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

Benjamin Kaduk <kaduk@mit.edu> Sat, 30 May 2020 21:03 UTC

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Date: Sat, 30 May 2020 14:03:31 -0700
From: Benjamin Kaduk <kaduk@mit.edu>
To: Alvaro Retana <aretana.ietf@gmail.com>
Cc: "Acee Lindem (acee)" <acee@cisco.com>, Peter Psenak <ppsenak@cisco.com>, "draft-ietf-isis-mpls-elc@ietf.org" <draft-ietf-isis-mpls-elc@ietf.org>, The IESG <iesg@ietf.org>, "lsr-chairs@ietf.org" <lsr-chairs@ietf.org>, "lsr@ietf.org" <lsr@ietf.org>
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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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I think it will work for me as well -- thanks!

-Ben

On Tue, May 26, 2020 at 08:03:17AM -0700, Alvaro Retana wrote:
> Just for the record, I’m ok with the latest text.
> 
> Thanks!
> 
> Alvaro.
> 
> On May 26, 2020 at 10:25:38 AM, Peter Psenak (ppsenak@cisco.com) wrote:
> 
> Hi Acee,
> 
> updated the text based on your comments.
> 
> thanks,
> Peter
> 
> 
> On 26/05/2020 16:07, Acee Lindem (acee) wrote:
> > Hi Peter,
> >
> > This is in response to the previous Email on your suggested text.
> >
> > On 5/26/20, 4:26 AM, "Peter Psenak" <ppsenak@cisco.com> wrote:
> >
> > Hi Alvaro,
> >
> > please see inline (##PP)
> >
> > On 22/05/2020 16:59, Alvaro Retana wrote:
> > > On May 21, 2020 at 3:39:03 PM, Benjamin Kaduk wrote:
> > >
> > >
> > > Peter:
> > >
> > > Hi!
> > >
> > >
> > >> With respect to Alvaro's clarification, your answer for (1) makes
> sense;
> > >> thanks! I think Alvaro has offered to help work out what (if any)
> > >> additional text we might want to be sure that the answer to (2) is
> clear in
> > >> the document.
> > >
> > > I think that #1 is where some clarification could be useful. :-)
> > >
> > >
> > > I'm including both ISIS and OSPF suggestions below to consolidate the
> > > discussion.
> > >
> > >
> > > ...
> > >>> 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.
> > >
> > > Right.
> > >
> > > For ISIS I think the current text is in line with the specification of
> > > the other bits in rfc7794. No changes are needed.
> > >
> > > If anything, you may want to change the order of this sentence to
> > > address Ben's comment:
> > >
> > > OLD>
> > > When a router propagates a prefix between ISIS levels ([RFC5302], it
> > > MUST preserve the ELC signaling for this prefix.
> > >
> > > NEW>
> > > The ELC signaling MUST be preserved when a router propagates a prefix
> > > between ISIS levels ([RFC5302]).
> > >
> > > [Similar for OSPF.]
> >
> > ##PP
> > done.
> >
> >
> > >
> > >
> > >
> > > I think that for OSPF it is not that simple...
> > >
> > > For OSPFv2: rfc7684 says that the "scope of the OSPFv2 Extended Prefix
> > > Opaque LSA depends on the scope of the advertised prefixes", which I
> > > assume means that for intra-area prefixes the scope will be
> > > area-local...so the ABR wouldn't simply propagate it; it would have to
> > > originate a new LSA.
> >
> > ##PP
> > correct. It is always a new LSA that ABR needs to generate. Here it's
> > actually two LSAs.
> >
> > >
> > > Suggestion (Add to 3.1)>
> > > When an OSPFv2 Area Border Router (ABR) distributes information between
> > > connected areas it SHOULD originate an OSPFv2 Extended Prefix Opaque
> LSA
> > > [RFC7684] including the received ELC setting. If the received
> information
> > > is included in an LSA with an AS-wide scope, then the new LSA is not
> needed.
> >
> > Here's my suggestion for OSPFv2 ABR related text:
> >
> > "The ELC signaling MUST be preserved when an OSPF Area Border Router
> > (ABR) distributes information between connected areas. To do so, ABR
> > MUST originate an OSPFv2 Extended Prefix Opaque LSA [RFC7684] including
> > the received ELC setting."
> >
> > Ok - I change "connected areas" to "areas" and "ABR MUST" to "an ABR
> MUST".
> >
> > Here's my suggested text for OSPFv2 ASBR case:
> >
> > "When an OSPF Autonomous System Boundary Router (ASBR) redistributes a
> > prefix from another instance of OSPF or from some other protocol, it
> > SHOULD preserve the ELC signaling for the prefix if it exists. To do so,
> > ASBR SHOULD originate Extended Prefix Opaque LSA [RFC7684] including the
> > ELC setting of the redistributed prefix. The flooding scope of the
> > Extended Prefix Opaque LSA MUST match the flooding scope of the LSA that
> > ASBR originates as a result of the redistribution. The exact mechanism
> > used to exchange ELC between protocol instances on the ASBR is outside
> > of the scope of this document."
> >
> > Sure - replace "ASBR SHOULD" with "an ASBR SHOULD", "that ASBR" with
> "that an ASBR", and "the ASBR is" with "an ASBR is" to be consistent.
> > Also, "originate Extended" with "originate an Extended".
> >
> >
> >
> > >
> > >
> > > For OSPFv3: The PrefixOptions are *in* the LSA, but I couldn't find
> > > anything in rfc5340 saying that the received values should be copied
> > > into the Inter-Area-Prefix-LSA (nor that they should not).
> > >
> > > Suggestion (Add to 3.2)>
> > > When an OSPFv3 Area Border Router (ABR) distributes information between
> > > connected areas, the setting of the ELC Flag in the
> Inter-Area-Prefix-LSA
> > > MUST be the same as the received value.
> >
> > Here's my suggestion for OSPFv3 ABR and ASBR:
> >
> > "The ELC signaling MUST be preserved when an OSPFv3 Area Border Router
> > (ABR) distributes information between connected areas. The setting of
> > the ELC Flag in the Inter-Area-Prefix-LSA [RFC5340] or in the
> > Inter-Area-Prefix TLV [RFC8362], generated by ABR, MUST be the same as
> > the value the ELC Flag associated with the prefix in the source area."
> >
> > Same change - replace "connected areas" with "areas" and "by ABR" with
> "by an ABR".
> >
> > "When an OSPFv3 Autonomous System Boundary Router (ASBR) redistributes a
> > prefix from another instance of OSPFv3 or from some other protocol, it
> > SHOULD preserve the ELC signaling for the prefix if it exists. The
> > setting of the ELC Flag in the AS-External-LSA [RFC5340] or in the
> > External-Prefix TLV [RFC8362], generated by ASBR, MUST be the same as
> > the value the ELC Flag associated with the prefix in the source domain.
> > The exact mechanism used to exchange ELC between protocol instances on
> > the ASBR is outside of the scope of this document.
> >
> > Add "NSSA-LSA" as a case. Replace "by ASBR" with "by an ASBR" and "value
> the ELC" with "value of the ELC".
> >
> > Thanks,
> > Acee
> >
> > thanks,
> > Peter
> >
> >
> > >
> > >
> > >
> > >
> > >>> (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.
> > >
> > > I think that text is ok.
> > >
> > >
> > > Thanks!
> > >
> > > Alvaro.
> > >
> > >
> >
> >
> >
> >