We propose the following changes to the [Tor Specification], once this proposal is implemented. Adding a new Relay subprotocol version lets testing relays identify other relays that support IPv6 extends. It also allows us to eventually recommend or require support for IPv6 extends on all relays. For the draft text, see proposal 311, section 5.1: https://gitweb.torproject.org/torspec.git/tree/proposals/311-relay-ipv6-reachability.txt#n608