[tor-bugs] #15358 [Tor]: Provide control port event+command for network connectivity info

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#15358: Provide control port event+command for network connectivity info
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 Reporter:  mikeperry              |          Owner:
     Type:  enhancement            |         Status:  new
 Priority:  normal                 |      Milestone:
Component:  Tor                    |        Version:
 Keywords:  tor-client, tbb-needs  |  Actual Points:
Parent ID:                         |         Points:
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 For #11222, it would be useful to have a control port event and GETINFO
 command to learn Tor's current opinion about overall network connectivity,
 so we can try to differentiate censorship from general network outage.

 One potential source of this network liveness info is via
 circuit_build_times_network_is_live() and
 circuit_build_times_network_close(). There may be other pieces of Tor that
 try to decide if the network is up too, though. I seem to recall athena
 working on some piece of code that needed to do this, but I can't remember
 the ticket.

 Alternatively, there may be more accurate platform-specific mechanisms we
 could use instead, I bet. Most OSes have a notion of if it's interfaces
 are connected and sending/receiving packets recently. OS info may prove
 more useful for this use case than the CBT code is.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15358>
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