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

Peter Psenak <ppsenak@cisco.com> Tue, 26 May 2020 12:22 UTC

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To: "Acee Lindem (acee)" <acee@cisco.com>, Alvaro Retana <aretana.ietf@gmail.com>, Benjamin Kaduk <kaduk@mit.edu>
Cc: "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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From: Peter Psenak <ppsenak@cisco.com>
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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 Acee,

have you looked at the texts that I suggested in my response to Alvaro 
earlier today?


Please see inline:

On 26/05/2020 13:49, Acee Lindem (acee) wrote:
> Hi Alvaro,
> 
> See inline.
> 
> On 5/22/20, 10:59 AM, "Alvaro Retana" <aretana.ietf@gmail.com> 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.]
> 
> 
> 
>      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.
> 
> I agree with the changes but have suggested alternate text.
> 
>      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.

when would ABR do inter area propagation of what is advertised in AS 
scope LSA? I can not think of such a case.

thanks,
Peter


> 
> I'd suggest:
> 
>        When an OSPFv2 Area Border Router (ABR) advertises prefix information between
>        areas and ELC information is was advertised for the prefix in the source area, the
>        ABR SHOULD originate an OSPFv2 Extended Prefix Opaque LSA [RFC7684] propagating
>        the prefix's source area setting. If the ELC setting, is also advertised in an OSPFv2
>        Extended Prefix Opaque LSA with AS-wide scope, the additional LSA origination
>        Is not needed.
> 
> 
>      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.
> 
> I'd suggest:
>        
>        When an OSPFv3 Area Border Router (ABR) advertises information between
>        areas, the setting of the ELC flag in the Inter-area-prefix-LSA MUST be the
>        propagated unchanged.
> 
> Thanks,
> Acee
> 
> 
> 
> 
>      > > (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.
> 
> 
>