Subject: IGMP Proxy and PIM-SM

From: Narayanan Vidya-CVN065 <[email-masked]>
Date: Thu, 18 May 2000 16:24:07 -0500

Hi,
I have a question on the interaction of IGMP Proxy and PIM-SM. IGMP proxy is
meant to extend the tentacles of the host one hop away from the multicast
routing domain. It seems like it is really meant only for receivers, when we
take PIM-SM behavior into account. PIM-SM source DRs apparently do not
encapsulate packets that do not originate from the local subnet to the RP.
This means, if I have a router A running IGMP proxy connected to a router B
that is running PIM-SM and is connected to a bunch of other routers running
PIM-SM also, I can only hang multicast receivers off router A. Consider the
following scenario:

Host A ----- Router A (IGMP Proxy) ----- Router B (PIM-SM; non-RP) -----
Router C (PIM-SM; RP) ----- Downstreams running PIM-SM

Assume Router A is connected to Router B through a LAN. Also, assume the
host A is on a different LAN. Assume WAN connectivity between Routers B and
C. Now, if Host A was a receiver, everything works fine. If host A were to
send multicast UDP packets, then, the source address of the packets that hit
Router B will belong to a subnet different from its local subnet. It seems
from various implementations that Router B will not encapsulate these
packets to the RP and instead drop it, since the source address check fails
for PIM-SM. 

Are there any negatives for why Router B must not just encapsulate these
packets to the RP? In a very restricted network, where I don't worry about
security, what could be the disadvantages of doing this? I am not sure if
this was just not considered in the IGMP proxy model or was just left open. 

Any comments would be of great help.

Thanks,
Vidya

---

From: "John M. Zwiebel" <[email-masked]>
Date: Thu, 18 May 2000 16:19:01 -0700

On the "are there any negatives" question only:
	Imagine a PIM domain running only dense-mode but with hundreds of
	sources.  Also, there have been problems with some failure
	topologies resulting in duplicate register packets arriving
	at the RP from different first-hop routers.

	It is better to specifically have to configure an interface
	to register for sources that are not directly connected.
	
	FWIW, IOS can do this.

 ^ Hi,
 ^ I have a question on the interaction of IGMP Proxy and PIM-SM. IGMP proxy is
 ^ meant to extend the tentacles of the host one hop away from the multicast
 ^ routing domain. It seems like it is really meant only for receivers, when we
 ^ take PIM-SM behavior into account. PIM-SM source DRs apparently do not
 ^ encapsulate packets that do not originate from the local subnet to the RP.
 ^ This means, if I have a router A running IGMP proxy connected to a router B
 ^ that is running PIM-SM and is connected to a bunch of other routers running
 ^ PIM-SM also, I can only hang multicast receivers off router A. Consider the
 ^ following scenario:
 ^ 
 ^ Host A ----- Router A (IGMP Proxy) ----- Router B (PIM-SM; non-RP) -----
 ^ Router C (PIM-SM; RP) ----- Downstreams running PIM-SM
 ^ 
 ^ Assume Router A is connected to Router B through a LAN. Also, assume the
 ^ host A is on a different LAN. Assume WAN connectivity between Routers B and
 ^ C. Now, if Host A was a receiver, everything works fine. If host A were to
 ^ send multicast UDP packets, then, the source address of the packets that hit
 ^ Router B will belong to a subnet different from its local subnet. It seems
 ^ from various implementations that Router B will not encapsulate these
 ^ packets to the RP and instead drop it, since the source address check fails
 ^ for PIM-SM. 
 ^ 
 ^ Are there any negatives for why Router B must not just encapsulate these
 ^ packets to the RP? In a very restricted network, where I don't worry about
 ^ security, what could be the disadvantages of doing this? I am not sure if
 ^ this was just not considered in the IGMP proxy model or was just left open. 
 ^ 
 ^ Any comments would be of great help.
 ^ 
 ^ Thanks,
 ^ Vidya
 ^ 

------------------------------------------------------------------------------
	John Zwiebel                       Phone: 408-526-5303
	Cisco Systems Inc.                 
	IP Multicast Group