Re: [Idr] Benjamin Kaduk's No Objection on draft-ietf-idr-bgp-ls-segment-routing-msd-17: (with COMMENT)

Benjamin Kaduk <kaduk@mit.edu> Wed, 06 May 2020 21:33 UTC

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Date: Wed, 6 May 2020 14:33:27 -0700
From: Benjamin Kaduk <kaduk@mit.edu>
To: Alvaro Retana <aretana.ietf@gmail.com>
Cc: Jeff Tantsura <jefftant.ietf@gmail.com>, Susan Hares <shares@ndzh.com>, idr-chairs@ietf.org, draft-ietf-idr-bgp-ls-segment-routing-msd@ietf.org, The IESG <iesg@ietf.org>, idr@ietf.org
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Subject: Re: [Idr] Benjamin Kaduk's No Objection on draft-ietf-idr-bgp-ls-segment-routing-msd-17: (with COMMENT)
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On Wed, May 06, 2020 at 02:22:45PM -0700, Alvaro Retana wrote:
> On May 6, 2020 at 1:56:07 PM, Benjamin Kaduk wrote:
> 
> 
> Ben:
> 
> Hi!
> 
> ...
> > It sounds like you are intending this to work the way that I was hoping it
> > would work, which is reassuring. That said (and this relates to a later
> > comment as well), I think we may want to double-check the wording in one
> > spot: in Section 3 we define the "Node MSD is the smallest MSD supported by
> > the node on the set of interfaces configured for use." This definition
> > seems to disallow advertising a Node MSD that is larger than any specific
> > Link MSD for that node. My question here was about whether a receiving
> > node was expected to enforce this (surprising) aspect of the definition.
> > If, instead, the definition should be changed to say only that the Node MSD
> > is the MSD to be used for interfaces on this node in the absence of an
> > interface-specific value, then there is no longer any enforcement needed.
> 
> §5 does say this: "When Link MSD is present for a given MSD-type, the
> value of the Link MSD MUST take precedence over the Node MSD."
> 
> Is that what you're looking for.

That's what I expect, yes.
But right now the document seems to be inconsistent, as the Node MSD is
claimed to be "the smallest MSD supported" but can be other values.

> Keep in mind that the expectation of BGP-LS is to simply transport
> information; the rules around that information and how the consumer
> should process it come form the IGP documents.  IOW, in this case
> we're bound by what those published documents already say (see RFC8491
> and RFC8476).

So the published documents are also inconsistent?

It still looks like we are defining usage of a field that contradicts what
that field is defined to carry.  I am not sure whether the contradiction is
across documents, within multiple documents, or only within this current
document.

If we're locked into the already-defined semantics that's fine (albeit
unfortunate), but we shouldn't then claim that it's allowed to violate the
already-defined semantics.

-Ben