Subject: Re: Re: DVMRP Forwarding

From: [email-masked]
Date: Tue, 20 Jun 2000 15:50:18 +0530

Hello,
 Thanks for the reply.

 What I thought was if IGMP v3 is not deployed( Finding this out is again a problem) then the 
(S1,G) Forwarding cache entry could be used for forwarding the first (S2,G) packet. The 
(S2,G) cache entry could be created by duplicating the (S1,G) entry, of course with the 
appropriate upstream detail. This way the first (S2,G) packet will not be unnecessarily 
forwarded on all interfaces.If IGMPv3 is deployed then this cannot be done. Is this approach 
correct. 

Regards,
Vijay

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From: Sundeep Singatwaria <[email-masked]>
Date: Tue, 20 Jun 2000 09:53:22 -0700

vijay, hope this figure helps you

      ----------------------------------
      | (S1,G)           (S2,G)                 |
    [R0] --------- [R1] -----------  [R3]

If at R1 you duplicate (S2,G) entry into (S1,G) then you would have multiple copies of same
packet.

sundeep

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From: Steve Deering <[email-masked]>
Date: Tue, 20 Jun 2000 09:07:16 -0700

At 3:50 PM +0530 6/20/00, [email-masked] wrote:
>Hello,
> Thanks for the reply.
>
> What I thought was if IGMP v3 is not deployed( Finding this out is again
>a problem) then the (S1,G) Forwarding cache entry could be used for
>forwarding the first (S2,G) packet. The  (S2,G) cache entry could be
>created by duplicating the (S1,G) entry, of course with the appropriate
>upstream detail. This way the first (S2,G) packet will not be unnecessarily
>forwarded on all interfaces.  If IGMPv3 is deployed then this cannot be
>done. Is this approach correct. 

No, it is not, as Bill Fenner and I already told you.  Here is an example
to show why.

Consider the following topology:

           N1        N2        N3 (member of G)
           o.........o.........o
           :                   :
           :                   :
           :                   :
           :                   :
           o...................o.........o
           N4                  N5        N6 (member of G)

Assume nodes N3 and N6 are members of the group G.
Assume the DVMRP distance metric is in terms of hops.
Recall that DVMRP constructs shortest reverse-path trees, which is the
same as shortest-path trees when the distance metrics are symmetric.

Now assume that N1 has been sending packets to group G, so the following
delivery tree has been established for (N1, G):

           N1        N2        N3 (member of G)
           o---------o-------->o
           |                   :
           |                   :
           |                   :
           V                   :
           o------------------>o-------->o
           N4                  N5        N6 (member of G)

Then N4 starts sending packets to the same group, G.  The correct delivery
tree for (N4, G) would be:

           N1        N2        N3 (member of G)
           o.........o.........o
           :                   ^
           :                   |
           :                   |
           :                   |
           o------------------>o-------->o
           N4                  N5        N6 (member of G)

But following your proposal, N5 would fail to send a copy on the N5->N3
link, because that link is not included in the set of outgoing links for
the earlier (N1,G) tree.

Note that in this example, from N5's point-of-view, both the (N1,G) and
(N4,G) delivery trees have the same destination G and the same incoming
link, but different outgoing link sets.

Steve