[Cake] cake default target is too low for bbr?

David Lang david at lang.hm
Fri Apr 28 17:29:27 EDT 2017

On Fri, 28 Apr 2017, xnor wrote:

> As I understand it, increase in RTT due to queueing of packets is the 
> main feedback mechanism for BBR.
> So dropping packets, which I already consider harmful, is really harmful 
> with BBR because you're not telling the sender to slow down.

If BBR does not slow down when packets are dropped, it's too hostile to use on a 
public network. The only way for a public network to respond to a flood of 
traffic higher than what it can handle is to drop packets (with a possible 
warning via ECN shortly before packets get dropped). If BBR doesn't slow down, 
it's just going to be wasting bandwidth.

> Instead, with a controlled delay qdisc like cake or codel, you're 
> telling the sender to keep sending the data faster because the qdisc 
> keeps the increase in RTT minimal. To make things worse, you're throwing 
> away perfectly good packets with no effect other than wasting bandwidth.

you are mistaking how cake and codel work. They are working at the link endpoint 
adjacent to where the bandwidth is most limited. They drop packets before they 
get send over the most contrained link. The fact that the packets eat up some 
bandwidth on the non-constrained link prior to the bottleneck doesn't matter, 
the bandwidth is available by definition (otherwise it would be a constrained 
link to the endpoint before it)

you have the systems A-H sending connected in series. If the bandwidth between D 
and E is the bottleneck, it doesn't matter that you wast the bandwidth between 
A-B-C-D because the bandwidth would otherwise be idle. D drops packets only when 
there is not bandwidth between D and E to handle them.

David Lang

> I'm not sure how BBR handles jitter or how small of an increase in RTT 
> it detects as "congestion" but from my limited testing, I've noticed no 
> problems with steam downloads in the bulk tin with 15 ms target in cake.
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