RE: BANDWIDTH TO MULTICAST
RE: BANDWIDTH TO MULTICAST
Sent: 12/2/96; 1:03:14 PM
From: gnu@toad.com (John Gilmore)
In the big Internet, there's enough bandwidth to pull it off. The protocol that does it is called multicast IP, and the subset of the Internet that runs it is called the MBone (multicast backbone). The MBone folks are working on rolling it out to the entire Internet.
A multicast IP address is an IP address in the range 224.0.0.0 upward. Applications can listen on sockets in that range, and the multicast IP protocols route multicast packets destined for address X so that they only go to subsets of the Internet that have someone listening on address X. The idea is that the packet is transmitted exactly once over any physical link in the Internet, and replicated several times if necessary at the far end of that link, to feed it down several other links. This is what conserves bandwidth.
The main problem as I see it is that the unicast (normal) Internet backbone is made up largely of Cisco routers, and Cisco's implementation of multicast is screwed up. So most people who are on the MBone are running "tunnels" between Unix machines rather than running it "native" on their Ciscos. I've talked with Cisco about this numerous times, and they're in complete denial. They don't have a problem, it's not their problem, it all works if you know what you're doing, etc. But it's simply not so.
Check out the MBone Information Web at www.mbone.com.
The other problem is how much bandwidth you have in "the last mile" to your house or office. That one the MBone can't solve. It doesn't work for dialup users (too much traffic swamps the line), it's marginal for 56K or ISDN users, it works fine on T1 or better.
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