Subject: [pim] Q: Receipt of a pkt at the DR

From: Sudhindra Bengeri <[email-masked]>
Date: Mon, 22 Oct 2001 11:12:46 -0400

Hi,

We have an implementation of PIM-SM based on RFC-2362, and I have a 
question about one of the statements in the RFC.

The RFC on page 18, point (4) says that "Receipt of a packet at the DR from 
a directly connected source S, on the subnet containing the address S, 
triggers a Join/Prune message towards the RP with S in the prune list and 
the RPT-bit set under the following conditions: a) there is no matching 
(S,G) state, and b) there exists a (*,G) or (*,*,RP) for which the DR is 
not the RP"

The statement does not say if a (S,G)RPT-bit route entry should be added to 
the multicast route table. If a (S,G)RPT-bit entry is not added each packet 
arriving packet from the source S, triggers a (S,G)RPT-bit message. Clearly 
the first (S,G)RPT-bit message would trigger the required actions. I want 
to know if installing a (S,G)RPT state would have any side effect (and if 
that's the reason the RFC does not mention that).

Also I could not locate a statement in draft-ietf-pim-sm-v2-new-03.ps that 
direclty addresses the problem. I will be glad if anyone could point me to 
the appropriate sections in the draft.

TIA.

Regards,
Sudhin.

--
Sudhindra S. Bengeri
Modeling Engineer
OPNET Technologies Inc.,
(240) 497-3000 x2414

---

From: John Zwiebel <[email-masked]>
Date: Mon, 22 Oct 2001 09:18:52 -0700

> that's the reason the RFC does not mention that).

WRT 2362, whether or not you create state in your router is up to you
and the type of state you create is also up to you.  It would also be
reasonable to install (S,G) state here.  Since the RPF neighbor to the 
source and the RPF to the RP are not the same the router would include
(S,G,R) prunes in messages sent toward the RP.

> the appropriate sections in the draft.

There is no single statement in the new draft that describes this.
However, if you look through the state machines on when to send (S,G,R)
prunes, you'll see that this case is also covered.  In this case, we
have a situation where the shortest-path intersects the X-line (the
shortest path from the source to the RP).  Of course you don't want
two copies of the data to show up on the LAN, so of course, you send
a (S,G,R) prune upstream.