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

["Les Ginsberg (ginsberg)" <[e-mail]>]

(Hard to follow Acee’s post – especially for entertainment value)

Bruno –

I think that we do want some less awkward text. So I am proposing to add the following into the Introduction:

“Label Imposition is the act of modifying and/or adding labels to the outgoing label stack associated with a packet.
This includes:

o  replacing the label at the top of the label stack with a new label
o  pushing one or more new  labels onto the label stack.

The number of labels imposed is then the sum of the labels which are replaced and the labels which are pushed.
See [RFC3031] for further details.”

The BMI definition then becomes:

“Base MPLS Imposition MSD (BMI-MSD) signals the total number of MPLS
   labels which can be imposed, including all service/transport/special
   labels.”

Does this work??

    Les


From: Acee Lindem (acee)
Sent: Wednesday, October 03, 2018 2:05 PM
To: Bruno Decraene <[e-mail]>
Cc: Routing Directorate <[e-mail]>; [e-mail]; [e-mail]; Alvaro Retana <[e-mail]>; [e-mail]; Jeff Tantsura <[e-mail]>; Les Ginsberg (ginsberg) <[e-mail]>; MEURIC Julien IMT/OLN <[e-mail]>
Subject: Re: [Lsr] [RTG-DIR] RtgDir Review: draft-ietf-isis-segment-routing-msd-15

Hi Bruno,


On Oct 3, 2018, at 10:07 AM, [e-mail]<mailto:[e-mail]> wrote:

Hi Acee,


From: Acee Lindem (acee) [mailto:[e-mail]]



Hey Bruno, Jeff, Les,

Have we agreed on the precise definition of “label imposition”?
Thanks for asking.
Not so far.
We don’t necessarily need to agree on a precision definition of “label imposition”. In my latest email (a few hours ago), I proposed to reuse the phrasing from RFC 3031, which does not use that term. If we are fine with using RFC 3031 terms, that would be fine for me.

Since the MSD type has always been defined in terms of “Imposition” in both the OSPF and IS-IS MSD drafts, I think it would be better to clarify any ambiguities the text Les quotes below.

Of course, we don’t want to get too bogged down in semantics as has happened in the past: https://www.youtube.com/watch?v=-P8IYKxpqG0

Thanks,
Acee




Thanks,
--Bruno


Thanks,
Acee

From: Lsr <[e-mail]<mailto:[e-mail]>> on behalf of Bruno Decraene <[e-mail]<mailto:[e-mail]>>
Date: Wednesday, October 3, 2018 at 4:37 AM
To: Jeff Tantsura <[e-mail]<mailto:[e-mail]>>
Cc: Routing Directorate <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>, Alvaro Retana <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>, "Les Ginsberg (ginsberg)" <[e-mail]<mailto:[e-mail]>>, MEURIC Julien IMT/OLN <[e-mail]<mailto:[e-mail]>>
Subject: Re: [Lsr] [RTG-DIR] RtgDir Review: draft-ietf-isis-segment-routing-msd-15

Jeff,

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

Gents,

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

Cheers
--Bruno



Cheers,
Jeff
On Oct 2, 2018, 11:20 AM -0700, Les Ginsberg (ginsberg) <[e-mail]<mailto:[e-mail]>>, 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
   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.
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.

https://tools.ietf.org/html/rfc3031#section-3.10 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.

   Les



Thanks,
--Bruno


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