[tor-bugs] #27816 [Core Tor/Tor]: Rendezvous failures could distinguish "unknown cookie" from "outdated cookie"

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#27816: Rendezvous failures could distinguish "unknown cookie" from "outdated
cookie"
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     Reporter:  arma          |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 dgoulet spoke of getting a bunch of these on his relay:
 "(protocol warn): [warn] Rejecting RENDEZVOUS1 cell with unrecognized
 rendezvous cookie"

 His theory was that maybe jerks are sending introduction requests when
 they never made the rendezvous circuit at all (and #25066 would address
 this possibility), but I think the explanation is much simpler: a real
 client *had* the rendezvous point established, but closed it (gave up)
 before the service got around to trying to connect to it.

 This problem happens because of a timing issue, and when the network or
 the onion service is under load, the timing issue gets worse, because it
 takes longer between the introduction attempt and the rendezvous response.

 So the patch to consider here is: if we remembered recently-established
 cookies, we could distinguish between "unknown cookie" or "cookie that
 would have worked earlier but it's too late now". That would help us know
 what the problem actually is right now, and also if these messages ramp up
 suddenly, it would let us distinguish which situation is causing it.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/27816>
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