CMSC628: Introduction to Mobile Computing

1. CMSC628: Introduction to Mobile Computing Nilanjan Banerjee University of Maryland Baltimore County, MD Mobile Systems Programming (Acknowledgment to DeepaShinde and Cindy Atheron

2. Socket Programming TCP and UDP

3. a host-local, application-created, OS-controlled interface (a “door”) into which application process can both send and receive messages to/from another application process socket Socket programming Socket API • introduced in BSD4.1 UNIX, 1981 • explicitly created, used, released by apps • client/server paradigm • two types of transport service via socket API: • unreliable datagram • reliable, byte stream-oriented Goal: learn how to build client/server application that communicate using sockets

4. TCP

5. process process TCP with buffers, variables TCP with buffers, variables socket socket Socket-programming using TCP Socket: a door between application process and end-end-transport protocol (UCP or TCP) TCP service: reliable transfer of bytesfrom one process to another controlled by application developer controlled by application developer controlled by operating system controlled by operating system internet host or server host or server 2: Application Layer

6. Client must contact server server process must first be running server must have created socket (door) that welcomes client’s contact Client contacts server by: creating client-local TCP socket specifying IP address, port number of server process When client creates socket: client TCP establishes connection to server TCP When contacted by client, server TCP creates new socket for server process to communicate with client allows server to talk with multiple clients source port numbers used to distinguish clients TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server application viewpoint Socket programming with TCP 2: Application Layer

7. A stream is a sequence of characters that flow into or out of a process. An input stream is attached to some input source for the process, eg, keyboard or socket. An output stream is attached to an output source, eg, monitor or socket. Stream jargon

8. Example client-server app: 1) client reads line from standard input (inFromUser stream) , sends to server via socket (outToServer stream) 2) server reads line from socket 3) server converts line to uppercase, sends back to client 4) client reads, prints modified line from socket (inFromServer stream) Socket programming with TCP Client process client TCP socket

9. create socket, connect to hostid, port=x create socket, port=x, for incoming request: clientSocket = Socket() welcomeSocket = ServerSocket() TCP connection setup wait for incoming connection request connectionSocket = welcomeSocket.accept() send request using clientSocket read request from connectionSocket write reply to connectionSocket read reply from clientSocket close connectionSocket close clientSocket Client/server socket interaction: TCP Server (running on hostid) Client

10. Example: Java client (TCP) import java.io.*; import java.net.*; class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence; BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); Socket clientSocket = new Socket("hostname", 6789); DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream()); Create input stream Create client socket, connect to server Create output stream attached to socket

11. Example: Java client (TCP), cont. Create input stream attached to socket BufferedReaderinFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); sentence = inFromUser.readLine(); outToServer.writeBytes(sentence + '\n'); modifiedSentence = inFromServer.readLine(); System.out.println("FROM SERVER: " + modifiedSentence); clientSocket.close(); } } Send line to server Read line from server

12. Example: Java server (TCP) import java.io.*; import java.net.*; class TCPServer { public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence; ServerSocketwelcomeSocket = new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept(); BufferedReaderinFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream())); Create welcoming socket at port 6789 Wait, on welcoming socket for contact by client Create input stream, attached to socket

13. Example: Java server (TCP), cont DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream()); clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n'; outToClient.writeBytes(capitalizedSentence); } } } Create output stream, attached to socket Read in line from socket Write out line to socket End of while loop, loop back and wait for another client connection

14. UDP

15. UDP: no “connection” between client and server no handshaking sender explicitly attaches IP address and port of destination to each packet server must extract IP address, port of sender from received packet UDP: transmitted data may be received out of order, or lost UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server application viewpoint Socket programming with UDP

16. Client create socket, port=x, for incoming request: serverSocket = DatagramSocket() create socket, clientSocket = DatagramSocket() Create, address (hostid, port=x, send datagram request using clientSocket read request from serverSocket write reply to serverSocket specifying client host address, port number read reply from clientSocket close clientSocket Client/server socket interaction: UDP Server (running on hostid)

17. Example: Java client (UDP) Client process Input: receives packet (TCP received “byte stream”) Output: sends packet (TCP sent “byte stream”) client UDP socket

18. Example: Java client (UDP) import java.io.*; import java.net.*; class UDPClient { public static void main(String args[]) throws Exception { BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); DatagramSocket clientSocket = new DatagramSocket(); InetAddress IPAddress = InetAddress.getByName("hostname"); byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024]; String sentence = inFromUser.readLine(); sendData = sentence.getBytes(); Create input stream Create client socket Translate hostname to IP address using DNS

19. Example: Java client (UDP), cont. Create datagram with data-to-send, length, IP addr, port DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); clientSocket.send(sendPacket); DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData(),0,receivePacket.getLength()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); } } Send datagram to server Read datagram from server

20. Example: Java server (UDP) import java.io.*; import java.net.*; class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(9876); byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024]; while(true) { DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); serverSocket.receive(receivePacket); Create datagram socket at port 9876 Create space for received datagram Receive datagram

21. Example: Java server (UDP), cont String sentence = new String(receivePacket.getData()); InetAddressIPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase(); sendData = capitalizedSentence.getBytes(); DatagramPacketsendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); serverSocket.send(sendPacket); } } } Get IP addr port #, of sender Create datagram to send to client Write out datagram to socket End of while loop, loop back and wait for another datagram 2: Application Layer

