Internet Core Protocols: The Definitive Guide

The User Datagram Protocol

Not every application requires guaranteed delivery, and these applications typically use UDP for transport services. Unlike TCP, UDP sends only the data it has received from the application, and makes no pretense towards guaranteed delivery or flow control or anything else. As such, UDP is much like IP, but is the protocol that applications use to communicate with each other, rather than using IP directly.

UDP is much like a postcard. If you were travelling around a foreign country, you might send postcards to friends and family from the different cities that you visit, informing them of recent events. You wouldn’t worry about the postcards getting delivered quickly, or even if they got lost entirely, since you’ll probably send more postcards from the next town anyway. You wouldn’t necessarily want the postcards to get lost, but at the same time you wouldn’t rely on the postcards for any urgent business (like “send money to the embassy”). For anything important, you’d use the telephone (TCP) to ensure that your message arrived intact and was processed correctly.

You may wonder why a UDP protocol exists, when it would seem that IP could serve the same function. The reason is simple: IP doesn’t do anything but get datagrams from one host to another. IP doesn’t provide any application interfaces or management services. UDP does provide these services, and it provides a consistent environment for developers to use when writing low-overhead network applications. ...