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

[Hal Rosenstock]

On Tue, 2006-08-15 at 17:26, Steve Wise wrote:
> On Tue, 2006-08-15 at 14:18 -0700, Sean Hefty wrote:
> > > > App 1:
> > > > Creates QP with QKey=22
> > > > Joins multicast group 1 with QKey=33
> > > >
> > > > App 2:
> > > > Creates QP with QKey=44
> > > > Joins multicast group 1
> > > > Sends to multicast group but with QKey=22
> > >
> > >I think that last send is technically an IBA spec violation.
> > 
> > I'll admit that this definitely seems like a hack, but I haven't found where it
> > is violation.  (I'll keep looking.)  From 10.5.2.1, a QP doesn't need to be
> > attached to a multicast group to initiate a send, so it seems like any potential
> > violation is only that the QKey and MLID wouldn't match.
> > 
> > - Sean
> 
> And I can't find in the IBTA spec where they talk about the qkey field
> of a mcast group at all.

Not so fast... I know the requirements are spread out through the spec
and it's hard to piece them all together. I cited where this one is
located.

>  qkeys are used to validate an incoming
> datagram.  If the qkey in the packet doesn't match the qkey of the qp,
> then its dropped. This is an HCA ingress issue, not a mcast issue
> really.
> 
> >From what I can gather, the sender:
> 
> 1) Doesn't need to be in the group (as sean pointed out)

Yes it does. There are send only ("non") members in the JoinState
component of the MCMemberRecord. This can be used to optimize the MC
routing (as that port never needs to receive but needs to be able to
send on the MLID).

-- Hal

> 2) posts a SEND WR with an address handle for the the mcast group, the
> remote_qpn == 0xfffff, and the remote_qkey == to remote qp's qkey.
> 
> 
> Now why C10-15 is there, I dunno...but i'm sure somebody around here
> does...
> 
> :)
> 
> 
> 
> 
> 
> 
>