Re: IGMP Unsolicited Report Interval too long for IGMPv3?

[Benjamin LaHaise <bcrl@kvack.org>]

On Mon, Jul 22, 2013 at 09:43:57PM +0100, William Manley wrote:
> If an IGMP join packet is lost you will not receive data sent to the 
> multicast group so if no data arrives from that multicast group in a 
> period of time after the IGMP join a second IGMP join will be sent.  The 
> delay between joins is the "IGMP Unsolicited Report Interval".
> 
> In the kernel this seems to be hard coded to be chosen randomly between 
> 0-10s.  In our use-case (IPTV) this is too long as it can cause channel 
> change to be slow in the presence of packet loss.
> 
> I would guess that this 10s has come from IGMPv2 RFC2236, which was 
> reduced to 1s in IGMPv3 RFC3376.

Reducing the timeout does not solve the problem you are encountering, as 
any packet loss will still result in a 1 second delay.  I've encountered 
similar issues dealing with LCP Echo request/replies for keepalive 
messages on PPP sessions.  The correct approach is to queue the IGMP 
multicast join with a higher priority than other traffic in the system 
so that the requests are not lost due to congestion of a single queue.  
Sending packets with an 802.1p header might be appropriate in your 
use-case, or perhaps using higher priority internal queues.

		-ben

> There was a thread about this on linux-rdma in 2010 in the context of IP 
> over Infiniband but it seems no patches got applied as a result of the 
> discussion:
> 
> http://www.mail-archive.com/linux-rdma@vger.kernel.org/msg05740.html
> 
> Would the right patch reducing the unsolicited report interval for 
> IGMPv3 be acceptable now?
> 
> Thanks
> 
> Will
> 
> 
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