AD Evaluation: draft-ietf-6man-segment-routing-header-21

[Suresh Krishnan <[e-mail]>]

Hi authors,
  I have completed my AD evaluation of draft-ietf-6man-segment-routing-header-21 and found the draft to be well written and easy to understand. There are a few issues I would like to see addressed before I send it off to IETF Last Call and I have listed them below.

Major:

* Section 2.1.

  TLVs are present when the Hdr Ext Len exceeds the Last Entry element
  in the Segment List.

This seems to be wrong. Say there are 5 entries in the Segment list and no TLVs. The Hdr Ext Len would be 10 and the Last Entry will be 4. 

* Section 2.1.2:

What is the purpose of being able to include a HMAC TLV with the Key ID set to 0 since there will be no HMAC?

* Section 2.1.2.2:

"SRH implementations can support multiple hash functions but MUST
 implement SHA-2 [FIPS180-4] in its SHA-256 variant.”

Since support for the HMAC TLV is optional, I think this MUST level requirement for SRH implementations needs to be relaxed or qualified.

* Section 5.3:

I think more descriptive text is needed here. Simply specifying a greater MTU value is not enough. Can you expand on your thinking here as to what you are trying to mitigate?

Minor:
=====

Section 2:

Not sure where this text is in Section 4.4 of RFC8200.

  In the SRH, the Next Header, Hdr Ext Len, and Routing Type fields are
  defined in Section 4.4 of [RFC8200] as not mutable.  The Segments
  Left field is defined as mutable in Section 4.4 of [RFC8200].

Section 2.1.1:

I think this section should also mention that the padding TLVs are used for alignment of TLVs themselves. Something like

NEW:
Padding TLVs are used for meeting the alignment requirement of the subsequent TLVs

Section 2.1.2.1:

This text in the beginning is really out of place. At this point “Text” is not even introduced.

"potentially provides an alternate composition of Text"

* Section 5.4:

Mentioning the 0th segment does not seem very obvious. Suggest rewording to

OLD:
Use the 0th segment in the segment list

NEW:
Use the SID at Segment List[0]

Editorial:

Section 1:

OLD:
Segment Routing can be applied to the IPv6 data plane using a new
type of Routing Extension Header (SRH)

NEW:
Segment Routing can be applied to the IPv6 data plane using a new
type of routing extension header called the Segment Routing Header (SRH)


OLD:
Terminology used within this document is defined in detail in [RFC8402]. 
Specifically, these terms: Segment Routing, SR Domain,
SRv6, Segment ID (SID), SRv6 SID, Active Segment, and SR Policy.

NEW:
This document uses the terms Segment Routing, SR Domain,
SRv6, Segment ID (SID), SRv6 SID, Active Segment, and SR Policy as defined in [RFC8402]. 

Please let me know if you have any questions or concerns.

Thanks
Suresh