After reading about multicasting, it seems to be the perfect solution for games. If you have the same data that needs to be send to more than one client you just multicast it. I''m writing a chat program that I''m going to incorporate into my game later, and I don''t see why I shouldn''t use multicasting to send chat messages to all the people in the same channel. Since no commercial games (seem to) use it, there must be something wrong. Are there not enough routers that support multicasting? Also, I heard that DirectPlay can use multicasting, is that right?

Because ISPs in general do not support multicasting.
Imagine that! You would be saving bandwidth, while they would still have to handle the full load (Think DDoS). Plus there are a limited number of multicast addresses.

Basically, unless you have strong influence ($$$) in the network world, odds are you won't be able to use multicast in anything but a private network.

[edited by - Fruny on April 1, 2002 12:29:33 AM]

I''m confused. Can you help me clear up some misconceptions?

Where do the packets for multicasting get divided? I thought it was as they travelled the internet, not necessarily at the ISP. My understanding is that when a multicast packet meets a multicasting router, the router copies the packet onto any branches of the net that contain one of the multicast IPs, so the routers themselves are responsible for duplicating the multicast packets. If they were all split at the ISP I don''t see how multicasting can be the big improvement over broadcasting that everyone touts.

I know some of the multicast IPs are reserved for special purposes, but I thought you could just pick one and use it. Thats what the multicasting tutorial on this site implied when I read it. That does seem odd to me though.

From looking at DirectPlay I also understand there are several kinds of multicasting. One kind is called "server message exploding" and is not true multicasting, but rather, what is done when true multicasting is not supported. "Exploding" is when the server is responsible for generating all the copies of the message itself (I think this is what you are discribing).

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If they were all split at the ISP I don''t see how multicasting can be the big improvement over broadcasting that everyone touts.

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No you are right about the technical part, it is not split in one single place. However, my point was that ISPs do not like you doing ''unusual'' stuff and not likely to support propagation of multicast messages. (Hence the ''exploding'' as you pointed out).

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I know some of the multicast IPs are reserved for special purposes, but I thought you could just pick one and use it. Thats what the multicasting tutorial on this site implied when I read it. That does seem odd to me though.

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Of course I could be completely wrong, but, as far as I understand, you have to apply for the use of a multicast address.

rfc 3171
Application form

Thanks!

No multicasting for me I guess