Real-Time Streaming Protocol
RealServer uses Real-Time Streaming Protocol (RTSP) to control the delivery of a video stream. The actual data representing the video stream is delivered out of band via another protocol (RTP, UDP, HTTP, as negotiated between the client and the server). I am only interested in finding out about the health of the video stream and do not need to sample the actual video stream itself; therefore, I can confine myself to working with RTSP. RTSP is an open standard originally developed and proposed by Real Networks, then handed over to the IETF standards committee years ago for approval and adoption (see http://www.ietf.org/rfc/rfc2326.txt). RTSP and HTTP share many behaviors but are necessarily different in the following aspects:
- An RTSP connection is a stateful connection, as opposed to HTTP that is stateless. An example of this is that a SETUP command must be issued from the client prior to a PLAY command (which initiates the delivery of the out of band video/audio stream).
- The client and server can initiate requests (related to the first point).
- The video/audio is carried out of band via a different protocol.
- The request URI is the absolute path rather than a relative path.
These differences are necessary to allow for delivery of a continuous stream of data (as opposed to HTTP requests for finite sets of fixed length documents). Essentially, RTSP is a control stream that allows the client and server to communicate events that affect the delivery of the out of band video/audio stream.
Listing Three contains the actual method used to probe the RTSP connection. I used the OPTIONS, DESCRIBE, and the GET_PARAMETER commands to check for the existence of a particular stream and parameters of that stream. Additional commands are available to control out of band data such as SETUP, PLAY, PAUSE, TEARDOWN, and RECORD.
The server can provide additional details on the performance of the stream such as bandwidth available, maximum bandwidth, packet size, and so on. These values are specific to the implementation of the server, some of which are captured by the StreamWatch object for reporting as auxiliary data.
— M.L.
