Prop 311: 4.4.1. Extend IPv6 From All Supported Second-Last Hops

This is an optional change.

This change has a zero points estimate, because I think it is the fastest way of implementing legacy/trac#33222 (moved). (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