Re: [Pce] PCE SID-algo draft extension

["Andrew Stone (Nokia)" <[e-mail]>]

Hi Samuel,

Replies inline below with [Andrew]

Thanks
Andrew

From: "Samuel Sidor (ssidor)" <[e-mail]>
Date: Thursday, March 30, 2023 at 8:22 AM
To: "Andrew Stone (Nokia)" <[e-mail]>, "[e-mail]" <[e-mail]>
Cc: "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>, "[e-mail]" <[e-mail]>
Subject: RE: [Pce] PCE SID-algo draft extension


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Hi Andrew,

Thanks for good comment.

There are really 3 things – optionA, optionB and L flag.

Option A + option B:
Option A cannot be combined with Option B as main difference here is source of optimization metric/constraints and topology attributes, which are supposed to be used in the path-computation (ASLA vs legacy).

[Andrew] Agreed

Option A + L flag:
I would say that option A can be combined with L flag as you are really doing path-computation based on “legacy” constraints specified in PCRpt. That will result in some path, which is translated into SID list and algo of those SIDs is not that important (if IGP path of those SIDs is congruent with computed path).

[Andrew] Agreed. My interpretation for a use case on the original adoption was that if PCE is setting up a path, it would be more ideal to set it up following a given Algo, so that way any native IGP convergence or protection mechanisms will still respect a metric/constraints differing from algo-0, and if you fail to resolve a SID list using the algo be permitted to use any SIDs available.

Option B + L flag:
Option B is implicitly restricting topology to only nodes/links with participation in that FA (PCE need to follow from path-computation what IGP is doing for that option). Constraints and metric-type in FAD are defining FA ASLA constraints, so even in the path-computation PCE is supposed to use FA ASLA link attributes. So if PCE would suddenly need to use links/nodes (if we would allow usage of non-FA topology for L=1), which does not advertise those attributes, then PCE would have to fallback into legacy (non-ASLA) link attributes and resulting path would have for example accumulated metric, which is combining ASLA latency of some links with non-ASLA latency of other links (it seems to me like mixing apples and oranges as it is not guaranteed that FA ASLA metric value for specific link is same as legacy metric value of same link). So I tend to say that topology should be restricted even with L=1 with option B.

[Andrew] Yes, agree that topology(edges in the graph) should be restricted with L=1. Topology must be restricted to links matching the flex algo, and thus any path programmed must only be for links within that flex algo, and If a given resource violates the FAD it must be pruned. But I do think there’s two sides to it, topology filtering vs SID selection to encode the selected the path in a given topology. If we take a simplistic case of a FAD with metric Delay without any constraints, assuming the entire network supports the Algo, Algo=0 and Algo=Delay are one-to-one with a difference of weights, so the concern for topological filtering is not as significant – what matters is encoding of the intended “best path” (FAD+LspConstraints+Rules imposed on PCE) using SIDs from Algo 0 or Algo=Delay.  (Secondary comment about MSD below)

I just described topology which must be used and not SIDs. I can still imagine that if L=1 is set, then PCE will use FA topology, but it can still fallback into Algo-0 Prefix SIDs (even if I think that there is higher change that adj SIDs + FA SIDs will be used) assuming that final computed path will still be shortest path of specified ASLA metric and it will satisfy ASLA constraints from FAD.

[Andrew] Yep agreed.

Btw for your other example with MSD – I assume that in most of the cases you will end up with smaller number of SIDs if you will use FA SIDs (as IGP forwarding will be more aligned with intended constraints in PCE path-computation) when compared with algo-0 SIDs.

[Andrew] While I generally agree with you, it could still be possible (likely outlier scenarios)  where the path constraints and behavior imposed by PCE may need to deviate from the Algo shortest path (ex: Delay) significantly enough that MSD becomes constraining. This would be more likely to occur with combination of factors imposed at PCE, such as de-congestion optimization and rules such as Bidirectionality and/or Diversity which by its nature generally requires avoiding the shortest path, potentially for each set of LSPs having diversity imposed on them.

I’ll think about it a little bit more, L flag is definitely introducing extra complexity into both cases, so maybe even dropping that flag may work (PCE can still compute path mix of FA and algo0 SIDs even without any constraint, so maybe added value of SID-algo constraint + L=1 is relatively small) or we can modify it to restrict it to combination of FA SIDs + adj SIDs.

[Andrew] ACK. Will think more about it as well. I don’t have a concrete suggestion at this moment. I do agree we need one or many knobs in the picture , and it seems reasonable to drop knob(s) into the FA SID TLV, but just want to make sure we’re covering exactly what scenarios these knobs are intending to cover/not cover.

