Subject: [rbridge] multicast router discovery: what's PIM got to do with it?

From: james.d.carlson at sun.com (James Carlson)
Date: Mon, 25 Jun 2007 12:43:43 -0400

I've read through the list archives, and I'd like some clarification
on the mechanism chosen for detecting multicast routers in the latest
(*-protocol-04) draft.

Section 4.5.1.2 has this text:

   If the frame is a PIM or MRD [RFC4286] message, RB1 learns that there
   is an IP multicast router (for the specified VLAN) on its link, and
   adds that information for the appropriate VLAN in its IS-IS link
   state.

and 4.7 has this:

   Until Multicast Router Discovery (MRD) [RFC4286] is universally
   deployed, RBridges discover IP multicast routers through their
   transmission of PIM messages. So an RBridge concludes there is an IP
   multicast router on its port if it either receives an MRD message or
   a PIM message on that port. A PIM message is recognized because the
   protocol type in the IP header is decimal 103.

This seems odd to me.  RFC 4541 (also referenced) doesn't talk at all
about listening to PIM as part of an IGMP snooping strategy, or any
need to tie IGMP L2 forwarding behavior to any particular multicast
routing protocol.

IGMPv0 (RFC 988) didn't have 'queries', but it also didn't have the
suppression logic that later versions have, and it seems to be dead,
so it shouldn't be an issue.

IGMPv1 (RFC 1112) requires multicast routers (regardless of what
higher-level routing protocol they're using -- PIM or otherwise) to
send periodic IGMP Host Membership Query messages.  You'll know that
they're present by seeing these messages.

Similarly, IGMPv2 (RFC 2236) and MLDv1 (RFC 2710) require routers to
send periodic IGMP Membership Query and MLD Multicast Listener Query
messages.

IGMPv3 (RFC 3376) and MLDv2 (RFC 3810) do the same.  It seems likely
that future versions will be similar.

IGMP and MLD version compatibility depends on having periodic queries
present on the link, so if there's any functioning multicast routing
at all, these messages will be available.  Perhaps more importantly,
listing to PIM "correctly" would mean paying attention not just to the
presence of PIM, but to PIM's DR election, so that IGMP/MLD Report
messages are sent to the right router for the network to make upstream
Joins happen -- sending to the wrong LANs if you misinterpret PIM DR
selection produces the same unintended result as not paying any
attention and forwarding IGMP everywhere.

I don't think it should be necessary to tie RBridge behavior to any
particular multicast routing protocol.  In fact, I don't see an
interoperability issue here at all in simply saying that bridges
SHOULD NOT send IGMP/MLD Report messages to host-only networks, MUST
set the link-state bit appropriately, and leaving it as an
implementation matter to determine how (and if) that's done in any
given implementation.

-- 
James Carlson, Solaris Networking              <james.d.carlson at sun.com>
Sun Microsystems / 1 Network Drive         71.232W   Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757   42.496N   Fax +1 781 442 1677

---

From: Donald.Eastlake at motorola.com (Eastlake III Donald-LDE008)
Date: Mon, 25 Jun 2007 23:14:48 -0400

You may be right. The provisions in the current draft were developed
based on earlier discussions in the working group and IETF meeting
hallways before RFC 4541 came out.

Donald 

-----Original Message-----
From: rbridge-bounces at postel.org [mailto:rbridge-bounces at postel.org] On
Behalf Of James Carlson
Sent: Monday, June 25, 2007 12:44 PM
To: TRILL/RBridge Working Group
Subject: [rbridge] multicast router discovery: what's PIM got to do with
it?

and 4.7 has this:

-- 
James Carlson, Solaris Networking              <james.d.carlson at sun.com>
Sun Microsystems / 1 Network Drive         71.232W   Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757   42.496N   Fax +1 781 442 1677
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