From: Radoslav Vasilev (deckland@gmail.com)
Date: Sat May 20 2006 - 15:06:38 ART
Hi Tony,
When listener A leaves the group, it uses 224.0.0.2(All multicast listners
address).
The message is seen by the Querier and a group-specifi query is sent out the
interface where the leave was received with the group address as destination
in the IP packet header. In this way all listeners for this group would
receive the query and one of them(if any left) would report membership with
which will override the leave sent by the listener A.
Group specific queries are good as they help reduce the leave latency and
are especially usefull in production multicast environments such as IPTV
networks.
This is sometimes not enough, especially in deployements where the segment
between listener and Querier are with restricted bandwidht - such as DSL
networks. In such cases so called "Immediate leave" or "Fast leave" is uses
where the Querier skips sending the group-specific query as it relies on a
CPE-based device to check if other listeners have joined the channel/group
in question. Of course there are situation where the customer CPE act as
IGMP querier and therefore the solution is different.
Rado
On 5/18/06, Tony Paterra <apaterra@gmail.com> wrote:
>
> Reading Beau's multicast book and I can't get a clear understanding of
> what happens when a multicast listener leaves a group in IGMPv2 (yes
> I'm sorry I'm slow)... If you have RouterA as the IGMP Query Router
> and a Host H1 on an Ethernet interface that is part of the multicast
> group 224.1.1.10... My understanding is when H1 wants to leave the
> group:
>
> 1. It sends a Leave Group message with the destination in the IP
> packet of 224.0.0.2 (All Routers) w/ the Group Address field set to
> 224.1.1.10
> 2. The IGMP Query Router responds by sending a Group Specific Query
> to 224.1.1.10 to see if there are still any active listeners out there
> (who will respond with a Membership Report if they exist).
>
> My question is what the packet looks like for the Group Specific
> Query. What is the IP destination of that packet? My guess is that
> it's 224.0.0.1 (All Hosts) with the Group Address of the IGMP layer
> set to 224.1.1.10. Sorry, don't really know how I could simulate this
> with debugs, etc... :-).
>
> Thanks in advance,
> --
> Tony Paterra
> apaterra@gmail.com
>
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