Subject: [rbridge] Should layer 2 or layer 3 IP Multicast addresses be advertised?

From: Radia.Perlman at sun.com (Radia Perlman)
Date: Tue, 02 May 2006 14:30:43 -0700

I'm trying to write up the RBridge/IP multicast interaction. What should 
a Designated RBridge put into
its link state information regarding IP multicast listeners?

The possibilities are:
a) IPv4 multicast addresses, and IPv6 multicast addresses
b) layer 2 multicast addresses derived from any of the groups the 
RBridge has heard in
IGMP Notification Messages from attached endnodes.
c) compressed layer 2 multicast addresses (i.e., leave off the top two 
bytes).

Either works. Does anyone care? The nice thing about b) is that it 
doesn't have to be different information
for IPv4 than IPv6. The nice thing about a) is that it is a little bit 
less information in the case of IPv4 (but
more information in the case of IPv6). The nice thing about c) is that 
it compresses the information
(does IPv6 have the same block of layer 2 multicast addresses derived 
from IPv6 multicast addresses
as for IPv4 multicast addresses?)

If the IPv6 and IPv4 layer 2 blocks for derived multicast addresses, I'd 
lean towards c). But does
anyone else have an opinion?

Radia

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From: shep at alum.mit.edu (Tim Shepard)
Date: Tue, 02 May 2006 23:27:50 -0400

Radia Perlman wrote:

> RBridge has heard in IGMP Notification Messages from attached endnodes.
> doesn't have to be different information for IPv4 than IPv6. The
> nice thing about a) is that it is a little bit less information in
> the case of IPv4 (but more information in the case of IPv6). The
> nice thing about c) is that it compresses the information (does IPv6
> have the same block of layer 2 multicast addresses derived from IPv6
> multicast addresses as for IPv4 multicast addresses?)

There seems to be an architectural flaw lurking in here.
What if someone wanted to make use of some protocol other than IPv4 or
IPv6 over an rbridge campus?  If it only uses unicast, then the
rbridge machinery should work just fine.

But if it uses ethernet multicast frames, then the rbridge system has
no way of determining who might be listening, other than to snoop into
higher level protocols that control the multicast routing.

Of course I'm assuming that there's no protocol somewhere in the 802.*
specifications that allows an ethernet host to announce to an ethernet
switch which ethernet multicast addresses it is interested in.  But it
seems there should be such a thing.

Should specifying that should happen in the IEEE 802 process somehow?
(I am assuming that it does not already exist.)

			-Tim Shepard
			 shep at alum.mit.edu

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From: Radia.Perlman at sun.com (Radia Perlman)
Date: Tue, 02 May 2006 23:12:41 -0700

Tim,

There isn't a problem. There's a block of layer 2 multicast addresses 
derived from IP multicast
addresses. These are the only multicast addresses that RBridges are 
attempting to optimize delivery of.
Other layer 2 multicast addresses just get delivered to all links, like 
bridges would do.

So there's no correctness issue, since when in doubt RBridges will 
deliver multicast packets to all links.
As an optimization issue, it's nice if they can be more specific, and 
only deliver to links with listeners.
I believe IEEE has somewhat recently standardized something similar to 
IGMP, but at layer 2, where
layer 2 endnodes can announce what layer 2 multicast addresses they wish 
to receive. But unless
it were universally implemented (and I've heard it is not very widely 
implemented, and certainly not universally
implemented) bridges would not be able to assume, from lack of an 
announcement, that nobody on the
link needs the packet, so they'd have to deliver everywhere.

So that is what RBridges will do...deliver multicasts to all links 
within the VLAN, other than the particular
l2 multicast block that is derived from IP multicast, where since some 
version of IGMP is implemented by all IP
endnodes that are receiving IP multicasts, lack of receipt of an IGMP 
announcement does mean it is
safe not to deliver a packet to that link.

Radia

Tim Shepard wrote:

>_______________________________________________
>rbridge mailing list
>rbridge at postel.org
>http://www.postel.org/mailman/listinfo/rbridge
>  
>

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From: gibanez at it.uc3m.es (Guillermo Ibáñez)
Date: Wed, 03 May 2006 09:42:16 +0200

>>(I am assuming that it does not already exist.)
Radia,
        I am not following the discussion, so excuse me if my reasoning 
is not correct. I understand that you refer to "IGMP snooping" and 
Generic Multicast Registration Protocol (GMRP) when talking about 
bridges.  I would say that it seems safer and simpler regarding 
compatibility, to follow the bridges approach of sniffing IGMP packets 
and distributing the information.
So option  b) seems to be the right election.
Guillermo
        
Guillermo