Julia Evans

Why do UDP packets get dropped?

There’s a joke about UDP. it goes like this: “Never mind, you probably wouldn’t get it.”

The first time I heard this joke I did not understand it because I didn’t really understand what UDP was. UDP is a network protocol. The deal is: I send you a network packet. Maybe you get it, maybe you don’t. I have no idea whether it arrived or not. UDP doesn’t care.

When you’re losing UDP packets, it’s sort of tempting to say “well, whatever, that’s what happens when you use UDP!” But UDP packets don’t get lost by magic.

I was pretty confused about some the details of dropping UDP packets (how do you know how many packets got dropped? what causes a packet to be dropped exactly?) Maggie Zhou (who is the best) explained some new things to me today! All the parts of this that are right are thanks to her and all the parts that are wrong are thanks to me.

This is all on Linux, as usual. There are going to be sysctls! It will be the best.

lost on the way out

Imagine you’re sending a lot of UDP packets. Really a lot. On every UDP socket, there’s a “socket send buffer” that you put packets into. The Linux kernel deals with those packets and sends them out as quickly as possible. So if you have a network card that’s too slow or something, it’s possible that it will not be able to send the packets as fast as you put them in! So you will drop packets.

I have no idea how common this is.

lost in transit

It’s possible that you send a UDP packet in the internet, and it gets lost along the way for some reason. I am not an expert on what happens on the seas of the internet, and I am not going to go into this.

lost on the way in

Okay, so a UDP packet comes into your computer. You have an application that is listening and waiting for a packet. Awesome! This packet goes into – maybe you guessed it – a socket receive buffer. How big is that buffer? Everything you might want to know about socket send and receive buffer sizes is helpfully explained in the man page for socket. Here’s the maximum receive buffer size on my computer:

# This prints the max OS socket receive buffer size for all types of connections.
$ sudo sysctl net.core.rmem_max
net.core.rmem_max = 212992
$ sudo sysctl net.ipv4.udp_mem 
net.ipv4.udp_mem = 181110	241480	362220

man udp says that that last number from net.ipv4.udp_mem (362220) means “Number of pages allowed for queueing by all UDP sockets.” 362220 pages is 1.7GB? That’s a lot of pages! Weird. Not sure what’s up with that.

Then your application reads packets out of that buffer and handles them. If the buffer gets full, the packets get dropped. Simple!

You can see how many packets have been dropped on your machine with netstat -suna. Mine has dropped 918 packets so far apparently (“918 packet receive errors”)

$ netstat -suna
IcmpMsg:
    InType3: 1072
    OutType3: 522
Udp:
    1828608 packets received
    568 packets to unknown port received.
    918 packet receive errors
    662721 packets sent
    RcvbufErrors: 918
    SndbufErrors: 1031
    IgnoredMulti: 659

This is cool! This means that if you have a machine which is trying to drop as few UDP packets as possible (for instance if you’re running statsd), then you can monitor the rate at which that machine is dropping packets!

buffers everywhere

After I published this blog post initially, @gphat and @nelhage very astutely pointed out that the OS socket send/receive buffers are not the only buffers.

EVERYTHING IS BUFFERS. Your network card has a buffer that can get full! There are a bunch of intermediate routers between your computer and my computer. All of those have buffers! Those buffers can get full! My current understanding is that most packet loss is because of full buffers one way or another.

If you’re interested in learning more details about the Linux networking stack, there is this huge post called Monitoring and Tuning the Linux Networking Stack: Receiving Data. I have not read it yet but it looks amazing.

Also everything here I said about UDP packets applies just as well to any kind of IP packet – TCP packets can get dropped just as easily, but they’ll get retried, so you’re not as likely to notice.