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

[Luca Muscariello <[e-mail]>]

On Mon, Mar 18, 2019 at 1:21 PM Tilmans, Olivier (Nokia - BE/Antwerp) <
[e-mail]> wrote:

> Hi Luca,
>
> Just to make sure, did I answer your question?
>

maybe. Let me ask a few more clarification questions below.


>
> If not the behavior of putting NQB flows in the L4S queue is dependent on
> the
> AQM used, hence the answer would be to look at
> draft-ietf-tsvwg-aqm-dualq-coupled for a discussion of the effects of
> doing so.
> DOCSIS adds on top of the AQM a protection scheme to sanction flows in the
> L4S that cause queue build up, and uses a WRR to control how packets get
> dequeued.
>
> The final answer would then be: yes, you can put non-ECN controlled flows
> in
> the L4S queue provided those are sparse enough. Otherwise, they would need
> to be wrapped in a scalable CC and/or interpret the received ECN feedback
> (as
>  the 2015 demo showing live video streaming zooming/zoomout did).
>

This is where things become unclear or just unspecified in the documents I
read so far.
Sometimes maths or code is simpler to read that a thousand words.

My interpretation of what you say is that the time sharing between the two
queues
depends on a closed loop mechanism that relies on classification first and
a compliant sender that
is sensitive to ECN according  to the mechanism described in
draft-ietf-tsvwg-l4s-arch.

Assuming that the control theoretic equilibrium you get requires a given
WRR weight,
anytime non conformant traffic is allowed into the low latency queue this
equilibrium can be
perturbed. Which is fine as long as the perturbation is small enough.

Non controlled traffic can generate a busy period in the low latency queue
that can grow
w/o necessarily generating large standing queues. That should not trigger
the queue protection
mechanism Greg mentioned.
>From what I understand WRR would  then make the equilibrium drift to
another value.
This value should be that the classic queue gets lower service time
compared to what
the original configuration was meant to provide.

Do I get this right?

Thanks
Luca


>
>
> Best,
> Olivier
>
>
> > -----Original Message-----
> > From: Luca Muscariello <[e-mail]>
> > Sent: Monday, March 18, 2019 12:21 PM
> > To: Tilmans, Olivier (Nokia - BE/Antwerp) <olivier.tilmans@nokia-bell-
> > labs.com>
> > Cc: Greg White <[e-mail]>; tsvwg IETF list <[e-mail]>
> > Subject: Re: [tsvwg] Fwd: New Version Notification for
> draft-white-tsvwg-lld-
> > 00.txt
> >
> >
> >
> > On Mon, Mar 18, 2019 at 9:56 AM Tilmans, Olivier (Nokia - BE/Antwerp)
> > <[e-mail] <mailto:olivier.tilmans@nokia-bell-
> > labs.com> > wrote:
> >
> >
> >       > Is there an assumption that the low latency queue is only used by
> > rate
> >       > controlled flows
> >       > that are sensitive to ECN feedback?
> >
> >       The L4S definitions in draft-ietf-tsvwg-l4s-arch state that L4S
> flow
> > have a
> >       scalable CC, in order to perform bandwidth pooling (fairly) between
> > the
> >       L4S flows and the classic ones.
> >       This can however be relaxed a bit, see below.
> >
> >       > 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.
> >
> >       This is what draft draft-white-tsvwg-nqb discusses, namely also
> > adding
> >       non-queue building flows in the L4S queue (e.g., non-capacity
> > seeking
> >       flows such as gaming, DNS traffic, pings, sparse CBR flows).
> >
> >       Note that the presence of such unresponsive flows in the L4S queue
> >       implicitly decreases the throughput of the scalable L4S flows (but
> not
> >       their classic counterpart), as they have to back off on behalf of
> the
> >       non-responsive flows (but can do so without fiddling with a magic
> > WRR
> >       number). This can be accounted for in the coupling factor of the
> > queue.
> >
> >
> >
> > Right, this is what I was trying to understand.
> > I checked draft-white-tsvwg-nqb but the mechanism to share traffic in the
> > two queues
> > is not described. This document refers back to draft-ietf-tsvwg-l4s-arch
> so I
> > cycled back to
> > my starting point as there is no description of how the queueing system
> > works when
> > non marked ECNed traffic is sent  to the low latency queue.
> >
> > I thought that that was described in the DOCSIS spec Greg shared, but did
> > not find an answer there either.
> >
> >
> >
> >
> >
> >
> >
> >       Best,
> >       Olivier
> >
> >
> >       > 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]
> > <mailto:[e-mail]>  <mailto:
> olivier.tilmans@nokia-
> > bell- <mailto:olivier.tilmans@nokia-bell->
> >       > labs.com <http://labs.com> > > 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
> >       >
> >
> >
>
>