Multicast in java

Question

I am trying to write a simple multicast trial.

I used a standard code (sender and reciever).

I tried a few different standard pieces of code. it appears that the receiving code is stuck on receive (as if it's not receving anything).

receive side:

        byte[] b = new byte[3];
    DatagramPacket dgram = new DatagramPacket(b, b.length);
    MulticastSocket socket =
      new MulticastSocket(4545); // must bind receive side
    socket.joinGroup(InetAddress.getByName("226.100.100.125"));

    while(true) {
      socket.receive(dgram); // blocks until a datagram is received
      System.err.println("Received " + dgram.getLength() +
        " bytes from " + dgram.getAddress());
      dgram.setLength(b.length); // must reset length field!
    }

sending side:

      DatagramSocket socket = new DatagramSocket();

  byte[] b = new byte[]{(byte)1,(byte)5,(byte)3};
  DatagramPacket dgram;

  dgram = new DatagramPacket(b, b.length,
    InetAddress.getByName("226.100.100.125"), 4545);

  System.err.println("Sending " + b.length + " bytes to " +
    dgram.getAddress() + ':' + dgram.getPort());
  while(true) {
    System.err.print(".");
    socket.send(dgram);
    Thread.sleep(1000);
  }

What is wrong with my code? *I tried alot of different IPs also*

thanks for the help.

Answer 1

Try receiving in from the same IP but sending to localhost. If this works, then it's your router that is the problem as it doesn't support multicasting. If this still doesn't work then it's the IP address. try something in the 233.x.x.x - 239.x.x.x range.

I ran your code on my computer and it works fine as is, and also works if I changed the send address to localhost. Sounds like this is a problem with your router and not your code.

Answer 2

The network 239.0.0.0/8 is designated for administrator multi-cast traffic. If all of your machines are on the same network segment, you can use an ip in this network to play around with multi-casting.

Here is the RFC defining these:

https://www.rfc-editor.org/rfc/rfc2365

As far as sending goes... Your code should look something like this:

            DatagramPacket p = ...
            MulticastSocket s = new MulticastSocket(LISTENPORT);
            InetAddress group = InetAddress.getByName(LISTENIP);
            s.joinGroup(group);
            s.send(p);
            s.leaveGroup(group);