Subject: RE: Solving the wrong problems

From: "Christian Huitema" <[email-masked]>
Date: Fri, 24 Jan 2003 09:16:39 -0800

> Question -  can you please provide a quatitative idea of large ?
> In the context of LL, how big a subnet do you think is large enough ?

How a large can a network be for multicast to become problematic? There
are really three parameters: number of hosts N, frequency of multicast
operation noted as 1/T where T is the average period, and reasonable
frequency of multicast messages. I would say that anything that results
in more than 100 multicast per second is unacceptable in most networks;
so you get a condition of N/T < 100. Now, you get to estimate T; if you
use multicast for name resolution, as basic NetBIOS does, T can be as
low as a few tens of seconds; for T = 30 sec, you get a problem if N >
3000.

There are also aggravating factors. In some configurations, networks are
bridged through point-to-point links; the multicast chat can in some
cases congest these links. There are also some situations in which the
responses to a multicast packet trigger a "packet storm", a very
undesirable event.

Note that NetBIOS is mostly used over IP nowadays, and that those large
networks that use NetBIOS tend to also use WINS, i.e. a centralized name
resolution service for NetBIOS.

-- Christian Huitema

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From: "s. goswami" <[email-masked]>
Date: Fri, 24 Jan 2003 13:04:03 -0500 (EST)

Thanks, that certainly is very helpful. So in a nework of N nodes
and f (mcast per second) frequencey, there would be Nf mcast messages.
If each multicast message has p bytes, then each host would receive
traffic of (N-1)fp bytes per second.  Substituting 3000 for N,
100 for f, and 1000 bytes for p. We get that each host would need
to have a pipe of capacity > 300,000,000 bytes per second. More than what
Fast Ethernet (100 Mbps) offers. So we would need to reduce N to 300,
f to 1 and p to 100 to get to 30,000 bps.

A LL network probably would not be larger than 300 hosts (?).

Subrata