Re: [ippm] [Rpm] lightweight active sensing of bandwidth and buffering

[rjmcmahon <[e-mail]>]

Most measuring bloat are ignoring queue build up phase and rather start 
taking measurements after the bottleneck queue is in a standing state.

My opinion, the best units for bloat is packets for UDP or bytes for 
TCP. Min delay is a proxy measurement.

Little's law allows one to compute this though does assume the network 
is in a stable state over the measurement interval. In the real world, 
this probably is rarely true. So we, in test & measurement engineering, 
force the standing state with some sort of measurement co-traffic and 
call it "working conditions" or equivalent. ;)

Bob
> Bob, Sebastian,
> 
> not being active on your topic, just to add what I observed on 
> congestion:
> - starts with an increase of jitter, but measured minimum delays still
> remain constant. Technically, a queue builds up some of the time, but
> it isn't present permanently.
> - buffer fill reaches a "steady state", called bufferbloat on access I
> think; technically, OWD increases also for the minimum delays, jitter
> now decreases (what you've described that as "the delay magnitude"
> decreases or "minimum CDF shift" respectively, if I'm correct). I'd
> expect packet loss to occur, once the buffer fill is on steady state,
> but loss might be randomly distributed and could be of a low
> percentage.
> - a sudden rather long load burst may cause a  jump-start to
> "steady-state" buffer fill. The above holds for a slow but steady load
> increase (where the measurement frequency determines the timescale
> qualifying "slow").
> - in the end, max-min delay or delay distribution/jitter likely isn't
> an easy to handle single metric to identify congestion.
> 
> Regards,
> 
> Ruediger
> 
> 
>> On Nov 2, 2022, at 00:39, rjmcmahon via Rpm 
>> <[e-mail]> wrote:
>> 
>> Bufferbloat shifts the minimum of the latency or OWD CDF.
> 
> 	[SM] Thank you for spelling this out explicitly, I only worked on a
> vage implicit assumption along those lines. However what I want to
> avoid is using delay magnitude itself as classifier between high and
> low load condition as that seems statistically uncouth to then show
> that the delay differs between the two classes;).
> 	Yet, your comment convinced me that my current load threshold (at
> least for the high load condition) probably is too small, exactly
> because the "base" of the high-load CDFs coincides with the base of
> the low-load CDFs implying that the high-load class contains too many
> samples with decent delay (which after all is one of the goals of the
> whole autorate endeavor).
> 
> 
>> A suggestion is to disable x-axis auto-scaling and start from zero.
> 
> 	[SM] Will reconsider. I started with start at zero, end then switched
> to an x-range that starts with the delay corresponding to 0.01% for
> the reflector/condition with the lowest such value and stops at 97.5%
> for the reflector/condition with the highest delay value. My rationale
> is that the base delay/path delay of each reflector is not all that
> informative* (and it can still be learned from reading the x-axis),
> the long tail > 50% however is where I expect most differences so I
> want to emphasize this and finally I wanted to avoid that the actual
> "curvy" part gets compressed so much that all lines more or less
> coincide. As I said, I will reconsider this
> 
> 
> *) We also maintain individual baselines per reflector, so I could
> just plot the differences from baseline, but that would essentially
> equalize all reflectors, and I think having a plot that easily shows
> reflectors with outlying base delay can be informative when selecting
> reflector candidates. However once we actually switch to OWDs baseline
> correction might be required anyways, as due to colck differences ICMP
> type 13/14 data can have massive offsets that are mostly indicative of
> un synched clocks**.
> 
> **) This is whyI would prefer to use NTP servers as reflectors with
> NTP requests, my expectation is all of these should be reasonably
> synced by default so that offsets should be in the sane range....
> 
> 
>> 
>> Bob
>>> For about 2 years now the cake w-adaptive bandwidth project has been
>>> exploring techniques to lightweightedly sense  bandwidth and
>>> buffering problems. One of my favorites was their discovery that ICMP
>>> type 13 got them working OWD from millions of ipv4 devices!
>>> They've also explored leveraging ntp and multiple other methods, and
>>> have scripts available that do a good job of compensating for 5g and
>>> starlink's misbehaviors.
>>> They've also pioneered a whole bunch of new graphing techniques,
>>> which I do wish were used more than single number summaries
>>> especially in analyzing the behaviors of new metrics like rpm,
>>> samknows, ookla, and
>>> RFC9097 - to see what is being missed.
>>> There are thousands of posts about this research topic, a new post on
>>> OWD just went by here.
>>> https://forum.openwrt.org/t/cake-w-adaptive-bandwidth/135379/793
>>> and of course, I love flent's enormous graphing toolset for
>>> simulating and analyzing complex network behaviors.
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