Subject: RE: Question on IGMP/RPF ?

From: Jesus Arango <[email-masked]>
Date: Fri, 15 Aug 1997 16:07:14 -0500

-----Original Message-----
From: Bill Fenner [SMTP:[email-masked]]
Sent: Friday, August 15, 1997 12:48 PM
Subject: Re: Question on IGMP/RPF ?

Routers never use IGMP reports to report group memberships on other
networks to other routers.  The exact mechanism depends on the
multicast routing protocol, for example with DVMRP the first packet
from any given source to a group is broadcasted to all routers in the
network, which then prune back the delivery tree if there are no local
group members.  With PIM-SM, the routers send Join messages towards the
RP to build up the tree.  The only case in which routers send IGMP
membership reports is when they are themselves members of a group.
(e.g. a router that runs NTP might want to join the well-known NTP
multicast group, NTP.MCAST.NET).

[Jesus Arango]
Thanks for the answer ... but wouldn't it be more efficient if routers
themselves issued IGMP reports? I think this would save a lot of
problems related to prunning, such as having to resend packets to the
whole network periodically in order to keep routing state up to date
(RPF). The only problem I see if IGMP is used is the join latency but
isn't it more important to save network resource? Or who knows, I might
be missing other problems you could point out.

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From: Bill Fenner <[email-masked]>
Date: Fri, 15 Aug 1997 16:58:51 PDT

Jesus Arango <[email-masked]> wrote:
>wouldn't it be more efficient if routers
>themselves issued IGMP reports?

I'd recommend checking out Deering's thesis
(ftp://gregorio.stanford.edu/vmtp-ip/sdthesis.part[123].ps.Z) for a
discussion of various styles of multicast routing.  Chapters 6 and 8
describe protocols that flood membership information, and chapter 7
describes DVMRP, the protocol the MBone uses today.  Also see the PIM
and CBT specs for protocols which require much less state than DVMRP.

In particular, chapter 6 describes a spanning-tree multicast routing
algorithm, in which routers send IGMP reports to describe group
membership.  However, building and maintaining a spanning tree is
expensive, so this protocol is only suited for use when you are
already building a spanning tree for other reasons.  Without the
spanning tree, membership reports can loop, which can cause traffic
to loop (catastrophic!) and/or membership information to live after
hosts have left the group.

  Bill

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From: Bill Fenner <[email-masked]>
Date: Fri, 15 Aug 1997 16:58:51 PDT

Bill