[openib-general] question regarding GRH flag in ib_ah_attr

[Hal Rosenstock]

On Thu, 2006-05-11 at 01:48, Jason Gunthorpe wrote:
> On Wed, May 10, 2006 at 09:56:58PM -0700, Roland Dreier wrote:
> >     Hal> Huh ? In this case, aren't the subnet prefixes are required
> >     Hal> to be different ?
> > 
> > It's kind of a crazy thing to do but I don't see anything in the IB
> > spec that forbids two subnets with the same subnet prefix, or any
> > reason why a router couldn't route between them.  The SMs would just
> > have to be smart enough to return the LID of the router for paths to
> > ports on the other subnet, and the routers would have to have explicit
> > routes rather than forwarding based on just GID prefix.
> 
> Hmm, this is an interesting point, you can do this in IP land using
> host routes.
> 
> How about this - the Path record (and related) SA responses include
> the Hop Limit fields and the spec says:
> 
> 8.3.6 Hop Limit: [..] Setting this value to 0 or 1 will ensure that
> the packet will not be forwarded beyond the local subnet.
> 
> So, it is within the spec to use HopLmt >= 2 as the GRH required flag.

That would be a simpler check but HopLimit is not a required component
of PathRecord but I think this may not be sufficient as just because a
HopLimit >= 2 doesn't mean that a packet would be forwarded off subnet.

> I'd propose that the combination of a non-link-local prefix and a >= 2
> Hop Limit should force a GRH. SM's that do not support routers should
> always fill in 0 for HopLmt.

Why is a request with just a non link local prefix (with HopLimit
wildcarded) not sufficient ?

-- Hal

> Jason