[tor-bugs] #33222 [Core Tor/Tor]: Prop 311: 4.2. Checking IPv6 ORPort Reachability

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#33222: Prop 311: 4.2. Checking IPv6 ORPort Reachability
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     Reporter:  teor          |      Owner:  teor
         Type:  defect        |     Status:  assigned
     Priority:  Medium        |  Milestone:  Tor: 0.4.4.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  ipv6, prop311
Actual Points:                |  Parent ID:  #33221
       Points:  6             |   Reviewer:
      Sponsor:                |
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 4.2. Checking IPv6 ORPort Reachability

 We propose that testing relays (and bridges) select some IPv6 extend-
 capable
 relays for their reachability circuits, and include their own IPv4 and
 IPv6
 ORPorts in the final extend cells on those circuits.

 The final extending relay will extend to the testing relay:
   * using an existing authenticated connection to the testing relay
     (which may be over IPv4 or IPv6), or
   * over a new connection via the IPv4 or IPv6 ORPort in the extend cell.

 The testing relay will confirm that test circuits can extend to both its
 IPv4 and IPv6 ORPorts.

 4.2.1. Selecting the Final Extending Relay

 IPv6 ORPort reachability checks require an IPv6 extend-capable relay as
 the second-last hop of reachability circuits. (The testing relay is the
 last hop.)

 IPv6-extend capable relays must have:
   * Relay subprotocol version 3 (or later), and
   * an IPv6 ORPort.
 (See section 5.1 for the definition of Relay subprotocol version 3.)

 The other relays in the path do not require any particular protocol
 versions.

 4.2.2. Extending from the Second-Last Hop

 IPv6 ORPort reachability circuits should put the IPv4 and IPv6 ORPorts in
 the extend cell for the final extend in reachability circuits.

 Supplying both ORPorts makes these extend cells indistinguishable from
 future client extend cells.

 If reachability succeeds, the testing relay (or bridge) will accept the
 final extend on one of its ORPorts, selected at random by the extending
 relay (see section 3.2.1).

 4.2.3. Separate IPv4 and IPv6 Reachability Flags

 Testing relays (and bridges) will record reachability separately for IPv4
 and IPv6 ORPorts, based on the ORPort that the test circuit was received
 on.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/33222>
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