[Cake] Upstream submission of dual-mode fairness patch

[Pete Heist]

> On Mar 4, 2019, at 5:22 AM, Ryan Mounce <ryan at mounce.com.au> wrote:
> 
> On Mon, 4 Mar 2019 at 13:47, Jonathan Morton <chromatix99 at gmail.com> wrote:
>> 
>>> On 4 Mar, 2019, at 4:55 am, Georgios Amanakis <gamanakis at gmail.com> wrote:
>>> 
>>> …the fairness logic wouldn't account for that "ingress traffic" and would yield fairer results.
>> 
>> Well there we have a quandary, since presently it enforces fairness of *load* on the bottleneck link, but the change would alter that to fairness of *delivered* traffic.  The current setup is arguably more robust against adversarial traffic, don't you think?

I think that’s the best argument for the current behavior.

> And it provides an incentive to use ECN so that congestion signals can
> be sent "for free" without dropping packets that have traversed the
> bottleneck. I'm firmly in favour of the current setup

Agreed. I was going to provide test results of aggressive UDP vs TCP with and without the change, but I’m seeing some odd behavior with UDP that I’ll investigate more and start in a separate thread if needed. :)