Re: [Lsr] [RTG-DIR] RtgDir Review: draft-ietf-isis-segment-routing-msd-15

<bruno.decraene@orange.com> Wed, 03 October 2018 08:34 UTC

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From: <bruno.decraene@orange.com>
To: "Les Ginsberg (ginsberg)" <ginsberg@cisco.com>, Alvaro Retana <aretana.ietf@gmail.com>, MEURIC Julien IMT/OLN <julien.meuric@orange.com>
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Subject: Re: [Lsr] [RTG-DIR] RtgDir Review: draft-ietf-isis-segment-routing-msd-15
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Les,


From: Les Ginsberg (ginsberg) [mailto:ginsberg@cisco.com]
Sent: Tuesday, October 02, 2018 8:20 PM
To: DECRAENE Bruno IMT/OLN; Alvaro Retana; MEURIC Julien IMT/OLN
Cc: rtg-ads@ietf.org; lsr@ietf.org; rtg-dir@ietf.org; draft-ietf-isis-segment-routing-msd@ietf.org
Subject: RE: [RTG-DIR] RtgDir Review: draft-ietf-isis-segment-routing-msd-15

Bruno –

Trimming the thread…

[Les2:] Label imposition is meant to cover both the SWAP operation and the PUSH operation. In the example you provided above where a label stack of “12” is replaced by a label stack of “14,15” the number of labels “imposed” is 2.
[Bruno2] In that case, I definitely think that the discussion was useful and that this point needs to be clarified in the document.
Whether you choose to call that (1 POP, 2 PUSH) or (1 SWAP, 1 PUSH)  or simply a SWAP isn’t relevant here (though it might matter to folks like the RFC 3031 authors).

With that ibn mind, here is proposed text:

“Base MPLS Imposition MSD (BMI-MSD) signals the total number of MPLS
   labels which can be imposed, including all service/transport/special
   labels.  Imposition includes swap and/or push operations.

If the advertising router performs label imposition in the context of
   the ingress interface, it is not possible to meaningfully advertise
   per link values.  In such a case only the Node MSD SHOULD be
   advertised.”

[Bruno2] Given that the term “imposition” does not seem to be defined within the IETF, I would still favor a formal definition not using it. e.g. “BMI-MSD advertises the ability to increase the depth of the label stack by BMI-MSD labels”.
Alternatively, I’d propose the following rewording which seems clearer to me:
OLD: Imposition includes swap and/or push operations.
NEW: A swap operation counts as an imposition of one label; just like one push operation.

[Les3:] This gets into implementation specific issues that I would really like to avoid.
[Bruno3] So am I
For example, some implementations perform one and only one  “operation”. Conceptually that may involve a swap and a push – but from the internal implementation POV it is simply one operation. And this may be true regardless of how many labels are involved. Other implementations might perform this in several discrete steps. The language we use here should not imply anything about how many labels are associated with a specific operation.
[Bruno3] Let’s not go into implementation specifics. Let’s reuse the terminology defined in the MPLS Architecture (RFC 3031) which does use the term “operation”.

The term “increase” isn’t accurate because in the case of a swap there is no increase, yet the label which is replaced is counted.

https://tools.ietf.org/html/rfc3031#section-3.10 is relevant here.
[Bruno3] Absolutely. So let’s use RFC 3031 terminology

The term “imposition” is generic – and as Alvaro has pointed out is used in RFC 4221. And the language proposed above does define the relationship between “swap and push” and “imposition”.
[Bruno3] AFAIK, the term “imposition” is not defined in the IETF (If I missed it, please points to its definition). RFC 4271 uses it without defining it and RFC 2171 is informational.

I appreciate your desire for clarity – and I am still open to new language – but at this point I still think what I proposed is  the most accurate.

[Bruno3] So let’s reuse RFC 3031 text verbatim
OLD: Imposition includes swap and/or push operations.

NEW: “When a LSR is capable of replacing the label at the top of the label stack with a
         specified new label, and then push N specified new labels onto the label stack, the LSR advertise a BMI-MSD of N+1.

Text is coming from the MPLS architecture which is the reference for everyone.
Thanks
--Bruno

   Les



Thanks,
--Bruno


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