o Major features (subprotocol versions):

    - Tor now uses "subprotocol versions" to indicate
      compatibility. Previously, versions of Tor looked at the declared Tor
      version of a relay to tell whether they could use a given feature.
      Now, they should be able to rely on its declared subprotocol versions.
      This change allows compatible implementations of the Tor protocol(s) to
      exist without declaring compatibility with pretending to be particular
      releases of Tor itself. Closes ticket 19958; implements part of
      proposal 264.

    - Tor directory authorities now vote on a set of recommended subprotocol
      versions, and on a set of required subprotocol versions. Clients and
      relays that lack support for a _required_ suprotocol version will not
      start; those that lack support for a _recommended_ subprotocol version
      will warn the user to upgrade. Closes ticket 19958; implements part of
      proposal 264.