3.2.3 Initialization
When the server is initialized, the higher-layer protocol passes the port to which the server is listening for the incoming BITS upload messages. The server MUST verify that there is an existing ServerPortListener (section 3.2.1.2) for the given port, and it MUST create a new instance of ServerPortListener otherwise. The server MUST register itself with ServerPortListener to receive the BITS upload messages (sent by the clients) from ServerPortListener.