[pim] IGMP v3 update - clarification about which router queries cause version fallback

[Phil Karn <[e-mail]>]

Hi. I'm been trying to figure out why Linux on my network keeps falling 
back to IGMPv2, and I've found out why. I may need a clarification to 
RFC3376.

At first, all works fine. My snooping switch issues an IGMPv3 general 
membership query and my Linux system answers with IGMPv3 membership 
messages. Great.

Then my Linux system sees a IGMPv2 group-specific query for 224.0.0.251 
(the multicast DNS group). Since I'm running avahi-daemon (the mDNS 
service daemon) this elicits an outgoing IGMPv2 membership report for 
224.0.0.251. Then subsequent IGMPv3 general membership queries elicits 
IGMPv2 (not v3) membership reports for all my other multicast group 
memberships.

I believe this is a bug in Linux, but I want to make sure RFC3376 says 
what I think it says.

RFC3376 says an IGMPv2 general query should change the interface version 
to IGMPv2 and start the IGMPv2 querier-present timer, after which the 
interface reverts to v3 if enough time goes by without another IGMPv2 
general query. But it does not explicitly say that ONLY a IGMPv2 GENERAL 
query should do this -- that a group-specific IGMPv2 query should NOT 
cause the interface to drop into IGMPv2 mode even if we're a member of 
that group.

The problem is that buggy Apple devices frequently and gratuitously join 
and leave 224.0.0.251, and these devices don't speak IGMPv3. Whenever 
one leaves, the querier correctly issues a group-specific query to see 
if any other members are left -- and this group-specific query knocks 
Linux back to IGMPv2. Since these Apple devices gratuitously join and 
leave more rapidly than the IGMPv2-querier-present timer expires, the 
situation persists. IGMPv3 becomes unusable on my network.

So is it the intent of RFC3376 that ONLY a *general* IGMPv2/v1 query 
should trigger the version fallback logic, or should a group-specific 
IGMPv2 query do the same?

To summarize:

1. RFC 3376 is arguably vague about exactly which membership queries 
should invoke a version fallback. All IGMPv2/v1 queries, or only general 
ones?

2. Linux interprets any IGMPv2 membership query - not just a general 
query - as triggering a fallback. Right now I don't know if this is 
correct behavior or an honest misunderstanding of a vague specification.

3. Apple really should get with the program and implement IGMPv3. It's 
only what, 22 years old?

4. Apple really should get with the program and stop explicitly joining 
a very popular link-local multicast address (224.0.0.251) that every 
host is supposed to be an implicit member of anyway.

5. Apple really should get with the program and stop gratuitously 
leaving and re-joining said link-local multicast group.

But I'm only asking this list to address point #1. Should group-specific 
IGMPv2 membership queries trigger a version fallback until the IGMPv2 
Querier Present timer expires? Or should only general queries do so?

--Phil