Date:	Mon, 27 Sep 2010 14:45:40 -0700
From:	David Stevens <dlstevens@...ibm.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	David Miller <davem@...emloft.net>, linux-rdma@...r.kernel.org,
	netdev@...r.kernel.org, netdev-owner@...r.kernel.org,
	rda@...con.com
Subject: Re: igmp: Allow mininum interval specification for igmp timers.

Christoph Lameter <cl@...ux.com> wrote on 09/27/2010 01:20:54 PM:

> 
> >         As I said before, I think per protocol, back-to-back is both
> > allowed and not a problem, even if both subsequent randomized reports
> > come out to 0 time. But if we wanted to enforce a minimum interval
> > of, say, X, then I think the better way to do that is to set the
> > timer to X + rand(Interval-X) and not a table of fixed intervals
> 
> The second patch sets the intervals to X .. X + Rand (interval) and not 
to
> a table of fixed intervals as you state here. I have pointed this out
> before.

        Sorry if I've misunderstood something you're proposing, but what
you describe above would be certainly technically incorrect. There are
really no circumstances for sending a report greater than <Interval>
that is protocol-compliant. You can enforce a minimum greater than 0,
which is a departure from both RFCs, though IGMPv2 uses wishy-washy
language. The intent for both was to explicitly allow 0, IMO.

> 
> > as in the original patch. For v2, X=1 or 2 sec and Interval=10
> > might work well, but for v3, the entire interval is 1 sec and I
> > think I saw that the set-up time for the fabric may be on the
> > order of 1 sec.
> 
> Again there is no knowledge about V2 or V3 without a query and this is
> during the period when no querier is known yet. You stated elsewhere 
that
> I can assume V3 by default? So 1 sec?

        Yes, without a querier or the tunable to force it to IGMPv2,
the default is IGMPv3. It appears there is a bug where IGMPv3 is also
using a 10sec interval (haven't verified that), but a 1 sec interval
as required makes your situation worse, not better. It makes it even
more likely that all the initial reports will occur before your set-up
is done.
 
> There can be any number of reasons that a short outage could prevent the
> packets from going through. A buffer overrun (that you mentioned
> elsewhere) usually causes lots of packets to be lost. Buffer overrun
> scenarios usually mean that all igmp queries are lost.

        You're arguing against protocol compliance. I didn't define
the protocol, I only implemented it. And your view is through the
IB lens, but I don't believe this is an actual problem in any way
for typical networks. If you wrote a standards-track RFC that modifies
IGMP for NBMA networks that require a delay or different parameters
there, I'd have no objection to implementing that. Unilaterally
changing linux's behavior on all network types without cause for
departing from RFC on the most common types is another matter.


> There is no solution on the IB layer since there is no notification when
> the fabric reconfiguration necessary for an multicast group is complete.

        Certainly that's not true; without notification, you can queue for
first use of a new hardware multicast address and send the queue after an
appropriate delay (1 sec? If that covers your set-up time). If you had
positive acknowledgement from the IB network, you'd know exactly when to
do it, but there's no need to change anything for non-IB networks here.

> The querier is of not use since (for the gazillionth of times) this is 
an
> unsolicited IGMP report. If there is a querier then the unsolicited igmp
> reports would not be used but the timeout indicated by the querier would
> be used.

        A querier affects unsolicited reports because it sets both the
query interval and the robustness value. If you want to send 10 reports,
you can cause that by having a querier that sets it to that many. The
initial join would then send 10 reports and the query interval can also
be as low as you like.
        But the linux code is not just for your particular problem or
particular configuration. You can solve your problem by adding a querier,
but I know you're trying to do it without. The mail I was responding to
referred also to the case of a querier present, which is actually the
"normal" case for using full IGMP is. I'm saying that for the non-querier
case, making those per-interface configurable is reasonable because
they *are* querier-changeable, but you can also use a querier to change
it _for_the_unsolicited_reports_, as well as making the querier interval
small enough that you don't have to care at all whether any or all of
the unsolicited reports are lost.

                                                                +-DLS

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