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

[Robert Raszuk <[e-mail]>]

Hi Ketan,

That is my main point. So we define something which is only local to the
area.

If this information will turn out to be very useful I am sure there is
going to be someone proposing to leak it :)  Remember the UPA discussions ?

If so my real question is - should such information belong in IGP ? Or
maybe rather in DROID (
https://datatracker.ietf.org/doc/html/draft-li-lsr-droid) ?

Honestly I am still a bit struggling to understand the need for it. And the
draft is not very helpful ...



*   The prefix may be configured as anycast and it is useful for other
 routers to know that the advertisement is for an anycast identifier.*

or







*2.  Use-case   In the absence of the N-flag, the node specific prefixes
need to be   identified from the anycast prefixs.  A prefix that is
advertised by   a single node and without an AC-flag MUST be considered
node   specific.*

Especially the "use-case" looks to me like copy and paste error :)

Thx,
Robert


On Thu, Mar 21, 2024 at 5:44 PM Ketan Talaulikar <[e-mail]>
wrote:

> Hi Robert,
>
> Summarization/aggregation does result in loss of individual prefixes'
> attributes.
>
> The draft does not intend to specify procedures for propagation of anycast
> attribute of individual prefixes to the summary since that is not something
> that is going to be robust.
>
> Thanks,
> Ketan
>
>
> On Thu, Mar 21, 2024 at 10:02 PM Robert Raszuk <[e-mail]> wrote:
>
>> Hi,
>>
>> Isn't this knowledge gone outside of the local area when ABR does
>> summarization ? If so, is this really practically useful ?
>>
>> Thx,
>> R.
>>
>>
>> On Thu, Mar 21, 2024 at 4:19 PM Ketan Talaulikar <[e-mail]>
>> wrote:
>>
>>> Hi Bruno,
>>>
>>> Please check inline below with KT2 for responses.
>>>
>>>
>>> On Thu, Mar 21, 2024 at 7:16 PM <[e-mail]> wrote:
>>>
>>>> Hi Ketan,
>>>>
>>>>
>>>>
>>>> Thanks for your quick reply.
>>>>
>>>> Please see inline [Bruno]
>>>>
>>>>
>>>>
>>>> *From:* Ketan Talaulikar <[e-mail]>
>>>> *Sent:* Thursday, March 21, 2024 2:18 PM
>>>> *To:* DECRAENE Bruno INNOV/NET <[e-mail]>
>>>> *Cc:* Acee Lindem <[e-mail]>; lsr <[e-mail]>;
>>>> [e-mail]; Dongjie (Jimmy) <
>>>> [e-mail]>; Tony Przygienda <[e-mail]>
>>>> *Subject:* Re: [Lsr] Working Group Adoption Poll for "Updates to
>>>> Anycast Property advertisement for OSPFv2" - draft-chen-lsr-anycast-flag-06
>>>>
>>>>
>>>>
>>>> Hi Bruno,
>>>>
>>>>
>>>>
>>>> Thanks for your feedback. Please check inline below for responses.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Thu, Mar 21, 2024 at 4:12 PM <[e-mail]> wrote:
>>>>
>>>> Hi all,
>>>>
>>>>
>>>>
>>>> I would also welcome a clear specification of the semantics.
>>>>
>>>> Such that the meaning and implications are clear on both the originator
>>>> and the receivers/consumers.
>>>>
>>>>
>>>>
>>>> e.g., from the originator standpoint:
>>>>
>>>> - The originator MAY advertise the Anycast Flag if CONDITIONS1 are met
>>>> (which allow for some useful features such as….)
>>>>
>>>> - The originator MUST advertise the Anycast Flag if CONDITIONS1 are met
>>>> (otherwise this breaks …)
>>>>
>>>>
>>>>
>>>> Please specify the CONDITIONS1.
>>>>
>>>>
>>>>
>>>> KT> Whether a prefix is anycast or not is configured by the operator.
>>>> This spec does not require implementations to detect that a prefix that it
>>>> is originating is also being originated from another node and hence may be
>>>> an anycast advertisement. We can clarify the same in the document.
>>>>
>>>>
>>>>
>>>> [Bruno] As an operator, why would I configure this? What for? What are
>>>> the possible drawbacks? (i.e., can this be configured on all prefixes
>>>> regardless of their anycast status)
>>>>
>>>
>>> KT2> If anycast property is configured on all prefixes, then it is an
>>> indication that none of those prefixes resolve to a unique node. That has
