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

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To: Jeff Tantsura <>
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Subject: Re: [Lsr] [RTG-DIR] RtgDir Review: draft-ietf-isis-segment-routing-msd-15
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From: Jeff Tantsura []
Sent: Tuesday, October 02, 2018 8:28 PM
To: DECRAENE Bruno IMT/OLN; Alvaro Retana; MEURIC Julien IMT/OLN; Les Ginsberg (ginsberg)
Subject: RE: [RTG-DIR] RtgDir Review: draft-ietf-isis-segment-routing-msd-15


I’m 100% with Les here, going into platform/asic specifics within this document would inevitably create ambiguity.
And nobody is asking for this.


On Oct 2, 2018, 11:20 AM -0700, Les Ginsberg (ginsberg) <>, wrote:

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

[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.
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.

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. is relevant here.

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”.

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.




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