[tor-bugs] #15942 [Core Tor/Tor]: Make tor connection failures random-exponential-backoff

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#15942: Make tor connection failures random-exponential-backoff
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 Reporter:  teor                                 |          Owner:  andrea
     Type:  defect                               |         Status:
 Priority:  High                                 |  assigned
Component:  Core Tor/Tor                         |      Milestone:  Tor:
 Severity:  Normal                               |  0.2.9.x-final
 Keywords:  SponsorS, 028-triage, tor-dos,       |        Version:
  TorCoreTeam201606                              |     Resolution:
Parent ID:  #17293                               |  Actual Points:
 Reviewer:                                       |         Points:  3
                                                 |        Sponsor:
                                                 |  SponsorU-can
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Comment (by andrea):

 Failed directory connections are handled in
 connection_dir_request_failed(),
 which calls:
   - networkstatus_consensus_download_failed() for a consensus
     - calls download_status_failed() /
 update_consensus_networkstatus_downloads()
       - download_status_failed() is macro for
 download_status_increment_failure()

   - connection_dir_download_cert_failed() for a certificate
     - calls authority_cert_dl_failed() / update_certificate_downloads()

     - this ultimately uses download_status_t too just like the consensus
 download;
       see download_status_is_ready_by_sk_in_cl() and friends in
 routerlist.c

   - connection_dir_download_routerdesc_failed()

  890   /* No need to relaunch descriptor downloads here: we already do it
  891    * every 10 or 60 seconds (FOO_DESCRIPTOR_RETRY_INTERVAL) in
 main.c. */

     - The mechanism here is in
 launch_descriptor_fetches_callback()/reset_descriptor_failures_callback();
       we can realize exponential backoff by suitable adjustments

   - connection_dir_bridge_routerdesc_failed()
     - calls connection_dir_retry_bridges()
       - calls retry_bridge_descriptor_fetch_directly()
         - calls launch_direct_bridge_descriptor_fetch()

 At minimum, it should be easy to implement exponential backoffs for
 consensus and certificate downloads through the download_status_t
 mechanism, since they already notify it of their successes/failures and
 ask it whether we're ready to attempt a new download yet.  Further
 ivestigation of the right approach for the bridge descriptor and router
 descriptor download cases pending.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15942#comment:14>
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