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

["Tilmans, Olivier (Nokia - BE/Antwerp)" <[e-mail]>]

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