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

Tony Li <tony.li@tony.li> Wed, 03 March 2021 18:14 UTC

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From: Tony Li <tony.li@tony.li>
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Cc: Robert Raszuk <robert@raszuk.net>, 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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To: Peter Psenak <ppsenak@cisco.com>
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Subject: Re: [Lsr] New draft on Flex-Algorithm Bandwidth Constraints
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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.


>> 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