[Cake] dual-src/dsthost unfairness, only with bi-directional traffic

[Jonathan Morton]

>> Yes, exactly. Would be interesting to hear what Jonathan, Toke and
>> others think. I want to see if fairness is preserved in this case with
>> sparse flows only. Could flent do this?
> 
> Well, sparse flows are (by definition) not building a queue, so it
> doesn't really make sense to talk about fairness for them. How would you
> measure that?
> 
> This is also the reason I agree that they shouldn't be counted for host
> fairness calculation purposes, BTW...

The trick is that we need to keep fairness of the deficit replenishments, which occur for sparse flows as well as bulk ones, but in smaller amounts.  The number of active flows is presently the stand-in for this.  It's possible to have a host backlogged with hundreds of new flows which are, by definition, sparse.

I'm still trying to get my head around how the modified code works in detail.  It's possible that a different implementation would either be more concise and readable, or better model what is actually needed.  But I can't tell until I grok it.

 - Jonathan Morton