Regards,
Samuel

From: Andrew Stone (Nokia) <[e-mail]>
Sent: Wednesday, March 29, 2023 4:24 PM
To: [e-mail]; Samuel Sidor (ssidor) <[e-mail]>
Cc: [e-mail]; [e-mail]; [e-mail]; [e-mail]
Subject: Re: [Pce] PCE SID-algo draft extension

Hi Samuel, PCE WG

I think your comparison points cover the differences well. Comparing/contrasting the two scenarios and behaviors should probably be in the updated document, too.

It does seem a need to signal the different behavior intention in some form or another (whether flag or inclusion/exclusion of constructs). Something not remarked in (B), is PCE implicitly restricted to using only SIDs found from the Flex Algo Tree? Or is it still permitted to use any SID it wants (existing draft L=1) if the path is using resources respecting the FAD. As an example, Let's say PCE computes a path based on FAD constraints but needs to work around constraints defined outside of the algo, such as known planned maintenance or other impairments/rules. Due to MSD, maybe it can't encode this path within the confines of the Algo specified. However, if it used Algo-0 or another SIDs it can encode the path. I would assume this should be permitted, but Is there a need to prohibit this and restrict (B) to also use only the SIDs from the same algo? I think I’m looking to clarify, if (A) is filtering strictness and (B) metric/constraint, is the behavior needed actually A||B, or is it A=true/false, B=true/false?

Thanks
Andrew

From: "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>
Date: Wednesday, March 29, 2023 at 5:46 AM
To: "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>
Cc: "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>, "[e-mail]<mailto:[e-mail]>" <[e-mail]<mailto:[e-mail]>>, "Andrew Stone (Nokia)" <[e-mail]<mailto:[e-mail]>>
Subject: Re: [Pce] PCE SID-algo draft extension


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Hi Samuel, WG,



Thanks for the effort work to get the consensus about path computation according to the content of FAD.

An explicit flag based on the existing SID-algo constraint for the purpose of behavior b, seems good to me.



Regards,

PSF




Original
From: SamuelSidor(ssidor) <[e-mail]<mailto:[e-mail]>>
To: [e-mail]<mailto:[e-mail]> <[e-mail]<mailto:[e-mail]>>;'pce-chairs' <[e-mail]<mailto:[e-mail]>>;
Cc: [e-mail]<mailto:[e-mail]> <[e-mail]<mailto:[e-mail]>>;'Dhruv Dhody' <[e-mail]<mailto:[e-mail]>>;彭少富10053815;Stone, Andrew (Nokia - CA/Ottawa) <[e-mail]<mailto:[e-mail]>>;
Date: 2023年03月29日 17:10
Subject: RE: [Pce] PCE SID-algo draft extension
Hi all,

Thanks all for discussion, which happened during PCE session and thanks for supporting this extension.

I think that we agreed that it is necessary to consider FAD in the path-computation on PCE side if SID-algo constraint was specified, but we were not able to finish discussion whether there is a need to introduce new flag, which will control whether original behavior (SID-algo filtering) or new behavior should be used, so re-opening this mail thread to finish that discussion.

I would say that there are really at least two usecases/behaviors for SID-algo constraint and we need new flag in SID-algorithm constraint to allow PCC to pick required behavior.


  1.  SID-filtering - already exists in the draft (valid for all algorithms)

  *   Path-computation should occur on the topology associated with specified SID-algo
  *   Computed path can have only SIDs of specified algo value (+ adjacency SIDs)
  *   PCE path-computation is done based on metric-type and constraints from PCRpt
  *   Flex-algo specific part:

     *   PCE still has to make sure that IGP path of FA SID is congruent with computed path

  1.  Path-computation based on FAD (only valid for Flex-algo)

  *   Metric-type and constraints are primarily retrieved from FAD
  *   Path-computation follow IGP Flex-algo path-computation logic

     *   Flex-algo participation, ASLA attributes,...

  *   Metric-type from FAD is overriding metric-type from PCRpt
  *   PCUpdate will use metric-type from FAD
  *   PCC can send metric-type in PCRpt and it does not have to be same as metric-type from FAD, but it is recommended to do not advertise any optimization metric
  *   Other constraints from PCRpt:

     *   PCE implementation can decide based on flags in PCEP object
     *   It is not recommended to specify constraints in PCRpt, which are already specified in FAD
     *   PCE is not supposed to include constraints from FAD in PCUpdate

Description here is slightly different then the proposal presented in original slides, but main idea is still same and more details is provided now. Please provide any comments.

