Subject: IGMPv3 older version host present question

From: Todd Hayton <[email-masked]>
Date: Mon, 8 Oct 2001 12:51:35 -0400 (EDT)

Hello,

I was reading throught the IGMPv3 draft and had some questions regarding
the older version host present processing. Specifically the draft states
that:

"When Group Compatibility Mode is IGMPv2, a router acts in IGMPv2
compatibility mode, using *only* the IGMPv2 protocol for that group.  When
Group Compatibility Mode is IGMPv1, a router acts in IGMPv1 compatibility
mode, using *only* the IGMPv1 protocol for that group."

The phrases "using *only* the IGMPv1/v2 protocol for that group" suggest
that the router should be ignoring any IGMPv3 messages it hears for a
group if that group is in v1/v2 mode. However, if that is the case, then
the only time the group timer is going to be set is when a v1 or v2
report is received. This is also when the v1/v2 host present timer is
set. Since these values are both set to GMI then the older version host
present timer and the group timer are always going to expire at the same
time.

The remedy for this it seems would be to still receive and process IGMPv3
messages for the group so that the group timer could still be updated
without also updating the v1/v2 host present timers. This would allow a
router to transition up to v3 mode after all v1/v2 hosts stopped
reporting.

One side effect of this would be that if all of the v3 hosts stopped
reporting and the group filter timer and source timers went down to 0, you
wouldn't necessarily delete the group record because there might be v2/v1
hosts still interested in receiving traffic from that group.

Todd H

---

From: Bill Fenner <[email-masked]>
Date: Mon, 15 Oct 2001 16:37:51 -0700

Todd,

  You're right, the old wording didn't effectively convey what was meant.
How does this sound instead:

When Group Compatibility Mode is IGMPv3, a router acts using the IGMPv3
protocol for that group.  When Group Compatibility Mode is IGMPv2, a
router acts in IGMPv2 compatibility mode, treating all IGMPv3 messages
mentioning a group as membership in that group, except IS_INC( {} ) and
TO_INC( {} ), which are treated as IGMPv2 Leave messages.  ALLOW() and
BLOCK() messages are ignored.  When Group Compatibility Mode is IGMPv1,
a router acts in IGMPv1 compatibility mode, treating IGMPv3 messages as
above but adittionally ignoring IGMPv2 Leave messages.  Note that IGMPv3
messages that are treated differently due to the Group Compatibility
Mode do not affect the older verison host present timers.

Thanks,
  Bill