[Codel] interval target relation ship question

Andrew McGregor andrewmcgr at gmail.com
Thu Mar 13 05:04:56 EDT 2014

My intuition suggests something like:
Target = 4ms + MTU sized packet duration
Interval = target * 20

(But only if the resulting interval is more than 100ms)

Possibly the packet duration may need a small factor (2 or 3) to get the
balance right.
On 13 Mar 2014 19:41, "Sebastian Moeller" <moeller0 at gmx.de> wrote:

> Dear Experts,
> Codel and especially fq_codel have massively improved
> snappiness/interactivity of typical residential internet connections, as
> shown in the cerowrt testbed and also in the french ISP free's roll-out of
> coddled xddl modems. One observation has been that at low bandwidth the
> latency/bandwidth trade-off does not seem to be ideal and an empirical
> solution to this problem has been to increase the target as a function of
> the available bandwidth. I realize that codel tries to accommodate for
> low-bandwidth links by always allowing at least one packet in the queue.
> But empirically that does not seem to be enough for good behavior on slow
> links (I think the issue is that the bandwidth sacrifice seems a bit to
> large)...
>         Currently we try to model what we know about free's approach in
> cerowrt, basically we increase target as a function of bandwidth and also
> increase interval be the same amount as target. Now having read section
> "3.2 Setpoint" of
> https://datatracker.ietf.org/doc/draft-nichols-tsvwg-codel/?include_text=1makes a strong point that target should be in the range of 5-10% of
> interval. So would it make more sense to increase interval so that after
> adjustments new_target = 0.05*new_interval still stays true? Or would you
> recommend to do something along the lines of:
>         new_interval = 100ms + known DSL link latency (can be in the range
> of dozens of ms)
>         new_target = new_interval * 0.05 or new_interval * 0.1
> I guess I will try to actually test the different approaches in the near
> future, but would be delighted to get help establishing a decent hypothesis
> before hand which modification actually will work best.
> Bet Regards
>         Sebastian
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