Re: [PATCH v2.53] datapath: Add basic MPLS support to kernel

On Wed, Feb 05, 2014 at 05:54:38PM +0900, Simon Horman wrote:

* Ben has explained to me that there has been a change of direction
  at the ONF with regards to MPLS and VLAN tag ordering. As per
  his changelog for "Always insert MPLS labels after VLAN tags",
  the situation is now as follows:

  * OpenFlow 1.1 and 1.2 always inserted MPLS labels after VLAN tags.
  * OpenFlow 1.3 and 1.4 insert MPLS labels before VLAN tags.
  * OpenFlow 1.3.4 and 1.5, both in preparation, recognize that the change
    in 1.3 was an error and revert it.

  With this in mind only OF1.4 specifies a requirement for inserting MPLS
  labels before VLAN tags. This appears to be an abbertation at this
  point.

  Ben's suggestion as per his patch "Always insert MPLS labels after VLAN
  tags" is that Open vSwtich should only support inserting MPLS labels
  after VLAN tags. I agree with this. And to this end I have
  updated the MPLS code for the kernel datapath (this patch).

I spoke to Jesse briefly about MPLS versus VLANs a few days ago.  He
expressed a concern about the larger problem of figuring out how all the
various forms of tags should interact: VLAN, MPLS, PBB, and others that
I can't think of at the moment.  It would be more consistent from this
higher point of view if "push" and "pop" actions always acted in a
predictable place.

I agree with that point, but it still stands that I don't know of a
well-defined way for VLAN-inside-MPLS to function.  So here is my
proposal for you and Jesse to consider:

        - The datapath push_mpls action is well-defined when no VLAN is
          present.  Great.

        - Declare the behavior of the datapath push_mpls action to be
          undefined in the presence of a VLAN for now, since we don't
          really know what should happen.  For now, make the datapaths
          reject any flow that does push_mpls in the presence of a VLAN.
          Later, we can redefine that case as we please since userspace
          should not depend on the previous behavior.

        - Modify userspace to pop_vlan, push_mpls, push_vlan if
          necessary to avoid the undefined behavior.

Jesse, is that reasonable?