Re: [RTG-DIR] Rtgdir early review of draft-ietf-mpls-rfc6374-sr-08

[Tarek Saad <[e-mail]>]

Thank you Jeffrey for the review and comments and to the authors for trying to address them.
Was there other outstanding issues that still need follow-up? If not, can the review result state be updated. Thanks.

Regards,
Tarek (as document Shepherd)


From: Rakesh Gandhi (rgandhi) <[e-mail]>
Date: Friday, February 9, 2024 at 1:20 PM
To: Zhaohui Zhang <[e-mail]>, [e-mail] <[e-mail]>
Cc: [e-mail] <[e-mail]>, Tarek Saad <[e-mail]>
Subject: Re: Rtgdir early review of draft-ietf-mpls-rfc6374-sr-08
Thanks, Jeffrey, for the detailed review. Apology for the delay in replying.

We have posted a revised draft with suggested changes.

HTML:     https://www.ietf.org/archive/id/draft-ietf-mpls-rfc6374-sr-09.html
Diff:     https://author-tools.ietf.org/iddiff?url2=draft-ietf-mpls-rfc6374-sr-09

Please see in line with replies tagged with <RG>….


From: Zhaohui Zhang via Datatracker <[e-mail]>
Date: Tuesday, August 29, 2023 at 4:50 PM
To: [e-mail] <[e-mail]>
Cc: [e-mail] <[e-mail]>, [e-mail] <[e-mail]>
Subject: Rtgdir early review of draft-ietf-mpls-rfc6374-sr-08
Reviewer: Zhaohui Zhang
Review result: Has Issues

General comment:

The document name and title/subject (including the short title at each
pager header) should have RFC6374 removed. It's better to be named as
"Performance Measurement for SR-MPLS", because it is not only using
RFC6374 but also 7876 and 9341.
<RG> Updated.

Some specific commnets:

   [RFC6374] specifies protocol mechanisms to enable the efficient and
   accurate measurement of performance metrics in MPLS networks using
   query and response messages.  [RFC7876] specifies mechanisms for
   sending and processing out-of-band responses over an UDP return path
   when receiving RFC 6374 based query messages.  These mechanisms are
   also well-suited in SR-MPLS networks.

   This document utilizes the mechanisms defined in [RFC6374] for
   Performance Delay and Loss Measurements in SR-MPLS networks, for both
   SR-MPLS links and end-to-end SR-MPLS paths including Policies.  In
   addition, this document defines Return Path TLV and Block Number TLV
   extensions for [RFC6374].

Should mention RFC7876 and RFC9341 as well.
<RG> Updated.

   For delay and loss measurement in SR-MPLS networks, the procedures
   defined in [RFC6374] are used in this document.  Note that the one-
   way, two-way and round-trip delay measurements are defined in
   Section 2.4 of [RFC6374] and are further described in this document
   for SR-MPLS networks.  Similarly, the packet loss measurement is
   defined in Section 2.2 of [RFC6374] and is further described in this
   document for SR-MPLS networks.

My personal preference is not to repeat what is already described/specified
in RFC6374. An implementation can't solely rely on this document but need
to refer to RFC6374 and other RFCs anyway, so this document can just refer
to those and point out differences. This will highlight
the differences and reduces the chance of copy and paste errors - a good
thing for authors, reviewers and implementors.
<RG> Updated to remove various repetitions.
I did not try to match the description between this document and the "base"
RFCs. So far I have not seen many differences besides the following:

- TTL
- Return Path TLV, presumably only applicable to SR-MPLS
- Block Number TLV, presumably not specific to SR-MPLS
- SR-P2MP

Am I missing anything?
<RG> FYI, the draft also describes:

  1.  the one-way/two-way/LB modes as applied to SR-MPLS.
  2.  destination and source TLVs as applied to SR-MPLS.
  3.  Path segment for loss.

   SR is enabled with MPLS data plane on nodes Q1 and R1.  The nodes Q1
   and R1 may be directly connected via a link enabled with MPLS
   (Section 2.9.1 of [RFC6374]) or a Point-to-Point (P2P) SR-MPLS path
   [RFC8402].  The link may be a physical interface, virtual link, or
   Link Aggregation Group (LAG) [IEEE802.1AX], or LAG member link.  The
   SR-MPLS path may be an SR-MPLS Policy [RFC9256] on node Q1 (called
   head-end) with destination to node R1 (called tail-end).

The paragraph mentions "link". Is that the same as the "SR-MPLS link" used
throughout the document? Is there a difference between "MPLS link" and
"SR-MPLS" link when it comes to performance measurement?
<RG> Updated to say link.

   It may be desired in SR-MPLS networks that the same path (same set of
   links and nodes) between the querier and responder be used in both
   directions of the measurement.  This is achieved by using the SR-MPLS
   Return Path TLV extension defined in this document.

Should "It may be ..." be changed to "If it is ..."? I don't see justification
for "it may be desired".
<RG> Updated.

   The packet loss measurement using Alternate-Marking Method defined in
   [RFC9341] requires collecting Block Number of the traffic counters.
   This is achieved by using the Block Number TLV extension defined in
   this document.

In RFC9341, the wording is MAY not MUST or "required":
<RG> Updated.

   The Alternate-Marking Method described in this document literally
   splits the packets of the measured flow into different measurement
   blocks.  An implementation MAY use a Block Number (BN) for data
   correlation.

Is it that Block Number TLV extension defined here is not SR-MPLS specific?
Better to clarify.
<RG> Updated for MPLS.

   A query message as shown in Figure 2 is sent over the SR-MPLS links
   for both delay and loss measurement using the procedures described in
   [RFC6374].  For SR-MPLS links, the TTL value MUST be set to 255 in
   the SR-MPLS header.  SR-MPLS encapsulation (e.g., using adjacency SID
   of the link) can be added for transmitting the query messages for SR-
   MPLS links.

Can you elaborate how adjacency SID can/should be used? If Q1 and R1 are
"directly" connected via an (SR-)MPLS link, do you need any MPLS
encapsulation, and in particular what adjacency SID does Q1 use?
<RG> Updated to remove it.

   Note that in addition to the P2P SR-MPLS paths, the SR-MPLS Segment
   List Sub-TLV is also applicable to the P2MP SR-MPLS paths.  For
   example, for P2MP SR-MPLS paths, it may only carry the Node Segment
   Identifier of the querier in order for the response to follow an SR-
   MPLS path back to the querier.

Shojld the "may only carry" be "MUST only carry"?
<RG> Updated to remove it.

SR-P2MP policy documents are not mature yet. draft-ietf-pim-sr-p2mp-policy
is only about general concept and  draft-ietf-idr-sr-p2mp-policy is only at
-00 revision - not ready for WGLC. It's best to leave P2MP outside the scope
of this document.

<RG> Updated to remove it.
Thanks,
Rakesh