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

Peter Psenak <ppsenak@cisco.com> Wed, 03 March 2021 10:06 UTC

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To: Robert Raszuk <robert@raszuk.net>
Cc: Tony Li <tony.li@tony.li>, Gyan Mishra <hayabusagsm@gmail.com>, DECRAENE Bruno IMT/OLN <bruno.decraene@orange.com>, Shraddha Hegde <shraddha@juniper.net>, Rajesh M <mrajesh@juniper.net>, "lsr@ietf.org" <lsr@ietf.org>, William Britto A J <bwilliam=40juniper.net@dmarc.ietf.org>
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From: Peter Psenak <ppsenak@cisco.com>
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Date: Wed, 3 Mar 2021 11:06:03 +0100
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Archived-At: <https://mailarchive.ietf.org/arch/msg/lsr/CacfbgorXhHjzNJxB5Xv8wAtFbk>
Subject: Re: [Lsr] New draft on Flex-Algorithm Bandwidth Constraints
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Robert,

On 03/03/2021 10:58, Robert Raszuk wrote:
> Hi Peter,
> 
> To your last point ...
> 
> Authors stated: "Whether egress queueing delay is included in the link 
> delay depends on the measuring mechanism."

I disagree with that statement - the Min Unidirectional Link Delay is 
the value that does not include the queueing delay - that's why it is 
called Min.

> 
> So sure there will be thresholds etc ... but this may very well depend 
> on the traffic.

no.

thanks,
Peter

> 
> Thx,
> R.
> 
> 
> 
> On Wed, Mar 3, 2021 at 10:34 AM Peter Psenak <ppsenak@cisco.com 
> <mailto:ppsenak@cisco.com>> wrote:
> 
>     Hi Tony,
> 
>     On 01/03/2021 21:47, Tony 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.
> 
>     TWAMP provided Min Unidirectional Link Delay is a dynamic one. On the
>     other side this value is calculated based on multiple measurements over
>     period of time and an average is used. Also, smart implementations can
>     normalize the value so that a small fluctuation of the delay is not
>     causing the traffic to shift or cause ECMP loss.
> 
>     What is important here is that the Min Unidirectional Link Delay is a
>     link characteristic, not something that is affected by the amount of
>     traffic on the link or subject to queuing delay. Same applies to
>     Maximum
>     link bandwidth.
> 
>     thanks,
>     Peter
> 
> 
>      >
>      > Regards,
>      > Tony
>      >
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