[tor-dev] Bandwidth scanner: request for feedback

teor teor at riseup.net
Mon Aug 27 03:50:01 UTC 2018


Juga and pastly have been working hard on sbws.

Sbws' results are now similar to torflow's results:

Now that sbws is close to torflow, we want some feedback on its
design. We’ll work on the design at the tor meeting in September.

Please feel free to give feedback by email, or on the tickets:

What happens when sbws doesn't match torflow?


We suggest this rule:

If an sbws deployment is within X% of an existing bandwidth
authority, sbws is ok. (The total consensus weights of the
existing bandwidth authorities are within 25% - 50% of each
other, see #25459.)

How long should sbws keep relay bandwidths?


Torflow uses the latest self-reported relay observed bandwidth
and bandwidth rate.

Torflow uses a complex feedback loop for measured bandwidths.
We think sbws can use a simple average or exponentially
decaying weighted average.

How should we scale sbws consensus weights?


If sbws' total consensus weight is different to torflow's total
consensus weight, how should we scale sbws?

(The weights might differ because the measurement method is
different, or because scanners and servers are in different

In the bandwidth file spec, we suggest linear scaling.

How should we round sbws consensus weights?


Torflow currently rounds to 3 significant figures (which is a maximum
of 0.5%). But I suggest 2 significant figures for sbws (or max 5%),
- tor has a daily usage cycle that varies by 10% - 20%
- existing bandwidth authorities vary by 25% - 50%

Proposal 276 contains a slightly more complicated rounding algorithm,
which we may want to implement in sbws or in tor:


Does sbws need a maximum consensus weight fraction?


Torflow uses 5%, but I suggest 1%, because the largest relay right
now is only 0.5%.



Please reply @torproject.org
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