[tor-bugs] #27346 [Core Tor/sbws]: Improve sbws bandwidth accuracy

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#27346: Improve sbws bandwidth accuracy
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     Reporter:  teor           |      Owner:  (none)
         Type:  defect         |     Status:  new
     Priority:  Medium         |  Milestone:  sbws 1.0 (MVP nice)
    Component:  Core Tor/sbws  |    Version:
     Severity:  Normal         |   Keywords:
Actual Points:                 |  Parent ID:  #27338
       Points:                 |   Reviewer:
      Sponsor:                 |
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 Better designs SHOULD:
 * use at least 4 measurements that are at least 6 hours apart, because:
   - there is a daily cycle
   - each day contains 2 similar points in the cycle (it is an up and down
 cycle)
   - if all 4 measurements happen within a few hours, they will still be
 biased
 * use at least 3 days of observed bandwidths, because:
   - a single download at the changeover point can affect 2 days
 * weight bandwidths based on the time since the last bandwidth, because:
   - if we only record bandwidths when they change, bandwidths that are
 updated soon after the last bandwidth are weighted too high
   - we can either:
     * record the bandwidths every hour, even if they haven't changed
     * weight each bandwidth by the time since the last bandwidth
 * use a decaying average for measured and observed bandwidths, because:
   - recent bandwidths are closer to the relay's current capacity
   - and we want accurate results

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/27346>
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