>>> consequences in terms of usage. E.g., taking the TI-LFA repair path
>>> use-case, we won't find the Node SID to be used to form the repair
>>> segment-list.
>>>
>>>
>>>> I would propose those points be discussed in the operation
>>>> considerations section of this draft.
>>>>
>>>> In the absence of reason, this is not likely be configured IMHO.
>>>>
>>>
>>> KT2> Sure. Thanks for that feedback. We can certainly do that in the
>>> draft. I hope this isn't blocking the adoption in your view though, right?
>>>
>>>
>>>>
>>>>
>>>> e.g., from the receiver standpoint:
>>>>
>>>> What does this mean to have this Anycast Flag set? What properties are
>>>> being signaled? (a priori this may be already specified by CONDITIONS1
>>>> above)
>>>>
>>>>
>>>>
>>>> KT> In addition to the previous response, for the receiver this means
>>>> that the same prefix MAY be advertised from more than one node (that may be
>>>> happening now or may happen in the future). This can be clarified as well.
>>>>
>>>>
>>>>
>>>> [Bruno] OK. If this is happening now, this is a priori already visible
>>>> in the LSDB.
>>>>
>>>
>>> KT2> This is tricky. If the prefix is originated in a different domain,
>>> it gets tricky to determine if the prefix is anycast or dual-homed since
>>> the LSDB has a local area/domain view.
>>>
>>>
>>>> Any reason to duplicate the info (I would guess that’s easier for
>>>> implementation but since this is not guaranteed to be implemented one would
>>>> need to also check in the LSDB. So doubling the work).
>>>>
>>>
>>> KT2> This extension brings in simplicity for the receivers provided that
>>> operators can configure this property. Like I mentioned above, this starts
>>> to get more complicated in multi-domain scenarios. Perhaps we can think of
>>> this as the complexities that we experience in determining this property
>>> via an LSDB/topology-db that motivate us to bring forth this easier and
>>> more robust way.
>>>
>>>
>>>> Any specific reason requiring the knowledge of the future?
>>>>
>>>
>>> KT2> Perhaps at time T1, there are two nodes originating the prefix.
>>> Then at time T2, one of them goes down (or becomes disconnected), do we
>>> assume that the prefix is now not anycast? Then what happens if that other
>>> node comes back up again. For certain use-cases where anycast prefix is not
>>> desired, it may be helpful to completely avoid use of this prefix. The
>>> operator knows their design and addressing and perhaps is able to provision
>>> this prefix property correctly from the outset.
>>>
>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> If this is specific to SR,  please say so.
>>>>
>>>>
>>>>
>>>> KT> It is not specific to SR, it is a property of an IP prefix.
>>>>
>>>>
>>>>
>>>> But even in this sub-case, SR anycast has some conditions, both for
>>>> SR-MPLS and SRv6.
>>>>
>>>>
>>>>
>>>> KT> This document does not discuss either SR-MPLS or SRv6 anycast. It
>>>> covers an OSPFv2 extension to simply advertise the anycast property of any
>>>> IP prefix. The discussion of SR anycast belongs to some other (SPRING)
>>>> document ;-)
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> SR-MPLS:  https://datatracker.ietf.org/doc/html/rfc8402#section-3.3.1
>>>>
>>>> “determining the second label is impossible unless A1 and A2 allocated the same label value to the same prefix.”
>>>>
>>>> “Using an anycast segment without configuring identical SRGBs on all
>>>>
>>>>    nodes belonging to the same anycast group may lead to misrouting (in
>>>>
>>>>    an MPLS VPN deployment, some traffic may leak between VPNs).”
>>>>
>>>>
>>>>
>>>> So for SR-MPLS, where we did not have anycast flag at the time, the
>>>> burden of respecting the conditions seems to be on the receiver. In which
>>>> case, Anycast flag didn’t seem to be required.
>>>>
>>>>
>>>>
>>>> KT> True. But that was also beyond the anycast property of the prefix -
