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

[[e-mail]]

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.

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)


If this is specific to SR,  please say so. But even in this sub-case, SR anycast has some conditions, both for SR-MPLS and SRv6.



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.

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.


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



“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)


So please specify the semantic.
This may eventually lead to further discussion (e.g., on SR-MPLS)

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]<mailto:[e-mail]>> wrote:
Hi Ketan,


On Mar 20, 2024, at 12:07, Ketan Talaulikar <[e-mail]<mailto:[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]<mailto:[e-mail]>> wrote:


> On Mar 20, 2024, at 11:17 AM, Ketan Talaulikar <[e-mail]<mailto:[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]<mailto:[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]<mailto:[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]<mailto:[e-mail]>> On Behalf Of Acee Lindem
> > Sent: Wednesday, March 20, 2024 4:43 AM
> > To: lsr <[e-mail]<mailto:[e-mail]>>
> > Cc: [e-mail]<mailto:[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]<mailto:[e-mail]>
> > https://www.ietf.org/mailman/listinfo/lsr
>

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