Subject: [magma] IGMP snooping, item 2.1.2 (7)

From: Toerless Eckert <[email-masked]>
Date: Sun, 24 Nov 2002 19:11:27 -0800

>     7)  The "include source" and "exclude source" options in IGMPv3 do
>         not work on shared segments. These options are used to register
>         for multicast traffic only from certain senders, or from all
>         except certain senders.  On shared segments, if one host has
>         registered to receive a multicast data stream but has used the
>         "include source" or "exclude source" option, any additional
>         hosts that later request membership for that same multicast
>         group must accept the restrictions issued in the first host's
>         request.

I consider this description to be misleading and would appreciate if
it could be rewritten. One possible interpretation for the text above 
could for example be:

     - First host A signals EXCLUDE(G, {S1}}) membership
     - Second host B signals INCLUDE (G, {S1,S2})

     -> From reading 2.1.2(7) one could think that the IGMP snooping
        switch would now forward  in mode EXCLUDE(G, {S1}) because
        host A requested for (S1,G) not to be forwarded, and according
        to the text - "any additional host that later request membership for
	that same multicast group must accept the restrictions issued in 
	the first host's request" - host B would need to suffer for (S1,G)
	not being forwarded.

The correct behavior of an IGMP snooping switch is instead as follows:

     7) In the face of IGMPv3 "include source" and "exclude source"
	memberships reports on shared segments, the switch needs to forward
	the superset of all received membership reports onto the shared
	segment. Forwarding of traffic from a particular source S to a
	group G MUST happen if at least one host on the shared segment
	reports an IGMPv3 membership of the type INCLUDE(G, Slist1) or
	EXCLUDE(G, Slist2) where S is element of Slist1 and not element
	of Slist2.

Thanks
	Toerless
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