>>>> it also involves checking the Prefix SID associated with it (plus other
>>>> considerations) and that is something quite different.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> SRv6:
>>>> https://datatracker.ietf.org/doc/html/rfc9352#name-advertising-anycast-propert
>>>>
>>>> “All the nodes advertising the same anycast locator MUST instantiate
>>>> the exact same set of SIDs under that anycast locator. Failure to do so may
>>>> result in traffic being dropped or misrouted.”
>>>>
>>>>
>>>>
>>>> So for SRv6 the burden is on the originator, and we felt the need to
>>>> define an anycast flag.
>>>>
>>>>
>>>>
>>>> KT> Note that RFC9352 does not restrict the advertisement of anycast
>>>> property of the prefix to SRv6. It applies to all IPv4/IPv6 prefixes -
>>>> irrespective of SRv6, SR-MPLSv4, SR-MPLSv6 or plain old IP. This is the
>>>> same for RFC9513 - since OSPFv3 supports IPv4/IPv6 prefixes as well as
>>>> SRv6, SR-MPLSv4, and SR-MPLSv6.
>>>>
>>>> [Bruno] Indeed. And note that  RFC9352 did specify some specific
>>>> conditions (MUST) before a node may advertise this anycast flag. A priori
>>>> there is a reason for this. A priori the same reason would apply to
>>>> SR-MPLS, no? So why this sentence has not also been copied from RFC9352 and
>>>> adapted for SR-MPLS? (the sentence is “All the nodes advertising the same
>>>> anycast locator MUST instantiate the exact same set of SIDs under that
>>>> anycast locator. Failure to do so may result in traffic being dropped or
>>>> misrouted.”)
>>>>
>>>
>>> KT2> You have a good point. All I can say is that RFC9352/9513 were
>>> focussed on SRv6 extensions and therefore covered only those aspects. This
>>> document is not an SR extension and I feel it is better that these aspects
>>> related to SR anycast (SR-MPLS or SRv6) are covered in a separate document
>>> in a more holistic manner.
>>>
>>>
>>>>
>>>>
>>>>
>>>>
>>>> Interestingly, the conditions seem different…
>>>>
>>>> Authors seems to use RFC9352 and RFC9513 as a justification. I’m not
>>>> familiar with OSPFv2 but my understanding is that it does not advertise
>>>> SRv6 SID. So presumably there are some differences
>>>>
>>>>
>>>>
>>>> KT> I hope the previous responses clarify.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> “The prefix may be configured as anycast”
>>>>
>>>> Putting the burden on the network operator is not helping clarifying
>>>> the semantic. We need the receivers/consumers and the network operators to
>>>> have the same understanding of the semantic. (not to mention all
>>>> implementations on the receiver side)
>>>>
>>>>
>>>>
>>>> KT> I hope again the previous responses have clarified.
>>>>
>>>> [Bruno] Not yet. Cf my first point about an operation considerations
>>>> section.
>>>>
>>>
>>> KT2> Ack for introducing operational considerations.
>>>
>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> So please specify the semantic.
>>>>
>>>> This may eventually lead to further discussion (e.g., on SR-MPLS)
>>>>
>>>>
>>>>
>>>> KT> That discussion is important and we've had offline conversations
>>>> about that. However, IMHO, that is beyond the scope of this document and
>>>> this thread.
>>>>
>>>> [Bruno] Too early to tell on my side.
>>>>
>>>
>>> KT2> How about now? :-)
>>>
>>> Thanks,
>>> Ketan
>>>
>>>
>>>>
>>>>
>>>> Thanks,
>>>>
>>>> --Bruno
>>>>
>>>>
>>>>
>>>> Thanks,
>>>>
>>>> Ketan
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> Thank you
>>>>
>>>> --Bruno
>>>>
>>>>
>>>>
>>>> *From:* Lsr <[e-mail]> *On Behalf Of *Tony Przygienda
>>>> *Sent:* Wednesday, March 20, 2024 5:44 PM
>>>> *To:* Acee Lindem <[e-mail]>
>>>> *Cc:* Ketan Talaulikar <[e-mail]>; Dongjie (Jimmy) <
>>>> [e-mail]>; lsr <[e-mail]>;
>>>> [e-mail]
>>>> *Subject:* 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 <[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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