Injecting DTMF event in the midst of RTP media streaming?

Question

Hi all, I am attempting to implement a mechanism that allows me to inject DTMF RTP events in an RTP media stream. This is useful in telephony applications where the users are prompted to enter digits while being served by automated voice services such as answerinng services or tele-bankings etc. So basically my approach is to extend Sun's RTP packetizer and intercept outgoing packets when necessary and replace them with the appropriate DTMF RTP (RFC 2833) packets. Sounds simple enough. So I derived my custom packetizer from com.sun.media.codec.audio.ulaw.Packetizer, and over-ridden method process(). Normally my packetizer's method process() simply delegates the functionality to its parent class's process() method. When a DTMF digit is required I'd take the outputbuffer generated by the parent's process() and modify its header and payload to turn it into a DTMF RTP packet. The problem is that the class RTPHeader is so limited, there is no way to set the payload type, sequence number, ... And the documentation is precious few and far in between. If someone has solved this issue, or if you have a reference to some documentation that describes the inner working of JMF's codec chaining, I would appreciate some pointers. What I need to know is:
- What JMF does with the Buffer objects between stages (from one codec to the next)?
- The data portion of Buffer is an Object of arbitray class, what on earth does JMF do with that?
- How does JMF take a Buffer object and turn it into a UDP packet?
- How do I go about creating an RTP header and fill it with the information (payload type, timestamp, sequence number, ...) that I want?
Thanks in advance,

Answer 1

I don't know what the heck RTPHeader represents but it sure doesn't seem to conform to RFC1889. And also, it seems the UDP RTP packets are formed somewhere after the packetizer and before the RTP connector, someone oughta jot all this down in a book. So anyway, using my own RTPConnector implementation I have some control over the outgoing/incoming RTP packets, to access and manipulate the real RTP header (not RTPHeader), I devised a new class that takes the RTP packet buffer and provides an API to examine and manipulate some RTP info directly on the buffer without doing any unecessary data copy (you guys can extend it to do more as per your requirement):
package com.mycompany.media;
import javax.media.rtp.RTPConnector;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.io.IOException;
import javax.media.protocol.ContentDescriptor;
import javax.media.protocol.SourceTransferHandler;
import java.net.DatagramPacket;
import javax.media.protocol.PushSourceStream;
import javax.media.rtp.OutputDataStream;
import java.net.SocketException;
import mitel.utilities.MiQueue;
import java.nio.ByteBuffer;
import java.util.LinkedList;
class MiRtpHeader
     byte[] data;
     int myoffset;
     public MiRtpHeader(byte[] buf, int offset, int len) throws ArrayIndexOutOfBoundsException
          if(len < 12)
               throw new ArrayIndexOutOfBoundsException("Buffer not large enough to contain a basic RTP header");
          data = buf;
          myoffset = offset;
     public boolean getExtension()
          return (0 != (data[myoffset] & 0x10));
     public void setExtension(boolean state)
          if(state)
               data[myoffset] = (byte)(data[myoffset] | 0x10);
          else
               data[myoffset] = (byte)(data[myoffset] & 0xef);
     public boolean getMarker()
          return (0 != (data[myoffset + 1] & 0x80));
     public void setMarker(boolean state)
          if(state)
               data[myoffset + 1] = (byte)(data[myoffset + 1] | 0x80);
          else
               data[myoffset + 1] = (byte)(data[myoffset + 1] & 0x7f);
     public int getTs()
          ByteBuffer tsBuf = ByteBuffer.wrap(data, myoffset + 4, 4);
          return tsBuf.getInt();
     public void setTs(int ts)
          ByteBuffer tsBuf = ByteBuffer.wrap(data, myoffset + 4, 4);
          tsBuf.putInt(ts);
     public int getPayloadType()
          ByteBuffer ptBuf = ByteBuffer.wrap(data, myoffset + 1, 1);
          return ptBuf.get();
}