[openib-general] IB mcast question

[Sean Hefty]

>In my IP-centric mind, the sender specifies the ip mcast address and a
>remote port.  All hosts with subscribers to the ip mcast address get the
>packet, and all sockets on those hosts who are bound to the dst_port
>receive a copy.   Other sockets on those hosts that joined the ipmcast
>group but are bound to different ports will _not_ get a copy of the
>packet.  In addition, the sender's local port number doesn't matter at
>all in the equation.   Now how does that translate to qkeys, udqops, and
>ib mcast?

Currently, the IP address is mapped to an MGID.  Senders and receivers are
required to subscribe to the multicast group in order to receive packets from
the multicast group.  (The UD QPs must be attached to the group to get the
packet.)  The port number is not used.

Is it possible for an IP socket to receive packets from multiple multicast
groups?

>It sounds to me like the remote_qkey is used to identify the mcast group
>when sending a mcast -and- to identify the set of qps on each host that
>should receive the incoming mcast packets.  Is this true?

I think the QKey usage in the RDMA CM needs to be redone.  If we look at just UD
QP transfers, in order to support one-to-many data transfers, all of the QPs
need to have the same QKey.

- Sean