Subject: Re: parallel p2p links

From: Bill Fenner <[email-masked]>
Date: Tue, 3 Nov 1998 17:39:43 PST

In message <[email-masked]>you write:
>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.

Just out of curiosity, why doesn't sending an Assert make sense?  You
received a packet on an outgoing interface, right?

  Bill

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From: Tom Pusateri <[email-masked]>
Date: Wed, 04 Nov 1998 00:53:48 -0500

>received a packet on an outgoing interface, right?

I was waiting to verify this in my code but since you brought it up...

What I should have said was it doesn't make sense in the
context of Asserts in the current spec. Asserts are only mentioned
in the current spec with respect to multi-access networks and
the outcome of the assert is that one router forwards and one
(or more) doesn't.

I think asserts ARE the right answer on p2p links since asserts are
what PIM uses to remove unnecessary downstream interfaces.
Prunes are NOT the right answer since they should only be sent upstream.

The difference with Asserts in p2p links is that you should always
remove the interface from the downstream interface list when a prune
is received on a p2p link. There's no such thing as an assert winner
or loser (both sides lose).

I think the spec should be updated to include this.

Thanks,
Tom

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From: Tom Pusateri <[email-masked]>
Date: Wed, 04 Nov 1998 01:31:57 -0500

>remove the interface from the downstream interface list when a prune
                                                                ^^^^^
Arghh! I said prune here but meant assert. 

Thanks,
Tom

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From: Bill Fenner <[email-masked]>
Date: Tue, 3 Nov 1998 22:46:35 PST

>or loser (both sides lose).

This isn't different from the what draft -00 says (other than talking
about LANs):

  Asserts received on outgoing interface:
...
 3    If you have won the election but there are no  directly  connected
      members on the LAN, schedule to prune the interface. The LAN might
      be a stub LAN with no members (and no downstream routers).  ...

The loser will lose and prune the interface.  The winner will prune the
interface unless there are group members or the loser sends a prune-override
(which it won't, since it's not the incoming interface).

  Bill

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From: Tom Pusateri <[email-masked]>
Date: Wed, 04 Nov 1998 02:17:15 -0500

Yes, you are correct. So this just needs to be generalized for p2p links.

Thanks,
Tom