Comments on: Networking for Physics Programmers

By: Glenn Fiedler

Glenn Fiedler — Wed, 25 Mar 2009 20:59:28 +0000

OK liam so sorry i missed your first comment somehow. you are right about all you say, I just simplified things to give people a very simple overview of the problem.

So this is why I told a white lie and said 64kbit/sec is 256 byte packets 30 times a second. It’s a nice round number. In truth this *is* slightly more than 64kbit/sec, so in reality you need packets that are a bit smaller. To cope with this what I do instead is drop to 10pps under bad network conditions. This way I’m hitting a half-decent bandwidth (nearish to 64kbit/sec) but then I drop to something around ~20kbit/sec when network conditions are poor.

Regarding the other points, the topic of the lecture was a very limited subset of the problem of networking physics. I only covered how to VIEW a physics simulation on a remote computer, no mention of topology, client side prediction, latency compensation, and all that – I wanted to keep it simple so people could get started with it.

There is definitely a whole other series of lectures where I cover the points you mentioned…

In fact, my friday lecture will touch on all of them – this is where I show the generalization of this technique to 4 players in peer-to-peer topology.

That’s all I have to say for now, I have to get back to work on the demo for friday

Thanks for your valuable feedback as always!

By: Glenn Fiedler

Glenn Fiedler — Wed, 25 Mar 2009 04:20:12 +0000

Yeah bytes are so scarce we think in bits instead

By: Pawel Stopinski

Pawel Stopinski — Tue, 24 Mar 2009 22:41:06 +0000

That’s what I was afraid of (this confusing convention being actually used by network programmers ). Thanks anyway for the answer!

By: Glenn Fiedler

Glenn Fiedler — Tue, 24 Mar 2009 18:32:38 +0000

kbits/sec is (I think) standard for networking, xbox360 TCRs are specified in it – that’s where I learned it, so that’s what I use

cheers

By: Pawel Stopinski

Pawel Stopinski — Tue, 24 Mar 2009 18:21:04 +0000

Maybe not the biggest issue in a world but I wonder why do you use kbits/sec instead of kbytes/sec? I understand why ISPs are doing that (so they can show 8 times bigger numbers in advertisements, and that annoys me so much), but what’s your excuse?
Even if one uses for example a compression “thinking in bits” (Huffman), it’s still stored and send in bytes. Is there a reason in networking to talk about sending bits, other than that’s how ISPs show connection speed?

By: Glenn Fiedler

Glenn Fiedler — Tue, 24 Mar 2009 17:55:55 +0000

yes i will briefly touch on authority scheme and how it works, then finish the talk with a Q&A session – i will show this in the demo, without slides

you didn’t like the pipes joke? waaaaaaah

By: gizard

gizard — Tue, 24 Mar 2009 15:28:52 +0000

Maybe you could talk about the problem to solve interactions (collisions) between rigid bodies in different timelines, or better said, under different authorities. I think that’s a very exciting thing for physics guys who love tech demos with brick stacks melting down, and bouncing balls runing down a valley

…Too much pipes.

By: Glenn Fiedler

Glenn Fiedler — Tue, 24 Mar 2009 15:01:21 +0000

yeah i think i’ll add something on that – maybe a quick section on topology after the UDP/TCP/IP comparison

what i’m going to do is explain initially in the context of just two computers, so it’s kind of moot — but i will have to touch on authority issues, client/server vs. distributed peer-to-peer

my goal here is to show folks how to send their physics state over the network, do smoothing and extrapolation – then leave them with resources that explain further client side prediction, authority scheme, and my networking articles – so people can go read more and network their own sims

but yeah, i TOTALLY glossed over topology, so i’ll add that

thanks liam

By: Liam

Liam — Tue, 24 Mar 2009 11:48:38 +0000

Just some quick thoughts.
Is this for a peer to peer model or client server model?
You do not seem to mention UDP frames overhead which adds to the upstream rate.
I am a little confused with your diagram of a packet, does this imply that input is sent even if the server is the sender?
Your maths seems wrong (or mine)64kBit is not 256 at a rate of 30 per second.
256*8*30 = 61440 bits per second
28*8*30 =6720 bits per second UDP overhead.
Users connections :
Reducing send rate for bad connections when packet loss above a threshold or the ping becomes high.
99 percentile is 8kbs – Bungie studios research.
Delta compression: is this used to snap an object or is this how position etc are send, as this may require knowledge of a packet which never arrived and was not reliable therefore not resent.
Compression: I assume you are going to talk about packing floats into uint16′s ie within a range?
When an object is close to the user sending high precession updates instead of compressed?
Client side reaction to events before server has authorised like your steering example?

I need some work doing in the bathroom do you have any more of the pipes