Fw: [openib-general] IPv6 All Router Multicast Group

[Hal Rosenstock]

Hal Rosenstock wrote:
> Roland Dreier wrote:
>>     Hal> A host needs to tell the multicast router when it is joining
>>     Hal> and leaving a group so the router knows when to join the
>>     Hal> multicast tree for that group or prune the tree.
>> 
>> If the host does not need to receive any multicast messages, then
>> only the multicast routers would need to join the group.
> 
> In IGMP, the host needs to both listen (for queries) and send
> (reports, leave).
> The inverse is true for mrouters. It seems like they both need to
> join the group
> if they are running IGMP of some version. IGMPv1 does not support
> leave. 

In thinking about this further, it depends on whether the messages
are multicast or not and in what direction(s). 

The router to host general query is multicast (which means a host 
would need to receive as well as send) but this is on the all
systems multicast address (224.0.0.1). Specific queries are
sent on the group being queried.

Routers send to these groups and also listen to the all routers group 
(224.0.0.2) for IGMPv2 (leaves are sent on this group from hosts) and
all other groups (which is where the reports come back on).

Only Version 3 Reports are sent with an IP destination address of
224.0.0.22, to which all IGMPv3-capable multicast routers listen.

So hosts send only on this group.

-- Hal