[tor-bugs] #24228 [Core Tor/Tor]: Tor keeps on creating new circuits even when it's idle

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#24228: Tor keeps on creating new circuits even when it's idle
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 Reporter:  asn                                  |          Owner:  (none)
     Type:  defect                               |         Status:  new
 Priority:  Very High                            |      Milestone:  Tor:
                                                 |  0.3.2.x-final
Component:  Core Tor/Tor                         |        Version:  Tor:
                                                 |  0.3.1.1-alpha
 Severity:  Normal                               |     Resolution:
 Keywords:  tor-circuit, regression,             |  Actual Points:
  backport-031                                   |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
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Comment (by asn):

 Replying to [comment:10 mikeperry]:
 > The goal is to learn a circuit build timeout within 30 minutes, so that
 unused orconn connections aren't padded for too long while we learn this
 timeout (which wastes bandwidth for clients that want less padding). It
 sounds like we may actually learn it within 10. We could make this 3X
 slower I suppose.
 >
 > But I don't really think new clients are going to put that much of a
 strain on the network with this. The ntor handshake completes in tens of
 microseconds, IIRC. And the rate of new clients arriving is not that high.

 Hmm, not sure if it's just new clients. IIRC, CBT is per-guard, so when a
 client switches to a new guard (or its current guard gets
 offline/unreachable), it will start learning CBT of its next guard, aka
 destroy and create tons of idle circs over time.

 Why is it important to learn CBT fast? What would happen if we learned CBT
 over a longer period of time, and used a bigger idle timeout value so that
 we don't destroy so many idle circuits?

 Alternatively, perhaps we could disable the predictive circuit building
 while we area learning CBT for a guard? Or is this too much effort?

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24228#comment:13>
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