Generally, if your application requires moving bulk data to far away places, you should be using TCP. For many other applications, TCP is also appropriate just because its reliability makes it easy to use. TCP is so easy to use because it provides so much functionality. The price paid for ease of use is performance. TCP does a good job of moving bulk data from one side of the planet to another. For data which will only be sent across one physical network (one LAN), or for data sent in small pieces, TCP doesn't perform so well. A much more specific set of functionality can always provide better performance than the most general set can. For performance-critical applications which don't move bulk data, UDP is usually the protocol of choice. Unfortunately, since UDP doesn't provide reliability, the application protocol must. This means a much more complicated application protocol. It isn't nearly as bad as it sounds, though, and Stevens (1990) offers two examples.
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