Re: [Cake] [Bloat] dslreports and inbound rate shaping

[Jonathan Morton <chromatix99@gmail.com>]

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When Codel is applied on the upstream side of a link, a burst arrives in it
almost instantly, and thus it only takes 5ms to detect that 5ms of queue
has developed. The interval parameter then delays action on that detection
until it is certain that it's a standing queue and not simply a burst.

When applied on the downstream side of a link, however, it takes longer for
a burst to filter through to where Codel can see it. If the shaper is set
to 90% of the link rate, it takes at least 45ms to build a 5ms queue,
during which the receiver is acking data without any clue that congestion
is in place. At 95%, it requires at least 95ms. The delay in detection
might be even longer under some circumstances.

This means Codel has to be more aggressive at responding to a detected 5ms
queue on ingress in order to provide control of the flow comparable to
egress. I'm proposing using a reduced interval parameter to do that. A
drawback is that the response will be stronger than designed, and this may
have an impact on throughput, but the same is true (and more definitely so)
of a policer.

On the one hand, this might lead to an interim solution while Bobbie gets
fleshed out. On the other, it should provide more information on whether
Bobbie is likely to work.

- Jonathan Morton

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