Tox provides a reasonable base set of features, but new features are slow to
implement, and may not be wanted by all clients. This is reasonable as any
protocol changes are API breaking and force clients to update to use them. Once
the features are in they are there forever meaning quality of the API and the
network layer have to be very strong.

An extension library with negotiation allows more experimentation with less risk
resulting in more features for tox clients.