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

[Dave Taht <dave.taht@gmail.com>]

On Wed, Nov 2, 2022 at 12:29 PM rjmcmahon via Rpm
<rpm@lists.bufferbloat.net> wrote:
>
> Most measuring bloat are ignoring queue build up phase and rather start
> taking measurements after the bottleneck queue is in a standing state.

+10. It's the slow start transient that is holding things back. If we
could, for example
open up the 110+ objects and flows web pages require all at once, and
let 'em rip, instead of 15 at a time, without destroying the network,
web PLT would get much better.

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

bytes, period. bytes = time. Sure most udp today is small packets but
quic and videconferencing change that.

>
> 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. ;)

There was an extremely long, nuanced debate about little's law and
where it applies, last year, here:

https://lists.bufferbloat.net/pipermail/cake/2021-July/005540.html

I don't want to go into it, again.

>
> 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
> >> <rpm@lists.bufferbloat.net> 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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> >
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