[openib-general] IPv6 and IPoIB scalability issue

[Hal Rosenstock]

On Fri, 2006-12-01 at 13:37, Jason Gunthorpe wrote:
> On Fri, Dec 01, 2006 at 11:20:15AM -0500, Hal Rosenstock wrote:
> > > For IPv6 only the lower 24 bits of each assigned IPv6 address are
> > > used to construct a solicited node multicast in the range 
> > > FF02::1:FF00:0/104. The Solicited Node Multicast address it not
> > > expected to be uniquely subscribed.
> > 
> > Any idea on how many would subscribe ? What does this depend on ?
> 
> Each node subscribes to a SNM on an interface for each IPv6 address on
> that interface. In most cases that should mean 1 subscription per
> interface, but more is possible..

> Generally IPv6 addresses should be constructed based on the EUI64 of
> the IB interface. In this case the lower 24 bits of the SNM will be
> the lower 24 bits of the EUI64. Thus in many cases the SNMs will be
> cluster-unique..

It seems to depend on the low 24 bits of the IPv6 addresses in the
subnet being the same (as to whether there is more than 1 member of
these groups).

> Here is another thought.. Is there anything in the spec that says a
> MGID must map to a MLID?

Yes. Here's the first one:
p.149 line 3-8
The multicast LID range is a flat identifier space defined as 0xC000 to
0xFFFE.
The DLID for any packet which contains a multicast GID shall be within
the above specified multicast LID range.

I'm sure there are others in the spec if I looked further...

>  If there is a single subscription why not
> just do away with the MLID and return a unicast LID of the only
> subscriber?

The current spec requirements :-( But this is an interesting idea and
may warrant further consideration.

-- Hal

>  That would probably solve 90% of the IPv6 issue Todd
> pointed out. MGID compression would take care of the rest..
> 
> Jason
>