Java Network Programming, 4th Edition

Chapter 12. UDP

Previous chapters discussed network applications that run on top of the TCP transport layer protocol. TCP is designed for reliable transmission of data. If data is lost or damaged in transmission, TCP ensures that the data is resent. If packets of data arrive out of order, TCP puts them back in the correct order. If the data is coming too fast for the connection, TCP throttles the speed back so that packets won’t be lost. A program never needs to worry about receiving data that is out of order or incorrect. However, this reliability comes at a price. That price is speed. Establishing and tearing down TCP connections can take a fair amount of time, particularly for protocols such as HTTP, which tend to require many short transmissions.

The User Datagram Protocol (UDP) is an alternative transport layer protocol for sending data over IP that is very quick, but not reliable. When you send UDP data, you have no way of knowing whether it arrived, much less whether different pieces of data arrived in the order in which you sent them. However, the pieces that do arrive generally arrive quickly.

The UDP Protocol

The obvious question to ask is why anyone would ever use an unreliable protocol. Surely, if you have data worth sending, you care about whether the data arrives correctly? Clearly, UDP isn’t a good match for applications like FTP that require reliable transmission of data over potentially unreliable networks. However, there are many kinds of applications in which raw ...