[Babel-users] Congestion-aware routing metrics [was: Setting ECN...]

[Juliusz Chroboczek]

> I can think of several other more silent metrics (like leveraging
> information from deep within minstrel and mac80211's states regarding
> transmission rates - or from within codel as to it's drop states) that
> would probably be more useful to use a base for metrics.

Yes, you've mentioned that before.  Dave, I do listen, and I find the
way you have of explaining the same thing time and again somewhat
patronising.

There's a lot of ideas floating around for better metrics for mesh
routing protocols.  In no particular order:

  - using link-layer indications (e.g. your idea with minstrel);
  - using delay to measure congestion;
  - using explicit network-layer indications to measure congestion;
  - etc.

The trouble is -- done carelessly, most of those cause massive jitter
and/or unbounded amounts of oscillation ("route flapping").  Babel is
particularly susceptible to that, since it tends to proactively send
packets when reconverging -- it will oscillate much faster than other
mesh protocols.

You'll recall that this whole discussion started with my announcement of
a new hysteresis algorithm in Babel which is supposed to (a) limit the
speed of oscillations, and (b) cause Babel to converge to an equilibrium
when one exists even when the equilibrium is only locally attractive.
However, the maths behind that is way over my head, so we'll need to
experiment in order to find more.

-- Juliusz