RE: IGMP/MLD-Proxying: broken upstream forwarding rule? (RFC4605 section 4.2)

Mark wrote:
 
>I can live with that, but then I wonder what's the forwarder selection (for
>downstream forwarding) is good for. If there's always only a single router
>towards the root (as it is the case in a tree topology), there is no need for
>selecting a designated forwarder.
 
The tree topology restriction is only for IGMP proxys but multiple IGMP routers (not proxies) can be connected to the same shared LAN. I think this is the reason why DR election is needed.
 
I hope this can help you
Regards

Alvaro
 
 
 

________________________________

De: Mark Doll [mailto:[email protected]]
Enviado el: sáb 29/09/2007 14:42
Para: Brian Haberman
CC: [email protected]
Asunto: Re: [magma] IGMP/MLD-Proxying: broken upstream forwarding rule? (RFC4605 section 4.2)



Hi Brian!

Thanks for your prompt answer.

Brian Haberman wrote:
>
> Mark Doll wrote:
>> This seems to suggest an unconditional forwarding in the upstream
>> direction, which will lead to forwarding loops in any non loop-free
>> physical topology (any topology other than a tree).
>
> RFC 4605 only supports loop-free topologies.  The following text is in
> the Introduction:
>
>    This document applies spanning tree multicast routing [MCAST] to an
>    Internet Group Management Protocol (IGMP) or Multicast Listener
>    Discovery (MLD)-only environment.  The topology is limited to a tree,
>    since we specify no protocol to build a spanning tree over a more
>    complex topology.  The root of the tree is assumed to be connected to
>    a wider multicast infrastructure.

I can live with that, but then I wonder what's the forwarder selection (for
downstream forwarding) is good for. If there's always only a single router
towards the root (as it is the case in a tree topology), there is no need for
selecting a designated forwarder.

Furthermore, the example in RFC 4605, section 3, is misleading, as it
suggests, that the displayed topology is possible with IGMP/MLD-Proxying:

           LAN 1  --------------------------------------
                  Upstream |              | Upstream
                           A              B
                Downstream |              | Downstream
           LAN 2  --------------------------------------

But it is not. Suppose that there are, on LAN 2, one receiver, that joined a
multicast group, and one source sending to that group. With the current spec,
as I understand it, both routers A and B will unconditionally forward traffic
onto LAN 1. A second receiver on LAN 1 will thus receive every packet twice.
Worse, one of the routers, the one, that is the querier on LAN 2, will forward
all traffic, that was forwarded by the other router onto LAN 1, back in the
downstream direction onto LAN 2, thus creating a loop.

The simple solution to this "bug" is not to forward unconditionally in
upstream direction but to forward only if one is the querier on the incoming
(downstream) interface. In the displayed topology, only one of the routers
will then forward traffic from LAN 2 onto LAN 1 an no traffic will loop back.

Since this solution is so straightforward, I wonder, if implementations
already fix this bug (does anyone know about a freely available implementation
of IGMP/MLG-Proxying?). To be honest, I always thought, upstream forwarding
would take queriership into account until I recently re-read RFC 4605 and
stumbled upon the formulation in the packet forwarding in section 4.2.

Mark.

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