Subject: [Lsr] Rtgdir last call review of draft-ietf-isis-mpls-elc-12

From: Dhruv Dhody via Datatracker <[email-masked]>
Date: Tue, 05 May 2020 02:43:23 -0700

Reviewer: Dhruv Dhody
Review result: Has Issues

Hello,

I have been selected as the Routing Directorate reviewer for this draft. The
Routing Directorate seeks to review all routing or routing-related drafts as
they pass through IETF last call and IESG review, and sometimes on special
request. The purpose of the review is to provide assistance to the Routing ADs.
For more information about the Routing Directorate, please see
​http://trac.tools.ietf.org/area/rtg/trac/wiki/RtgDir

Although these comments are primarily for the use of the Routing ADs, it would
be helpful if you could consider them along with any other IETF Last Call
comments that you receive, and strive to resolve them through discussion or by
updating the draft.

Document: draft-ietf-isis-mpls-elc-12
Reviewer: Dhruv Dhody
Review Date: 2020-05-05
IETF LC End Date: 2020-05-05
Intended Status: Proposed Standard

Summary:
I have some minor concerns about this document that I think should be resolved
before publication.

Comments:
Disclaimer: I reviewed an earlier version (-08) as part of the early
directorate review, which could be located at -
https://datatracker.ietf.org/doc/review-ietf-isis-mpls-elc-08-rtgdir-early-dhody-2019-09-12/

I have reviewed this and the OSPF I-D together and you will find similar
comments for both I-Ds. You could discuss them in one place.

Major Issues:
None

Minor Issues:

(1) Introduction

   Recently, mechanisms have been defined to signal labels via link-
   state Interior Gateway Protocols (IGP) such as IS-IS [RFC8667].

   Is there a better way to introduce IS-IS extension for SR (than saying that
   it is just an example to signal labels)?

(2) Section 3

    Query: The text says that the ABR "MUST" preserve the ELC setting where as
    the ASBR "SHOULD" preserve it. What is the reason for using SHOULD in case
    of ASBR? Maybe we can spell out in which case ASBR might not preserve the
    ELC setting.

(3) Section 4

   The absence of ERLD-MSD advertisements indicates only that the
   advertising node does not support advertisement of this capability.

   Do you mean to differentiate between support for the capability itself v/s
   support for 'advertisement' only. But RFC 8662 says that ERLD value is
   advertised only when following conditions are met:

   *  MUST be entropy label capable and, as a consequence, MUST apply
      the data-plane procedures defined in [RFC6790].

   *  MUST be able to read an ELI/EL, which is located within its ERLD
      value.

   *  MUST take into account an EL within the first ERLD labels in its
      load-balancing function.

   Thus, I am not sure about this sentence. Maybe you mean to say that the
   absence only indicates that the ERLD-MSD value of the node is unknown (and
   it might still be capable of handling ELI/EL)?

   I see similar language in RFC8491 but I think we could be clearer in this
   I-D for ERLD.

(4) Section 4

    What would be the behavior if an IS-IS router receives an ERLD of the node
    but no ELC set for the corresponding prefix? That would be an error as per
    RFC 8662, we should specify how one handles it within IS-IS. If it is to
    just ignore the ERLD, we should explicitly say that.

(5) Section 4

    We need to clearly state that this new MSD Type is carried in Node MSD
    sub-TLV as described in [RFC8491]. And then I guess we don't really need
    figure 2? The format is as per RFC 8491!

Nits:

(1) Abstract - use term LSP instead of tunnel for consistency

   OLD:
   An ingress Label
   Switching Router (LSR) cannot insert ELs for packets going into a
   given Label Switched Path (LSP) unless an egress LSR has indicated
   via signaling that it has the capability to process ELs, referred to
   as the Entropy Label Capability (ELC), on that tunnel.
   NEW:
   An ingress Label
   Switching Router (LSR) cannot insert ELs for packets going into a
   given Label Switched Path (LSP) unless an egress LSR has indicated
   via signaling that it has the capability to process ELs, referred to
   as the Entropy Label Capability (ELC), on that LSP.
   END

(2) Expand BGP-LS on first use!

Thanks!
Dhruv

---

From: Peter Psenak <[email-masked]>
Date: Wed, 6 May 2020 13:40:19 +0200

Hi Dhruv,

thanks, for your comments, please see inline:

>     it is just an example to signal labels)?

What about:

Segment Routing with the MPLS Data Plane relies on Interior Gateway 
Protocols (IGP) such as IS-IS [RFC8667] to signal labels.

>      of ASBR? Maybe we can spell out in which case ASBR might not preserve the

redistribution is a local matter on the box and is not standardized, so 
it's quite hard to mandate a specific behavior. That's why SHOULD.

>     advertised only when following conditions are met:

What is meant is that even though all the below conditions are set, if 
the node does not support the advertisement, one can not conclude what 
its ERLD is.

If the node supports the advertisement of the ERLD, but the below 
conditions are not met, the node should not advertise the ELC capability 
in a first place.

>      just ignore the ERLD, we should explicitly say that.

the behavior is specified in the RFC 8662.  ISIS is just a messenger, 
not the consumer of this information.

>      figure 2? The format is as per RFC 8491!

done.

>     as the Entropy Label Capability (ELC), on that LSP.

fixed

> (2) Expand BGP-LS on first use!

fixed.

thanks,
Peter