[tor-bugs] #15775 [Tor]: Add IPv4 Fallback Directory List to tor, active by default

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#15775: Add IPv4 Fallback Directory List to tor, active by default
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     Reporter:  teor     |      Owner:  teor
         Type:           |     Status:  needs_revision
  enhancement            |  Milestone:  Tor: 0.2.7.x-final
     Priority:  major    |    Version:  Tor: 0.2.4.7-alpha
    Component:  Tor      |   Keywords:  tor-dist, 027-triaged-1-in,
   Resolution:           |  TorCoreTeam201508
Actual Points:           |  Parent ID:  #15228
       Points:           |
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Changes (by teor):

 * status:  needs_information => needs_revision


Comment:

 Currently, our top fallback directories are weighted around 2%-3% of the
 total eligible fallback directory weight. It's possible to reduce that to
 around 1.6% by redistributing some of the consensus weight across the rest
 of the relays (or, equivalently, reducing the consensus weight of those
 relays to some maximum). But making the maximum less than about 1.5% tends
 to warp the distribution of weights too much.

 The updated code is in my teor2345 github branch `feature15775-fallback-
 update-v2`.
 I still need to implement whitelisting and blacklisting of fallback
 directories in the script.

 Is it OK if a fallback directory sees 1 in 50 client bootstraps (2%)?
 (The final figure may change depending on how many relays opt-in to be
 fallback directories.)
 We'd need to widen eligibility criteria a bit to get it down to 1%.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15775#comment:37>
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