[mpls] RtgDir review: draft-ietf-mpls-mna-ps-hdr-08

[Tony Przygienda <[e-mail]>]

I have been selected as the Routing Directorate reviewer for this draft.

Document:  draft-ietf-mpls-mna-ps-hdr-08
Reviewer: Tony Przygienda
Intended Status: Standards Track

Summary:

I do think the document needs to clarify how it would work with multicast
and specifically BIER before it can be progressed, especially on standards
track.

Comments:

Major Issues:

    - I struggle to understand how post stack data will work with deployed
RFC8296 which mandates S bit being set to BoS which would make 2 labels
having S set
      which AFAIS has no valid semantics. Even if we could somehow assume
that it's possible I don't see how a reasonable offset can be chosen
      for the post stack data. It cannot be in front of the BIER label
since BIER label is in the case of MPLS a label with S set itself.
      It cannot follow the BIER label since a reserved NIBBLE is expected
after BIER label.
      Then, BIER itself indicates the carried PROTO
      which can be MPLS again so if the intention is to carry it after
whatever BIER carries (which may be MPLS with post stack data again) it
would
      mandate parsing of post stack possibly at 9K+ boundaries which seems
infeasible in any reasonable silicon. I didn't analyze any further use
      cases but it seems to me that anything that can encapsulate a further
frame behind a BoS label
      will push the post stack data very far back into the stack (assuming
the two BoS problem can be architecturally resolved at all).

    - The draft does not explain any of the implications of multicast but
neither does mna-hdr so it is unclear whether in case of a label
      triggering multicast replication it is expected that it's BoS and
hence stack before it (plus any data is discarded) or in case of
      multicast label producing multiple copies with a stack behind it the
data is supposed to be preserved.

  - I omit further comments where the document contradicts IMO other drafts
progressing on standards track since they seem to have undergo
    discussions on the list already

--- tony