[openib-general] question regarding GRH flag in ib_ah_attr

[Jason Gunthorpe]

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.

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.

Jason