[openib-general] question regarding GRH flag in ib_ah_attr

[Jason Gunthorpe]

On Thu, May 11, 2006 at 07:20:19AM -0400, Hal Rosenstock wrote:

> 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 was thinking of the other direction: How does the requestor/client
know if a Path requires a GRH.

To allow what Roland is talking about you need an unambiguous
mechanism where the SA can signal to the client that the path
needs a GRH.

The only field I can see that could be used for that is HopLimit..

Think of it the other way, HopLimit < 2 means it _can't_ be forwarded
off subnet, so that result from the SA should _always_ cause the
requesting client to not use a GRH for that path.

Any test beyond HopLimit could be done in the SA prior to returning
the path records to the client. If further tests are put in the client
they only limit the routing configurations that are possible.

Note:
Although 8.3.6 specifies that 0 and 1 don't let the packet off
the subnet table 60 says that CA's should set the HopLimit
to 0 and the 'first' router should fill it in. Hmm..
 
> Why is a request with just a non link local prefix (with HopLimit
> wildcarded) not sufficient ?

I think it wouuld be best of the SA had full control over what headers
the CA's put on their packets on a path by path basis. That allows for
the most flexability down the road.

Jason