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

[Ruediger.Geib at telekom.de]

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 at 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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> Rpm at lists.bufferbloat.net
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