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copied!<p>Others have given perfectly good answers, but I think an analogy may help.</p> <p>Imagine that UDP is a bit like a mailman - putting letters through people's doors, but never checking that they actually <em>see</em> the letter, or that the person even exists. This is cheap, and works well for junk mail send to many people (read: broadcast packets).</p> <p>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.</p> <p>Don't try to read too much into my analogy - it's not like there's <em>actually</em> 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.</p> <p>(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 <em>media</em> solutions use UDP, despite it not giving a reliable actual stream.)</p>

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