[tor-bugs] #33225 [Core Tor/Tor]: Prop 311: 4.4.1. Extend IPv6 From All Supported Second-Last Hops

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#33225: Prop 311: 4.4.1. Extend IPv6 From All Supported Second-Last Hops
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     Reporter:  teor           |      Owner:  teor
         Type:  enhancement    |     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:  #33222
       Points:  0              |   Reviewer:
      Sponsor:  Sponsor55-can  |
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 This is an optional change.

 This change has a zero points estimate, because I think it is the fastest
 way of implementing #33222. (The alternative design requires creating
 different circuit types for IPv4 and IPv6 reachability checks.)

 4.4.1. Extend IPv6 From All Supported Second-Last Hops

 The testing relay (or bridge) puts both IPv4 and IPv6 ORPorts in its final
 extend cell, and the receiving ORPort is selected at random by the
 extending relay (see sections 3.2.1 and 4.2). Therefore, approximately
 half
 of IPv6 ORPort reachability circuits will actually end up confirming IPv4
 ORPort reachability.

 We propose this optional change, to improve the rate of IPv6 ORPort
 reachability checks:

 If the second-last hop of an IPv4 ORPort reachability circuit supports
 IPv6
 extends, testing relays may put the IPv4 and IPv6 ORPorts in the extend
 cell for the final extend.

 As the number of relays that support IPv6 extends increases, this change
 will increase the number of IPv6 reachability confirmations. In the ideal
 case, where the entire network supports IPv4 and IPv6 extends, IPv4 and
 IPv6
 ORPort reachability checks would require a similar number of circuits.

 See proposal 311, section 4.4.1:
 https://gitweb.torproject.org/torspec.git/tree/proposals/311-relay-
 ipv6-reachability.txt#n439

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/33225>
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