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UDP - Packet Loss in Real Life?
PacketStorm asks: "There's always an argument between TCP and UDP users. TCP users complain that UDP is non-determinstic and lossy, while UDP users complain that TCP is slower and nobody needs all the features anyway. So the question is - has anyone actually seen/experienced UDP loss in high-traffic environments? Is the degeneration of UDP any better or worse than TCP in a congested environment? TCP also craps out in times of congestion, but at least you know - or do you? Experiences?"
UDP is commonly used in games and other time sensitive environments precisely because it lacks reliabilty. With time sensitive data (such as streaming video or unit positions in a game), if the data gets dropped it's not worth it to retransmit, because it woudl be out of date. Therefore, the program just transmits the next update and the user sees a small skip. This is better than getting "out of sync".
TCP is designed to make unreliable networks (like the internet, which only gives "best effort delivery") reliable by ensuring that a stream can be reassembled, in order, with no missing pieces. Read the RFC for more info here. This reliability makes it good for things that need zero corruption (file transfers for example), and aren't time criticial.
Hope this helps.
--MonMotha
I run a cluster of high volume mail servers that use syslog-ng to log to a remote syslog server. This reduces disk IO for each message and actually gives a very noticable performance increase. However, I noticed under periods of very high volume, I was losing nearly 50% of my log entries. Syslog-ng has the ability to use TCP for logging, so I switched, and I haven't lost any entries yet.
UDP is great for things you that aren't useful if retransmitted later, like real time apps (Quake, video streams, audio streams, etc.) But to ensure no loss at all, you need to use TCP.
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About the only reason for using UDP is if you deliberately want to circumvent the congestion avoidance protocols that are built into TCP. So, if you are playing a game, and you need the packets to get through at all costs, but you aren't sending many packets- by using UDP you can agressively defend the small amount of bandwidth you need- any TCP connections around will tend to back off and get out of your way; and that's reasonable if you code it carefully. But writing the protocol to do that is hard, you have to understand not only UDP but also TCP, as well as your game requirements.
And that's the real problem. In most cases people think that waving UDP at the problem will solve their problems- in fact it makes them worse; and TCP has solutions to problems only PhDs have even thought of, and the solutions are built in.
As an example, somebody I know implemented a tftp protocol using UDP. The guy is off the chart in his software abilities (trust me the guy is amazing, he's in the top 2 percent of software engineers according to the tests). Anyway in a back-back comparison against a standard ftp protocol- the tftp protocol loses by a factor of 10 or more (on a network with some congestion, I expect a quiescent network would have been much more level). Of course tftp isn't supposed to handle congestion. But that's the problem- UDP can't handle network congestion out of the box... indeed if anything it tends to create network congestion.
The main algorithms in tcp include 'slow start' and 'exponential backoff'. Both of these are missing in UDP, and both improve the network performance enormously. If your application doesn't affect network performance and doesn't worry about packet loss much, then UDP may be the way to go, otherwise stay away from UDP.
-WolfWithoutAClause
"Gravity is only a theory, not a fact!"This is a very interesting paper on why things don't always work like we think they should...
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