Subject: Re: IGMP v3 / snooping suggestion

From: Isidor Kouvelas <[email-masked]>
Date: Sun, 18 Nov 2001 23:10:01 -0800

What if you have more than one switches on the same router interface?
Doesn't the use of a fixed address break explicit tracking?

thanks
I

Toerless Eckert writes:
>Opinions about the following please: useful, necessary ?:
>
>IGMP Snooping switches may want to implement "proxy-reporting",
>eg: summarizing reports received from downstream hosts, building internal
>forwarding state and then simply reporting out of their own state in
>response to IGMP queries received from the upstream DR. This very
>much helps to offload the total number of IGMP reports the router needs
>to see, especially with IGMPv3 where there's no report suppression.
>While this feature is certainly something that better be optional,
>it is certainly worthwhile.
>
>When something like this is to be implemented, the question arises as
>to which reporter ip address the switch should use. In general, switches
>do _not_ have ip addresses on all of their VLANs.
>
>So the idea Hugh had here is to use the address 0.0.0.0 for reports in these
>cases. Question: Would this be an implicitly allowed case because 0.0.0.0
>is always a valid source-ip-address on an ip-subnet (at least for certain
>operations ?), or would that need to be / should this be explicitly noted
>as a valid source address for reports - in the IGMPv3 spec ? And would it
>be sufficient (not in violation of some other RFC) ?
>
>Beside the fact that this is very useful to make life easier for certain
>products (IGMP Snooping switches), i also think this is just from the
>pure service model quite valid - Unfortunately, RFC1112 (service model) 
>does not discuss the case of a host that _only_ supports receipt of
>ip multicast - but for such a host, what would be the need to have
>an assigned ip address ?
>
>Summary: i'd like to have some note about 0.0.0.0 being a valid reporter
>source ip address in the IGMPv3 spec, unless this would already be
>implicitly deducable from whatever other RFCs there are, and i'd like to
>know if this is sufficient to be able to use this then on hosts/switches
>whenever there's an IGMPv3 querier.
>
>Cheers
>	Toerless

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From: Hugh Holbrook <[email-masked]>
Date: Sun, 18 Nov 2001 23:34:03 -0800 (PST)

A router doing explicit tracking would have to treat 0.0.0.0
specially, recognizing that it might correspond to multiple hosts.
For example, it could use the L2 source address as the key for
explicit membership tracking when the reporter's IP source address is
0.0.0.0.

-Hugh

---

From: Falko Dressler <[email-masked]>
Date: Mon, 19 Nov 2001 09:13:12 +0100 (MET)

The switch as a 'proxy reporter' interacts with the router / other
switches just as a servant for the hosts which originated the IGMP report.
Also, all interactions between layer-2 switches are based on the layer-2
address of these devices. So, the router may recognise the 'proxy-switch'
on the basis its MAC address.

The second question of Toerless is many more interesting. Currently, there
is no way to have hosts for multicast receiving only. You always have to
have an IP address assigned to it. We should find a way to avoid this
requirement.

Falko.

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--
Falko Dressler
Universitaet Erlangen-Nuernberg, RRZE
EMail: [email-masked]
Tel: +49 9131 85-27802 / Fax: +49 9131 302941
WWW: http://bsd.rrze.uni-erlangen.de/~fd/

---

From: Toerless Eckert <[email-masked]>
Date: Sun, 18 Nov 2001 18:04:06 -0800

Cheers
	Toerless

---

From: Toerless Eckert <[email-masked]>
Date: Mon, 19 Nov 2001 09:01:21 -0800

> explicit membership tracking when the reporter's IP source address is

I am glad that i did not have to say this, because the "layer violation"
police force already has me on parole ;-), but yes, that's the way to
go for explicit tracking - but remember, the explicit tracking then on
the router is only good for zero leave latency, but not for actually
keeping track of all reporters - that property is one that you MUST loose
on the router when you enable such a feature on the switch for the reason
of scaling you LAN for more reporters than you want to be able to see
on the router itself.

And by the way, "stealing" an ip address from one of the downstream reporters
as someone else in this thread suggested is really ugly especially when that
downstream reporter decides to move to some other port in the LAN (as
might happen continuoously in a LAN hosting several 802.11 access points).

Cheers
	Toerless