[openib-general] IB routing discussion summary

[Sean Hefty]

Here's a first take at summarizing the IB routing discussion.

The following spec references are noted:

9.6.1.5 C9-54. The SLID shall be validated (for connected QPs).
12.7.11. CM REQ Local Port LID - is LID of remote router.
13.5.4: Defines reversible paths.

The main discussion point centered on trying to meet 9.6.1.5 C9-54.  This
requires that the forward and reverse data flows between two QPs traverse the
same router LID on both subnets.  The idea was made to try to eliminate this
compliance statement for packets carrying a GRH, but this is viewed as going
against the spirit of IBA.

Ideas were presented around trying to construct an 'inter-subnet path record'
that contained the following:

   - Side A GRH.SGID = active side's Port GID
   - Side A GRH.DGID = passive side's Port GID
   - Side A LRH.SLID = any active side's port LID
   - Side A LRH.DLID = A subnet router
   - Side A LRH.SL   = SL to A subnet router

   - Side B GRH.SGID = Side A GRH.DGID
   - Side B GRH.DGID = Side A GRH.SGID
   - Side B LRH.SLID = any passive side's port LID
   - Side B LRH.DLID = B subnet router
   - Side B LRH.SL   = SL to B subnet router

It is still unclear how such a record can be constructed.  But communication
with remote SAs might be achieved by using a well-known GID suffix.  It's also
unclear whether the fields in a path record are relative to the SA's subnet or
the SGID.

It's anticipated that SAs will need to interact with routers, but in an
unspecified manner.