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=1
makes 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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