[openib-general] question regarding GRH flag in ib_ah_attr

[Jason Gunthorpe]

On Fri, May 12, 2006 at 08:11:17AM -0400, Hal Rosenstock wrote:

> > 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.
> 
> Ah, you are referring to the SA path record response not the request.

Yes.. Though I think we are still talking about different things in a
few places ;>

How about this, how do you see this scenario:

1) Client gets a DGID from 'someplace'
2) Client sends a SA query to resolve the DGID to a Path Record
3) Client configures a QP based on the Path Record

Now, the question I'm interested in is this:
  During step #3 what test should the client apply to determine if a 
  GRH should be used with the QP.

Other issues around the GRH like management MAD responses use and
multicast I feel are well specified and don't need more consideration.
 
> > 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.
> 
> Not always true in terms of local subnet (multicast and management MAD
> response exceptions).

Yes, but these are well specified. Multicast must always have a GRH.
MAD requests are covered under my scenario above and MAD responses
to MAD requests with GRH's are specified to use the GRH and set the
HopLimit = 0xFF.

Also, I would assume when building a router that multicast packets
with a hop limit of 0 are non-forwardable based on the rules in IBA.

> Are you saying HopLimit is supplied to the SA in the request ? It could
> be but it's optional in general. In the router case, an off subnet DGID
> should be sufficient. I would think the HopLimit (as well as the other
> GRH fields) would need to be returned by the SA to the client.

Talking about a request for a Path to the SA from a client now:
I would suggest that if the client wishes to restrict itself to paths
that are only on-link then it could send a SA request with the
path record HopLimit=0. A SA request with HopLimit=* (masked out
of component mask) should let the SA return routed paths.

I also think that the SA response should have a HopLimit of 0 for
local paths and a HopLimit >= 2 for routed paths.

However, I can't find any wording in IBA that would require this
behavior.

> Not sure exactly what you mean by full control over the routing header
> (GRH). The SA supplies the info for the headers to the client and the
> client is responsible for putting the correct info in the headers. Do
> you mean supplies sufficient info for the client to do this correctly ?
> If so, I agree.

As far as I can see IBA includes all header information for the GRH
and LRH in the PathRecord response. It does not define a how to
determine if the path described by a PathRecord response requires
a GRH or not.

Thanks,
Jason