Re: [Lsr] Working Group Adoption Poll for "Updates to Anycast Property advertisement for OSPFv2" - draft-chen-lsr-anycast-flag-06

I think the draft is somewhat superfluous and worse, can generate
completely unclear semantics

1) First, seeing the justification I doubt we need that flag. if the only
need is for the SR controller to know it's anycast since it computes some
paths this can be done by configuring the prefix on the controller itself.
It's all centralized anyway.
2) OSPF today due to SPF limitations has a "baked-in weird anycast" since
if prefixes are ECMP (from pont of view of a source) they become anycast,
otherwise they ain't. I think the anycast SID suffers from same limi8ation
and is hence not a "real anycast" (if _real anycast_ means something that
independent of metrics balances on the prefix). Hence this draft saying
"it's anycast" has completely unclear semantics to me, worse, possibly
broken ones. What do I do as a router when this flag is not around but two
instances of the prefix are ECMP to me? What do I do on another router when
those two instances have anycast but they are not ECMP? What will happen if
the ECMP is lost due to ABR re-advertising where the "flag must be
preserved" .
3) There is one good use case from my experience and this is to
differentiate between a prefix moving between routers (mobility) and real
anycast. That needs however far more stuff in terms of timestamping the
prefix. pascal wrote and added that very carefully to rift if there is
desire here to add proper anycast semantics support to the protocol.

So I'm not in favor in adopting this unless the semantic is clearly written
out for this flag and the according procedures specified (mobility?
behavior on lack/presence of flag of normal routers etc). Saying "

It
   is useful for other routers to know that the advertisement is for an
   anycast identifier.

" is not a use case or justification for adding this.

if this is "anycast in case of SR computed paths that are not ECMP" then
the draft needs to say so and call it "SR anycast" or some such stuff. If
it is something else I'd like to understand the semantics of this flag
before this is adopted.

-- tony

On Wed, Mar 20, 2024 at 5:10 PM Acee Lindem <acee.ietf@gmail.com> wrote:

> Hi Ketan,
>
> On Mar 20, 2024, at 12:07, Ketan Talaulikar <ketant.ietf@gmail.com> wrote:
>
> Sure, Acee. We can take that on :-)
>
> I hope it is ok that this is done post adoption?
>
>
> Yup. I realize this is a simple draft to fill an IGP gap but I did ask the
> question below. Hopefully, we can get to WG last call quickly.
>
> Thanks,
> Acee
>
>
>
>
> Thanks,
> Ketan
>
>
> On Wed, Mar 20, 2024 at 9:35 PM Acee Lindem <acee.ietf@gmail.com> wrote:
>
>>
>>
>> > On Mar 20, 2024, at 11:17 AM, Ketan Talaulikar <ketant.ietf@gmail.com>
>> wrote:
>> >
>> > Hi Acee/Jie,
>> >
>> > The most common users of the anycast property of a prefix are external
>> controllers/PCE that perform path computation exercises. As an example,
>> knowing the anycast prefix of a pair of redundant ABRs allows that anycast
>> prefix SID to be in a SRTE path across the ABRs with protection against one
>> of those ABR nodes going down or getting disconnected. There are other use
>> cases. An example of local use on the router by IGPs is to avoid picking
>> anycast SIDs in the repair segment-list prepared for TI-LFA protection -
>> this is because it could cause an undesirable path that may not be aligned
>> during the FRR window and/or post-convergence.
>> >
>> > That said, since ISIS (RFC9352) and OSPFv3 (RFC9513) didn't have the
>> burden of this justification of an use-case, I hope the same burden would
>> not fall on this OSPFv2 document simply because it only has this one
>> specific extension.
>>
>> But they also weren't added in a draft specifically devoted to the
>> Anycast flag. It would be good to list the examples above as  potential use
>> cases.
>>
>>
>> Thanks,
>> Acee
>>
>>
>>
>> >
>> > Thanks,
>> > Ketan
>> >
>> >
>> > On Wed, Mar 20, 2024 at 8:16 PM Acee Lindem <acee.ietf@gmail.com>
>> wrote:
>> > Hi Jie,
>> >
>> > I asked this when the flag was added to IS-IS and then to OSPFv3. I
>> agree it would be good to know why knowing a prefix is an Anycast address
>> is "useful" when the whole point is that you use the closest one (or some
>> other criteria).
>> >
>> > Thanks,
>> > Acee
>> >
>> > > On Mar 20, 2024, at 9:09 AM, Dongjie (Jimmy) <jie.dong@huawei.com>
>> wrote:
>> > >
>> > > Hi authors,
>> > >
>> > > I just read this document. Maybe I didn't follow the previous
>> discussion, but it seems in the current version it does not describe how
>> this newly defined flag would be used by the receiving IGP nodes?
>> > >
>> > > Best regards,
>> > > Jie
>> > >
>> > > -----Original Message-----
>> > > From: Lsr <lsr-bounces@ietf.org> On Behalf Of Acee Lindem
>> > > Sent: Wednesday, March 20, 2024 4:43 AM
>> > > To: lsr <lsr@ietf.org>
>> > > Cc: draft-chen-lsr-anycast-flag@ietf.org
>> > > Subject: [Lsr] Working Group Adoption Poll for "Updates to Anycast
>> Property advertisement for OSPFv2" - draft-chen-lsr-anycast-flag-06
>> > >
>> > >
>> > > This starts the Working Group adoption call for
>> draft-chen-lsr-anycast-flag. This is a simple OSPFv2 maintenance draft
>> adding an Anycast flag for IPv4 prefixes to align with IS-IS and OSPFv3.
>> > >
>> > > Please send your support or objection to this list before April 6th,
>> 2024.
>> > >
>> > > Thanks,
>> > > Acee
>> > >
>> > >
>> > > _______________________________________________
>> > > Lsr mailing list
>> > > Lsr@ietf.org
>> > > https://www.ietf.org/mailman/listinfo/lsr
>> >
>>
>>
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