[tor-bugs] #20272 [Core Tor/Tor]: constraint broken in case 1 of consensus weight calculation

[Tor Bug Tracker & Wiki]

#20272: constraint broken in case 1 of consensus weight calculation
------------------------------+-----------------
     Reporter:  pastly        |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
------------------------------+-----------------
 [https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n2648 dir-
 spec] specifies the constraint `Wmg == Wmd` in case 1, but also that

 {{{
 Wmg = (weight_scale*(2*G-E-M))/(3*G)
 Wmd = weight_scale/3
 }}}

 This constraint is impossible to satisfy unless it just happens that
 `(2G-E-M)/G == 1`.

 Indeed, in my testing of `networkstatus_compute_bw_weights_v10`, I found
 that `Wmg` and `Wmd` were calculated as above, but the constraint was
 ignored.

 The easy solution is to change the spec, but that would ignore the logic
 that went into having that constraint in the first place. I do not know
 the logic that went into designing the consensus weight calculations, so I
 do not know if this solution is appropriate.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/20272>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online