CODEWITHSUNDEEP
androidudp
ForeignerBR
ForeignerBR

Reputation: 2469

getSocketAddress() method causes delay which leads to communication lag in Android

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

Upvotes: 2

Views: 803

Answers (2)

JimR
JimR

Reputation: 16143

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.

Upvotes: 1

Seth Noble
Seth Noble

Reputation: 3303

The most likely problem is that getSocketAddress() is trying to resolve the DNS name of the IP address, which is timing out either due to it being a multicast address or just general DNS lag.

The InetSocketAddress class has a constructor option needResolved which can control this behavior. Unfortunately, it does not appear that DatagramPacket.getSocketAddress() allows you to specify that you want that set to false.

This is apparently a known issue, with some recent discussion of it here: Issue 12328: DatagramChannel - cannot receive without a hostname lookup

The thread suggests that this has been fixed in Android 3.0, and offers a couple of workarounds for Android 2.0 which may or may not work.

In your case, you could try creating an InetSocketAddress set to INADDR_ANY and port 0 with needsResolved set to 0, and then pass that in when you create receivedPack. Hopefully receive() will reuse that and remember the setting.

Upvotes: 2

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