[Cake] Upstream submission of dual-mode fairness patch

[Pete Heist]

> On Mar 3, 2019, at 5:10 PM, Jonathan Morton <chromatix99 at gmail.com> wrote:
> 
>> On 3 Mar, 2019, at 6:07 pm, Pete Heist <pete at heistp.net> wrote:
>> 
>> Does it say anything to you that enabling ECN makes the asymmetry go away?
> 
> Yes - it tells me that it has something directly to do with how dropped packets are accounted for, not with the congestion response by TCP.  ECN changes the number of dropped packets, and shouldn't change the congestion response.

That suggests it’s possible to fix. It almost seems like in ingress mode, dropped packets are being counted as bytes transferred, when they weren’t.

In the four pcaps I sent, enabling ECN resulted in 0 dropped packets for the N flows down, so it didn’t get to the point where a drop was necessary. Also, the rate of CWR bits set was a bit lower than the rate of drops, and total aggregate throughput was a bit higher. so enabling ECN is a win in this test.