Data point: Annex M ADSL2 can be approximated as 10M down, 2M up in
practice. Throw BitTorrent at that, and round-robin delay absolutely is
relevant. ADSL1 connections will be even more so. Not everyone lives in a
city in Scandinavia.

So a simple tiered scheme which can distinguish VoIP from BitTorrent and
both from general traffic, and applies fq_codel to each tier, is a good
idea.

- Jonathan Morton
 On 8 Dec 2013 20:49, "Sebastian Moeller" <moeller0@gmx.de> wrote:

> Hi Juliusz,
>
> On Dec 8, 2013, at 14:25 , Juliusz Chroboczek <
> jch@pps.univ-paris-diderot.fr> wrote:
>
> >>> The promise of fq_codel is that we can get rid of our prioritising
> >>> hacks -- if we need that kind of features, then fq_codel has
> >>> failed.
> >
> >> Is that really true? given enough concurrent flows, critical flows
> >> might be delayed purely be the round robin scheduling of equally
> >> "worthy" packets in fq_codel
> >
> > At 100 Mbit, one full-size Ethernet frame is 120us.  This means that
> > if you want your VoIP traffic to have less than 30ms delay, you should
> > in principle reach your deadline as long as you have fewer than 250
> > congestion-limited flows at a given time.
>
>         Well, is 250 enough and are the 250 really realistic in a
> residential setting? Currently not doing much of anything my router has 142
> active connections (according to conntrack) so 250 might be on the low size
> for a device that routes multiple devices. Plus, unfortunately, most
> residential internet connections are asymmetric, so on the upload will
> allow fewer congestion-limited concurrent flows before the round robin
> delay will impede the VOIP session. (In Germany residential VDSL with
> 100Mbit/s downlink will run at 40Mbit/s uplink, so hopefully not a big
> issue, but most cable providers keep the upload below 10Mbit/s, typically
> 5Mbit/s for 100Mbit/s download).  So we talk about an order of magnitude
> fewer flows required to make phone calls "interesting".
>         So I still think that for VoIP prioritizing might still be
> required until supplied minimum bandwidth gets higher.
>
> >
> >> so some residual priory system might still make sense...
> >
> > For throughput-sharing reasons, perhaps.  For latency reasons, hopefully
> not.
>
>         Even at 1000 symmetric I still think it would be a good idea to
> isolate really latency critical traffic from the rest, even if under normal
> circumstances there should be no problem, I guess a "better safe than
> sorry" approach. But, hey I do not do this for a living so I might be on
> the wrong track here.
>
> best
>         Sebastian
>
> >
> > -- Juliusz
>
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