UDP protocol to minimize latency?

Christoffer Lernö · 2013-07-31T04:47:04

This is sort of a follow-up to this question: http://www.gamedev.net/topic/645872-responsive-mobile-multiplayer-udp-or-tcp/ Experimental results After a lot of testing I managed to determine that the bad 3G latency times I initially got for UDP on my phone was due to the phone saving power. In particular, I was seeing roundtrips of 2000 ms for UDP over 3G when sending 1 message every 2 seconds. When I instead sent a new message within 50 ms of receiving the return message, that number would dive to a steady 80 ms, with occasional hiccups of 400 ms. TCP got around 120-200 ms with the same setup, but when it encountered packet loss, the roundtrip would hit 2000-4000 ms! This verifies that there are indeed serious stuttering with TCP that would be extremely hard to cover with animations. Looking for a good reliable UDP protocol I've been looking at various existing libraries. Most of them are way too high level - I prefer to write all the serialization myself thank you. But even for products like ENet, it looks like the strategy is to push a TCP-like reliable layer on top of UDP and I don't think I want that. What about something like thie? I remember reading about one of those early Lucasart space combat games (X-wing vs Tie-fighter?), that they used an extremely simple scheme - basically sending the packet n together with packet n + 1, so that only on two consequent packet losses would there actually be a packet loss. Our particular game doesn't have much action. In fact, as I've described in the other posting, it's about 80 packets sent per player total. In order to keep the phone awake though, one would have to send pings, and send pings often. Assuming we send pings every 100 ms we could imagine simply piggy bagging un-ack:ed messages until they're acked, it could look something like this: 1. Client sends action request with message n 2. Client sends ping with message n + 1, and adds message n 3. Client sends ping with message n + 2, and adds ping n + 1, message n 4. Client receives pong from server, ack-ing up to n + 1 5. Client sends ping with message n + 3, and adds ping n + 2 etc. We could then prune old pings from the resends, so we only send resend actions, and vice versa for the server obviously. This looks like it could be much faster in recovering from latency than any resend mechanism that relies on requesting missing packets, like TCP does. The biggest worry I have is that I'd move from TCP with very conservative bandwidth-requirements to something which constantly bombards the server with packets.

Networking and Multiplayer Programming

Started by lerno July 27, 2013 08:16 PM

11 comments, last by hplus0603 10 years, 8 months ago

lerno

269

Author

July 30, 2013 04:18 PM

Just want to put this out there, I am developing a client-server 2D TCP networked game, I tried connecting to my game server (hosted at my home) on my laptop though my phones WiFi hotspot (I was further out in the country at the time aswell). That version's build had liquid animation that was compressed -> It ended up running pretty well and latency was within 200ms and this is a game that is not developed for internet through the phone.

It would be pointless to use UDP if your going to just reimplement TCP. If some packets can be dropped and/or you want a custom error detection design other than the way TCP handles it then UDP would make more sense, otherwise in my opinion TCP saves time and is optimized well.

Well, for my gameplay I think the lag in TCP when moving is sufficiently annoying to detract from the gameplay. Opportunities in my case for predictively animating responses is minimal, so I'm willing to try anything that can reduce the latency.

I agree that it's meaningless to implement reliable UDP for a game like this. However, given that my packets are typically on the order of a few tens of bytes, I have more opportunities with UDP. More specifically I can keep resending the all the non-acked data until I receive an ack for them. That way I don't need any ack-timeouts (which is what TCP and many "reliable UDP"-solutions are using).

snacktime

457

July 31, 2013 04:34 AM

I prefer not having ordering at the protocol layer at all and let individual systems send requests again if they need extra reliability. Reliable, ordered messaging over unreliable networks just ain't going to happen,and trying to force a square peg into a round hole doesn't work. I've seen more brittle, unmanageable code because dev's try to create architectures that rely on ordered messaging.

Be wary of larger messages crossing packet boundaries, which is an issue with udp.

As for serialization, I'm firmly convinced that bit level protocols should be a thing of the past. There is very little to gain from operating at that level, if at all. I've been using protocol buffers with good success. They are sparse(no schema data in the message), performant, and cross platform.

hplus0603

11,916

July 31, 2013 04:47 AM

If you don't have some way to support reliable-in-order where it's really needed, then each dev will try to build it alone in each application system, which is a much worse place to put it than in the network layer proper.

enum Bool { True, False, FileNotFound };

UDP protocol to minimize latency?

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