[openib-general] [Fwd: RE: IB mcast question]

[Steve Wise]

accidentally dropped off the list...



-------- Forwarded Message --------
From: Sean Hefty <sean.hefty at intel.com>
To: 'Steve Wise' <swise at opengridcomputing.com>
Subject: RE: [openib-general] IB mcast question
Date: Tue, 15 Aug 2006 11:59:12 -0700

FYI - your reply dropped off the list.

>For type SOCK_DGRAM (UDP), the socket will receive packets from multiple
>subscribed ip mcast groups iff the dst_port of the incoming packet
>matches the port to which the socket is bound...

This is what I was referring to.  I'm really not familiar with IP multicast
beyond what I read in a book while coding the RDMA CM.  It sounds like we might
be able to use the QKey as the port number for the QP to mimic the behavior.

The RDMA CM sets the QKey for UD QPs to the port number, but sets the QKey of a
multicast group to the IPv4 address.

>NOTE: I'm just trying to understand how this works in IB.  I'm not
>necessarily advocating it should behave exactly like ip mcast/udp.

Clients need to create an UD QP.  When they join a multicast group, they get an
MGID, MLID, and QKey.  The UD QP needs to attach to the MGID / MLID, and have a
matching QKey.  Today, the RDMA CM assigns a QKey to a UD QP when it's created;
it doesn't know if it will join a multicast group or not.

>And if you want to support a single UD QP receiving from multiple
>subscribed groups, you'll have to have the same Qkey for all the groups
>and QPs.  Right?

I believe that this will be the case.  A send can specify the remote QKey, so it
may be that UD QP transfers are okay, and only multicast has an issue.

- Sean