[Bloat] Bloat goes away, but with ~25% speed loss?

[Jonathan Morton]

I think he may be seeing a complex interaction of various different queue
components and bottlenecks, which in total manages to confuse TCP
congestion control sufficiently to cause reduced throughput.

I suspect that there is a shaper at the ISP end which limits the bandwidth
available to any single subscriber. This is separate from the queue serving
the tower itself, which is what has been admitted to be periodically
overloaded. However, "overloaded" in network engineer parlance just means a
multi-user link that is saturated. There might well be enough elasticity to
allow one subscriber their full allocation by pushing others out of the
way. In this condition, the tower queue will be full and so will the shaper
queue. I imagine the shaper queue contributes the most to induced latency.

However, this "pushing others out of the way" on a drop-tail queue is done
by allowing the congestion window to grow to large values, facilitated by
the deep ISP shaper queue. This effect is defeated (by design) by running
an ingress AQM shaper, which keeps the ISP shaper queue empty.

A useful exercise might be to log the idle latency over a long period of
time, and correlate it to peak load periods, as A&A do.
http://aa.net.uk/kb-broadband-cqm.html .

- Jonathan Morton
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