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

[Peter Psenak <[e-mail]>]

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" <[e-mail]> 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.
>      >
>      >
> 
> 
> 
>