The first stream opened is a client-initiated bidirectional control stream where the endpoints exchange Setup messages (Section 9.3), followed by other messages defined in Section 9.
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 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.