[Cake] Cake3 - source code and some questions

[Jonathan Morton]

This is a question worth discussing. There is a certain amount of
controversy over the actual meaning, utility and constraints of the target
parameter, although the interval parameter is fairly well understood as a
rough (order of magnitude) estimate of the prevailing RTT.

Note that even with FTTP, while the RTT to the head end may be unusually
low, the RTTs to interesting servers will still be in roughly the same
range as on a good quality ADSL link. This is especially true if the
interesting servers tend to be at the other end of the country/continent or
on the other side of an ocean. This variability is within Codel's capacity.

Due to the Diffserv and flow isolation features of cake, the latency
minimization feature provided by Codel also isn't as critical to tune as it
is when standalone, or with a lesser flow isolation system such as
fq_codel's collision prone hash function. I think this is sufficient to
make further tuning unnecessary up to 1 gigabit, whether on a LAN or over
the internet, and since I haven't seen any home affordable gear for more
than a gigabit yet - marketing tricks by Wi-Fi vendors aside - I don't
think it's worth thinking too hard about pushing that higher in the home
use case. Fq_codel also works quite well on a LAN already.

The difference in a datacentre is that typical native RTTs are measured in
microseconds, well outside the range that Codel is by default tuned for.
The bandwidths involved also mean that the standard 5ms target invokes a
large amount of buffered data. Additionally, we're inherently talking about
a wholly local environment, so there is no need to adapt to internet scale
RTTs.

For those cases where you do have a datacentre like environment connected
to an internet like environment, the solution is obvious. Deploy datacentre
tuned AQM (which might be fq_codel with altered parameters) within the
datacentre, and put cake at the gateway(s) to the internet. Job done.

- Jonathan Morton
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