[Cake] Fighting bloat in the face of uncertinty

Jonathan Morton chromatix99 at gmail.com
Sun Sep 8 13:27:03 EDT 2019

>> You could also set it back to 'internet' and progressively reduce the 
>> bandwidth parameter, making the Cake shaper into the actual bottleneck. 
>> This is the correct fix for the problem, and you should notice an 
>> instant improvement as soon as the bandwidth parameter is correct.
> Hand tuning this one link is not a problem. I'm searching for a set of settings that will provide generally good performance across a wide range of devices, links, and situations. 
> From what you've indicated so far there's nothing as effective as a correct bandwidth estimation if we consider the antenna (link) a black box. Expecting the user to input expected throughput for every link and then managing that information is essentially a non-starter. 
> Radio tuning provides some improvement, but until ubiquiti starts shipping with Codel on non-router devices I don't think there's a good solution here. 
> Any way to have the receiving device detect bloat and insert an ECN?

That's what the qdisc itself is supposed to do.

> I don't think the time spent in the intermediate device is detectable at the kernel level but we keep track of latency for routing decisions and could detect bloat with some accuracy, the problem is how to respond.

As long as you can detect which link the bloat is on (and in which direction), you can respond by reducing the bandwidth parameter on that half-link by a small amount.  Since you have a cooperating network, maintaining a time standard on each node sufficient to observe one-way delays seems feasible, as is establishing a normal baseline latency for each link.

The characteristics of the bandwidth parameter being too high are easy to observe.  Not only will the one-way delay go up, but the received throughput in the same direction at the same time will be lower than configured.  You might use the latter as a hint as to how far you need to reduce the shaped bandwidth.

Deciding when and by how much to *increase* bandwidth, which is presumably desirable when link conditions improve, is a more difficult problem when the link hardware doesn't cooperate by informing you of its status.  (This is something you could reasonably ask Ubiquiti to address.)

I would assume that link characteristics will change slowly, and run an occasional explicit bandwidth probe to see if spare bandwidth is available.  If that probe comes through without exhibiting bloat, *and* the link is otherwise loaded to capacity, then increase the shaper by an amount within the probe's capacity of measurement - and schedule a repeat.

A suitable probe might be 100x 1500b packets paced out over a second, bypassing the shaper.  This will occupy just over 1Mbps of bandwidth, and can be expected to induce 10ms of delay if injected into a saturated 100Mbps link.  Observe the delay experienced by each packet *and* the quantity of other traffic that appears between them.  Only if both are favourable can you safely open the shaper, by 1Mbps.

Since wireless links can be expected to change their capacity over time, due to eg. weather and tree growth, this seems to be more generally useful than a static guess.  You could deploy a new link with a conservative "guess" of say 10Mbps, and just probe from there.

 - Jonathan Morton

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