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

[Tony Przygienda <[e-mail]>]

hmm, Peter, thanks for clarification but wasn't A the attach flag is OSPF
in 7684?

In ISIS it's in the SR support draft and hence I would ask here as well to
somehow clarify that  "this is for SR purposes"  and call the draft
accordingly before we get it confused with today's semantics. And possibly
call the flag the "SR-anycast flag" or some such thing that clarifies it
does not have anything to do with "normal" OSPF pseudo-anycast

I buy your 2nd use case

I don't get the first, are we talking TI-LFA with SR only? because
otherwise it's a tunnel and hence doesn't matter whether it traversed an
anycast . So for me it looks like SR specific thing again.

thanks

-- tony

On Wed, Mar 20, 2024 at 6:22 PM Peter Psenak <[e-mail]> wrote:

> Tony,
>
> there are two use cases:
>
> 1. Your application wants to exclude address that is anycast - an example
> of where this can be used internally by IGP is a TI-LFA or uloop, when
> picking up the address of the node over which we want to do the
> enforcement. There is a N bit as well, but in case there is no address with
> the N-bit, you want to exclude anycast addresses.
>
> 2. Your application want to use only anycast addresses - inter-domain SRTE
> with anycast address for ASBRs. SRTE is using the IGP topology provided by
> BGP-LS.
>
> BTW, the A-bit exists in ISIS and OSPFv3. We are just filling the gap with
> this draft.
>
> thanks,
> Peter
>
>
>
> On 20/03/2024 17:44, Tony Przygienda wrote:
>
> 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.
>
> please see the TI-LFA, uloop use case that is internal to IGP.
>
>
> 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 <[e-mail]> wrote:
>
>> Hi Ketan,
>>
>> On Mar 20, 2024, at 12:07, Ketan Talaulikar <[e-mail]>
>> 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 <[e-mail]> wrote:
>>
>>>
>>>
>>> > On Mar 20, 2024, at 11:17 AM, Ketan Talaulikar <[e-mail]>
>>> 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 <[e-mail]>
>>> 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) <[e-mail]>
>>> 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 <[e-mail]> On Behalf Of Acee Lindem
>>> > > Sent: Wednesday, March 20, 2024 4:43 AM
>>> > > To: lsr <[e-mail]>
>>> > > Cc: [e-mail]
>>> > > 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
>>> > > [e-mail]
>>> > > https://www.ietf.org/mailman/listinfo/lsr
>>> >
>>>
>>>
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