Thanks,
Samuel

From: Pce <[e-mail]<mailto:[e-mail]>> On Behalf Of Samuel Sidor (ssidor)
Sent: Thursday, January 12, 2023 10:12 AM
To: [e-mail]<mailto:[e-mail]>; 'Dhruv Dhody' <[e-mail]<mailto:[e-mail]>>
Cc: [e-mail]<mailto:[e-mail]>; 'pce-chairs' <[e-mail]<mailto:[e-mail]>>
Subject: Re: [Pce] PCE SID-algo draft extension

Hi Dhruv,

Thanks for feedback. I completely agree – I would like to hear from WG if they can see added value in both (or they can specify even other) use-cases – using SID-algo constraint just for SID filtering and using it also for specification of constraints from FAD (I agree with Stephane here – computation based on in FAD seems to be even more important use-case to me and it is not covered in current version of that draft).

For constraint conflict solving – there are multiple possible solutions, but I would prefer to ignore metric-type from PCRpt (as metric-type would be retrieved from FAD) or reject PCEP Metric object completely (that may have potential issues with backward compatibility). Do not block usage of other constraints on top of SID-algo constraint explicitly in the draft – actual PCE implementation can still reject any combination of constraints, which PCE cannot handle (with PCUpdate with empty ERO or with some specific PCError) That would allow usage of some specific constraints like metric bounds on top of path computed with constraints from FAD. I would like to clearly specify in the draft that PCC is not supposed to reflect constraints from FAD in PCRpt as intended/requested attributes (only constraints, which should be used on top of FAD should be specified).

For SID-algo constraint signaling – can you please specify benefit of using association in this case? FAD with constraints is part of topology information received from IGP/BGP-LS, so we need to encode only algorithm number (and potentially source IGP, but that is separate story).

Thanks,
Samuel

From: [e-mail]<mailto:[e-mail]> <[e-mail]<mailto:[e-mail]>>
Sent: Tuesday, January 10, 2023 5:34 PM
To: 'Dhruv Dhody' <[e-mail]<mailto:[e-mail]>>; Samuel Sidor (ssidor) <[e-mail]<mailto:[e-mail]>>
Cc: [e-mail]<mailto:[e-mail]>; 'pce-chairs' <[e-mail]<mailto:[e-mail]>>
Subject: RE: [Pce] PCE SID-algo draft extension

Hi

Happy new year guys !

IMO, from a use case point of view, the SID filtering use case is far more limited and niche (e.g.: plane selection…) vs the interdomain FA path computation which is widely required. For large networks that are multidomain, there must be a PCE based solution for interdomain FA path computation.

Brgds,

Stephane

From: Pce <[e-mail]<mailto:[e-mail]>> On Behalf Of Dhruv Dhody
Sent: mardi 10 janvier 2023 14:00
To: Samuel Sidor (ssidor) <[e-mail]<mailto:[e-mail]>>
Cc: [e-mail]<mailto:[e-mail]>; pce-chairs <[e-mail]<mailto:[e-mail]>>
Subject: Re: [Pce] PCE SID-algo draft extension

Hi Samuel,

As a WG participant --- Assuming the WG agrees with the usecase, we need a clear way to signal when the Algo is a constraint along with others (current) v/s when Algo is a shorthand to refer to the constraints as per the IGP definition (proposed).

This could be a flag in the SID Algorithm TLV or could be a brand new mechanism (such as a new dynamic association type for FlexAlgo). More importantly, we need to be clear on how other PCEP constraints interact with the constraints referred in the IGP. The easiest thing would be to not allow other PCEP constraints to be encoded at all and rely only on IGP; or have flags to signal how to handle the complexity of combining them including mismatch! This needs to be handled with care!

Thanks!
Dhruv

On Tue, Jan 10, 2023 at 3:51 PM Samuel Sidor (ssidor) <[e-mail]<mailto:[e-mail]>> wrote:
Hi all,

I would like to get feedback from PCE WG for one extension proposed for existing SID-algo draft<https://datatracker.ietf.org/doc/html/draft-tokar-pce-sid-algo-05#name-sid-algorithm-constraint-2> (currently expired), which is trying to cover all existing algorithm types as defined in IGP – that includes SPF (algo 0), Strict-SPF (algo 1) and Flex-algo (algo 128-255)
It introduced SID-algo constraint, which currently can be used for filtering SIDs used in path computed by PCE.
To be able to compute inter-domain Flex-algo path, PCE Flex-algo path-computation must be aligned with path-computation done by IGP (Use ASLA attributes, honor FAD lookup priorities,…). This use-case is different one from SID filtering we need to use constraints/metric-type from Flex-algo definition that is bound to SID algo number specified in constraint.

Before we modify the draft, we would like to know if WG has any objection.

Thanks,
Samuel