Re: [tsvwg] Fwd: New Version Notification for draft-white-tsvwg-lld-00.txt

[Luca Muscariello <[e-mail]>]

Is there an assumption that the low latency queue is only used by rate
controlled flows
that are sensitive to ECN feedback?

 This goes back to my question to Greg about serving non marked flows in
the low latency queue.

I would assume that a low latency queue is open to flows that are non
necessarily controlled by ECN but just exogenously rate limited by say a
 codec rate.

But now it seems like this is not the case.

Thanks
Luca

On Sun 17 Mar 2019 at 18:32, Tilmans, Olivier (Nokia - BE/Antwerp) <
[e-mail]> wrote:

> Hi Luca,
>
> > The protection mechanism assumes that one queue has soft priority over
> > the other. Strict priority would be stupid, so there must be a wfq
> weight to
> > schedule the classic queue less frequently. I did not find the magic
> number
> > that  is set in the DOCSIS specs but whatever number is chosen I wonder
> > which opitimization objective would be the foundation of that.
> > 10%, 20%, 30% or any number would imply that if the priority queue is
> used
> > at 100% utilization the other apps would get a small fraction of the link
> > capacity.
> >
> > What number is chosen and based on which calculations?
>
> The protection mechanism only affect how competing packets within a RTT
> gets
> dequeued. In this instance, a WRR protects the classic flows from
> unresponsive/misclassified L4S flows, at the expense of the other L4S
> flows.
>
> It does not affect the overall throughput of the flows when measuring it
> over a
> timeframe of a few Tupdate interval (e.g., 16ms). Indeed, the core of the
> DualPI2 AQM is a PIE2 controller computing the random drop/marking
> probability
> based on the classic queue. This probability is then also used as base to
> mark L4S
> flows, albeit with a quadratic relationship (i.e., for the same base
> probability, the
> L4S flows are marked much more often than the classic ones).
>
> Consequently, whenever L4S traffic causes the classic queue to build up
> (even
> slightly above the PIE2 target), its marking probability drastically
> increases as the
> PIE2 controller reacts. As the L4S flows have scalable CCs (i.e.,
> understand how to
> leverage multiple CE marks per RTT), they reduce their cwnd accordingly
> within
>  a RTT (which is much smaller than the classic flows' RTT).
>
> In other words, L4S senders end up slowing down themselves (to a point
> where
> the L4S queue is empty for most of the transmission slots) to allow the
> classic
> queue to drain, and this reaction is the result of a much faster
> control-loop than
> the one used by the classic sender.
>
> There are thus no magic number per say for the WRR ratio. §4.1.4 of
>  draft-ietf-tsvwg-aqm-dualq-coupled explains the guidelines to pick one.
>
> Note that the exact throughput ratio between L4S and classic flows can be
> tweaked using a coupling factor, as explained in appendix C. of
>  draft-ietf-tsvwg-aqm-dualq-coupled.
>
> I hope this was clear enough.
>
>
> Best,
> Olivier
>