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

Julien Meuric <> Fri, 05 October 2018 16:18 UTC

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To: "Les Ginsberg (ginsberg)" <>
CC: "" <>, "Acee Lindem (acee)" <>, Benjamin Kaduk <>, Routing Directorate <>, "" <>, "" <>, Alvaro Retana <>, "" <>, Jeff Tantsura <>
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From: Julien Meuric <>
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Subject: Re: [Lsr] [RTG-DIR] RtgDir Review: draft-ietf-isis-segment-routing-msd-15
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Hi Les,

Indeed, this version resolves my initial comments.

Thank you.


Oct. 05, 2018 -

Bruno/Julien/Benjamin –


V18 of the draft has been published. I believe this addresses all outstanding comments.


Thanx very much for your input.





From: <>
Sent: Thursday, October 04, 2018 12:56 AM




Thanks for the proposed text.

It’s crystal clear. It works for me.




From: Les Ginsberg (ginsberg) []
Sent: Wednesday, October 03, 2018 11:27 PM


(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



Does this work??





From: Acee Lindem (acee)
Sent: Wednesday, October 03, 2018 2:05 PM


Hi Bruno, 


On Oct 3, 2018, at 10:07 AM, wrote:


Hi Acee,



From: Acee Lindem (acee) [] 


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:" rel="nofollow">














From: Lsr <> on behalf of Bruno Decraene <>
Date: Wednesday, October 3, 2018 at 4:37 AM



From: Jeff Tantsura [] 
Sent: Tuesday, October 02, 2018 8:28 PM




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.



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.