Subject: [tsvwg] tests of L4S through tunnels traversing fq_codel bottlenecks

From: Pete Heist <[email-masked]>
Date: Sun, 15 Nov 2020 13:48:39 +0100

Yesterday we had an unofficial "virtual hackathon" and added new tests
of L4S traffic through Wireguard and ipfou tunnels to our current test
batch:

https://github.com/heistp/l4s-tests/#unsafety-in-tunnels-through-rfc3168-bottlenecks

In the past, we've discussed how L4S flows can dominate non-L4S flows
at RFC3168 bottlenecks, but often the counter-argument is that this
doesn't happen much in practice, because the most significant
deployment of those bottlenecks is likely to be fq_codel, which has FQ
to isolate flows (hash collisions aside).

What these tests show is that since tunneled traffic usually shares the
same five-tuple, it bypasses the flow isolation that FQ provides, so
that L4S traffic dominates non-L4S traffic in the same tunnel,
regardless of whether FQ is available or not.

It might be worth considering the practical implications of this, given
the prevalence of both tunnels and fq_codel deployments. I think of
devices that may be hard to add L4S support to, and share an fq_codel
bottleneck with potentially L4S capable traffic in the same VPN.

Regards,
Pete