[Babel-users] Tuning Babel-RTT [was: Babel with VPP]

[Juliusz Chroboczek]

> 1) Is there any advice you could offer for rtt cost/min/max/decay values when
> using Bird2 ?


1. RTT-MIN 

In the ideal case, your network consists of a number of interconnected
clusters.  For example, if you have routers in Berlin, Paris and Warsaw,
then each of the cities constitutes a cluster.  Within each cluster, it
doesn't make sense to chose routes based on RTT, since small RTT values
tend to be noisy and cause instability.

Rtt-min should ba a value that is more than the intra-cluster latency but
less than the inter-cluster latency.  For example, if latency within each
cluster is on the order of 5ms, and the inter-cluster latency is 20ms,
then the deault value of 10ms is fine.

Large values of rtt-min improve stability in the presence of bufferbloat.


2. RTT-MAX

Symmetrically to rtt-min, rtt-max is the value above which links are
considered bad.  It should be slightly larger than the largest RTT in your
network.  Set it as small as possible in your network, since it has
a dramatic effect on stability in the presence of bufferbloat.

The default is 120ms, which is very conservative, but already has
a big effect on improving stability in bufferbloated networks.


3. MAX-RTT-PENALTY (rtt cost in BIRD)

This is the maximum cost penalty that will be applied to high-RTT links.
The default (96) is rather conservative, it will cause one high-RTT link
to be equivalent to two low-RTT links.  Increase this value if you see
traffic following non-local routes.  There's probably no reason to
decrease it, except if you want to minimise the number of hops at the cost
of taking non-local routes.


Hope that helps,

-- Juliusz