[Codel] CoDel and Phantom Queues?

[Richard Scheffenegger]

Hi,

I assume some of the members of this list, working with AQM schemes are also 
familiar with the work at Stanford around DCTCP.

In a recent paper of Balaji Prabakhan (DCTCP/ QCN), Phantom Queues are 
mentioned as another means to significantly reduce (real queue) occupancy. I 
think the concept (as virtual queues) stems from ATM times.However, phantom 
queues are only a logical (accounting) entitiy, and unlike VQs they are set 
in series with the real queue, but drained at a rate (1-eps), slightly 
slower than the real queue (i.e. at 95-98 of the link capacity - when there 
is a defined link capacity).

The idea, if I can repeat it properly, is that any standing queue would form 
in the phantom queue first, before that standing queue would actually show 
on the real queue.

 I was wondering if phantom queues and CoDel would work synergistically 
together.  Or if Phantom Queues should rather be regarded as a workaround 
for the (so far) poor AQM schemes proposed.

As you are probably aware, DCTCP achieves more than an order of magnitude 
better network delays by adjusting both the end host TCP stack (alike 
ECN(alpha,beta) to adjust the CWnd dynamically based on the signaled 
congestion / queue depth), and by utilizing a step-function marking scheme 
in the network, instead of a probabilistic marking scheme.

According to the latest paper, 
http://www.stanford.edu/~balaji/papers/nsdi12-final187.pdf, the utilization 
of Phantom Queues in a DCTCP environment cut the network latency once more 
by one order of magnitude. Effectively, all buffers run virtually empty 
(except for slow-start / initial window) bursts, at only a very minuscle 
loss of effective network capacity.

I would like to learn your thoughts around that!
If anyone has a proper ns2 / ns3 environment (mine is currently broken) 
where Phantom Queues plus CoDel can be simulated, if any odd interactions 
arise...

Best regards,