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RTSP allows a media player to control the tranmission of a media stream. Control actions include pause/resume,
repositioning of playback, fast-forward, and rewind. RTSP is an out-of-band protocol. The RSTP
messages are sent out-of-band, whereas the media stream, whose packet structure is not defined by RTSP, is considered "in-band."
RTSP message use a different port number, 544, from the media stream.
The file transfer protocol (FTP) also uses the out-of-band notion. FTP uses tow client/server pairs of sockets,
each pair that transports control information; the other client/server socket pair supports a TCP connection that actually
transports the file. The RTSP channel is in many ways similar to FTP's control channel.
The Web browser first rrequests a presentation description file from a Web server. The presentation description
file can have references to serveral continuous-media files as well as directives fro synchronization of the continuous-media
files. Each reference to a continuous-media fiel begins withthe URL method, rtsp://.
The Web server encapsulates the presentation description file in an HTTP response message and sends the message to the
browser. When the browser receives the HTTP response message, the browser invokes a media player based on the content-type
field of the message. The presentation description file includes references to media streams, using the URL method rtsp://.
It is interesting to note the similarities between HTTP and RTSP. All request and response message are in ASCII
text, the client employs standardized methods, and the server responds with standardized reply codes. One important
defferece, however, is that the RTSP server keeps track of the state of the client fro each ongoing RTSP session.
The client initiates the session with the SETUP request, providing the URL of the file to be streamed and the RTSP version.
The setup message includes the client port number to which the media should be sent over UDP using the RTP packetization protocol.
RTSP has facilities that allo clients to stream toward the server.
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