Question

I am writing a class in Java which is used to greatly simplify the process of multicasting. However, I am having two big problems with it:

1. The class sends data (I can verify this with my net monitor, Wireshark) but the data is not received by any others in the same group.
2. On some machines, the sending packet TTL is exceeded in transit (again, according to Wireshark).

Could anyone please help me? I’ve been trying and searching for answers for hours, and it appears that my code follows all of the basic procedures for connecting to, joining, sending, and receiving data from a multicast host.

Here is a snippet of the relevant portions of the class:

Multicaster class:

public class Multicaster {
  public int port = 5540;

  protected String IPAddress;

  private MulticastSocket msConn;
  private InetAddress netAddr;

  public Multicaster(String IPAddress) {
    this.IPAddress = IPAddress;
  }

  public String recieveData() {
    byte[] buf = new byte[1000];
    DatagramPacket pack = new DatagramPacket(buf, buf.length);

    try {
      this.msConn.receive(pack);
      new Message(pack);
      String out = new String(pack.getData());
      return out.substring(0, pack.getLength());
    } catch (IOException e) {
      return new String("");
    }
  }

  public void joinGroup() { 
    try {
      this.msConn.joinGroup(this.netAddr);
    } catch (IOException e) {
    //This error shouldn't occur since it would caught by the connect() method during initial connection to the host
    }
  }

  public void connect() throws MulticasterInitException {
  //Try to create a multicast connection on the given IP address and port
    try {
      try {
      //Create a multicast connection on a given port, throws UnknownHostException
        this.msConn = new MulticastSocket(this.port);

      //If all goes well, then create a connection to a given IP address using the above port number, throws IOException and SecurityException
        this.netAddr = InetAddress.getByName(this.IPAddress);
      }
    }

  /**
   * Here all of the possible exceptions that are thrown above
   * are caught and handled here. This works just fine.
   */
  }

  public void sendData(String data) throws MulticasterSendException {
    DatagramPacket packet = new DatagramPacket(data.getBytes(), data.length(), this.netAddr, this.port);

    try {
      this.msConn.send(packet);
    } catch (IOException e) {
      throw new MulticasterSendException("Java could not communicate with the server. Please check your network connections.", e);
    }
  }
}

Sample usage to send data:

Multicaster multicast = new Multicaster("239.0.0.0");

try {
  multicast.connect();
} catch (MulticasterInitException e) {
  //Handle exception...
}

multicast.joinGroup();

try {
  multicast.sendData("Hi");
} catch (MulticasterSendException e) {
  //Handle exception... 
}

Sample usage to receive data:

Multicaster multicast = new Multicaster("239.0.0.0");

try {
  multicast.connect();
} catch (MulticasterInitException e) {
  //Handle exception...
}

multicast.joinGroup();
System.out.print(multicast.recieveData());

Answer 1

I’ve run into similar problems before and had to ensure that the NetworkInterface was specified on the receiving side.

SocketAddress socketAddress = new InetSocketAddress(groupIp, groupPort);
NetworkInterface networkInterface = NetworkInterface.getByName(interfaceName);

socket.joinGroup(socketAddress, networkInterface);

Where interfaceName is one of the interface names shown when running ifconfig on Linux or Mac.