[tsvwg] qdisc hash customization as a path to shared RFC3168 queues

[Pete Heist <[e-mail]>]

In the L4S safety discussion we've talked about ways that L4S and non-
L4S traffic can wind up in the same RFC3168 queue, even with FQ:

https://github.com/heistp/l4s-tests/#unsafety-in-shared-rfc3168-queues

I don't think we've mentioned that the hash used for Linux's FQ qdiscs
(fq_codel and cake included) can be customized, for example to provide
host fairness, "subscriber" fairness, or otherwise. I wrote this up
with a test script showing some ways to use it:

https://github.com/heistp/qdisc-custom-classification/

It's easy to see how this could lead to L4S and non-L4S traffic sharing
the same queue. If an operator does not want flow-fairness, but wants
fairness e.g. among destination hosts, this can be done by attaching a
flow filter to fq_codel (where 10: is the fq_codel handle):

tc filter add dev enp1s0 protocol all parent 10: handle 1 \
        flow hash keys dst divisor 1024

That would cause all traffic to a given IP to share one queue.

This has been in the wild for some years, so it might be worth adding
to the l4sops draft.

Pete