这并不总是明确的。然而，如果您需要保证数据包以正确的顺序无丢失地传递，那么TCP可能是您想要的。

另一方面，UDP适用于传输信息的短数据包，其中信息的顺序不太重要，或者数据可以放入单个数据包中 包。

当您希望向许多用户广播相同的信息时，这种方法也很合适。

其他时候，当您正在发送序列数据时，它是合适的，但如果有些数据丢失了 错过你不太关心的(例如VOIP应用程序)。

有些协议更复杂，因为需要的是TCP的一些(但不是全部)功能，但比UDP提供的功能更多。这就是应用层必须做到的 实现附加功能。在这些情况下，UDP也是合适的(例如，互联网广播，顺序很重要，但不是每个数据包都需要通过)。

它在哪里/可以被使用的例子 1)时间服务器向局域网上的一堆机器广播正确的时间。 2) VOIP协议 3) DNS查找 4)请求局域网服务，例如:where are you? 5)网络电台 还有许多其他的……

在unix上，您可以输入grep udp /etc/services以获得实现的udp协议列表 今天……有几百个。

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。

关键的问题是“在什么情况下UDP是更好的选择[而不是tcp]”

上面有很多很好的答案，但是缺少的是对传输不确定性对TCP性能影响的正式、客观的评估。

随着移动应用程序的大量增长，以及“偶尔连接”或“偶尔断开”的范式，在很难获得连接的情况下，TCP试图维持连接的开销肯定会导致UDP及其“面向消息”的性质的强烈情况。

现在我没有数学/研究/数字，但我制作的应用程序使用ACK/NAK和UDP上的消息编号比使用TCP更可靠，当时连接通常很差，可怜的旧TCP只是花费了时间和客户的金钱来尝试连接。在许多西方国家的地区和农村地区都有这种情况....

UDP是完美的VoIP地址，其中数据包必须发送不考虑其可靠性… 视频聊天是UDP的一个例子(你可以在任何视频聊天期间通过wireshark网络捕获来检查它)。 而且TCP不能与DNS和SNMP协议一起使用。 UDP没有任何开销，而TCP有很多开销

比较TCP和UDP，像UDP这样的无连接协议可以保证速度，但不能保证数据包传输的可靠性。 例如，电子游戏通常不需要可靠的网络，但速度是最重要的，在游戏中使用UDP具有减少网络延迟的优势。

关于这个问题，我所知道的最好的答案之一来自Hacker News的用户zAy0LfpBZLC8mAC。这个答案太好了，我就原原本本地引用它吧。

TCP has head-of-queue blocking, as it guarantees complete and in-order delivery, so when a packet gets lost in transit, it has to wait for a retransmit of the missing packet, whereas UDP delivers packets to the application as they arrive, including duplicates and without any guarantee that a packet arrives at all or which order they arrive (it really is essentially IP with port numbers and an (optional) payload checksum added), but that is fine for telephony, for example, where it usually simply doesn't matter when a few milliseconds of audio are missing, but delay is very annoying, so you don't bother with retransmits, you just drop any duplicates, sort reordered packets into the right order for a few hundred milliseconds of jitter buffer, and if packets don't show up in time or at all, they are simply skipped, possible interpolated where supported by the codec. Also, a major part of TCP is flow control, to make sure you get as much througput as possible, but without overloading the network (which is kinda redundant, as an overloaded network will drop your packets, which means you'd have to do retransmits, which hurts throughput), UDP doesn't have any of that - which makes sense for applications like telephony, as telephony with a given codec needs a certain amount of bandwidth, you can not "slow it down", and additional bandwidth also doesn't make the call go faster. In addition to realtime/low latency applications, UDP makes sense for really small transactions, such as DNS lookups, simply because it doesn't have the TCP connection establishment and teardown overhead, both in terms of latency and in terms of bandwidth use. If your request is smaller than a typical MTU and the repsonse probably is, too, you can be done in one roundtrip, with no need to keep any state at the server, and flow control als ordering and all that probably isn't particularly useful for such uses either. And then, you can use UDP to build your own TCP replacements, of course, but it's probably not a good idea without some deep understanding of network dynamics, modern TCP algorithms are pretty sophisticated. Also, I guess it should be mentioned that there is more than UDP and TCP, such as SCTP and DCCP. The only problem currently is that the (IPv4) internet is full of NAT gateways which make it impossible to use protocols other than UDP and TCP in end-user applications.