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

Robert Raszuk <robert@raszuk.net> Wed, 03 March 2021 19:57 UTC

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From: Robert Raszuk <robert@raszuk.net>
Date: Wed, 3 Mar 2021 20:57:10 +0100
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To: Peter Psenak <ppsenak@cisco.com>
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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Subject: Re: [Lsr] New draft on Flex-Algorithm Bandwidth Constraints
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Peter,

>  that differ by few microsecond

Really you normalize only single digit microseconds ???

What if link delay changes in milliseconds scale ? Do you want to compute
new topology every few milliseconds ?

Out of curiosity as this is not a secret -  What are your default min delay
normalization timers (if user does not overwrite with their own). Likewise
how Junos or Arista normalizes it today ?

Thx,
R.

On Wed, Mar 3, 2021 at 7:41 PM Peter Psenak <ppsenak@cisco.com> wrote:

> Tony,
>
> On 03/03/2021 19:14, Tony Li wrote:
> >
> > Peter,
> >
> >>> There are several link types in use that exhibit variable delay:
> satellite links (e.g., Starlink), microwave links, and ancient link layers
> that deliver reliability through retransmission.
> >>> Any of these (and probably a lot more) can create a noticeable and
> measurable difference in TWAMP. That would be reflected in an FA metric
> change. If you imagine a situation with multiiple parallel paths with
> nearly identical delays, you can easily imagine an oscillatory scenario.
>  IMHO, this is an outstanding concern with FA.
> >> yes, and that is what I referred to as "delay normalization", which can
> avoid that oscillation.
> >
> >
> > It can also negate the benefits of the feature. One might well imagine
> that Starlink would want to follow a min-delay path for optimality.  If the
> delay variations are “normalized” out of existence, then the benefits are
> lost.  The whole point is to track the dynamics.
>
> for all practical purposes that we use it for, the two values of min
> delay that differ by few microsecond can be treated as same without any
> loss of functionality. So it's about the required normalization interval
> - something that can be controlled by the user.
>
> thanks,
> Peter
>
>
>
> >
> >
> >>> Please note that I’m NOT recommending that we back away. Rather, we
> should seek to solve the long-standing issue of oscillatory routing.
> >>
> >> not that I disagree. History tells us that the generic case of
> oscillation which is caused by the traffic itself is a hard problem to
> solve.
> >
> >
> > Any oscillation is difficult to solve.  Positive feedback certainly can
> exacerbate the problem. But solving hard problems is why we are here.
> >
> > Yours in control theory,
> > Tony
> >
> >
> >
>
>