[openib-general] IB mcast question

[Steve Wise]

[adding back to list]

On Tue, 2006-08-15 at 11:59 -0700, Sean Hefty wrote:

> 
> >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.
> 

Looking at the mckey code, I see that the code calls rdma_get_dst_attr()
to get the remote qpn/qkey + the ah_attrs for the mcast group (which is
the dst addr in this case).  Then it creates an ibv_ah.  Later when
sending, the SEND WR contains both the ah and the remote qpn/qkey.  

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?

Trying to understand how ah's relate to qpn/qkeys...