[openib-general] IPv6 and IPoIB scalability issue

[Roland Dreier]

 > IPV6 defines that each node will have a Solicited Node Multicast
 > address.  This address is unique per node and is constructed from the
 > IPV6 unicast address of the node.  (see RFC 2373 for more details).

 > IP over IB defines that IPV6 multicast addresses map to IB multicast
 > GIDs in a one to one manner.

 > IB defines a multicast address space limit of 4095 LIDs.

Yes, this is all correct...

 > Proposed solution:
 > - add an IPoIB configuration parameter.  This parameter could redirect
 > the Solicited Node Multicast traffic to the IPv6 All Nodes multicast
 > address (IB GID 0xff01601B.....0000001)

This is silly however.  For one thing you are now not following the
RFC, and compliant IPv6 over IPoIB stacks will send neighbour
discovery messages to the solicited node address, so they won't be
received since the node didn't join.

There's no requirement that a SM assign a unique MLID to each
multicast group.  The obvious solution to the problem is simply that
the SM reuse MLIDs for solicited node multicast groups, perhaps even
collapsing all of them down to 1 MLID.

 - R.