Prop 311: 4. Ensure Relay and Bridge IPv6 ORPorts are Reachable

This is a parent ticket.

We propose that relays (and bridges) check their own IPv6 ORPort reachability.

To check IPv6 ORPort reachability, relays (and bridges) extend circuits via other relays (but not other bridges), and back to their own IPv6 ORPort.

If IPv6 reachability checks fail, relays (and bridges) should refuse to publish their descriptors, if they believe IPv6 reachability checks are reliable, and their IPv6 address was explicitly configured. (See [Proposal 312: Relay Auto IPv6 Address] for the ways relays can guess their IPv6 addresses.)

Directory authorities always publish their descriptors.

From Proposal 311, section 4: https://gitweb.torproject.org/torspec.git/tree/proposals/311-relay-ipv6-reachability.txt#n246