Subject: [spring] Comments on draft-ietf-spring-segment-routing-policy (ver 03)

From: Vishnu Pavan Beeram <[email-masked]>
Date: Wed, 24 Jul 2019 05:41:49 -0500

Authors, Hi!

There are some use-cases where the candidate-path (multipath) needs to be
constructed in such a way that a part of the multipath (a set of
segment-lists) uses one set of constraints, while the other part (another
set of segment-lists) uses another set of constraints. Consider the
scenario where the traffic needs to be steered onto a policy in such a way
that a specified portion of it uses paths that traverse only blue links
while the rest uses paths that traverse only red links.

The current semantics of a candidate-path as defined in Section 2.2 of
version 3 preclude such use-cases. It should be possible to let the
semantics be a tad more flexible than they currently are and cater to the
above scenario (and the likes of it).

There are a few different ways of addressing this, but I would like to
propose one that seems least disruptive.

In addition to the currently specified semantics:
-- A candidate path may comprise of a set of sub-candidate paths where each
each sub-candidate path is either dynamic or explicit. The sub-candidate
path is the unit of signaling of an SR policy between a headend and a
controller.

Regards,
-Pavan

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From: Przemyslaw Krol <[email-masked]>
Date: Wed, 24 Jul 2019 14:23:01 -0400

Hi Vishnu,

Clarification question, if I may.
Are you envisioning weights to be normalized for all sub-candidate-paths
(regardless of the candidate-path they belong to), or rather propose to
have a 2-level split:
- weight for candidate-path grouping specific sub-candidate-paths defining
"global" share of traffic, in comparison with other candidate-paths
- weight for specific sub-candidate-path defining "local" share of the
traffic within specific candidate-path, in comparison with other
sub-candidate-paths in that particular candidate-path

thanks,
pk

On Wed, Jul 24, 2019 at 6:42 AM Vishnu Pavan Beeram <
[email-masked]> wrote:

> -- A candidate path may comprise of a set of sub-candidate paths where
> each each sub-candidate path is either dynamic or explicit. The
> sub-candidate path is the unit of signaling of an SR policy between a
> headend and a controller..
>
> Regards,
> -Pavan
> _______________________________________________
> spring mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/spring
>

-- 
Przemyslaw "PK" Krol |  Strategic Network Engineer ing | [email-masked]

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From: Vishnu Pavan Beeram <[email-masked]>
Date: Thu, 25 Jul 2019 22:29:50 -0500

I synced up with PK in-person this morning and clarified his questions. I'm
reproducing my response here on the list to keep record.

The weights are specified per sub-candidate path.

If the sub-candidate path is dynamic, the specified weight is distributed
across the set of segment-lists as dynamically computed by the
compute-engine. So for example, if the specified weight for the dynamic
sub-candidate path is 3 and the computation result yielded 3 equal cost
segment-lists, then the weight associated with each segment-list would be
1.

If the sub-candidate path is explicit, the specified weight translates to
the weight of the corresponding segment-list.

Regards,
-Pavan