Aijun,
On 04/03/2021 10:16, Aijun Wang wrote:
Hi, Peter:
If we use such unpredicted parameters for the dynamic IGP calculation,
I'm not sure what unpredicted parameters you are talking about.
will the network be operated automatically in non-consistent manner and let the
operator stuck in a mess, and busy to find which semi static value was changed
and what's it the cause?
no, if it is done properly. We have examples of deployments in the field
that prove it to work.
Is this the right direction for network automation?
delay based traffic optimization has been requested from the field for
years.
thanks,
Peter
Best Regards
Aijun Wang
China Telecom
-----Original Message-----
From: lsr-boun...@ietf.org <lsr-boun...@ietf.org> On Behalf Of Peter Psenak
Sent: Thursday, March 4, 2021 5:07 PM
To: Robert Raszuk <rob...@raszuk.net>
Cc: Gyan Mishra <hayabusa...@gmail.com>; Rajesh M <mraj...@juniper.net>; Shraddha Hegde
<shrad...@juniper.net>; DECRAENE Bruno IMT/OLN <bruno.decra...@orange.com>; Tony Li
<tony...@tony.li>; lsr@ietf.org; William Britto A J <bwilliam=40juniper....@dmarc.ietf.org>
Subject: Re: [Lsr] New draft on Flex-Algorithm Bandwidth Constraints
Robert,
On 03/03/2021 20:57, Robert Raszuk wrote:
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 ?
let me repeat again.
Min delay is not something that changes every few milliseconds significantly.
It's a semi static variable that reflects the property of the underlying
physical path. It only changes when the physical path properties changes - e.g.
the optical path reroutes, etc. We deliberately picked Min delay for flex-algo
purposes for this semi static property.
The small, non significant changes can be filtered by the normalization.
If the min delay changes significantly every few milliseconds there's something
wrong with the link itself - we have standard dampening mechanisms in LS
protocols to deal with unstable links that would kick in. Similar to what we do
if the link flaps 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).
there is no timer needed for the normalization itself.
You are likely referring TWAMP computation interval which is 30 sec,
with probes being sent every 3 seconds in our implementation by default,
if I'm not mistaken.
Normalization is applied to the value that come from the above.
thanks,
Peter
Likewise how Junos or Arista normalizes it today ?
Thx,
R.
On Wed, Mar 3, 2021 at 7:41 PM Peter Psenak <ppse...@cisco.com
<mailto:ppse...@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
>
>
>
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