P2P message sending using NAT punch-through in Android

Question

I try to send a message from one phone to another using Kryonet (that uses socket communication) and I intend to use NAT punch-through by storing the public address of the clients.

The following code is used:

public class MainActivity extends Activity implements View.OnClickListener
{
  /**
   * Called when the activity is first created.
   */
  @Override
  public void onCreate(Bundle savedInstanceState)
  {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    Button button = ((Button) findViewById(R.id.btnTest));
    button.setOnClickListener(this);
    TextView textViewOwn = ((TextView) findViewById(R.id.lblOwnIP));
    textViewOwn.setText(getLocalIpAddress()+"\n"+GetGateway());
    try
    {
      Server server = new Server();
      server.start();
      server.bind(54555, 54777);
      Log.i("DEBUG", "Server is listening");
      final TextView textView = ((TextView) findViewById(R.id.txtMessage));
      server.addListener(new Listener()
      {
        public void received(Connection connection, Object object)
        {
          if (object instanceof String)
          {
            String request = (String) object;
            Log.i("DEBUG", request);
            ShowMessage(request, textView);
            String response = "Thanks";
            connection.sendTCP(response);
          }
        }
      });

    }
    catch (Exception e)
    {
      e.printStackTrace();
    }
  }

  public static String getLocalIpAddress() {
    try {
      for (Enumeration<NetworkInterface> en = NetworkInterface.getNetworkInterfaces(); en.hasMoreElements();) {
        NetworkInterface intf = en.nextElement();
        for (Enumeration<InetAddress> enumIpAddr = intf.getInetAddresses(); enumIpAddr.hasMoreElements();) {
          InetAddress inetAddress = enumIpAddr.nextElement();
          if (!inetAddress.isLoopbackAddress() && inetAddress instanceof Inet4Address) {
            return inetAddress.getHostAddress();
          }
        }
      }
    } catch (SocketException ex) {
      ex.printStackTrace();
    }
    return null;
  }

  private String GetGateway()
  {
    DhcpInfo d;
    WifiManager wifii;
    wifii= (WifiManager) getSystemService(Context.WIFI_SERVICE);
    d=wifii.getDhcpInfo();
    int gatewayip = d.gateway;
    String mask = Formatter.formatIpAddress(gatewayip);
    return mask;
  }

  private void ShowMessage(final String request, final TextView textView)
  {
    runOnUiThread(new Runnable()
    {
      @Override
      public void run()
      {
        textView.setText(request);
      }
    });
  }

  @Override
  public void onClick(View view)
  {
    final String ip = ((EditText) findViewById(R.id.txtIP)).getText().toString();
    if (TextUtils.isEmpty(ip))
    {
      Toast.makeText(this, "No IP address", Toast.LENGTH_SHORT).show();
      return;
    }
    Runnable runnable = new Runnable()
    {
      public void run()
      {
        try
        {
          Client client = new Client();
          client.start();

          client.connect(5000, ip, 54555, 54777);
          Log.i("DEBUG", "Client is sending: "+ip);
          runOnUiThread(new Runnable()
          {
            @Override
            public void run()
            {
              Toast.makeText(MainActivity.this, "Client is sending: "+ip, Toast.LENGTH_SHORT).show();
            }
          });

          String request = "Here is the request";
          client.sendTCP(request);
        }
        catch (final Exception e)
        {
          e.printStackTrace();
          runOnUiThread(new Runnable()
          {
            @Override
            public void run()
            {
              Toast.makeText(MainActivity.this, "Client has errors: "+e.toString(), Toast.LENGTH_SHORT).show();

            }
          });
        }
      }
    };
       Thread thread=new Thread(runnable);
    thread.start();
  }
}

If the phones are on the same WIFI, it works okay. If one of them is on 3G, I get an external IP address (like: 100.65.96..), and a local address as gateway. As knowing the external IP is the key for NAT PT, I thought it is enough to get the message, but I get a timeout.

What else should I implement to make the 3G phone get the message?

Answer 1

You can do it by using a mediator server that helps establishing the connection between the phones.

1. Create a server program that can accept UDP sockets and can store the public address of the phone that connects to it
2. Connect to that server with phone A to register its public address
3. Use phone B to connect to the server for asking the public address of phone A
4. Once you have that address, you can create a direct socket from phone A to phone B by using its public address. The NAT will do the rest, and the router will send the packages to the phone's private address