[tor-dev] Understanding bwauth data in Stem?

Anna Kornfeld Simpson aksimpso at cs.washington.edu
Fri Nov 21 22:34:21 UTC 2014

I'm trying to understand how the bandwidth authorities work, and reading
the spec [0] got me only partway to an understanding, so I'm trying to use
Stem to see what the bwauth measurements look like in practice.  I'm
working off the "Votes by Bandwidth Authorities" example on the Stem
webpage [1].  The query there returns a RouterStatusEntryV3, so I've been
looking at the "bandwidth" and "measured" fields to try to understand
what's going on [2].  "Bandwidth" maxes out at 10,000 (kb/s??) but
"measured" doesn't seem to have the same ceiling, which made me realize
that they aren't in the same units.  Damian mentioned on IRC that
"measured" might be returning the bwauth weight rather than a bandwidth,
but what is the meaning of that weight?  Does a higher "measured" value
mean a higher bandwidth, or a higher bandwidth relative to what the relay
advertises?  In other words, if I sorted the descriptors by "measured"
value, what would that order mean?

Separately, is there a way (using Stem or some other tool) to see the raw
bwauth measurements rather than the weights?  Is that a calculation I can
reverse?  I haven't looked into the historical data on CollecTor yet, but
ideally, I would like to use the historical data to figure out how
effective the bwauth measurements seem to be in different situations (for
example, the misconfigured to very high bandwidth relay this past February
seems to have produced confusing bwauth measurements [3]).  If I'm looking
for interesting events in the historical bwauth data, would I be looking
for high "measured" values, rapid changes, or ...?

PhD Student, University of Washington

[2] https://stem.torproject.org/api/descriptor/router_status_entry.html
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