Subject: [Lsr] Rtgdir last call review of draft-ietf-ospf-mpls-elc-13

From: Dhruv Dhody via Datatracker <[email-masked]>
Date: Tue, 05 May 2020 02:44:54 -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-ospf-mpls-elc-13
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 (-09) as part of the early
directorate review, which could be located at -
https://datatracker.ietf.org/doc/review-ietf-ospf-mpls-elc-09-rtgdir-early-dhody-2019-09-12/

I have reviewed this and the IS-IS 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 OSPFv2 [RFC8665] and
   OSPFv3 [RFC8666].

   Is there a better way to introduce OSPF 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)?

(4) Section 4

    What would be the behavior if an OSPF router receives a 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 OSPF. If it is to
    just ignore the ERLD, we should explicitly say that.

Nits:
(1) Change OSPF Working group to LSR Working group in the metadata (first line
of the I-D)

(2) 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

(3) Section 4 s/Node MSD sub-TLV/Node MSD TLV/

(4) 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 OSPFv2 [RFC8665] and OSPFv3 [RFC8666] 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.  OSPF is just a messenger, 
not the consumer of this information.

> (1) Change OSPF Working group to LSR Working group in the metadata (first line

fixed.

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

fixed.

> (3) Section 4 s/Node MSD sub-TLV/Node MSD TLV/

fixed.

> (4) Expand BGP-LS on first use.

done.

thanks,
Peter

---

From: Dhruv Dhody <[email-masked]>
Date: Wed, 6 May 2020 21:10:08 +0530

Hi Peter,

Thanks for your reply, snipping to points that need further discussion...

> Protocols (IGP) such as OSPFv2 [RFC8665] and OSPFv3 [RFC8666] to signal

Much better.

> conditions are not met, the node should not advertise the ELC capability

There are two things here - (a) the actual load balancing capability
of a node (b) the capability to advertise the ELC/ERLD. Usually
capability and the advertisement of the capability go together. In
this case we want to be explicit that the absence of ERLD-MSD
indicates just (b) and not (a).

You do use the word "only", so may its all fine! I will leave it to
you/shepherd.

> not the consumer of this information.

Is there some text in RFC 8662 the clarify what one does on the
receiving side? I found only the sending conditions in section 4. How
does a receiving node behaves when he receives conflicting
information, which one does he trust (no ELC present or ERLD=10). We
could have interop issues here if you leave it open.

Thanks!
Dhruv

PS. Since the comments are the same for the IS-IS I-D, no need duplicate them.

---

From: Peter Psenak <[email-masked]>
Date: Wed, 6 May 2020 17:59:55 +0200

Hi Dhruv,

please see inline:

> You do use the word "only", so may its all fine! I will leave it to

yes, the absence of ERLD-MSD advertisements only indicates that a node 
does not support advertisement of (b).

It can not be interpreted that (b) is not supported.  Old nodes that do 
not advertise ERLD-MSD can not be assumed not to support non-zero ERLD.

The "only" is there to express the above.

> could have interop issues here if you leave it open.

well, if the node does not support ELC, then ERLD value is irrelevant. 
It's like having a speed limit for a road with no entry.

But that is something that does not belong to protocol drafts.

thanks,
Peter

---

From: "Acee Lindem (acee)" <[email-masked]>
Date: Wed, 6 May 2020 17:06:31 +0000

Hi Peter, Dhruv,

On 5/6/20, 12:00 PM, "Peter Psenak" <[email-masked]> wrote:

    Hi Dhruv,

    please see inline:

As document shepherd, I think this aligns with other OSPF functional capabilities.

Thanks,
Acee

    thanks,
    Peter