Multicast Networking Thread

Hello all,

Ive been doing some network programming for a game recently and I came across a bit of an interesting piece of technology called multicast. Basically it bypasses a common problem for Server-Client architecture games such as HL2 which is the exponential increase in load on the servers upload bandwidth. Usually you have to send a copy of everyones position to everyone else in a game so you get an (n-1)^2 system where n is the number of players. This can be reduced a little by some optimisations but it cant really be changed that much using a unicast system.

However multicast gets round this by having the server only send 1 copy of everyones position out. Routers on the net itself then copy this data and distribute it themselves so the servers bandwidth only follows an n-1 system. This reduction in power means that you can have huge increases in player count even on home broadband ADSL. Im not going to go into how I got these numbers but I estimated that on a game Im making I could host maybe 10 players with regular unicast on my bandwidth, this is probably optimistic as well. With multicast this increases to a comfortable 50 players which is slightly insane.

With this technology we could see an end to servers as players with average bandwidths would be able to host server size matches, this could really allow a lot of games to take off as well as giving people more individual control over their hosting. Proper gaming servers would also see an increase with this technology and we could see enormous numbers of players in game simultaneously.

The only downside is the lack of support currently available for multicast through ISP's. But given the exponential increase in bandwidth savings the technology provides it seems likely that support will only increase.

So does anyone have any experience with multicast and know any more about it and what do you think the implications of this would be on servers and gaming in general ?

Yeah, it will be awesome, but don't hold your breath wating for it. Internet infrastructure progresses at a glacially slow pace.

Gibberstein wrote: Yeah, it will be awesome, but don't hold your breath wating for it. Internet infrastructure progresses at a glacially slow pace.

Aint that the god darned unfortunate truth!

Its not as good as its cracked up to be. Games atm only send info to the player that is related to where the player is in the current map. Under multicast you would send all info and this is bad for a couple of reasons. All info is going to be many times larger than the local info thus choking client connections and getting all info allows cheat makers to cheat easier.

It looks like a silver bullet but it really isint.

Edited by: lodle

While there would certainly be more information going to each client I doubt with the bandwidths that are available now that this would be a problem. Usually in a game you would expect to get updated frequently (at least 10 fps) on peoples positions who are in your line of sight or are near to you. Players outside this might get updated much less frequently or not at all. In any case it is reasonable to assume that sometimes quite a lot of players will be close to each other, in this case you would be sending everyones data anyway. But either way Im betting you wouldnt see even a 2-3x increase on peak download bandwidth requirements which probably wouldnt get close to requiring your full bandwidth.

As for the cheating issue, yes I suppose someone could use it for finding out peoples positions easier but I personally think that risk is worth taking myself and is a small con for a lot of very large pros.

he only downside is the lack of support currently available for multicast through ISP's.

The problem is a technical one rather than a bandwidth one
Support.3com.com

The best way to solve it is with a virtual mesh or grid networking layer.

Gibberstein wrote: Yeah, it will be awesome, but don't hold your breath wating for it. Internet infrastructure progresses at a glacially slow pace.

Sometimes i hate you for being realistic ^^
Sad but true :/