[Codel] Worst case conditions - large RTT

[Jim Gettys]

On 05/09/2012 04:41 PM, Dave Taht wrote:
> On Wed, May 9, 2012 at 1:36 PM, Simon Barber <simon at superduper.net> wrote:
>> Codel assesses the minimum delay over a fixed time period (the interval) -
>> what happens when the interval is much smaller that the RTT. For example a
>> tcp stream is experiencing a 2 second RTT (perhaps due to bufferbloat at
>> some point), yet the interval is 100ms. The TCP implementation is a poor one
>> that will ACK a window at a time. Here codel will drop excessively, and
>> cause low throughput?
>>
>
One test I'd love to see is to compute from first principles what the
latency and jitter we should now be observing in a single ideal TCP
flow, at several RTT's.  Then we can compare with that latency and
jitter we *actually* observe.

I'd expect a slightly higher value right now, as BQL's buffering is
being added to the queue that CoDel is managing, but we can at least see
if the implementation we have is behaving exactly as we would predict.
                    - Jim