[Babel-users] Basic settings to enable RTT metric.

[Juliusz Chroboczek]

> Hi, I was wondering what is the best way to enable the RTT metric with
> sensible defaults. It looks like max-rtt-penalty is the option that turns it
> on. What is a good value to set this at starting out? 

Yeah, the RTT metric is fiddly.  For very good reasons -- it's an
intrinsically unstable metric (it contains a feedback loop).  I suggest
you have a look at Section III.D of

  https://arxiv.org/pdf/1403.3488.pdf

The most important parameters is rtt-max.  It should be configured as
a sufficiently large value, but small enough that the latency of highly
congested bottleneck links is higher than rtt-max.  The default value of
120ms works well in the global-scale overlay networks we've experimented
with.

Rtt-min and max-rtt-penalty don't matter much -- if rtt-max is chosen
wisely, the algorithm will be reasonably stable whatever the other values.
Rtt-min should be a value slightly larger than the latency of completely
uncongested low-latency links; the default value of 10ms should be fine
for most applications.  max-rtt-penalty determines how much high-latency
links are penalised; any value between 128 and 384 should yield decent
results, I suggest starting with 197 (which happens to be a prime number).

-- Juliusz