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?"

Comments

Why UDP in games, TCP for bulk tranfer

[MonMotha]

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video or unit positions in a game), if the data gets dropped it's not worth it to retransmit, because it woudl be out of date.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

Definitely

[austad]

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.

Re:UDP packet loss

[Jeremi]

However, with UDP, it's laughably easy. You just do one recvfrom() and you get a packet and it also fills in a data structure to tell you where it came from.

... or maybe you don't get the packet, because a router was loaded down and had to drop it. Now you gotta implement a timeout and retransmit protocol. Not to mention that packets may arrive out-of-order from the way they were sent, so you if ordering is important (and it usually is) you have to implement some sort of sequencing tag systems too. A few dozen hours later, you find out you've implemented something that looks suspiciously like a primitive version of TCP... :^)

Re:UDP Experience QWZX

[Anonymous Coward]

/me bangs his head on his desk over and over at the sheer stupidity of this.

Dude, I hate sound flame-y, but do you have any understanding of what you implemented? That you got lucky and it "works" is totally irrelevent to the fact that it's completely unreliable. All it takes is one flakey piece of Ethernet dropping packets to SCREW UP FREAKING TIMESHEETS.

This reminds me of the morons who use MySQL for financial transactions (i.e., no transactions, no foreign keys, etc) who justify their ignorance with "well, it works so far!!"

They point isn't whether it works or not, the point is that when it fails, it fails spectacularly. Like your system. Just because you can keep spinning the chamber and the russian roulette gun never goes off doesn't mean it never will.

Seriously, you screwed up bad. These are the kinds of stories that really make me think that programmers should have some sort of licensing.