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

2 Socket Programming TCP and UDP

3 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 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 Goal: learn how to build client/server application that communicate using sockets

4 TCP

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

2: Application Layer 6 Socket programming with TCP 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

7 Stream jargon 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.

8 Socket programming with TCP 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) Client process client TCP socket

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

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. BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); sentence = inFromUser.readLine(); outToServer.writeBytes(sentence + '\n'); modifiedSentence = inFromServer.readLine(); System.out.println ("FROM SERVER: " + modifiedSentence ); clientSocket.close(); } Create input stream attached to socket 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; ServerSocket welcomeSocket = new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept(); BufferedReader inFromClient = 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); } Read in line from socket Create output stream, attached to socket Write out line to socket End of while loop, loop back and wait for another client connection

14 UDP

15 Socket programming with UDP 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 application viewpoint UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server

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

17 Example: Java client (UDP) Output: sends packet (TCP sent “byte stream”) Input: receives packet (TCP received “byte stream”) Client process 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. 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(); } Create datagram with data-to-send, length, IP addr, port 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

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

Required Packages

How does the Bluetooth protocol work? discovery pairing RFComm Scanning for other BT Devices --- inquiry scan Followed by page scan. Take about seconds Authentication process where two devices exchange a pin. Once paired the info is maintained in service discovery db 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

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

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.

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); }

Discovering devices Set 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 BroadcastReceiver mReceiver = 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. –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

Enabling Discovery Intent discoverable = new Intent(BluetoothAdapter.BLUETOOTH_ACTION_DISCOVERABLE); Discoverable.putExtras(BluetoothAdapter.EXTRA_DISCOVERABLE_DURATION, 300); startActivity(discoverable); –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, 3600 – default is taken.

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 public class AcceptConnection extends Thread{ private final BluetoothServerSocket soc; public AcceptConnection() { try { soc = adapter.listenUsingRfcommWithServiceRecord(NAME, UDID); } catch(IOException e){} } public void run() { BluetoothSocket socket = null; while(true) { try { soc.accept(); } catch(IOException e) { break; } if(soc != null) { //spawn another thread to manage the connection } } Name of the service Unique ID for the service

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(Exception e) { } } public void run() { adapter.cancelDiscover(); try { temp.connect(); } catch(Exception e) { } //manage the connection }

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(Exception e) { } } public void run() { byte[] buffer = new byte[1024]; int numbytes; adapter.cancelDiscover(); try { numbytes = temp.read(buffer); //do whatever you want with the bytes } catch(Exception e) { } //manage the connection }