Re: [Lsr] New draft on Flex-Algorithm Bandwidth Constraints

Jeff Tantsura <> Mon, 01 March 2021 21:48 UTC

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Date: Mon, 1 Mar 2021 13:45:53 -0800
From: Jeff Tantsura <>
To: Robert Raszuk <>, Tony Li <>
Cc: Gyan Mishra <>, DECRAENE Bruno IMT/OLN <>, Shraddha Hegde <>, Rajesh M <>, "=?utf-8?Q?" <>, William Britto A J <>
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Subject: Re: [Lsr] New draft on Flex-Algorithm Bandwidth Constraints
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In ol’ good RSVP-TE days we already used “severity/relevance indicator” to decide whether changes in link  attributes (BW/etc) are significant enough and should be propagated in into TED and trigger re-optimization/rerouting, this is no different,  define your threshold for a trigger.
Note - flex-also requires contiguous topology to work, self isolation as the result of (dynamic) topology re-computation would not be a great thing.

On Mar 1, 2021, 12:48 PM -0800, Tony Li <>li>, wrote:
> Robert,
> > Constructing arbitrary topologies with bw constrain is useful work. For example I want to create a topology without links of the capacity less then 1 Gbps. All cool. Of course if I have a case where two nodes have 10 L3 1Gbps links nicely doing ECMP I will not include those which may be a problem.
> I agree that it may be a problem. Maybe it’s not the right tool for the job at hand. That doesn’t make it a bad tool, just the wrong one. I try not to turn screws with a hammer. And I try not to drive nails with a screwdriver.
> I will happily stipulate that we need more tools and that these are not enough. We should not reject a tool simply because it doesn’t solve all problems. Let’s work towards the right set of tools. Linear algebra tells us that we want an orthogonal set of basis vectors. What are they? Adding them one at a time is not horrible progress.
> > However my observation is precisely related to your last sentence.
> >
> > Is this extension to be used with static or dynamic data ? If static all fine. But as William replied to me earlier link delay may be dynamically computed and may include queue wait time. That to me means something much different if Flex-Algo topologies will become dynamically adjustable. And I am not saying this is not great idea .. My interest here is just to understand the current scope.
> Link delay was dynamic before this draft. As William mentioned, TWAMP can already be used to provide a dynamic measurement of link delay. That, coupled with the link delay metric already gave us dynamic path computation requirements and the possibilities of oscillation and instability. We have chosen to charge ahead, without addressing those concerns already.
> Regards,
> Tony
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