NET0183 Networks and Communications Lecture 31 The Socket API 8/25/20091 NET0183 Networks and Communications by Dr Andy Brooks Lecture powerpoints from the recommended textbook are by Lami Kaya, Lecture powerpoints are © 2009 Pearson Education Inc. Their content has sometimes been edited by Andy Brooks. socket/tengill

8/25/2009 NET0183 Networks and Communications by Dr Andy Brooks 2 The recommended textbook is Computer Networks and Internets by Douglas E. Comer (for additional discounts and offers)

Webopedia 31. mars /25/2009 NET0183 Networks and Communications by Dr Andy Brooks 3 “(1) In UNIX and some other operating systems, a software object that connects an application to a network protocol. In UNIX, for example, a program can send and receive TCP/IP messages by opening a socket and reading and writing data to and from the socket. This simplifies program development because the programmer need only worry about manipulating the socket and can rely on the operating system to actually transport messages across the network correctly. Note that a socket in this sense is completely soft - it's a software object, not a physical component.”

4 Socket: a door between an application process and a transport protocol (e.g. TCP) 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 from Kurose and Ross

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Network Programming and the Socket API The interface an application uses to specify communication is known as an Application Program Interface (API). One particular API has emerged as the de facto standard for software that communicates over the Internet. This API is known as the socket API, commonly abbreviated sockets. The socket API is available for many OS such as Microsoft's Windows systems as well as various UNIX systems, including Linux. sockets/tenglar

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Parameters and the Socket API Socket programming differs from conventional I/O. An application must specify details such as : –the address of a remote computer –the protocol port number –whether the application will act as a client or as a server To avoid having a single socket function with many parameters, designers of the socket API chose to define many functions. An application creates a socket, and then invokes functions.

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.7 Figure 3.7 A summary of the major functions in the socket API.

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved Socket Calls in a Client and Server Figure 3.8 illustrates the sequence of socket calls made by a typical client and server that use a stream connection. –The server waits for the client to send data. In practice, some applications arrange for the server to send first (i.e. send and recv are called in the reverse order).

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.9 Figure 3.8 Illustration of the sequence of socket functions called by a client and server using the stream paradigm.

10 How does this all work in Java? A brief overview follows... 8/25/2009 NET0183 Networks and Communications by Dr Andy Brooks

Simple, example client-server in Java. 11 AdditionClient.java RunAdditionClient.java AdditionServer.java RunAdditionServer.java The source code files are on NET0183 website. 8/25/2009 NET0183 Networks and Communications by Dr Andy Brooks The client sends two numbers to the server. The server computes the addition and sends the result back to the client. Window dialogues are used to report behaviour to let us see what is happening.

Socket and ServerSocket classes in Java Socket knows about: – opening a connection – sending data – receiving data – closing a connection ServerSocket knows about: – binding to a port – listening for incoming data – accepting connections 8/25/2009 NET0183 Networks and Communications by Dr Andy Brooks 12

8/25/2009 NET0183 Networks and Communications by Dr Andy Brooks 13 import java.net.*; import java.io.*; import javax.swing.*; public class AdditionServer extends JFrame { private JTextArea textWindow = new JTextArea(); private int port; // the constructor public AdditionServer(int portIn) { port = portIn; setTitle("Addition Server"); add("Center",textWindow); setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); setSize(400, 300); setVisible(true); startServer(); } AdditionServer.java

8/25/2009 NET0183 Networks and Communications by Dr Andy Brooks 14 AdditionServer.java private void startServer() { // declare a "general" socket and a server socket Socket connection; ServerSocket listenSocket; // declare low level and high level objects for input InputStream inStream; DataInputStream inDataStream; // declare low level and high level objects for output OutputStream outStream; DataOutputStream outDataStream; // declare other variables String client; int first, second, sum; boolean connected;

8/25/2009 NET0183 Networks and Communications by Dr Andy Brooks 15 AdditionServer.java while(true) { try { // create a server socket listenSocket = new ServerSocket(port); textWindow.append("Listening on port " + port + "\n"); // listen for a connection from the client connection = listenSocket.accept (); connected = true; // create an input stream from the client inStream = connection.getInputStream(); inDataStream = new DataInputStream(inStream); // create an output stream to the client outStream = connection.getOutputStream (); outDataStream = new DataOutputStream (outStream ); // wait for a string from the client client = inDataStream.readUTF(); textWindow.append("Connection established with " + client + "\n" );

8/25/2009 NET0183 Networks and Communications by Dr Andy Brooks 16 AdditionServer.java while(connected) { //read an integer from the client first = inDataStream.readInt(); textWindow.append( "First number received: " + first + "\n"); //read an integer from the client second = inDataStream.readInt(); textWindow.append( "Second number received: " + second + "\n"); sum = first + second; textWindow.append( "Sum returned: " + sum + "\n"); //send the sum to the client outDataStream.writeInt(sum); } catch (IOException e) { connected = false; }}}}

8/25/2009 NET0183 Networks and Communications by Dr Andy Brooks 17 RunAdditionServer.java public class RunAdditionServer { /** args */ public static void main(String[] args) { new AdditionServer(8901); }

8/25/2009 NET0183 Networks and Communications by Dr Andy Brooks 18 Simple, example client-server in Java. Extracts from the Java API. listenSocket = new ServerSocket(port); – “Creates a server socket, bound to the specified port. A port of 0 creates a socket on any free port. The maximum queue length for incoming connection indications (a request to connect) is set to 50. If a connection indication arrives when the queue is full, the connection is refused. “ connection = listenSocket.accept (); – “Listens for a connection to be made to this socket and accepts it. The method blocks until a connection is made. A new Socket s is created and, if there is a security manager, the security manager's checkAccept method is called with s.getInetAddress().getHostAddress() and s.getPort() as its arguments to ensure the operation is allowed. This could result in a SecurityException.”