Re: [Lsr] RFC 8362 and LSInfinity

Mon, 10 Oct 2022 00:55:58 -0700 

Aijun,

On 09/10/2022 07:44, Aijun Wang wrote:

Hi, Acee, Peter and Ketan:
I propose we limit the usage of LSInfinity within the network. That is to say, we should depreciate its usages, not enhance it.
As defined in RFC2328, the sole purpose of LSInfinity is to let the receiver bypass the SPF calculation for the associated LSA: 

a)In case the advertisement of LSA for some special aim.

b)Another is for the premature aging the LSA (which is not encouraged).
There is few application for the a) usage until now, same situation for b) usage. 

definition of LSInfinity is very exact - it means unreachability.
The reason for the above situations may be the definition within the RFC2328 is counterintuitive----the maximum value of the metric should be used for the last resort of the reachability, no other more meanings. Or else, it will complex the implementation and deployment, for example: 

a)For OSPFv2, the LSInfinity is defined as 0xffffff
b)For IS-IS, the equivalent variable is MAX_PATH_METRIC, which is defined as 0xFE000000 

you are comparing apples to oranges. These are two different protocols.
c)For OSPFv3, which value will you be defined, especially for the Intra-Area-Prefix? Considering the metric for the intra-area and inter-area are all 24-bit long?
OSPFv3 inherits all the constants defined for OSPFv2, it's explicitly mentioned in the OSPFv3 RFC. And LSInfinity is 24 bits long.
d)And, for the metric in ”IP Algorithm Prefix Reachability” , https://datatracker.ietf.org/doc/html/draft-ietf-lsr-ip-flexalgo#section-6.3 <https://datatracker.ietf.org/doc/html/draft-ietf-lsr-ip-flexalgo#section-6.3>, its length is again 32-bit, will you define another LSInfinity value later?
0xFFFFFFFF seems like a good candidate for the unreachable metric for the IP flex-algo. 



Won’t you think the above special rule complex the whole situation?


not at all.



I think we should seek other methods to achieve the necessary goals.


I do not see a problem

thanks,
Peter


Best Regards

Aijun Wang

China Telecom
*From:* lsr-boun...@ietf.org <lsr-boun...@ietf.org> *On Behalf Of *Acee Lindem (acee) 
*Sent:* Saturday, October 8, 2022 4:03 AM
*To:* Ketan Talaulikar <ketant.i...@gmail.com>; Peter Psenak <ppsenak=40cisco....@dmarc.ietf.org> 
*Cc:* lsr@ietf.org
*Subject:* Re: [Lsr] RFC 8362 and LSInfinity

Hi Peter, Ketan,
We’ll do another WG last call on the updated IP Flex Algo document and it will update RFC 8362. As you probably surmised, this is useful for OSPFv3 IP Flex Algorithm when you want don’t want to use the prefix with the base algorithm.
*From: *Lsr <lsr-boun...@ietf.org <mailto:lsr-boun...@ietf.org>> on behalf of Ketan Talaulikar <ketant.i...@gmail.com <mailto:ketant.i...@gmail.com>> 
*Date: *Thursday, October 6, 2022 at 3:35 AM
*To: *Peter Psenak <ppsenak=40cisco....@dmarc.ietf.org <mailto:ppsenak=40cisco....@dmarc.ietf.org>> *Cc: *"lsr@ietf.org <mailto:lsr@ietf.org>" <lsr@ietf.org <mailto:lsr@ietf.org>> 
*Subject: *Re: [Lsr] RFC 8362 and LSInfinity

Hi Peter,
I support this "update" - not sure if it qualifies as a "clarification". Also, this obviously is doable only when the network has migrated to use only Extended LSAs (i.e., legacy LSAs are removed) as indicated in https://www.rfc-editor.org/rfc/rfc8362.html#section-6.1 <https://www.rfc-editor.org/rfc/rfc8362.html#section-6.1>
In sparse-mode, the legacy LSAs are used. So if you want a prefix to be unreachable with the base algorithm, simply omit it from the legacy Intra-Area-Prefix LSA. 

Thanks,
Acee

Thanks,

Ketan
On Wed, Oct 5, 2022 at 3:01 PM Peter Psenak <ppsenak=40cisco....@dmarc.ietf.org <mailto:40cisco....@dmarc.ietf.org>> wrote: 

    Hi Folks,

    metric of LSInfinity (0xFFFFFF) has been defined in RFC2328:

    LSInfinity
              The metric value indicating that the destination described
    by an
              LSA is unreachable. Used in summary-LSAs and
    AS-external-LSAs as
              an alternative to premature aging (see Section 14.1). It is
              defined to be the 24-bit binary value of all ones: 0xffffff.

    RFC5340 inherited it from RFC2328:

    Appendix B.  Architectural Constants

         Architectural constants for the OSPF protocol are defined in
    Appendix
         B of [OSPFV2].  The only difference for OSPF for IPv6 is that
         DefaultDestination is encoded as a prefix with length 0 (see
         Appendix A.4.1).

    Both RFC2328 and RFC5340 used 16 bits metric for intra-area prefix
    reachability, so the LSInfinity was not applicable for intra-area
    prefixes.

    RFC8362 defines 24-bit metric for all prefix reachability TLVs -
    Intra-Area-Prefix TLV, Inter-Area-Prefix TLV, External-Prefix TLV.
    Although it is silent about the LSInfinity as such, it is assumed that
    such metric means unreachability for Inter-Area-Prefix TLV and
    External-Prefix TLV. Given that Intra-Area-Prefix TLV now has 24 bits
    metric as well, it would make sense to define the LSInfinity as
    unreachable for Intra-Area-Prefix TLV as well.

    Would anyone object such a clarification in RFC8362?

    thanks,
    Peter

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