Streams and UDP

Question

Why does the UDP protocol not support streams like TCP protocol does?

Answer 1

Well ...

UDP is a datagram protocol. It is intended to let applications send invidual datagrams, without a high-level protocol controlling what goes on.

A "stream" is a high-level concept, and thus not possible for UDP to support.

Many applications end up re-implementing parts of TCP on UDP, to get the particular mixture of latency, order independence, error handling, and bandwidth that they need.

Answer 2

Others have given perfectly good answers, but I think an analogy may help.

Imagine that UDP is a bit like a mailman - putting letters through people's doors, but never checking that they actually see the letter, or that the person even exists. This is cheap, and works well for junk mail send to many people (read: broadcast packets).

TCP is more like a messenger who will knock on your door and deliver a message in person. If you're not in, he'll come back and try a bit later. Eventually, you'll get the message - or the messenger will know that he hasn't delivered it. You can also send a message back to the sender via the messenger.

Don't try to read too much into my analogy - it's not like there's actually a single "messenger" in TCP - but it might help in terms of thinking about things. In particular, imagine you were sending a whole series of letters to someone, which they had to read in order - that can't work (reliably) with a mailman, as you might come down to find 10 letters sitting on your doormat, with no idea what order to read them in. The mailman might have dropped a few on the way, as well - and no-one will know. That's not a suitable basis for a reliable stream.

(On the other hand, it's a fine model for newspaper distribution. If you happen to miss a few, that's not a problem - you'll still get the later ones, which will be more interesting to you by then. That's why some streaming media solutions use UDP, despite it not giving a reliable actual stream.)

Answer 3

The reason is simple: It doesn't, because it doesn't, because that wasn't one of the design goals for the protocol.

Making UDP work as a stream would mean adding almost every feature that TCP has. You'd end up with two identical protocols. What'd be the point?

UDP was meant to be the minimal lightweight protocol. It was meant to complement TCP.

Answer 4

If you define a 'stream' as a 5-tuple of IP(& L4) header, UDP does support streams. I guess what you mean to convey is whether UDP supports connections (where in each stream, aka, 5-tuple, is treated as a logical connection) and reliability of delivery of packets belonging to that stream is more or less guaranteed. Whereas, in case of UDP, althrough the packets are part of a stream (5-tuple), there's no effort to ensure reliability, instead relying on the underlying network layer to make proper delivery. If it fails, so be it. Jon Skeet's analogy makes perfect sense.