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From: cire@CISCO.COM (cire|eric)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: IP Multicasting in SunOS
Message-ID: <9001092322.AA15750@ucbvax.Berkeley.EDU>
Date: 9 Jan 90 16:41:53 GMT
References: <472@berlioz.nsc.com>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
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Well its not that Token Ring doesn't support multicasting ...  it does.
It just does so in a different way.  The Group Address is the form that
the Ether community is used to thinking as Multicasting.  However as you
pointed out there is only one of those in the current implementations.

The useable form of multicasting though is the Functional address.  These
are bit significant addresses.  A Token Ring interface can have upto 31
of these assigned.  For example, if I had an interface assigned the
functional address c000.0080.1000, this interface would receive packets
with the destination addresses that had either of those bits set.  ie.
c000.0080.0000 or c000.0000.1000 or c000.0080.1000 or c000.07f0.d000
etc.

This provides essentially a superset of the ethernet multicast capability.
The biggest dissadvantage however is the small address space.  However you
don't have to convince the various hardware vendors to implement n
multicast address slots.  Which in practice is as much as a limitation.

Another argument is that they are not universal and thus will lead to
address collisions.  I maintain that this can ge dealt with via 
appropriate naming authority mechanisms.

-c
cire|eric

"I could have done it in a much more complicated
way", said the Red Queen, immensely proud.
			-- Lewis Carrol, Alice in Wonderland
Eric B. Decker
Token Ring Development
cisco Systems - engineering
Menlo Park, California

email:	cire@cisco.com
uSnail: 1360 Willow Rd.,  Menlo Park, CA  94025
Phone : (415) 326-1941