Lost UDP datagrams when sending in socket.send callback

Linux 4.4.0-17763-Microsoft #253-Microsoft x86_64 GNU/Linux
node versions tried: v8.15.0 v10.15.0 v11.7.0

Use this server or any tool you can listen to UDP with:

require('dgram')
.createSocket('udp4')
.on('message', (msg, rinfo) => {
    msg = `${msg}`.split(' ')[0];
    console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
})
.bind(41234);

Run this to demonstrate the problem:

const dgram = require('dgram');

const WOULD_YOU_LIKE_ME_TO_DROP_BUFFERS = true; // TOGGLE THIS

const PORT = 41234;
const GRAMS = 1000;
const FILL = '##################################################'

function sendgrams() {
    let pos = 0;
    const socket = dgram.createSocket('udp4');
    function funkytown() {
        if (pos >= GRAMS) {
            socket.close((e) => {
                if (e) {
                    console.log(e);
                }
                console.log('Closed.');
            });
            return;
        }
        socket.send(`${pos++} ${FILL}${FILL}${FILL}${FILL}${FILL}${FILL}${FILL}`, PORT, (e) => {
            if (e) {
                console.log(e);
            }
            if (WOULD_YOU_LIKE_ME_TO_DROP_BUFFERS) {
                funkytown();
            } else {
                setTimeout(funkytown, 0);
            }
        });
    }
    funkytown();
}

sendgrams();

If I call socket.send from within its own callback, some UDP datagrams never show up at the destination. The workaround is to let the callback exit first. Here we see server output showing missing datagrams where we expect the log to finish up with "997, 998, 999".

server got: 958 from 127.0.0.1:61866
server got: 988 from 127.0.0.1:61866
server got: 992 from 127.0.0.1:61866

That's not unexpected and not a Node.js issue. UDP is lossy, unreliable. If you transmit faster than the operating system can send out the datagrams, or faster than the intermediate hops can retransmit them, some of them will end up being dropped.

If you have reason to believe it is a Node.js issue let me know but until then I'm closing this out.

This is a node bug.

This is still broken in node 12. I know UDP is lossy. That's not the problem. If this is the intended behavior then it needs to be documented because dropping back to the event queue in this situation isn't obvious.

You're failing to make a case for why this should be a Node.js issue.

@bnoordhuis

Let's try a slightly different way: as @bnoordhuis points out, the current dgram/udp stack in Node.js makes no guarantees that datagram sending is reliable. The code is working as intended. Making udp more reliable is possible, and if it's something that's important to you, a PR will always be considered. In the meantime, we are busy working to implement QUIC support that does add a layer of reliability on top of UDP. In the future, that may be worth exploring.

This is a node bug.

yeah, its a bug, we use setTimeout aswell to solve it,

This is still broken in node 12. I know UDP is lossy. That's not the problem. If this is the intended behavior then it needs to be documented because dropping back to the event queue in this situation isn't obvious.

I tested your code on node 16, it is not an issue anymore, I think somewhere along they way it was solved (not tested on 13,14,15 versions)

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Lost UDP datagrams when sending in socket.send callback #25646

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Lost UDP datagrams when sending in socket.send callback #25646

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bnoordhuis commented on Jan 23, 2019

AE9RB commented on Jan 23, 2019

AE9RB commented on Aug 22, 2019

bnoordhuis commented on Aug 22, 2019

jasnell commented on Aug 22, 2019

jacobbogers commented on Jul 25, 2021

jacobbogers commented on Jul 25, 2021

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