consider the single most important protocol in the world of networking.

Of course, there's a bit of architectural distance between the network layer and the applications that run at the layers well above. While IP is the protocol that performs the bulk of the functions needed to make an internetwork, it does not include many capabilities that are needed by applications. In TCP/IP these tasks are performed by a pair of protocols that operate at the transport layer: the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).

Of these two, TCP gets by far the most attention. It is the transport layer protocol that is most often associated with TCP/IP, and, well, its name is right there, “up in lights”. It is also the transport protocol used for many of the Internet's most popular applications, while UDP gets second billing. However, TCP and UDP are really peers that play the same role in TCP/IP. They function very differently and provide different benefits and drawbacks to the applications that use them, which makes them both important to the protocol suite as a whole. The two protocols also have certain areas of similarity, which makes it most efficient that I describe them in the same overall section, highlighting where they share characteristics and methods of operation, as well as where they diverge.

In this section I provide a detailed examination of the two TCP/IP transport layer protocols: the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). I begin with a quick overview of the role of these two protocols in the TCP/IP protocol suite, and a discussion of why they are both important. I describe the method that both protocols employ for addressing, using transport-layer ports and sockets. I then have two detailed sections for each of UDP and TCP. I conclude with a summary quick-glance comparison of the two.

Incidentally, I describe UDP before TCP for a simple reason: it is simpler. UDP operates more like a classical message-based protocol, and in fact is more similar to IP itself than is TCP. This is the same reason why the section on TCP is much larger than that covering UDP: TCP much more complex and does a great deal more than UDP.

TCP/IP Transport Layer Protocols 1 TCP and UDP Overview and Role In TCP/IP