Subject: Grafts and Graft-ACKs in PIM-DM

From: Bill Fenner <[email-masked]>
Date: Fri, 4 Dec 1998 10:42:42 PST

Some issues with Grafts and Graft-ACKs in PIM-DM:

1. If a router isn't sure that it knows who the RPF neighbor is
   (i.e. it sent its Prune message with an Upstream Neighbor of
   0.0.0.0) then to whom does it send its Graft message?

2. Shouldn't the graft retransmission back off, instead of being
   fixed at every 3 seconds?

3. Section 5.3 doesn't put any conditions on sending a Graft-Ack;
   it basically says that the Graft message gets turned into a
   Graft-Ack and gets sent back.  Section 6.8 says that you don't
   send a Graft-Ack if the Graft came in on the RPF interface.
   I'd suggest removing this restriction; the Graft-Ack should be
   treated as an acknowledgment of reliable transmission of the
   Graft message, not as an acknowledgment that some action was
   taken.  Otherwise, a buggy router that has incorrect RPF info
   could retransmit Grafts to the wrong neighbor forever.

   (Or is this the intent -- that the router with the wrong RPF
   info will constantly retransmit and someday he might get the
   right RPF info so will eventually retransmit to the right router?)

  Bill

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From: Liming Wei <[email-masked]>
Date: Fri, 4 Dec 1998 11:01:06 -0800 (PST)

> From: Bill Fenner <[email-masked]>
> 

Good find.

>    fixed at every 3 seconds?

When a downstream sends a graft, and it doesn't receive a graft-ack,
it is more concerned about getting data in time (to reduce join
latency), than to save a graft message once every 3 seconds.
The tradeoff here needs to gear toward reducing join latency.

>    right RPF info so will eventually retransmit to the right router?)

This is a good suggestion. Since sending a graft-ack does not mean
that data must be fowarded too.

-Liming