[mpls] On the use of GAL in MPLS-SFC OAM

[Greg Mirsky <[e-mail]>]

Hi Tarek,
thank you for your comment on our draft at the MPLS WG meeting earlier this
week. If I captured your comment correctly, you've pointed out that RFC
5586 defined that GAL MUST be at the bottom of the stack. And, because of
that, it can appear only once in the label stack. I agree with you that
that is the definition of GAL in RFC 5586 but I have several clarifications
to the current GAL definition:

   - firstly, the requirement that GAL MUST be at the bottom of the stack
   in RFC 5586 is applicable only to the MPLS-TP network. For other MPLS
   environments RFC 5586 "places no restrictions on where the GAL may appear
   within the label stack". Obviously, for any MPLS environment, the
   presence of GAL in the label stack means that ACH immediately follows the
   bottom-of-the-stack label.
   - also, will note that RFC 6423 updated the requirement of where in the
   label stack GAL is placed to the following:

         In MPLS-TP, the GAL MUST be used with packets on a G-ACh on
         LSPs, Concatenated Segments of LSPs, and with Sections, and MAY
         be used with PWs.  The presence of a GAL indicates that an ACH
         immediately follows the MPLS label stack.

As I interpret the text, the requirement for placing GAL as BoS in the
MPLS-TP environment has been lifted by RFC 6423.


To conclude, I don't find in the current normative documents related to the
use of GAL any requirements to use it only as the BoS label or that it
cannot appear more than once in the label stack. Perhaps I've missed
something in documents that specify the applicability of GAL. I much
appreciate your thoughts, comments on the use of GAL proposed in our draft.

Regards,
Greg