First of all, I apologize for the tardiness of this review; my bad.
Now onto the boilerplate:

     I have reviewed this document as part of the security directorate's
     ongoing effort to review all IETF documents being processed by the
     IESG.  These comments were written primarily for the benefit of the
     security area directors.  Document editors and WG chairs should treat
     these comments just like any other last call comments.

     The summary of the review is Ready-with-(late)-Nits

The nits are as follows:

   - instead of refering to a bit as the one that "follows" a field or is
     "right in front of" a field I suggest saying the bit is "adjacent"
     to the field.

   - it's not clear how this document updates RFC 6754. Type 11 still has
     8 reserved bits, nothing changes.

The Security Considerations are very light but that seems fine given that
the document is just codifying existing practice (sections 4.1 to 4.4) and
future-proofing (section 5) a limited resource.

   regards,

   Dan.