[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: 0.2.8.x-final
     Priority:  major    |    Version:  Tor: 0.2.4.7-alpha
    Component:  Tor      |   Keywords:  0.2.8.x-triage, tor-dist,
   Resolution:           |  027-triaged-1-in, DoS, SponsorU,
Actual Points:  20       |  TorCoreTeam201510, 028-triaged
       Points:           |  Parent ID:  #15228
  medium/large           |    Sponsor:  SponsorU
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Comment (by teor):

 The current behaviour of Fallback Directories is:
 * tor contacts fallback directories *before* the directory authorities
 * tor will try 2 fallback directories, then wait for a minute or two
 before trying another fallback directory

 Do we want to:
 * increase the number of fallbacks that are tried quickly to 3?
 * try the authorities before the fallback directories?
   * for the trial?
   * for the feature release?
 * try N fallback directories quickly, then use the authorities?

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15775#comment:46>
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