[Live-devel] Defintion of the ServerMediaSession and SeverMediaSubsession classed and their usage in relation with doGetNextFrame()

[Mehran Ansari]

I am trying to understand the relationship and interaction of different Live555 classes so that I can implement an OnDemandRTSPServer to be able to send live video/audio frames that are generated by my encoder associated to different video port channels.

To start, I derived myOnDyanmicRTSPServer from RTSPServer. I also subclassed myDyanmicRTSPClientSession from RTSPServer::RTSPClientSession  within myOnDyanmicRTSPServer and implemented handlCmd_PLAY and handleCmd_TEARDOWN vitrual methods. Additionally, I implemented lookupServerMediaSession() virtual method in  myOnDyanmicRTSPServer. Now in lookupServerMediaSession() I checked to see if the channel name in the URI passed to this method does not exist then I created a ServerMediaSession and added addSubsession() for the corresponding video codec and dataport on my video board for the channel specified in the URI. Now when handleCmd_PLAY() or handleCmd_TEARDOWN is called I start/stop my video encoder for that particular video data port. Lastly when doGetNextFrame() is called in my my sub-classed VideoStreamFramer, I get the frame provided by my video encoder and copy it to  fTo buffer and call afterGetting() to inform the sink the frame was obtained.

My Questions are:

1) What is the relationship between the ServerMediaSession and SeverMediaSubsession? What is the rule of each of these classes? My understanding is that their relationship in case video only is one to one. I create a ServerMediaSession for a particular video port channel requested by the RTSP URI DESCRIBE command, then add the associated video encoder which is my SeverMediaSubsession to this ServerMediaSession. I also think that if I had audio I would have added a audio SeverMediaSubsession. But is it possible that there are more than one audio or video SeverMediaSubsession to be associated to a ServerMediaSession. If yes, what is the purpose of it?

2) I need to know when to start/stop my video encoder. Currently, I do this by sub-classing RTSPServer::RTSPClientSession and handling the handlCmd_PLAY and handleCmd_TEARDOWN. Is this the correct place or should I sub-class the ServerMediaSubsession class and handle startStream() and deleteStream() virtual methods?

3) How does clientSessionId that is passed to various xxxStream() methods (i,e, startStream(), deleteStream()) relates to a particular URI associated to a ServerMediaSession?

4) Lastly, how do I relate doGetNextFrame(), that is called by the framer to get the next video frame, to the channel name as it was specified in the RTSP URI command Play?

Thanks for your help.