[ovs-dev] [mpls v2 1/2] Implement OpenFlow support for MPLS, for up to 3 labels.

[Jesse Gross]

On Sun, Dec 29, 2013 at 2:50 AM, Ben Pfaff <blp at nicira.com> wrote:
> I've been a little frustrated with the current approach to MPLS, because it
> seems quite difficult to understand.  One particularly difficult bit for
> me is the variables used during translation, e.g. mpls_depth_delta and
> pre_push_mpls_lse.  And what we end up with is support for a single MPLS
> label, which I don't think is going to make any real-world users happy.
>
> This commit attempts to implement something easier to understand and more
> powerful by just keeping track of all the labels in struct flow.
>
> Signed-off-by: Ben Pfaff <blp at nicira.com>
> Co-authored-by: Simon Horman <horms at verge.net.au>
> Signed-off-by: Simon Horman <horms at verge.net.au>

I have some general comments:

* I'm not sure that flow_count_mpls_labels() is megaflow-safe since it
reads fields without updating the wildcards. I think the only real way
to handle this is with automatic tracking.
* It seems like we might want a harder failure mechanism for cases
where we exceed the number of labels that we are able to handle. I
don't think that this is the same as trying to change L4 ports when
you don't have an L4 header because an MPLS packet could still be
interpreted, just differently.

Other things, assuming that the goal is to eventually hook this up to
the kernel patches:

* How do we handle cases where userspace supports parsing more labels
than the kernel (which is actually true at the moment)? I think that
it would just fail at flow installation time right now.
* Somewhat related to the above, the kernel's action validator doesn't
trust userspace to provide a valid EtherType for pop actions, so it
doesn't allow multiple consecutive pops in cases where there are more
tags than it knows about (which is 1).