netdev - Re: igmp: Staggered igmp report intervals for unsolicited igmp reports

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Date:	Thu, 23 Sep 2010 12:56:17 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
cc:	David Stevens <dlstevens@...ibm.com>, linux-rdma@...r.kernel.org,
	netdev@...r.kernel.org, Bob Arendt <rda@...con.com>
Subject: Re: igmp: Staggered igmp report intervals for unsolicited igmp
 reports

On Thu, 23 Sep 2010, Jason Gunthorpe wrote:

> > IPoIB creates a infiniband multicast group via the IB calls for a IP
> > multicast group. Then IGMP comes into play and the kernel sends the IP
> > based igmp report. This igmp report must be received by an outside router
> > (on an IP network) in order to for traffic to get forwarded into the IB
> > fabric. You can end up with a IB multicast configuration that is all fine
> > but with loss of the unsolicited packets due to fabric reconfiguration not
> > being complete yet. The larger the fabric the worse the situation.
>
> But my point is that IB has very limited multicast, if I create a IB
> group and then send IGMP into that group *it will not reach a router*.

The IPoIB routers automatically join all IP MC groups created.

> The only way this kind of scheme could work is if an IGMPv2 IPoIB
> router listens for IB MGID Create notices from the SA and
> automatically joins all groups that are created, so it can get IGMPv2
> membership reports. Which obviously adds more delay, lag, and risk.

Right that is how it works now.

> I'm *guessing* that the change in IGMPv3 to send reports to 224.0.0.22
> (all IGMPv3 multicast address) is related to this sort of problem, and
> it seems like on IB IGMPv2 is not a good fit and should not be used if
> v3 is available..

Existing routers do no support IGMPv3.

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