[Bloat] Comcast & L4S

[Jonathan Morton]

> On 1 Feb, 2025, at 7:06 pm, Sebastian Moeller <moeller0 at gmx.de> wrote:
> 
>> As with NQB, Cake already does essentially what L4S requires, except for default-configured Codel being less than ideal as an AQM for producing congestion signals for a DCTCP-type response.  I have no intention of modifying Cake to *specifically* accommodate L4S in any way.  If their crap doesn't work properly in a standards-compliant environment, that's THEIR problem.
> 
> Now, as advocatus diabolical, the way CoDel works we have interval and/or target as configurable parameters and a trade-off between maintaining utilisation over the wider internet and keeping the signalling reactive for closer by flows, maybe we could teach cake to allow a second set of interval/(automatically calculated) target to optimise for local and non local traffic, and use a proper (configurable and maskable) DSCP/TOS to steer packets into this? Maybe CS7 would do to signal its intent for local delivery?

Codel's default 5ms target is already pretty tight, about as tight as you can reasonably make it while still accommodating typical levels of link-level jitter.  And COBALT does already find and maintain the appropriate marking rate for DCTCP when required - it just takes a little while to ramp up, so there is a noticeable hump in the delay curve during flow startup.  I don't see any low-hanging fruit there; Codel is simply not designed for that congestion response style.

DelTiC is a bit more flexible in this respect.  I don't however plan to add DelTiC to Cake.  Rather, I'm building a new qdisc that does some of the same things as Cake, but using more advanced technology and generally learning some object lessons from the experience.

 - Jonathan Morton