The very fact that the TCP/IP protocol suite bears the name of the Internet Protocol and the Transmission Control Protocol suggests that these are the two key protocols in the suite: IP at the network layer and TCP at the transport layer. It's no wonder, therefore, that many people don't even realize that there is a second transport layer protocol in TCP/IP. Like a shy younger brother, the User Datagram Protocol (UDP) sits in the shadows while TCP gets the glory. Its fancier sibling deserves much of this limelight, since TCP is arguably the more important of the two transport layer protocol. However, UDP itself fills a critical niche in the TCP/IP protocol suite, allowing many applications to work at their best when using TCP would be less than ideal.

In this section I describe the simpler and lesser-known TCP/IP transport protocol: the User Datagram Protocol (UDP). I begin with an overview of the protocol and a discussion of its history and standards. I outline how UDP operates, and describe the format used for UDP messages. I conclude with a discussion of what sorts of applications use UDP, and the well-known or registered ports that are assigned to them.