Subject: parallel p2p links

From: Tom Pusateri <[email-masked]>
Date: Mon, 19 Oct 1998 16:33:57 -0400

If a router has parallel p2p links to another router, the upstream
router will forward multicast data over both links. The downstream
interface will do an RPF check and only one path will be selected.
This path can be pruned but the other path will fail the RPF
check. Sending a prune here doesn't make sense since it isn't
upstream. And sending an assert doesn't make sense either. Therefore,
you can never prune back the source.

1. Is there any option aside from the obvious one of not allowing
   parallel p2p links.

2. Can this be addressed in the spec, please? It may be there but
   I couldn't find it.

Thanks,
Tom

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From: [email-masked]
Date: Mon, 19 Oct 1998 13:49:19 -0700 (PDT)

> you can never prune back the source.

Previous versions of the pimdm draft had the following:
   When a multicast datagram is received on the incorrect LAN  interface
   (i.e. not the RPF interface), the packet is silently discarded if the
-> router does not have any state about the destination group. If it  is
-> received on an incorrect point-to-point interface, Prunes may be sent
-> in a rate-limited fashion.

I don't see this in the current version of the draft.  Was this left out
intentionally or overlooked (possibly by me as I'm reading it...)?  

Anyway, with the above (if it is still meant to be in the spec), the
non-RPF p2p path CAN be pruned back.

--Kurt

---------------------------------------------------------------------------
Kurt Windisch
Internet Engineering Group, LLC                              [email-masked]
Univ. Oregon - Advanced Network Technolgy Center     [email-masked]
ph 541-346-1698   ---  fax 541-346-4397  ---  http://ns.uoregon.edu/~kurtw/
PGP fingerprint: 33 2A 7E 38 24 36 03 11  DB BF 22 DA C4 28 CC 23

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From: Tom Pusateri <[email-masked]>
Date: Mon, 19 Oct 1998 16:44:59 -0400

By the way, I'm thinking of PIM dense mode here.
Sparse mode should be able to handle this ok.

Thanks,
Tom

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From: Tom Pusateri <[email-masked]>
Date: Mon, 19 Oct 1998 17:13:52 -0400

>non-RPF p2p path CAN be pruned back.

Sending prunes on a downstream interface seems to really confuse the
model. Remember, there is no way to tell that it is a parallel link.

How about including a router id in the hello message to detect
parallel links? Then both ends could agree to use a single path
(maybe the one with the highest remote IP address.) Or load
splitting could be done based on other criteria once the parallel
paths have been detected.

Thanks,
Tom

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From: Dave Thaler <[email-masked]>
Date: Mon, 19 Oct 1998 17:31:48 -0400 (EDT)

> 1. Is there any option aside from the obvious one of not allowing

This isn't a real solution, since you have the same problem
even without parallel links:

         S
         |
         A
        / \
       B---C

The p2p B-C link exhibits the problem you brought up.

I agree with Kurt's answer, that the spec should allow pruning.

-Dave

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From: Steve Deering <[email-masked]>
Date: Tue, 27 Oct 1998 17:29:04 -0800

DVMRP handles it OK, too.  :-)

Steve