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Re: [tor-bugs] #12595 [Tor]: Think of better data structures for guard nodes

#12595: Think of better data structures for guard nodes
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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
Actual Points:          |  Parent ID:
       Points:          |
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Comment (by asn):

 Some more thoughts on the new data structures:

 - Since we want our circuit guards to also be our directory guards (if
   possible), we should probably use a single entry guard list (like we
   do currently), instead of using separate lists for circuit guards
   and directory guards. Reasons for this can be found here:
   https://lists.torproject.org/pipermail/tor-dev/2014-May/006824.html

 - Furthermore, we want to make sure that if our directory guard claims
   that it doesn't have a microdescriptor, we will go ahead and ask
   other directory caches too:
   https://lists.torproject.org/pipermail/tor-dev/2014-May/006820.html

   I wonder if this behavior is any different from the corresponding
   behavior of circuit guards: "If our circuit guard fails our circuit,
   we have to go ahead and ask the next circuit guard"

   If not, maybe we could just switch NumDirectoryGuards to 1 too, and
   just make sure that if a microdescriptor gets denied we move to the
   next directory guard, till we have enough microdescriptor to be happy?
   (logic similar to compute_frac_paths_available()).

 - As a further point to the paragraph above, I uploaded an image of 3
   different possible entry guard lists:
   https://people.torproject.org/~asn/guards2/entry_guard_list.jpg

   With the current code, and if NumDirectoryGuards was 3, from the
   first entry guard list we would select a directory guard between
   (entry2, entry4, entry6). On the second list, we would select
   between (entry1, entry4, entry6).  On the third list, the worst case
   scenario, we would select between (entry4, entry5, ...).

   I'd argue that we should strongly prefer the *top* directory guard
   every time, and only move to the lower ones if the top one doesn't
   give us what we want.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/12595#comment:3>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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