这是我最喜欢的问题之一。UDP被误解了。

当你真的想快速地向另一个服务器得到一个简单的答案时，UDP是最好的选择。通常，您希望答案在一个响应包中，并准备实现自己的协议以提高可靠性或重新发送。DNS是这个用例的完美描述。连接设置的成本太高了(然而，DNS 不支持TCP模式以及)。

另一种情况是，当您交付的数据可能会丢失，因为新的数据将取代之前的数据/状态。天气数据、视频流、股票报价服务(不用于实际交易)或游戏数据浮现在脑海中。

另一种情况是，当您正在管理大量的状态时，您希望避免使用TCP，因为操作系统无法处理那么多会话。这在今天是一个罕见的案例。事实上，现在可以使用用户专用的TCP堆栈，以便应用程序编写人员可以对该TCP状态所需的资源进行更细粒度的控制。在2003年之前，UDP是唯一的游戏。

另一种情况是多播流量。UDP可以多播到多个主机，而TCP根本不能这样做。

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。

UDP can be used when an app cares more about "real-time" data instead of exact data replication. For example, VOIP can use UDP and the app will worry about re-ordering packets, but in the end VOIP doesn't need every single packet, but more importantly needs a continuous flow of many of them. Maybe you here a "glitch" in the voice quality, but the main purpose is that you get the message and not that it is recreated perfectly on the other side. UDP is also used in situations where the expense of creating a connection and syncing with TCP outweighs the payload. DNS queries are a perfect example. One packet out, one packet back, per query. If using TCP this would be much more intensive. If you dont' get the DNS response back, you just retry.

在某些情况下，如果丢失一些数据不会完全破坏正在传输的数据，则应该使用UDP而不是TCP。它的很多应用都是在实时应用中，比如游戏(例如FPS，你不需要知道每个玩家在特定时间的位置，如果你丢失了一些数据包，新的数据会正确地告诉你玩家在哪里)，以及实时视频流(一个损坏的帧不会破坏观看体验)。

There are already many good answers here, but I would like to add one very important factor as well as a summary. UDP can achieve a much higher throughput with the correct tuning because it does not employ congestion control. Congestion control in TCP is very very important. It controls the rate and throughput of the connection in order to minimize network congestion by trying to estimate the current capacity of the connection. Even when packets are sent over very reliable links, such as in the core network, routers have limited size buffers. These buffers fill up to their capacity and packets are then dropped, and TCP notices this drop through the lack of a received acknowledgement, thereby throttling the speed of the connection to the estimation of the capacity. TCP also employs something called slow start, but the throughput (actually the congestion window) is slowly increased until packets are dropped, and is then lowered and slowly increased again until packets are dropped etc. This causes the TCP throughput to fluctuate. You can see this clearly when you download a large file.

因为UDP没有使用拥塞控制，它可以更快，并且经历更少的延迟，因为它不会寻求最大化缓冲区直到丢弃点，也就是说，UDP数据包在缓冲区中花费的时间更少，到达那里的速度更快，延迟更少。由于UDP不采用拥塞控制，但TCP采用拥塞控制，因此它可以从TCP中占用生成UDP流的容量。

UDP仍然容易受到拥塞和数据包丢失的影响，所以你的应用程序必须准备好以某种方式处理这些问题，可能使用重传或错误纠正代码。

结果是UDP可以:

Achieve higher throughput than TCP as long as the network drop rate is within limits that the application can handle. Deliver packets faster than TCP with less delay. Setup connections faster as there are no initial handshake to setup the connection Transmit multicast packets, whereas TCP have to use multiple connections. Transmit fixed size packets, whereas TCP transmit data in segments. If you transfer a UDP packet of 300 Bytes, you will receive 300 Bytes at the other end. With TCP, you may feed the sending socket 300 Bytes, but the receiver only reads 100 Bytes, and you have to figure out somehow that there are 200 more Bytes on the way. This is important if your application transmit fixed size messages, rather than a stream of bytes.

总之，UDP可以用于TCP可以使用的任何类型的应用程序，只要您还实现了适当的重传输机制。UDP可以非常快，有更少的延迟，在连接的基础上不受拥塞的影响，传输固定大小的数据报，并可用于组播。

当TCP可以工作时，我有点不情愿建议使用UDP。问题是，如果TCP由于某种原因不能工作，因为连接太延迟或拥塞，将应用程序更改为使用UDP不太可能有帮助。一个坏的连接对UDP也不好。TCP在减少拥塞方面已经做得很好了。

我能想到的唯一需要UDP的情况是广播协议。在应用程序涉及两个已知主机的情况下，UDP可能只会提供边际的性能优势，而代码复杂性的成本则会大幅增加。