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

xnor xnoreq at gmail.com
Fri Apr 28 17:11:36 EDT 2017


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.

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.

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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