Experiments in Streaming Content in Java ME(2)

Creating an RTSP Protocol Handler

Recall that RTSP is the actual protocol over which streaming commands are initiated, through which the RTP packets are received. The RTSP protocol is like a command initiator, a bit like HTTP. For a really good explanation of a typical RTSP session, please see these specifications for a simple RTSP client. For the purposes of this article, I am going to oversimplify the protocol implementation. Figure 1 shows the typical RTSP session between a client and a streaming server.

 

 

 

Figure 1. A typical RTSP session between a RTSP client and a streaming server (click for full-size image).

In a nutshell, an RTSP client initiates a session by sending a DESCRIBE request to the streaming server which means that the client wants more information about a media file. An example DESCRIBE request may look like this:

 

DESCRIBE rtsp://localhost:554/media.3gp rtsp/1.0
CSeq: 1

 

The URL for the media file is followed by the RTSP version that the client is following, and a carriage return/line feed (CRLF). The next line contains the sequence number of this request and increments for each subsequent request sent to the server. The command is terminated by a single line on its own (as are all RTSP commands).

All client commands that are successful receive a response that starts with RTSP/1.0 200 OK. For the DESCRIBE request, the server responds with several parameters, and if the file is present and streamable, this response contains any information for any tracks in special control strings that start with a a=control:trackID= String. The trackID is important and is used to create the next requests to the server.

Once described, the media file's separate tracks are set up for streaming using the SETUP command, and these commands should indicate the transport properties for the subsequent RTP packets. This is shown here:

 

SETUP rtsp://localhost:554/media.3gp/trackID=3 rtsp/1.0
CSeq: 2
TRANSPORT: UDP;unicast;client_port=8080-8081

 

The previous command indicates to the server to set up to stream trackID 3 of the media.3gp file, to send the packets via UDP, and to send them to port 8080 on the client (8081 is for RTCP commands). The response to the first SETUP command (if it is okay) will contain the session information for subsequent commands and must be included as shown here:

SETUP rtsp://localhost:554/media.3gp/trackID=3 rtsp/1.0
CSeq: 3
Session: 556372992204
TRANSPORT: UDP;unicast;client_port=8080-8081

An OK response from the server indicates that you can send the PLAY command, which will make the server start sending the RTP packets:

 

PLAY rtsp://localhost:554/media.3gp rtsp/1.0
CSeq: 3
Session: 556372992204

 

Notice that the PLAY command is issued only on the main media file, and not on any individual tracks. The same is true for the PAUSE and TEARDOWN commands, which are identical to the PLAY command, except for the command itself.

The following listing contains the RTSPProtocolHandler class. The comments in the code and the brief information so far should help with understanding how this protocol handler works:

 

import java.util.Vector;
import java.io.InputStream;
import java.io.IOException;
import java.io.OutputStream;

public class RTSPProtocolHandler {

    // the address of the media file as an rtsp://... String
    private String address;

    // the inputstream to receive response from the server
    private InputStream is;

    // the outputstream to write to the server
    private OutputStream os;

    // the incrementing sequence number for each request
    // sent by the client
    private static int CSeq = 1;

    // the session id sent by the server after an initial setup
    private String sessionId;

    // the number of tracks in a media file
    private Vector tracks = new Vector(2);

    // flags to indicate the status of a session
    private boolean described, setup, playing;
    private Boolean stopped = true;

    // constants
    private static final String CRLF = "\r\n";
    private static final String VERSION = "rtsp/1.0";
    private static final String TRACK_LINE = "a=control:trackID=";
    private static final String TRANSPORT_DATA =
      "TRANSPORT: UDP;unicast;client_port=8080-8081";
    private static final String RTSP_OK = "RTSP/1.0 200 OK";

    // base constructor, takes the media address, input and output streams
    public RTSPProtocolHandler(
        String address, InputStream is, OutputStream Os) {

        this.address = address;
        this.is = is;
        this.os = Os;
    }

    // creates, sends and parses a DESCRIBE client request
    public void doDescribe() throws IOException {

        // if already described, return
        if(described) return;

        // create the base command
        String baseCommand = getBaseCommand("DESCRIBE " + address);

        // execute it and read the response
        String response = doCommand(baseCommand);

        // the response will contain track information, amongst other things
        parseTrackInformation(response);

