The first stream opened is a client-initiated bidirectional control stream where the endpoints exchange Setup messages (Section 8.3), followed by other messages defined in Section 8.
This draft only specifies a single use of bidirectional streams. Objects are sent on unidirectional streams. Because there are no other uses of bidirectional streams, a peer MAY currently close the session as a 'Protocol Violation' if it receives a second bidirectional stream.
The control stream MUST NOT be closed at the underlying transport layer while the session is active. Doing so results in the session being closed as a 'Protocol Violation'.