Subject: RSVP over network using IP-PIM multicast routing - Path msg received on more than 1 interface

From: Dana Znamova <[email-masked]>
Date: Mon, 24 Apr 2000 12:20:59 -0400

Hello,

I have the following question regarding RSVP over a network using PIM-SM 
routing (multicast scenario).
The problem is experienced before multicast routing switches to shortest 
path tree algorithm. In that case
all traffic is being forwarded to the randezvous point, and then it may be 
routed back towards the sender
(see the scenario below).

For a scenario:
Sender ------->Router1 --------->Router 2- Randezvous Point 
-------->Destination1
                           ----
                              ---> Destination2

Scenario in summary:
- sender sending to a multicast group (Destination1 and Destination2 joined 
that group),
- second router - Router2 is a randezvous point

Routing of Path messages using PIM-SM routing (before shortest-tree is 
being used):
- Path message from sender is recieved on Router2
- Router2 forwards the message to Router2-Randezvous Point
- Router2-Randezvous Point forwards the traffic to Destination1 and Router1
- Router1 forwards the traffic to Destination2

Problems:
1. Router1 receives Path message from 2 hops
              - Sender
              - Router2 - Randezvous Point
RFC 2209 it is assumed that Path message can be received only on 1 incoming 
interface. If there are 2
incoming interfaces, one of them is considered not to be used for routing 
of Resv messages.

2. Even if we dropped the assumption that Path message can be received on 1 
incoming interface, and
considered both states to be forwarding, what about forwarding of Resv 
messages?
RFC 2205 states that RSVP messages are never forwarded on the same 
interface they arrived.
But when Router1 receives Resv message , according to the Path states on 
Router1, the message should be forwarded to Router2 and Destination2.
Since Resv message was received from Router2, the message will not be 
forwarded to Router2.
This will, however, imply that no reservation will be made on the 
Router1---Router2 link in the direction Router2->Router1. But the traffic 
routed by PIM-SM also flows from Router2->Router1.

Does anybody know how the above case is handled? Can you please point me to 
the appropriate documentation?

If further clarification is needed, please ask.

Thanks you very much for your time.

Dana

************************************
Dana Znamova
OPNET Applications Engineer
OPNET Technologies, Inc.
www.mil3.com
202-364-4700
************************************