Julia Evans

What's interesting about UDP?

I asked on Twitter today “what’s interesting about UDP”? (this tweet) I got a bajillion replies. Here’s a rough summary because there was some really interesting stuff in there once I made it through all the UDP jokes and I don’t want to lose it.

First, we should talk about what UDP is for a second: UDP lets you send network packets. If a UDP packet you send gets dropped, you never find out about it and nobody will help you retry. Retrying is up to you.

Technically speaking it stands for “user datagram protocol” but I think “unreliable data protocol” is better because it’s funny and probably more accurate anyway.

Another fact about UDP is that if you mention UDP there will be 1000 jokes about dropping packets.

So! What is there to know about UDP? Here’s a list.

One interesting thing was that there’s a really common notion “video streaming/VOIP uses UDP” and “games use UDP” but I think the issues there are actually kind of subtle and sometimes these things actually end up using TCP instead. I don’t understand this very well yet but it doesn’t seem to be totally straightforward.

DNS uses UDP

This is possibly the most important protocol that works on top of UDP. I think the reason DNS uses UDP is probably that practically all DNS requests and responses fit into a single IP packet, so retry logic is relatively simple (you send your request, if you don’t get a response, you just.. try again.) You don’t need to assemble multiple packets.

When servers need to send a large DNS response, they use TCP instead (I actually ran into a bug at work recently related to this – in one case UDP DNS responses were working properly, and TCP responses weren’t).

statsd uses UDP

statsd is a metrics server from Etsy. The idea here is that statsd uses UDP because metrics reporting should be as cheap as possible (sending UDP packets is really fast, there is no connection to manage). Some extra factors.

  • the overhead of setting up a TCP connection is pretty high, so they don’t want to do that for every single statistics request
  • Etsy uses PHP, which I think means they can’t have long-lived persistent TCP connections

Also apparently sometimes people do logging (like syslog) over UDP. Here’s an RFC about that. It’s not clear to me that this is generally a good idea (it leads with “Network administrators and architects should be aware of the significant reliability and security issues of this transport”)

Packet size limits

The practical packet size limits for UDP are pretty important to understand.

This seems super important – with TCP you can kind of ignore the fact that internet packets can only be so big, because TCP will automatically combine packets for you. With UDP, it gets important really fast because there’s no automatic combining of packets. You need to manually split up your data into packets.

For example the reason there are only 13 root DNS servers is that DNS uses UDP and that is how many fit inside a single UDP packet! (according to wikipedia)

WebRTC uses UDP

The chapter in “High Performance Browser Networking” about WebRTC is super interesting and well written and very much worth a read. Also that whole book is great. Actually you should probably just go read that chapter instead of this blog post :).

Some games use UDP

Your Game Doesn’t Need UDP (Yet) is an article about this. Many real-time games use UDP because dropped frames are considered better than delayed frames. I know almost nothing about this.

@caitie summarized the reason some games use UDP pretty clearly:

UDP is used for video and some games because with TCP you can get huge delays for one dropped packet. Imagine you are sending 20 packets via TCP, and packet 3 goes missing. Due to network delay you don’t get the missing packet 3 msg until you’ve sent all 20 messages so now you have to send 3 through 20 again to guarantee in order delivery. So on very lossy networks you can waste a lot of bandwidth and cycles resending packets.

video streaming uses UDP, sometimes

  • “Most h.264 streams for live cameras and such are UDP as far as I know.” (here)
  • “I worked on the video delivery side more recently, and RTMP, RTSP, and obviously HLS etc. are all TCP now” (here)

probably don’t reimplement TCP on top of UDP

If you actually want reliable message delivery, you should probably just use TCP and not try to do any fancy UDP tricks. If you really actually do not care if your packets arrive or not and they are all basically independent of each other and the order does not matter at all (like with statsd), maybe that is a good time to use UDP.

It’s possible that, most of the time, the answer to “when should I use UDP” is “don’t, just use TCP instead, it’ll be easier”.

Google is maybe trying to reimplement TCP with UDP though

See QUIC, SPDY

multicast

Wikipedia says “The most common transport layer protocol to use multicast addressing is User Datagram Protocol (UDP).” I still don’t understand what’s up with multicast but many people mentioned it. Here’s the wikipedia article.

“udp lets you write stateless protocols. stateless protocols are great cause you can talk to millions of peers from 1 machine”

other interesting stuff

  • udp checksum is endian independent!
  • “how Bittorrent built uTP on top of it is interesting”
  • I’ve been confused before that you can still get a “connection refused” (via icmp) even though UDP is connectionless (this is a real, weird, thing)
  • UDP is used in a bunch of denial of service attacks in various ways (“you can write someone else’s address as the return address. A lot. This sucks.“)
  • UDP came about when tcp was split into ip and tcp
  • “It’s really hard to do load balancing well for UDP, but that can be really important (eg for server infrastructure for video calls).”
  • DHCP uses UDP
  • strictly speaking UDP carries “more data” than TCP because TCP headers take more space (more overhead) than UDP headers
  • I think there are some issues with UDP and network address translation (NAT) because UDP doesn’t have connections. I’m not super clear on this though.
  • It’s easier to spoof IPs for UDP traffic since no handshake is necessary

Questions

  • Why would you use TCP instead of UDP? Why would you use UDP instead of TCP?
  • Why is UDP considered ‘not blocking’?? What is configurable about it? Sendbuffer / receive buffer size??
  • “why would you use this? why is unreliable a better idea sometimes?”
  • how lossy is it in practice?

2016: Year in review How much memory is my process using?