[Babel-users] Fun with metrics [was: Babel questions]

[Juliusz Chroboczek]

> Well, I'm intending to use 802.11, but in infrastructure mode
> (AP/station) rather than ad-hoc.

I'm wondering if you're not over-complicating things.  Are the STAs
actually forwarding packets?  If they aren't, then why don't you just do
static routing on the STAs, and run Babel on the APs only?

> Packet loss happens, and with TCP/IP traffic, should be avoided -- but
> like city driving, sometimes the best route is congested and of
> questionable quality, and sometimes it's the only route to your
> destination.

Hmm... ETX assumes that the network is not congested, and that the only
cause of packet loss is due to the phy layer.  Are you trying to design
a routing metric that takes both congestion and phy-layer issues into
account simultaneously?

> If they match, default metric, otherwise 2x default.

ETX is sub-linear in the number of hops.  The first-order approximation
for ETX over two hops is

  256 + ETX over one hop

I'm too lazy to derive an accurate formula (the first-order approxi-
mation doesn't take into account correlated losses over the two links),
but I'm pretty sure the above is good enough.

> Good point. I think I also need it to be aware of traffic levels --

Heh.  Google for ``load-aware metrics''.  Careful about failure of
isotonicity.

> just a general feeling of if I should try.

You definitely should try.  Playing with metrics in this way certainly
counts as interesting, original research, and it's stuff that we can
only do because we know that Babel is robust in the presence of weird
metrics (i.e. the proof of loop-freedom of Babel doesn't depend on the
exact metric being used).

On a related note, please check babelz.

                                        Juliusz