[openib-general] question regarding GRH flag in ib_ah_attr

[Eitan Zahavi]

I agree with Hal. If you look for Path Record to ANOTHER subnet you
should provide the GRH in the sent packet address ...

Eitan Zahavi
Design Technology Director
Mellanox Technologies LTD
Tel:+972-4-9097208
Fax:+972-4-9593245
P.O. Box 586 Yokneam 20692 ISRAEL


> -----Original Message-----
> From: openib-general-bounces at openib.org [mailto:openib-general-
> bounces at openib.org] On Behalf Of Hal Rosenstock
> Sent: Thursday, May 11, 2006 2:20 PM
> To: Jason Gunthorpe
> Cc: Roland Dreier; openib-general at openib.org
> Subject: Re: [openib-general] question regarding GRH flag in
ib_ah_attr
> 
> 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
> 
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