22. Required Packages

23. How does the Bluetooth protocol work? Scanning for other BT Devices --- inquiry scan Followed by page scan. Take about 15-20 seconds discovery Authentication process where two devices exchange a pin. Once paired the info is maintained in service discovery db pairing Service Discovery Every server device publishes a set of service that client connect to After pairing the devices communicate amongst each other over a RF communication channel RFComm

24. Android implementation overview? Access to the local Bluetooth device and its properties BluetoothAdapter Access to any Bluetooth device (usually remote) BluetoothDevice Socket interface for the server-end BluetoothServerSocket Socket interface for the client-end BluetoothSocket

25. Bluetooth Permissions • Permission BLUETOOTH is used ONLY for communication • Requesting a connection, accepting a connection, and transferring data • Permission BLUETOOTH_ADMIN is used for controlling the device • Device discovery, changing the settings of the Bluetooth device etc. <manifest> <uses permission android:name=“android.permission.BLUETOOTH”> <uses permission android:name=“android.permission.BLUETOOTH_ADMIN”> </manifest>

26. Setting up the Bluetooth Adapter • Use BluetoothAdapter to get a reference to the Bluetooth device • If Bluetooth device is not supported the adapter returns a NULL • Enable Bluetooth device using an Intent and starting a new Activity with the Bluetooth device • It does ask the user whether he wants to enable the device • How do you know that the Bluetooth device is enabled? --- the resultcode in onActivityResult() callback will be RESULT_OK. Bluetooth adapter = BluetoothAdapter.getDefaultAdapter(); if(adapter == null) { //Device does not support Bluetooth. } if(!adapter.isEnabled()) { Intent enableBT = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE); startActivityForResult(enableBT, REQUEST_ENABLE_BT); }

27. Discovering devices • First step is to find devices that you have already paired with: these are devices you do not need to pair to get connected • Use a broadcast receiver discover new Bluetooth devices Set<BluetoothDevice> pairedDevices = adapter.getBondedDevices(); if (pairedDevices.size() > 0) { for(BluetoothDevice device: pairedDevices) { //get access to the devices name through device.getName(); //get access to the devices MAC address through device.getAddress(); } } //discovering devices adapter.startDiscover(); private final BroadcastReceivermReceiver = new BroadcastReceiver() { public void onReceive(Context context, Intent intent) { String action = intent.getAction(); if(BluetoothDevice.ACTION_FOUND.equals(action)) { BluetoothDevice device = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE); //get the name of the device through device.getName(); //get the MAC address of the device through device.getAddress(); } } IntentFilter filter = new IntentFilter(Bluetooth.ACTION_FOUND); registerReceiver(mReceiver, filte); //register for broadcast receiver when a BT device is found.

28. Enabling Discovery • Why do you need to set a device’s Bluetooth to Discoverable • If you are a server and you want client devices to connect to you • If you want other devices to see you in order to pair with you • You set it up using an Intent • A parameter that you can set up is the time that you want the device to be discoverable • Default = 120 seconds, 0  forever, max = 3600, < 0 or > 3600 – default is taken. Intent discoverable = new Intent(BluetoothAdapter.BLUETOOTH_ACTION_DISCOVERABLE); Discoverable.putExtras(BluetoothAdapter.EXTRA_DISCOVERABLE_DURATION, 300); startActivity(discoverable);

29. Connecting to a device (server-side) • Just like a TCP socket called BluetoothServerSocket • You wait on an accept() (blocking call) till you receive an incoming connection request • accept() is blocking so it should happen in a separate thread from the UI thread Name of the service Unique ID for the service public class AcceptConnection extends Thread{ private final BluetoothServerSocket soc; public AcceptConnection() { try { soc = adapter.listenUsingRfcommWithServiceRecord(NAME, UDID); } catch(IOExceptione){} } public void run() { BluetoothSocket socket = null; while(true) { try { soc.accept(); } catch(IOExceptione) { break; } if(soc != null) { //spawn another thread to manage the connection } } } }

30. Connecting to a device (client-end) • Connect() is a blocking call so needs to happen in a thread separate from the UI thread • From the remote device, create a Rfcomm channel for data transfer.  public class ClientThread extends Thread { BluetoothSocket temp = null; public ClientThread(Bluetooth device) { try { temp = device.createRfcommSocketToServiceRecord(UDID); }catch(Exceptione) { } } public void run() { adapter.cancelDiscover(); try { temp.connect(); } catch(Exceptione) { } //manage the connection } }

31. Data transfer using the server/client socket • Attach an InputStream and an OutputStream to the the socket • Use read(byte[]) and write(byte[]) to read and write --- both are blocking calls  public class ClientThread extends Thread { BluetoothSocket temp = null; public ClientThread(Bluetooth device) { try { temp = device.createRfcommSocketToServiceRecord(UDID); }catch(Exceptione) { } } public void run() { byte[] buffer = new byte[1024]; intnumbytes; adapter.cancelDiscover(); try { numbytes = temp.read(buffer); //do whatever you want with the bytes } catch(Exceptione) { } //manage the connection } }