[Codel] Do codel or fq-codel need to know the link bandwidth?

[Evan Mesterhazy <evan.mesterhazy@gmail.com>]

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As far as I can tell, the fq-codel algorithm (
https://www.rfc-editor.org/rfc/rfc8290.html) doesn't use link bandwidth as
a parameter. From reading the description, it seems like it should be able
to work without knowing the link bandwidth since it decides to drop packets
based on the amount of time they have waited in a queue. In fact, the
original codel rfc (https://www.rfc-editor.org/rfc/rfc8289#section-4.1)
even says that it's designed to be non-starving and work over variable
bandwidth links.

However, I have noticed that the commercial implementations for at least
two firewalls that I have used (OPNsense and a Unifi gateway) both require
the user to input the upload and download bandwidths of the link.

In OPNsense I tried setting the bandwidth to a large number above the
actual link bandwidth and then manually setting the fq-codel parameters to
their Linux defaults, but that resulted in poor performance. My concern, of
course, is that I'm leaving throughput on the table by artificially
limiting the bandwidth at a value that may be less than the actual
available bandwidth depending on the time of day, etc.

I'm not sure what exactly OPNsense is doing with the bandwidth parameters,
and I've asked on its forum (
https://forum.opnsense.org/index.php?topic=44501.0). My question for all of
you is whether I am misunderstanding something about fq-codel. Does it need
to be configured with the link bandwidth, or can it work nicely in its
default configuration with variable bandwidth links (let's consider a
typical home fiber connection between 300 and 1000 Mbps).

Thanks for any insight you can share.

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