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UDP vs TCP

gafferongames.com

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Re: UDP vs TCP

#71

Post by jlgaddis »

exDM69 wrote: post #8
mjevans wrote: post #6

The main difference between TCP and UDP, as this programmer discovered, relates to quality of realtime service. Times to use UDP over TCP * When you need the lowest latency * When LATE data is worse than GAPS (loss of) in data. * When you want to implement your own form of error correction to handle late/missing/mangled data. TCP is best when * You need all of the data to arrive, period. * You want to automatically m…

Another article on this site addresses TCP plus UDP mix. The tl;dr is that they should not be used at the same time because TCP retransmission will interfere with UDP (and routers give TCP priority). As long as TCP happens outside of the "hot" loop, it's fine.

I'm a network engineer and I think this is the first time I've ever heard the claim that "routers give TCP priority" (ICMP is often de-prioritized but that's on the control plane).

Exactly which routers are alleged to be doing this?

Re: UDP vs TCP

#72

Post by wtallis »

josephg wrote: post #69

Does anyone know if SCTP is suitable / in use by any games? It supports streams to work around the head-of-the-line blocking problem TCP runs into and it also supports opt-in unreliable delivery for game data. On the surface seems ideal for games, though I don't know if its getting much actual use.

Nobody who's in it for the money wants to be the first one to prove that SCTP can be deployed at scale. Application developers don't want to have to sort out getting a working SCTP implementation installed and configured on the proprietary operating systems they target, and past the firewalls of all the bottom of the barrel consumer routers.

Re: UDP vs TCP

#74

Post by chaosmonkey »

chii wrote: post #62
mikepurvis wrote: post #7

The recommendation to avoid TCP altogether is surprising to me. Having encountered a number of video-conferencing systems which are in a similar space, it seems pretty standard to have separate real-time and control sockets on UDP and TCP respectively. I skimmed the linked paper and didn't find it conclusive; can someone summarize how it is that having a TCP socket can affect UDP traffic on the same interface? All th…

the post also recommends that some http traffic is OK, but doesn't that directly contradict the advice to not mix tcp with udp (assuming that the http traffic sits on top of tcp)?

It's clarified in the next line. > "The point is, don’t split your game protocol across UDP and TCP."

There is always going to be data transmitted other than the game protocol. Most likely that is going to be limited and not at the same time as real time data is being transmitted.

Re: UDP vs TCP

#75

Post by wtallis »

macintux wrote: post #49
cpncrunch wrote: post #37

The reality is that these days there generally isn't any packet loss, so UDP vs TCP isn't such an issue as it might have been in the past. In fact TCP has a number of advantages these days such as easier firewall traversal, WebSockets, etc.

I'm....skeptical. https://queue.acm.org/detail.cfm?id=2655736

That article is about network failures and misconfigurations. It's looking at reliability on a vastly longer timescale than the domain of congestion control operating on traffic from interactive applications. Basically all of the problems described in that article would simply result in a game dropping the connection, whether it was using TCP or UDP.

Re: UDP vs TCP

#77

Post by amaranth »

z3t4 wrote: post #70

I think tcp have an unfair reputation. Our networks are better now then 30 years ago ... Worst case latency for tcp is like 3 seconds, compared to the packet never arriving. The trick is to hide the lag with animations. I think google, facebook, and world of warcraft use tcp for their real time apps !?

3 seconds is 2900ms too long, a game would typically rather lose the packet rather than have it delay all transmission for that long. World of Warcraft is a relatively simple game that as far as I know is heavily client authoritative so the server is mostly just serving content and telling other clients the result of your actions. In a game like Call of Duty, while they have some form of lag compensation, if you had all the players in your game freeze for 3 seconds while you shoot them the server will disregard your actions as impossible as the players weren't actually there. It would have been better for you to see the players slightly out of sync or have them teleport a short distance by just losing that one update and continuing to get the rest on time.

Re: UDP vs TCP

#78

Post by kijiki »

macintux wrote: post #48
wtallis wrote: post #28

Earlier quoted context omitted.

I'm well aware of how dumb home routers can be, but a general de-prioritizing of the transport protocol that DNS uses is both really dumb and pretty easy to detect. Are you aware of any router vendors that have actually shipped such a configuration? (Or for that matter, any consumer router that has shipped with any prioritization rules enabled out of the box?)

DNS strikes me as something relatively painless to deprioritize. Small packets, not all that latency-sensitive for most use cases.

That would be a disaster.

Without active queue management, a single bulk upload can break DNS. When the bottleneck link (usually your cable/DSL modem) is saturated by the bulk upload, it will start dropping new packets that come in while the queue is full.

When a single packet worth of space frees up, the chances are that the bulk flow will instantly take it, such that nearly all outbound DNS packets get dropped, and DNS queries time out.

The solution is active queue management (CODEL is a good choice) at every bottleneck.

Re: UDP vs TCP

#80

Post by Thaxll »

z3t4 wrote: post #70

I think tcp have an unfair reputation. Our networks are better now then 30 years ago ... Worst case latency for tcp is like 3 seconds, compared to the packet never arriving. The trick is to hide the lag with animations. I think google, facebook, and world of warcraft use tcp for their real time apps !?

Google is no real-time VS a FPS game running @60hz.
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