Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-isis-mpls-elc-12: (with DISCUSS and COMMENT)

Peter Psenak <ppsenak@cisco.com> Thu, 21 May 2020 12:15 UTC

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To: Alvaro Retana <aretana.ietf@gmail.com>, Benjamin Kaduk <kaduk@mit.edu>, The IESG <iesg@ietf.org>
Cc: Acee Lindem <acee@cisco.com>, draft-ietf-isis-mpls-elc@ietf.org, lsr-chairs@ietf.org, lsr@ietf.org
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From: Peter Psenak <ppsenak@cisco.com>
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Date: Thu, 21 May 2020 14:15:37 +0200
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Subject: Re: [Lsr] Benjamin Kaduk's Discuss on draft-ietf-isis-mpls-elc-12: (with DISCUSS and COMMENT)
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Hi Alvaro,

On 21/05/2020 13:44, Alvaro Retana wrote:
> On May 21, 2020 at 6:05:41 AM, Peter Psenak wrote:
> 
> 
> Peter:
> 
> Hi!
> 
> 
>>> ----------------------------------------------------------------------
>>> DISCUSS:
>>> ----------------------------------------------------------------------
>>>
>>> As for other reviewers, many of my comments duplicate those for the OSPF
>>> document; I expect that the analogous responses apply and am fine if
>>> they only appear for one document's review.
>>>
>>> Here, the question I have about normative language applies to the text
>>> in Section 3:
>>>
>>> When a router propagates a prefix between ISIS levels ([RFC5302], it
>>> MUST preserve the ELC signaling for this prefix.
>>>
>>> The scenario in question is analogous to the OSPF cross-area case: is
>>> the router propagating the prefix between ISIS levels required to
>>> implement this document; is preservation of the flag value a new
>>> requirement from this document vs. a preexisting property; and is this
>>> document trying to make normative requirements of devices that don't
>>> implement this document?
>>
>> ##PP
>> this is a new requirement and only applies to the routers that support
>> this document. We are not making normative requirements of devices that
>> don't implement this document, we cannot.
>>
>> Maybe we can add that it only applies to the routers that supports this
>> extension:
>>
>> "When a router supporting this extension propagates a prefix between
>> ISIS levels ([RFC5302], it MUST preserve the ELC signaling for this prefix."
>>
>> Would it work?
> 
> 
> You're right, we can only apply requirements to routers that support
> this specification.  IOW, adding the clarification is not necessary.
> 
> 
> My interpretation of Ben's question is two-fold:
> 
> (1) Would ISIS routers normally propagate the information to a
> different level?  The ELC is a new prefix attribute flag -- are prefix
> attributes always propagated (unchanged) to other levels?  If so, then
> the requirement (MUST) is not needed.  My reading of rfc7794 is that
> the propagation is optional...

depends on the attribute or a bit. Some are propagated some are not. 
That's why we are saying this one MUST be preserved.

> 
> (2) If the propagation is not automatic, and the L1L2 router doesn't
> support this specification, then what are the drawbacks/failure
> scenarios?  IOW, for multi-level operation is it a requirement that
> the L1L2 support this specification?

drawback are identical to what is mentioned in the Security 
Considerations section.

thanks,
Peter


> 
> 
> Thanks!
> 
> Alvaro.
> 
>