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Document: draft-ietf-idr-bgp-ls-sbfd-extensions-??
Reviewer: Thomas Fossati
Review Date: 2022-04-16
IETF LC End Date: 2022-04-27
IESG Telechat date: Not scheduled for a telechat

This document is compact and well written.  I think it is ready for
publication.  Thank you editors, and all the people involved in its
drafting.  I have left a few small editorial suggestions (caveat: I am
completely unfamiliar with the subject matter) in case you may find them
useful.

## Section 3, first para:

OLD
   The BGP-LS [RFC7752] specifies the Node NLRI for the advertisement of
   nodes and their attributes using the BGP-LS Attribute. 

NEW
   BGP-LS [RFC7752] specifies the Node NLRI for the advertisement of
   nodes and their attributes using the BGP-LS Attribute. 

## Section 3, second para:

I had to jump a couple of times between the textual definition and the
TLV layout to understand how the two actually match.  Specifically, I
think it's the "Length: variable." bit that triggered my confusion.

So, I have an editorial suggestion that would have saved me from a
(very) mild cognitive dissonance:

OLD
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Type             |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Discriminator 1                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Discriminator 2 (Optional)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               ...                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Discriminator n (Optional)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 1: S-BFD Discriminators TLV

     where:

   o  Type: 1032 (early allocation by IANA)

   o  Length: variable.  It MUST be a minimum of 4 octets and increments
      of 4 octets for each additional discriminator.

   o  Discriminator n: 4 octets each, carrying an S-BFD local
      discriminator value of the node.  At least one discriminator MUST
      be included in the TLV.

NEW
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Type             |          Length=4*n           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Discriminator 1                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Discriminator 2 (Optional)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               ...                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Discriminator n (Optional)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where Type is 1032, Length is 4*n, and n is the number of S-BFD local
   discriminator values of the node carried in the TLV.  n MUST be at least
   1.

## Section 3, ignorant hat on:

I am not sure what is the typical number of discriminators expected in a
TLV?  It looks to me that ~65K/4 is a decent magnitude.  What is the
expectation in case the number of discriminators is higher than the
maximum allowed by the TLV encoding?  Is this a valid scenario?  If so,
how is overflow handled?

## Section 4, Table 1:

The structure doesn't seem to match the layout of the "BGP-LS Node
Descriptor, etc." IANA registry.  In particular:
* The length field seems spurious?
* The IS-IS TLV/Sub-TLV is missing — but I guess this can be skipped, so
  not a problem;
* The first column heading should be "TLV Code Point";
* The Reference column is missing.

OLD
         +---------------+--------------------------+----------+
         |  Code Point   | Description              | Length   |
         +---------------+--------------------------+----------+
         |     1032      | S-BFD Discriminators TLV | variable |
         +---------------+--------------------------+----------+

NEW
         +----------------+--------------------------+-----------+
         | TLV Code Point | Description              | Reference |
         +----------------+--------------------------+-----------+
         |      1032      | S-BFD Discriminators TLV |  RFCthis  |
         +----------------+--------------------------+-----------+

## Section 5, first sentence:

OLD
   The new protocol extensions introduced in this document augment the
   existing IGP topology information that was distributed via [RFC7752].

NEW
   The new protocol extensions introduced in this document augment the
   existing IGP topology information that was distributed via BGP-LS
   [RFC7752].

## Section 6, first para:

OLD
   The new protocol extensions introduced in this document augment the
   existing IGP topology information that can be distributed via
   [RFC7752].

NEW
   The new protocol extensions introduced in this document augment the
   existing IGP topology information that can be distributed via BGP-LS
   [RFC7752].

# Section 5, third sentence:

Typo: "now encompasses" should be "now encompass"