[Cake] [Bloat] Fwd: [Galene] Dave on bufferbloat and jitter at 8pm CET Tuesday 23

Sebastian Moeller moeller0 at gmx.de
Fri Feb 26 12:14:07 EST 2021

Hi Roland,

are you sure that BBRv2 actually evaluates CE marks and responds to them? As far as I understood, BBR is simply not rfc3168 compliant, there was some talk about teaching BBRv2+ some still to be defined high frequency (low amplitude) ECN signaling.

The thing I fail to understand is, why BBR with its cavalier approach to drops did not grow ECN support on day one. While a drop could have a lot of reasons, including transient/stray wifi losses, a CE mark is less ambiguous.

Best Regards

> On Feb 26, 2021, at 17:59, Bless, Roland (TM) <roland.bless at kit.edu> wrote:
> Hi,
> On 26.02.21 at 16:27 Nils Andreas Svee wrote:
>> On 2/26/21 12:47 PM, Toke Høiland-Jørgensen wrote:
>>> Yeah, there would have to be some kind of probing to discover when the
>>> bandwidth goes up (maybe something like what BBR does?). Working out the
>>> details of this is still in the future, this is all just loose plans
>>> that I'll try to get back to once we have the measurement tool working
>>> reasonably well. Input and experiments welcome, of course!
>> I've kept to maintaining CAKE binaries for the EdgeRouters, so I have a lot to read up on if I'm gonna take a stab at this, should be fun though :)
>> I'll have a look at how BBR does it, and see if I can't figure out how that works at least.
> BBR increases its sending rate and looks whether the delivery rate
> increases. It assumes that the bottleneck limit hasn't been reached
> in case the delivery rate still increases. This is certainly true when
> it is the only flow at the bottleneck, but not true when multiple
> flows are present (the probing flow may simply steal other flows'
> shares adn thus get a higher delivery rate nevertheless).
> BBRv2 at least checks for packet loss and ECN
> signals and detects when it hits a hard limit. One could try to
> detect a correlated increase in RTT when probing for more bandwidth
> and then stop, because it seems that the queue is filled by the
> increased probing rate. However, getting that reliably detected out of
> the RTT measurement noise is sometimes difficult.
> Regards,
> Roland
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