[tor-relays] significance of different bandwidth values

[starlight.2013q4 at binnacle.cx]

At 16:19 10/15/2013 -0400, starlight at binnacle.cx wrote:
>Question:
>
>What the difference between the bandwidth
>value given by
>   getinfo ns/name/xxxxx
>   >>> w Bandwidth=297
>and
>   getinfo dir/server/authority
>   >>> bandwidth 300000 375000 335872

Since no one answered the question and
I now have an idea about it, I'll attempt
an answer for future web-searchers. . .


   getinfo ns/name/<node-name>

This appears to be a consensus bandwidth
calculation generated by five of the
nine Tor authority nodes.  These nodes
run a special relay control client that
accumulates and analyzes bandwidth
reporting by other Tor nodes via
"signal" updates from a daemon
control channel.  Think they are
called 'bwauth's.

The value can swing wildly, but appears
that the pattern is sinusoidal with an
average close to the true bandwidth
available from a relay.  After a few days
the swings in the value seem to dampen
somewhat.

This "consensus bandwidth" value appears
to be what is used by the client nodes'
relay path selection algorithm.


   getinfo dir/server/authority

This appears to the observed bandwidth
calculation derived from local node
statistics.  Is much more stable and
accurate, but is "informational" and is
not used in the path selection algorithms.