gstreamer/gst/rtsp

RTSP source 
-----------

The RTSP source establishes a connection to an RTSP server and sets up
the UDP sources and RTP session handlers.

An RTSP session is created as follows:

- Parse RTSP URL:

   ex:
     rtsp://thread:5454/south-rtsp.mp3

- Open a connection to the server with the url. All further conversation with
  the server should be done with this connection. Each request/reply has
  a CSeq number added to the header.

- Send a DESCRIBE request for the url. We currently support a response in
  SDP.

  ex:

    >> DESCRIBE rtsp://thread:5454/south-rtsp.mp3 RTSP/1.0
    >> Accept: application/sdp
    >> CSeq: 0
    >>
    << RTSP/1.0 200 OK
    << Content-Length: 84
    << Content-Type: application/sdp
    << CSeq: 0
    << Date: Wed May 11 13:09:37 2005 GMT
    <<
    << v=0
    << o=- 0 0 IN IP4 192.168.1.1
    << s=No Title
    << m=audio 0 RTP/AVP 14
    << a=control:streamid=0

- Parse the SDP message, for each stream (m=) we create an GstRTPStream. We need
  to allocate two local UDP ports for receiving the RTP and RTCP data because we
  need to send the port numbers to the server in the next request.

  In RTSPSrc we create two elements that can handle the udp://0.0.0.0:0 uri. This
  will create an udp source element. We get the port number by getting the "port"
  property of the element after setting the element to PAUSED.

    +-----------------+
    | +------------+  |
    | | udpsrc0    |  |
    | |  port=5000 |  |
    | +------------+  |
    | +------------+  |
    | | udpsrc1    |  |
    | |  port=5001 |  |
    | +------------+  |
    +-----------------+

- Send a SETUP message to the server with the RTP ports. We get the setup URI from
  the a= attribute in the SDP message. This can be an absolute URL or a relative
  url.

  ex:

    >> SETUP rtsp://thread:5454/south-rtsp.mp3/streamid=0 RTSP/1.0
    >> CSeq: 1
    >> Transport: RTP/AVP/UDP;unicast;client_port=5000-5001,RTP/AVP/UDP;multicast,RTP/AVP/TCP
    >>
    << RTSP/1.0 200 OK
    << Transport: RTP/AVP/UDP;unicast;client_port=5000-5001;server_port=6000-6001
    << CSeq: 1
    << Date: Wed May 11 13:21:43 2005 GMT
    << Session: 5d5cb94413288ccd
    <<

  The client needs to send the local ports to the server along with the supported 
  transport types. The server selects the final transport which it returns in the
  Transport header field. The server also includes its ports where RTP and RTCP
  messages can be sent to.

  In the above example UDP was choosen as a transport. At this point the RTSPSrc element
  will furter configure its elements to process this stream.

  The RTSPSrc will create and connect an RTP session manager element and will
  connect it to the src pads of the udp element. The data pad from the RTP session 
  manager is ghostpadded to RTPSrc.
  The RTCP pad of the rtpdec is routed to a new udpsink that sends data to the RTCP
  port of the server as returned in the Transport: header field.

    +---------------------------------------------+
    | +------------+                              |
    | | udpsrc0    |   +--------+                 |
    | |  port=5000 ----- rtpdec --------------------
    | +------------+   |        |                 |
    | +------------+   |        |  +------------+ |
    | | udpsrc1    ----- RTCP   ---- udpsink    | |
    | |  port=5001 |   +--------+  |  port=6001 | |
    | +------------+               +------------+ |
    +---------------------------------------------+

  The output type of rtpdec is configured as the media type specified in the SDP
  message. 

- All the elements are set to PAUSED/PLAYING and the PLAY RTSP message is
  sent.

    >> PLAY rtsp://thread:5454/south-rtsp.mp3 RTSP/1.0
    >> CSeq: 2
    >>
    << RTSP/1.0 200 OK
    << CSeq: 2
    << Date: Wed May 11 13:21:43 2005 GMT
    << Session: 5d5cb94413288ccd
    <<

- The udp source elements receive data from that point and the RTP/RTCP messages
  are processed by the elements.

- In the case of interleaved mode, the SETUP method yields:

    >> SETUP rtsp://thread:5454/south-rtsp.mp3/streamid=0 RTSP/1.0
    >> CSeq: 1
    >> Transport: RTP/AVP/UDP;unicast;client_port=5000-5001,RTP/AVP/UDP;multicast,RTP/AVP/TCP
    >>
    << RTSP/1.0 200 OK
    << Transport: RTP/AVP/TCP;interleaved=0-1
    << CSeq: 1
    << Date: Wed May 11 13:21:43 2005 GMT
    << Session: 5d5cb94413288ccd
    <<

  This means that RTP/RTCP messages will be send on channel 0/1 respectively and that
  the data will be received on the same connection as the RTSP connection.

  At this point, we remove the UDP source elements as we don't need them anymore. We
  set up the rtpsess session manager element though as follows:

    +---------------------------------------------+
    | +------------+                              |
    | | _loop()    |   +--------+                 |
    | |            ----- rtpses --------------------
    | |            |   |        |                 |
    | |            |   |        |  +------------+ |
    | |            ----- RTCP   ---- udpsink    | |
    | |            |   +--------+  |  port=6001 | |
    | +------------+               +------------+ |
    +---------------------------------------------+

  We start an interal task to start reading from the RTSP connection waiting
  for data. The received data is then pushed to the rtpdec element.

  When reading from the RTSP connection we receive data packets in the
  following layout (see also RFC2326)

    $<1 byte channel><2 bytes length, big endian><length bytes of data>