[tor-bugs] #13207 [Tor]: Is rend_cache_clean_v2_descs_as_dir cutoff crazy high?

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#13207: Is rend_cache_clean_v2_descs_as_dir cutoff crazy high?
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     Reporter:  arma    |      Owner:
         Type:  defect  |     Status:  assigned
     Priority:  major   |  Milestone:  Tor: 0.2.7.x-final
    Component:  Tor     |    Version:  Tor: 0.2.7
   Resolution:          |   Keywords:  SponsorR, tor-hs, 027-triaged-1-in
Actual Points:          |  Parent ID:  #13209
       Points:  medium  |
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Comment (by sysrqb):

 Handling clock skew is always tricky. 24 hours seems reasonable. In this
 case:
   1) HSDirs should only keep descriptors which were published most 24
 hours, and fewer hours if the current descriptor is overwritten with a
 newer version or to prevent running out of memory
   2) It doesn't seem like a client's cache needs this 24 hour cutoff (it
 could be less), but it doesn't seem like it will hurt.

 Assuming the client's clock is not skewed significantly, the descriptor
 that the client possesses is deleted from the cache around the same time
 the descriptor becomes invalid. If the clock is skewed by fewer than 3
 hours then this probably, maybe, likely won't impact onion service
 reachability. Any client running with a clock positively skewed by more
 than 3 hours will never have a usable recent consensus, and a client whose
 clock is negatively skewed by more than 3 hours will never have a usable
 consensus.

 Are there objections to setting the cutoff at 26 or 27 hours? Then we have
 two options:
   1) REND_CACHE_MAX_AGE = 24 hours, REND_CACHE_MAX_SKEW = 3 hours
   2) REND_CACHE_DESC_LIFETIME = 27 hours

 There may yet be an even better name.

 Interestingly, rend-spec agrees with this sentiment:

    Alice should cache the descriptor locally, but should not use
    descriptors that are more than 24 hours older than their timestamp.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/13207#comment:9>
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