Introduction to Socket Programming for Mobile Systems

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

Socket Programming TCP and UDP 2

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

TCP 4

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 controlled by application developer controlled by application developer controlled by operating system process socket process socket TCP with buffers, variables controlled by operating system TCP with buffers, variables internet host or server host or server 2: Application Layer 5

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 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 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 application viewpoint TCP provides reliable, in-order transfer of bytes ( pipe ) between client and server 2: Application Layer 6

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. 7

Socket programming with TCP keyboard monitor 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) inFromUser input stream Client Process process inFromServer outToServer output stream input stream client TCP clientSocket socket TCP socket to network from network 8

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

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

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

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

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

UDP 14

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 application viewpoint UDP provides unreliable transfer of groups of bytes ( datagrams ) between client and server UDP: transmitted data may be received out of order, or lost 15

Client/server socket interaction: UDP Server (running on hostid) 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 16

Example: Java client (UDP) keyboard monitor inFromUser input stream Client Process process Input: receives packet (TCP received byte stream ) Output: sends packet (TCP sent byte stream ) receivePacket sendPacket UDP packet UDP packet clientSocket client UDP socket UDP socket to network from network 17

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 18

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 19

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 20

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

Package Description Represents a number of packages that provide fine control and functions for HTTP communications. You might recognize Apache as the popular open source Web server. org.apache.* Contains additional network access sockets beyond the core java.net.* classes. This package includes the URI class, which is used frequently in Android application development beyond traditional networking. android.net android.net.http Contains classes for manipulating SSL certificates. Contains classes for managing all aspects of WiFi (802.11 wireless Ethernet) on the Android platform. Not all devices are equipped with WiFi capability, particularly as Android makes headway in the "flip- phone" strata of cell phones from manufacturers like Motorola and LG. Contains classes required for managing and sending SMS (text) messages. Over time, an additional package will likely be introduced to provide similar functions on non-GSM networks, such as CDMA, or something like android.t lephony.cdma. android.net.wifi android.telephony Required Packages

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 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 Service Discovery RFComm

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

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>

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 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 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.

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);

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(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 } } } }

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 } }