Internet Core Protocols: The Definitive Guide

Chapter 6. The User Datagram Protocol

Summary	The User Datagram Protocol provides a low-overhead transport service for application protocols that do not need (or cannot use) the connection-oriented services offered by TCP. UDP is most often used with applications that make heavy use of broadcasts or multicasts, as well as applications that need fast turnaround times on lookups and queries.
Protocol ID	17
Relevant STDs	2 (http://www.iana.org/); 3 (includes RFCs 1122 and 1123); 6 (RFC 768, republished)
Relevant RFCs	768 (User Datagram Protocol); 1122 (Host Network Requirements)

There are two standard transport protocols that applications use to communicate with each other on an IP network. These are the User Datagram Protocol (UDP), which provides a lightweight and unreliable transport service, and the Transmission Control Protocol (TCP), which provides a reliable and controlled transport service.

The majority of Internet applications use TCP, since its built-in reliability and flow control services ensure that data does not get lost or corrupted. However, many applications that do not require the overhead found in TCP—or that cannot use TCP because the application has to use broadcasts or multicasts—will use UDP instead. UDP is more appropriate for any application that has to issue frequent update messages or that does not require every message to get delivered.