Question

I’m developing a UDP responder to handle basic SSDP commands. The purpose of this piece of code is to do auto discovery, so when the server sends a multicast to a specific group all other subscribed devices should send back a UDP packet announcing its presence to the host and port of who sent the multicast. My android device receives and sends the packet just fine but because it takes too long to get back the SocketAddress object from getSocketAddress() method the server times out, closes the listening port and never gets a packet back from the android device.

Here’s my code:

    public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    MulticastSocket ms = null;
    byte[] packBuf = new byte[128];
    try {
        ms = new MulticastSocket(32410);
        ms.joinGroup(InetAddress.getByName("239.255.255.250"));
    } catch (IOException e3) {
        // TODO Auto-generated catch block
        e3.printStackTrace();
    }

    while (true)
    {
        DatagramPacket receivedPack = new DatagramPacket(packBuf, packBuf.length);
        try {
            ms.receive(receivedPack);
            Log.d(TAG, "Received data");

        } catch (IOException e3) {
            // TODO Auto-generated catch block
            e3.printStackTrace();
        }

        String responseStr = "HTTP/1.0 200 OK\n" + 
           "Content-Type: app\n" + 
           "Resource-Identifier: 945e7dd5913ab45f1db4f271a1620b9471fb7d4d\n" +
           "Name: Test App\n" +
           "Port: 8888\n" + 
           "Updated-At: 1319511680\n" +
           "Version: 0.9.3.4-29679ad\n" +
           "Content-Length: 23\n\n" + 
           "<message>test</message>";

        byte[] response = responseStr.getBytes();

        DatagramSocket sendSocket = null;
        try {
            sendSocket = new DatagramSocket();

        } catch (IOException e2) {
            // TODO Auto-generated catch block
            Log.e(TAG,"Erro",e2);
        }

        DatagramPacket outPack;
        try {
            outPack = new DatagramPacket(response, responseStr.length(), receivedPack.getSocketAddress());
            sendSocket.send(outPack);
        } catch (UnknownHostException e1) {
            Log.e(TAG,"Erro",e1);
        }
        catch (IOException e) {
            Log.e(TAG,"Erro",e);
        }
        catch (Exception e)
        {
            Log.e(TAG,"Erro",e);
        }
    }
}

Any ideas?

thanks in advance,

fbr

Answer 1

2 things come to mind…

1) What happens when you change:

outPack = new DatagramPacket(response, responseStr.length(), receivedPack.getSocketAddress());

to

outPack = new DatagramPacket(response, responseStr.length(), receivedPack.getAddress(), receivedPack.getPort());

2) I remember having this sort of problem with an embedded Java on a Home Automation system. Our short term solution was to put most of the machine and multicast addresses in the hosts file. Long term we ended up with a local DNS server.

There is a parameter somewhere in the Java Network stack that tells it how long to cache DNS failures in memory. We cranked that number up to, I think, 5 minutes instead of 10 seconds.