[tor-bugs] #19460 [Core Tor/Tor]: Improve consensus handling of clients with skewed clocks

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#19460: Improve consensus handling of clients with skewed clocks
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     Reporter:  asn           |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.2.???
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  029-proposed
Actual Points:                |  Parent ID:
       Points:  2             |   Reviewer:
      Sponsor:                |
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 It is my understanding that we aim to support clients with skewed clocks.

 I open this ticket for a few suggestions on improving consensus handling
 by clock skewed clients. It might be worth opening more tickets for better
 support of clock skewed clients in other parts of the protocol as well.
 The ideas of this ticket came up [https://lists.torproject.org/pipermail
 /tor-dev/2016-June/011068.html while working on prop224].

   a) We might want to start accepting consensuses whose `valid_after` date
 is slightly in the future (maybe an hour or two max). We are already
 flexible towards old consensuses (we accept consenuses up to 24 hours
 old), so maybe we should be flexible in the other direction as well. This
 should help with slightly backwards-skewed clients.

   Security wise, handling future consensuses is in some way safer than
 handling old consensuses (since these are replayable).

   b) We might want to improve our logging when receiving ultra old
 consensuses. Right now we fail bootstrap '''silently''' if we receive a
 consensus with a `valid_until` older than 24 hours. This affects clients
 with clocks skewed forward, and they never learn what the problem is.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19460>
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