Re: [Bloat] Solving bufferbloat with TCP using packet delay

[grenville armitage <garmitage@swin.edu.au>]

On 03/21/2013 02:36, Steffan Norberhuis wrote:
> Hello Everyone,
>
> For a project for the Delft Technical University myself and 3
> students are writing a review paper on the buffer bloat problem and
> its possible solutions.

My colleagues have been dabbling with delay-based CC algorithms,
with FreeBSD implementations (http://caia.swin.edu.au/urp/newtcp/)
if that's of any interest.

Some thoughts:

  - When delay-based TCPs share bottlenecks with loss-based TCPs,
	the delay-based TCPs are punished. Hard. They back-off,
	as queuing delay builds, while the loss-based flow(s)
	blissfully continue to push the queue to full (drop).
	Everyone sharing the bottleneck sees latency fluctuations,
	bounded by the bottleneck queue's effective 'length' (set
	by physical RAM limits or operator-configurable threshold).

  - The previous point suggests perhaps a hybrid TCP which uses
	delay-based control, but switches (briefly) to loss-based
	control if it detects the bottleneck queue is being
	hammered by other, loss-based TCP flows. Challenging
	questions arise as to what triggers switching between
	delay-based and loss-based modes.

  - Reducing a buffer's length requires meddling with the
	bottleneck(s) (new firmware or new devices). Deploying
	delay-based TCPs requires meddling with endpoints (OS
	upgrade/patch). Many more of the latter than the former.

  - But yes, in self-contained networks where the end hosts can all
	be told to run a delay-based CC algorithm, delay-based CC
	can mitigate the impact of bloated buffers in your bottleneck
	network devices. Such homogeneous environments do exist, but
	the Internet is quite different.

  - Alternatively, if one could classify delay-based CC flows into one
	queue and loss-based CC flows into another queue at each
	bottleneck, the first point above might not be such a problem.
	I also want a pink pony ;)  (Of course, once we're considering
	tweak the bottlenecks with classifiers and multiple queues,
	might as continue the surgery and reduce the bloated buffers too.)

cheers,
gja