Subject: [magma] IGMPv2 query processing by a v1 stack

From: Subrata Sa <[email-masked]>
Date: Sat, 18 Jul 2009 17:53:18 +0530

Hello,

An IGMPv1 stack has joined groups G1, G2 and G3 and it has received an
IGMPv2 group (G1) specific query where none of the groups is "all
hosts group". I can think of following possibilities as how the stack
can handle the Query:

1. Process it as an IGMPv1 general query

As per RFC 1112 (pages 11, 12)
"
Group Address
In a Host Membership Query message, the group address field is zeroed
when sent, ignored when received. (B)
"

So the IGMPv2 Query packet is treated as a general query and reports
for ALL groups are sent.

Cons: Groups (G2, G3) that are not queried are being reported.

2. Send report for only group G1

Cons: any?

3. Drop it

Cons: Router/ snooping switch cannot refresh its forwarding state for
group G1 though sending a periodic v2 general query would do so.

I'm tempted to go for the 3rd option. What to you think?

-Subrata

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From: Indranil Bhattacharya <[email-masked]>
Date: Sun, 19 Jul 2009 02:44:17 +0530

Hi Subrata,

                I think it is option 3. This is because once the V3 router
in the LAN receives the General Query from V1 router then it will switch
back to v1 mode and will start sending v1 query then onwards.

Thanks,
Indranil

On Sat, Jul 18, 2009 at 5:53 PM, Subrata Sa <[email-masked]> wrote:

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---

From: Subrata Sa <[email-masked]>
Date: Mon, 20 Jul 2009 13:05:34 +0530

> back to v1 mode and will start sending v1 query then onwards.

In my case there is only one router in the subnet and it supports v2.
Some hosts have v1 and others support v2. If a v1 host does not
respond to a v2 group (G1) specific query, the router is not going to
delete the group's forwarding state because:

A) v2 router would ignore leave messages sent by v2 members of the
same group (G1)

RFC 2236:
"
If a router receives a Version 1 Membership Report, it MUST set
a timer to note that there are version 1 hosts present which are
members of the group for which it heard the report. This timer
should be the same as the [Group Membership Interval].

If there are version 1 hosts present for a particular group, a
router MUST ignore any Leave Group messages that it receives for
that group.
"

B) v1 host would respond to periodic v2 general query and let router
refresh its forwarding states.

On a different note, it should be okay if the router in the subnet
runs v3.  It would operate in v1 compatibility mode for group G1 on
receipt of a v1 report joining G1.

Thanks,
Subrata