[tor-bugs] #11480 [Tor]: Implement the single guard node proposal

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#11480: Implement the single guard node proposal
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     Reporter:  asn     |      Owner:
         Type:  task    |     Status:  new
     Priority:  normal  |  Milestone:  Tor: 0.2.6.x-final
    Component:  Tor     |    Version:
   Resolution:          |   Keywords:  tor-client
Actual Points:          |  Parent ID:
       Points:          |
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Comment (by asn):

 Some notes on an implementation plan:

 == Switch to one guard per client ==
 * Look into `decide_num_guards()`.
 * See if setting `NumEntryGuards=1` breaks any assumption of the rest of
 the code.
 * Implementation depends on whether we pick alternative behavior of
 section 1.1.1 or not.

 == Increase guard rotation period ==
 * Either play with `MIN_GUARD_LIFETIME` or use the `GuardLifetime`
 consensus option.
 * Implementation depends on whether we pick alternative behavior of
 section 1.2.1 or not.

 == Raise bw thresholds for guard ==
 * See `set_routerstatus_from_routerinfo()` and `AuthDirGuardBWGuarantee`
 etc.
 * How to test: Make fake network and see which guards are eligible to
 become guards

 == Prioritize young guards for non-guard tasks ==
 * Implementation plan:
   Download/parse/verify old consensuses in an external script,
   write file with results,
   have little-t-tor read the results.

 * Bandwidth authorities: See `dirserv_read_measured_bandwidths()` and
 https://gitweb.torproject.org/torflow.git/blob/HEAD:/NetworkScanners/BwAuthority/README.spec.txt#l333

 * How to test the external script: Make artificial data sets of
 consensuses and see if the external script parses them properly.
 * How to test Tor: Create artificial guard age results file, give the test
 a fake network (some relays) and see if Tor generates the right weights
 for the relays.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/11480#comment:3>
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