[openib-general] IB mcast question

[Hal Rosenstock]

On Tue, 2006-08-15 at 16:07, Steve Wise wrote:
> On Tue, 2006-08-15 at 12:39 -0700, Sean Hefty wrote:
> > >Why are these separated?  Isn't an address handle needed for each
> > >destination QP?  If so, then why is the remote qpn/qkey also needed to
> > >transmit a datagram?
> > 
> > The address handle doesn't include QPN/QKey information.  Maybe think of them
> > more as specifying the path to some port.
> > 
> 
> Ok.
> 
> >From what I can tell via experimentation, the qkey of the mcast group
> doesn't need to have any relation to the qkeys of the qps. 

That may be what is happening but I don't think that is correct (per the
IBA spec).

> I was able to create a mcast group with the mc qkey==0xe00a0a0a,

Don't we need to be careful about controlled Q_Keys as well ?

-- Hal

> and 3 apps joined this group, but their qp qkeys were 0 (I changed
> ucma_init_ud_qp() to set the qp qkey to 0).  One app sent to the mcgroup
> ah/qkey/qpn and the other two received the packet.  Does that make
> sense?
> 
> So maybe all we need is the concept of REUSE_PORT to allow multiple
> librdma users to create cm_ids with the same local port.  currently this
> isn't allowed.  If we do this, then all processes that want to exchange
> mcast packets would create cm_ids and do rdma_resolve_addr() with the
> same src port number on all systems. 
> 
> Senders send to the ah/remote_qpn/remote_qkey of the mcast group.  This
> routes packets to all IB ports that have subscribers.  Then since the
> sender's qp has the same qkey as all the group participants each qp will
> receive a copy of the packet.
> 
> The mcast setup code in librdma doesn't need to change.  IE the qkey can
> remain the ip mcast address.
> 
> I think this will work.  It is similar to UDP/IP/MCAST...
> 
> Or am I all wet?
> 
> whatchathink?
> 
> 
> 
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