[Cake] Emulating Bufferbloat

[Stephen Hemminger]

On Thu, 22 Nov 2018 12:28:36 -0800
Fabian Ruffy <fruffy at cs.ubc.ca> wrote:

> Hello,
> 
> this is a somewhat esoteric question. I am trying to actually force bufferbloat in an emulation setup I am using. I set up a dumbbell topology and push traffic through it, causing congestion at the central link. I use this setup to compare congestion avoidance algorithms such as DCTCP to other solutions.
> This has worked nicely with the 4.18 kernel. However, after upgrading to 4.19 I cannot reproduce bufferbloat anymore. The traffic (even UDP packets) is perfectly rate limited and I never see any congestion happening. This is great, but in practice it prevents me from prototyping algorithms.
> 
> My interface configuration for bottlenecked links is:
> 
> qdisc tbf 5: dev OBcbnsw1-eth2 root refcnt 2 rate 10Mbit burst 15000b lat 12.0ms
> >  Sent 6042 bytes 51 pkt (dropped 0, overlimits 0 requeues 0)
> >  backlog 0b 0p requeues 0
> > qdisc netem 10: dev OBcbnsw1-eth2 parent 5:1 limit 500
> >  Sent 6042 bytes 51 pkt (dropped 0, overlimits 0 requeues 0)
> >  backlog 0b 0p requeues 0
> 
> 
> I have the suspicion that it is related to the CAKE changes in the 4.19 kernel, but I am not exactly sure. I am not using tc cake at all. Do you maybe know what could cause this behavior? Apologies if this is the wrong mailing list.

More likely it is a combination of TCP small queues and pacing support.
To emulate a network you need to have an intermediate box, otherwise the local feedback
in TCP will defeat what you are trying to do.