        // set flag
        described = true;
    }

    // creates, sends and parses a SETUP client request
    public void doSetup() throws IOException {

        // if not described
        if(!described) throw new IOException("Not Described!");

        // create the base command for the first SETUP track
        String baseCommand =
          getBaseCommand(
                "SETUP " + address + "/trackID=" + tracks.elementAt(0));

        // add the static transport data
        baseCommand += CRLF + TRANSPORT_DATA;

        // read response
        String response = doCommand(baseCommand);

        // parse it for session information
        parseSessionInfo(response);

        // if session information cannot be parsed, it is an error
        if(sessionId == null)
          throw new IOException("Could not find session info");

        // now, send SETUP commands for each of the tracks
        int cntOfTracks = tracks.size();
        for(int i = 1; i < cntOfTracks; i++) {
            baseCommand =
                getBaseCommand(
                    "SETUP " + address + "/trackID=" + tracks.elementAt(i));
            baseCommand += CRLF + "Session: " + sessionId + CRLF + TRANSPORT_DATA;
            doCommand(baseCommand);
        }

        // this is now setup
        setup = true;
    }

    // issues a PLAY command
    public void doPlay() throws IOException {

        // must be first setup
        if(!setup) throw new IOException("Not Setup!");

        // create base command
        String baseCommand = getBaseCommand("PLAY " + address);

        // add session information
        baseCommand += CRLF + "Session: " + sessionId;

        // execute it
        doCommand(baseCommand);

        // set flags
        playing = true;
        stopped = false;
    }

    // issues a PAUSE command
    public void doPause() throws IOException {

        // if it is not playing, do nothing
        if(!playing) return;

        // create base command
        String baseCommand = getBaseCommand("PAUSE " + address);

        // add session information
        baseCommand += CRLF + "Session: " + sessionId;

        // execute it
        doCommand(baseCommand);

        // set flags
        stopped = true;
        playing = false;
    }

    // issues a TEARDOWN command
    public void doTeardown() throws IOException {

        // if not setup, nothing to teardown
        if(!setup) return;

        // create base command
        String baseCommand = getBaseCommand("TEARDOWN " + address);

        // add session information
        baseCommand += CRLF + "Session: " + sessionId;

        // execute it
        doCommand(baseCommand);

        // set flags
        described = setup = playing = false;
        stopped = true;
    }

    // this method is a convenience method to put a RTSP command together
    private String getBaseCommand(String command) {

        return(
            command +
            " " +
            VERSION + // version
            CRLF +
            "CSeq: " + (CSeq++) // incrementing sequence
        );
    }

    // executes a command and receives response from server
    private String doCommand(String fullCommand) throws IOException {

        // to read the response from the server
        byte[] buffer = new byte[2048];

        // debug
        System.err.println(" ====== CLIENT REQUEST ====== ");
        System.err.println(fullCommand + CRLF + CRLF);
        System.err.println(" ============================ ");

        // send a command
        os.write((fullCommand + CRLF + CRLF).getBytes());

        // read response
        int length = is.read(buffer);

        String response = new String(buffer, 0, length);

        // empty the buffer
        buffer = null;

        // if the response doesn't start with an all clear
        if(!response.startsWith(RTSP_OK))
          throw new IOException("Server returned invalid code: " + response);

        // debug
        System.err.println(" ====== SERVER RESPONSE ====== ");
        System.err.println(response.trim());
        System.err.println(" =============================");

        return response;
    }

    // convenience method to parse a server response to DESCRIBE command
    // for track information
    private void parseTrackInformation(String response) {

        String localRef = response;
        String trackId = "";
        int index = localRef.indexOf(TRACK_LINE);

        // iterate through the response to find all instances of the
        // TRACK_LINE, which indicates all the tracks. Add all the
        // track id's to the tracks vector
        while(index != -1) {
            int baseIdx = index + TRACK_LINE.length();
            trackId = localRef.substring(baseIdx, baseIdx + 1);
            localRef = localRef.substring(baseIdx + 1, localRef.length());
            index = localRef.indexOf(TRACK_LINE);
            tracks.addElement(trackId);
        }

    }

    // find out the session information from the first SETUP command
    private void parseSessionInfo(String response) {

        sessionId =
          response.substring(
                response.indexOf("Session: ") + "Session: ".length(),
                response.indexOf("Date:")).trim();

    }

}