[tor-bugs] #12466 [Tor]: Possible primary guard node skip when some guards are down

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#12466: Possible primary guard node skip when some guards are down
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     Reporter:  asn     |      Owner:
         Type:  defect  |     Status:  new
     Priority:  normal  |  Milestone:  Tor: 0.2.6.x-final
    Component:  Tor     |    Version:
   Resolution:          |   Keywords:  tor-guard tor-client
Actual Points:          |  Parent ID:
       Points:          |
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Comment (by asn):

 I think part of the problem here, is that when we think of
 `NumEntryGuards` being `3`, we think "There should only be 3 possible
 first-hops for any circuit.".

 However, when `choose_random_entry_impl()` thinks of `NumEntryGuards`
 being `3`, it thinks "For every entry guard selection for a circuit, I
 need to have 3 possible node picks.".

 Since different circuits have different requirements (others need `Stable`
 nodes, other need `DirCache`, etc.), these two definitions are not
 equivalent (since the first 3 nodes in our entry guard list will probably
 not fulfil all those requirements.).

 I call the first 3 nodes in our entry guard list the `primary guards` of a
 client. The rest of the nodes in our entry guard are auxiliary guards,
 used only when the first 3 cannot satisfy us (because they are down, or
 because they are not `Fast`, etc.).

 To bring Tor's behavior closer to the first definition (which seems like
 the right one), maybe it makes sense to always try to pick a primary guard
 if it satisfies the requirements of a circuit, even if we don't have 3
 possible choices to pick from.

 Unfortunately, the implementation of this idea does not sound that easy,
 since the concept of a `primary guard` is not so clear in real life. For
 example, is a node that has been down for 2 weeks still a primary guard
 (even if it's on the top of the entry guard list)?

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/12466#comment:1>
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