[openib-general] ipoib send-only join to IGMP multicast group

[Hal Rosenstock]

On Tue, 2005-09-13 at 11:54, Jack Morgenstein wrote:
> I noticed that at startup, IPoIB attempts a send-only join to the MGID
> ff12:401b:ffff:0:0:0:0:16 (equivalent to IP 224.0.0.22 -- the IGMP
> multicast group -- see
> http://www.iana.org/assignments/multicast-addresses).
> 
> 1. Why is this a send-only join? Is this just so that the local host
> to "announce" itself to a router (for neighbor calculations)?

The router needs to create the group for the hosts to join.

> 2. Who is responsible for creating this multicast group in IPoIB? 

See above.

> (A send-only join will not cause a group to be created if it does not
> yet exist)

Correct.

BTW, I think there may be a difficiency in Linux multicast semantics v.
IB multicast semantics which has been commented on previously (a long
time ago).

-- Hal