Simple Service Discovery Protocol

According to wikipedia SSDP is a network protocol for advertisement and discovery of network services and presence information. In another words, SSDP is used to broadcast message in order to discover devices. Message which I am sending through SSDP looks like:

M-SEARCH * HTTP/1.1\r\n
HOST: 239.255.255.250:1900\r\n
MAN: "ssdp:discover"\r\n
MX: 3\r\n
ST: urn:milosev:com:Stanko:1\r\n

Detailed explanation of headers you can read in this document.

Code in Android which I am using for sending M-SEARCH request looks like this:

        StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
        StrictMode.setThreadPolicy(policy);

        try {
            MulticastSocket s = new MulticastSocket(1900);
            s.joinGroup(InetAddress.getByName("239.255.255.250") );
            DatagramPacket packet = new DatagramPacket(DISCOVER_MESSAGE.getBytes(), DISCOVER_MESSAGE.length(), getBroadcastAddress(), 1900);
            s.setBroadcast(true);
            s.send(packet);
        } catch (IOException e) {
            e.printStackTrace();
        }

Where DISCOVER_MESSAGE is constant which looks like this:

    private final static String DISCOVER_MESSAGE = "M-SEARCH * HTTP/1.1\r\n"
            + "HOST: 239.255.255.250:1900\r\n" + "MAN: \"ssdp:discover\"\r\n"
            + "MX: 3\r\n" + "ST: urn:milosev:com:Stanko:1\r\n";

getBroadcastAdress is like:

    InetAddress getBroadcastAddress() throws IOException {

        Context mContext = getApplicationContext();
        WifiManager wifi = (WifiManager) mContext.getSystemService(Context.WIFI_SERVICE);
        DhcpInfo dhcp = wifi.getDhcpInfo();
        // handle null somehow

        int broadcast = (dhcp.ipAddress & dhcp.netmask) | ~dhcp.netmask;
        byte[] quads = new byte[4];
        for (int k = 0; k < 4; k++)
            quads[k] = (byte) ((broadcast >> k * 8) & 0xFF);
        return InetAddress.getByAddress(quads);
    }

Permissions in \app\src\main\AndroidManifest.xml which we need:

 <uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />

Code to receive messages looks like this:

StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
StrictMode.setThreadPolicy(policy);

try {
	MulticastSocket s = new MulticastSocket(1900);
	s.joinGroup(InetAddress.getByName("239.255.255.250") );
	DatagramPacket packet = new DatagramPacket(DISCOVER_MESSAGE.getBytes(), DISCOVER_MESSAGE.length(), getBroadcastAddress(), 1900);
	s.setBroadcast(true);

	while(true) {
		byte[] buf = new byte[1024];
		DatagramPacket receivePacket = new DatagramPacket(buf, buf.length);

		s.receive(receivePacket);
		String msg = new String(receivePacket.getData(), 0, receivePacket.getLength());
	}
} catch (IOException e) {
	e.printStackTrace();
}

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Just one update, in order to receive answer it seems that I have to use same object for receiving, in my case I just placed code for receiving immediately under the code for sending request...

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One more update, it seems that getBroadcastAddress is not needed, we can simply write InetAddress.getByName("239.255.255.250")