Subject: Assert RPF

From: [email-masked] (Hal Sandick)
Date: Fri, 04 Sep 1998 08:45:30 -0400

Billy Ng and I found a slight ambiguity in RFC2362 (PIM-SM) regarding how
upstream routers do the RPF check during the Assert process.  Section 2.9
describes an upstream router doing a RPF check to the source for a (S,G)
entry  or to the RP for a shared tree entry: 

   "If a router receives a multicast data gram on a multi-access LAN from
   a source whose corresponding (S,G) outgoing interface list includes
   the interface to that LAN, the packet must be a duplicate.  In this
   case a single forwarder must be elected.  Using Assert messages
   addressed to `224.0.0.13' (ALL-PIM-ROUTERS group) on the LAN,
   upstream routers can resolve which one will act as the forwarder.
   Downstream routers listen to the Asserts so they know which one was
   elected, and therefore where to send subsequent Joins. Typically this
   is the same as the downstream router's RPF (Reverse Path Forwarding)
   neighbor; but there are circumstances where this might not be the
   case, e.g., when using multiple unicast routing protocols on that
   LAN. The RPF neighbor for a particular source (or RP) is the next-hop
   router to which packets are forwarded en route to that source (or
   RP); and therefore is considered a good path via which to accept
   packets from that source."

In section 3.5.1 on sending asserts, only the "source" of the data gram is
referred to:

   "The following Assert rules are provided when a multicast packet is
   received on an outgoing multi-access interface "I" of an existing
   active (S,G), (*,G) or (*,*,RP) entry:

      1 Do unicast routing table lookup on source address from data
        packet, and send assert on interface "I" for source address in
        data packet; include metric preference of routing protocol and
        metric from routing table lookup."

We assume that section 2.9 is correct and that section 3.5.1 should be
changed to indicate a RPF lookup for the source for a (S,G) entry and a RPF
lookup for the RP for a shared tree entry.

Regards,

Hal

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From: [email-masked]
Date: Fri, 4 Sep 1998 12:10:34 -0700 (PDT)

> lookup for the RP for a shared tree entry.

Hal, it looks like you are correct, i've made a note of this in

 http://www.isi.edu/~eddy/pim/pim-specv2-notes.html

- rusty

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From: Ahmed A-G Helmy <[email-masked]>
Date: Fri, 4 Sep 1998 15:53:30 -0700 (PDT)

> lookup for the RP for a shared tree entry.

Yes, I believe your assumption is correct,... depending on the whether 
the matching entry is S,G, or *,G/*,*,RP either the source address will 
be looked up or the RP address will be looked up, respectively.

thanks for the comment,

Regards,
-A