如果TCP数据包丢失，将重新发送。这对于依赖于以特定顺序实时处理数据的应用程序来说并不方便。

例如视频流，特别是VoIP(例如Skype)。然而，在这些情况下，一个掉落的包并不是什么大问题:我们的感官并不完美，所以我们甚至可能不会注意到。这就是为什么这些类型的应用程序使用UDP而不是TCP。

视频流是使用UDP的一个很好的例子。

UDP确实有更少的开销，适合做一些事情，比如流式实时数据，如音频或视频，或者在任何情况下，如果数据丢失是ok的。

UDP是一种无连接协议，用于SNMP和DNS等协议，在这些协议中，无序到达的数据包是可以接受的，数据包的即时传输很重要。

在SNMP中使用它是因为网络管理通常必须在网络处于压力状态时进行，即当可靠的、拥塞控制的数据传输难以实现时。

它用于DNS，因为它不涉及连接建立，从而避免了连接建立延迟。

干杯

We have web service that has thousands of winforms client in as many PCs. The PCs have no connection with DB backend, all access is via the web service. So we decided to develop a central logging server that listens on a UDP port and all the clients sends an xml error log packet (using log4net UDP appender) that gets dumped to a DB table upon received. Since we don't really care if a few error logs are missed and with thousands of client it is fast with a dedicated logging service not loading the main web service.

I work on a product that supports both UDP (IP) and TCP/IP communication between client and server. It started out with IPX over 15 years ago with IP support added 13 years ago. We added TCP/IP support 3 or 4 years ago. Wild guess coming up: The UDP to TCP code ratio is probably about 80/20. The product is a database server, so reliability is critical. We have to handle all of the issues imposed by UDP (packet loss, packet doubling, packet order, etc.) already mentioned in other answers. There are rarely any problems, but they do sometimes occur and so must be handled. The benefit to supporting UDP is that we are able to customize it a bit to our own usage and tweak a bit more performance out of it.

Every network is going to be different, but the UDP communication protocol is generally a little bit faster for us. The skeptical reader will rightly question whether we implemented everything correctly. Plus, what can you expect from a guy with a 2 digit rep? Nonetheless, I just now ran a test out of curiosity. The test read 1 million records (select * from sometable). I set the number of records to return with each individual client request to be 1, 10, and then 100 (three test runs with each protocol). The server was only two hops away over a 100Mbit LAN. The numbers seemed to agree with what others have found in the past (UDP is about 5% faster in most situations). The total times in milliseconds were as follows for this particular test:

1记录 IP: 390760 ms TCP: 416,903毫秒 10个记录 IP: 91,707 ms TCP: 95,662毫秒 100条记录 IP: 29,664 ms TCP: 30,968毫秒

IP和TCP传输的数据总量大致相同。我们在UDP通信方面有额外的开销，因为我们拥有一些与TCP/IP“免费”相同的东西(校验和，序列号等)。例如，Wireshark显示对下一组记录的请求在UDP中是80字节，在TCP中是84字节。