Re: [Babel-users] Follow-up on the new babel-rtt branch: µs resolution

[Juliusz Chroboczek <jch@pps.univ-paris-diderot.fr>]

Baptiste:

> - Babel sees a RTT that is 400 µs higher than ping6.  The babel-rtt
>   implementation timestamps outgoing message as late as possible, and
>   timestamps incoming messages as early as possible, but it's not
>   perfect.

The good news, of course, is that the offset is pretty constant, so
the samples computed by babeld are good enough to be input to the
metric computation.  The smoothed samples would appear to have
a precision of roughly 200us, which should be good enough for almost
any application.

Baptiste, I'm wondering if you can count the number of Ethernet
switches that way.

> I'm delighted to see this measurement as collected by babel

I knew you would be :-)

> my sekret plan was to be able to measure heavy traffic benchmarks vs
> various qdiscs like the new "fq" and older fq_codel ones...

Well, the reason we decided it's worthwile to improve the precision of
our measurements is that, according to Jim, you had a secret plan to
use RTT to measure link-layer congestion in Wifi meshes.  How many
more secret plans do you have?

-- Juliusz