Re: [Lsr] Last Call: <draft-ietf-lsr-isis-srv6-extensions-14.txt> (IS-IS Extension to Support Segment Routing over IPv6 Dataplane) to Proposed Standard

Hi Peter, All,

Could we update to "draft-ietf-lsr-isis-srv6-extensions" that the prefix-attribute tlv is mandatory when a locator is redistributed?

Why? 
*When calculating a LFA for an SRv6 End.SID we better know if the locator has been redistributed or not for a correct operation.

Reasoning:
* A locator has the D bit. This one is set when we redistribute from L2 to L1.
** So this end-sid will not be used as we know that it is redistributed.

* In the other direction (L1-L2), we only know that a locator is redistributed from L1 to L2 if the prefix-attribute sub-tlv is advertised.
** This means if the operator does not configure advertisement of the prefix-attribute tlv, ISIS could potentially use an end-sid which does not terminate on the expected  node.

* Compared to sr-mpls, a prefix-sid has the R flag indicating it is redistributed.
* We don't have that for locator end-sids.

Relevant snip from " draft-ietf-lsr-isis-srv6-extensions"

7.1.  SRv6 Locator TLV Format

   The SRv6 Locator TLV has the following format:

       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    |R|R|R|R|    MT ID              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type: 27

        Length: variable.

        R bits: reserved for future use. They MUST be
        set to zero on transmission and MUST be ignored on receipt.

        MT ID: Multitopology Identifier as defined in [RFC5120].
        Note that the value 0 is legal.

   Followed by one or more locator entries of the form:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Metric                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Flags       |  Algorithm    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Loc Size     | Locator (variable)...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Sub-TLV-len  |         Sub-TLVs (variable) . . .             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Metric: 4 octets. As described in [RFC5305].

        Flags: 1 octet. The following flags are defined

          0
          0 1 2 3 4 5 6 7
         +-+-+-+-+-+-+-+-+
         |D|    Reserved |
         +-+-+-+-+-+-+-+-+

         where:
           D-flag: Same as described in section 4.1. of [RFC5305].

G/

-----Original Message-----
From: Lsr <lsr-bounces@ietf.org> On Behalf Of The IESG
Sent: Thursday, April 22, 2021 10:14 PM
To: IETF-Announce <ietf-announce@ietf.org>
Cc: lsr@ietf.org; lsr-chairs@ietf.org; chopps@chopps.org; draft-ietf-lsr-isis-srv6-extensions@ietf.org; aretana.ietf@gmail.com
Subject: [Lsr] Last Call: <draft-ietf-lsr-isis-srv6-extensions-14.txt> (IS-IS Extension to Support Segment Routing over IPv6 Dataplane) to Proposed Standard

The IESG has received a request from the Link State Routing WG (lsr) to consider the following document: - 'IS-IS Extension to Support Segment Routing over IPv6 Dataplane'
  <draft-ietf-lsr-isis-srv6-extensions-14.txt> as Proposed Standard

The IESG plans to make a decision in the next few weeks, and solicits final comments on this action. Please send substantive comments to the last-call@ietf.org mailing lists by 2021-05-07. Exceptionally, comments may be sent to iesg@ietf.org instead. In either case, please retain the beginning of the Subject line to allow automated sorting.

Abstract

   The Segment Routing (SR) allows for a flexible definition of end-to-
   end paths by encoding paths as sequences of topological sub-paths,
   called "segments".  Segment routing architecture can be implemented
   over an MPLS data plane as well as an IPv6 data plane.  This document
   describes the IS-IS extensions required to support Segment Routing
   over an IPv6 data plane.

   This documents updates RFC 7370 by modifying an existing registry.

The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-lsr-isis-srv6-extensions/

The following IPR Declarations may be related to this I-D:

   https://datatracker.ietf.org/ipr/3796/
   https://datatracker.ietf.org/ipr/4486/

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