Lecture 3: Socket Programming

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1 Lecture 3: Socket Programming Prof. Shervin Shirmohammadi SITE, University of Ottawa Fall 2005 CEG4183/ELG

2 Sockets From a logical perspective, a Socket is a communication end point. It is not a physical entity, such as a connection on your network card. Sockets have an address consisting of IP address for the machine (e.g ), plus a port number. Port numbers 0 through 1023 are well known, and are reserved for standard services. Fall 2005 CEG4183/ELG

3 Programming Most programming languages provide an Application Programming Interface (API) to deal with networking and communications. A programmer uses this API to write the necessary code for the networking part of his/her application. The socket interface is the centerpiece of such an API. Typical steps involve opening a socket, read from and write to the socket according to the application s protocol, and close the socket. Fall 2005 CEG4183/ELG

4 The Java Socket Constructors Socket() Creates an unconnected socket Socket(InetAddress address, int port) Creates a socket and connects it to the specified port number at the specified IP. Socket(String host, int port) Creates a stream socket and connects it to the specified port number on the named host. Methods getinputstream() Returns an input stream for this socket. getoutputstream() Returns an output stream for this socket. connect(socketaddress endpoint) Connects this socket to the server. connect(socketaddress endpoint, int timeout) Connects this socket to the server with a specified timeout value. close() Closes this socket. Socket API Fall 2005 CEG4183/ELG

5 Example: Client import java.io.*; import java.net.*; public class TCPClient { public static void main (String[] args) { // establish connection Socket MySocket = new Socket( site0.site.uottawa.ca", 71); // get output and input streams OutputStream os = MySocket.getOutputStream(); InputStream is = MySocket.getInputStream(); // use advanced streams for easier manipulation PrintWriter out = new PrintWriter(os, true); BufferedReader in = new BufferedReader(new InputStreamReader(is)); // send data out.println( Time ); // receive response String T = in.readline(); System.out.println( according to the server, the time is: " +T); out.close(); in.close(); MySocket.close(); Fall 2005 CEG4183/ELG

6 Example: Server import java.io.*; import java.net.*; public class TCPServer { public static void main (String[] args) { // Create a server socket ServerSocket server = new ServerSocket(71); while (true) { // wait for incoming connection requests Socket client = server.accept(); // get output and input streams and advanced streams OutputStram os = client.getoutputstream(); InputStream is = client.getinputstream(); PrintWriter out = new PrintWriter(os, true); BufferedReader in = new BufferedReader(new InputStreamReader(is)); // read token if (in.readline()== Time ) // send the time out.println(system.currenttimemillis( )); out.close(); in.close(); client.close(); Fall 2005 CEG4183/ELG

7 The Java Datagram Socket Constructors DatagramSocket() Creates an unconnected socket DatagramSocket(int port) Creates a socket and binds it to the specified port number on local host. DatagramPacket(byte[] buf, int length) Constructs a DatagramPacket for receiving packets of length length. DatagramPacket(byte[] buf, int length, InetAddress address, int port) Constructs a datagram packet for sending packets of length length to the specified port number on the specified host. Methods (DatagramSocket) receive(datagrampacket p) Receives a datagram packet from this socket. send(datagrampacket p) Sends a datagram packet from this socket. DatagramSocket API Fall 2005 CEG4183/ELG

8 Example: Client import java.io.*; import java.net.*; public class UDPClient { public static void main (String[] args) { DatagramSocket socket = new DatagramSocket(); // establish socket // prepare data to be sent String Message = Time ; byte[] Buffer_out = Message.getBytes(); DatagramPacket packet_out = new DatagramPacket(Buffer_out, Buffer_out.length, \ , 71); // send data socket.send(packet_out); // receive response byte[] Buffer_in = new byte[256]; DatagramPacket packet_in = new DatagramPacket(Buffer_in, Buffer_in.length); socket.receive(packet_in); String Response = new String(packet_in.getData()); System.out.println( according to the server, the time is: " +Response); socket.close(); Fall 2005 CEG4183/ELG

9 Example: Server import java.io.*; import java.net.*; public class UDPServer { public static void main (String[] args) { DatagramSocket socket = new DatagramSocket(71); // establish socket and listen byte[] Buffer_in = new byte[256]; DatagramPacket packet_in = new DatagramPacket(Buffer_in, Buffer_in.length); // wait for incoming data while (true) { socket.receive(packet_in); String IncomingData = new String(packet_in.getData()); if (IncomingData == Time ) { // prepare data to be sent String TheTime = Long.toString(System.currentTimeMillis( )); byte[] Buffer_out = TheTime.getBytes(); InetAddress Add = packet_in.getaddress(); int P = packet_in.getport(); DatagramPacket packet_out = new DatagramPacket(Buffer_out, \ Buffer_out.length, Add, P); socket.send(packet_out); // send data Fall 2005 CEG4183/ELG

10 Notes Socket is reliable and connection-oriented DatagramSocket is unreliable and connectionless. The program waits at the point where a packet/connection is expected to be received/made, and doesn t proceed until that s done. Socket: server.accept(); in.readline(); DatagramSocket: socket.receive(packet_in); In programming, this is known as a blocking call (not specific to sockets). Buffers: we can see that while a packet is being processed, others might arrive. Packets not currently being processed will wait in a buffer until their turn to be serviced. Same as the queue concept from queuing theory (we shall see this later). Fall 2005 CEG4183/ELG

11 Application Message Semantics This example was easy: send time, receive time. Others can be more complicated, imagine a media player with commands like pause, play, stop, load,. We need to distinguish between different messages. Solution: use tokens before each message to signify its meaning. Fall 2005 CEG4183/ELG

12 Whiteboard Example Fall 2005 CEG4183/ELG

13 Whiteboard Capabilities Several users collaborate in real-time. What is real-time? Access control allows only one user to perform an action at a time. Features: Open image, slide show, or video. Annotate image. Annotate slide show and video in pause mode. Send text messages. Fall 2005 CEG4183/ELG

14 Whiteboard Messaging Scheme Open image OPEN file URL Annotate DRAW Color X0 Y0 X1 Y1 Send text CHAT User message What is the main problem with such scheme? Fall 2005 CEG4183/ELG

15 Other Approaches Remote Procedure Call: instead of a messaging scheme, let s call the corresponding function of each client. RCP in C/C++, Remote Method Invocation (RMI) in Java. Also possible between different programming languages, if CORBA or SOAP is used (to be covered in future lectures). Fall 2005 CEG4183/ELG

16 Other Issues How can we have one server support multiple clients at the same time? Connectionless service Multithreading How to handle sender crashes gracefully? How to handle receiver crashes gracefully? How to send the same message to multiple receivers? Fall 2005 CEG4183/ELG

CSCD 330 Network Programming Spring 2018

CSCD 330 Network Programming Spring 2018 Lecture 7 Application Layer Socket Programming in Java Reading: Chapter 2, Java links Relevant Links page Some Material in these slides from J.F Kurose and K.W.