Re: [Bloat] SQM tuning question

[John D <j.w.r.dexter@gmail.com>]

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Thanks for the detail. It makes sense but it kind of feels like in some
(maybe many) cases the router could know the internet link performance.
Particularly home router-modems often monitor this already. Maybe that's
just not exposed in any standardised way? I'm guessing if I was into
openwrt I could maybe do something, but I prefer just to find something off
the shelf with half decent SQM... If "auto configuration" isn't a feature
then that answers my question and I can get on choosing the best option.

On Sat, Jun 3, 2023, 16:44 Jonathan Morton <chromatix99@gmail.com> wrote:

> > On 3 Jun, 2023, at 4:56 pm, John D via Bloat <
> bloat@lists.bufferbloat.net> wrote:
> >
> > On the website it says the following:
> >
> > CoDel is a novel “no knobs”, “just works”, “handles variable bandwidth
> and RTT”, and simple AQM algorithm.
> >
> >       • It is parameterless — no knobs are required for operators,
> users, or implementers to adjust.
> >       • It treats good queue and bad queue differently - that is, it
> keeps the delays low while permitting bursts of traffic.
> >       • It controls delay, while insensitive to round-trip delays, link
> rates, and traffic loads.
> >       • It adapts to dynamically changing link rates with no negative
> impact on utilization.
> >
> > But everywhere I have read about about hardware which implements SQM
> (including the bufferbloat website) it describes the need to tune based on
> actual internet connection speed.
> > These seem to conflict especially that "handles variable bandwidth" bit.
> Have I misunderstood or do the algorithms used in modern hardware just not
> provide this part typically? My connection performance is quite variable
> and I'm worried about crippling SQM to the lowest speed seen.
>
> SQM in practice requires three components:
>
> 1: Flow isolation, so that different flows don't affect each others'
> latency and are delivered fairly;
>
> 2: Active Queue Management (AQM) to signal flows to slow down
> transmissions when link capacity is exceeded;
>
> 3: Bandwidth shaping to match the queue to the available capacity.
>
> CoDel is, in itself, only the AQM component.  It does indeed work pretty
> well with no additional tuning - but only in combination with the other two
> components, or when applied directly to the actual bottleneck.
> Unfortunately in most consumer internet links, the actual bottleneck is
> inaccessible for this purpose.  Thus an artificial bottleneck must be
> introduced, at which SQM is applied.
>
> The most convenient tool for applying all three SQM components at once is
> Cake.  This includes implementations of advanced flow isolation, CoDel AQM,
> and a deficit-mode bandwidth shaper.  All you really need to do is to tell
> it how much bandwidth you have in each direction, minus a small margin to
> ensure it becomes the actual bottleneck and can exert the necessary control.
>
> When your available bandwidth varies over time, that can be inconvenient.
> There are methods, however, of observing how available capacity tends to
> change over time (typically on diurnal and weekly patterns, if the
> variations are due to congestion in the ISP backhaul or peering) and
> scheduling adjustments on that basis.  If you have more information on your
> situation, we might be able to give more detailed advice.
>
>  - Jonathan Morton

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