Support simplex transports where the recipient makes the connection
For the simplex transports we've considered so far, such as USB sticks and radio broadcasts, the sender of a stream is the one who creates the underlying transport connection. But it's possible for a transport to operate the other way round: the recipient opens a connection and receives a stream. Downloading from a web server would be an example.
The plugin architecture should support such transports. This can be deferred until we actually want to implement such a transport.