Introduction to Sockets

• “A socket is one endpoint of a two-way communication link between two programs running on the network. A socket is bound to a port number so that the TCP layer can identify the application to which data is destined to be sent.”

Networking Review

• TCP/IP Protocol Suite• Application

programming interface to TCP/IP (API)

• BSD Unix• Java.net.Socket• Platform

Independence

Ports

• TCP and UDP use ports to map incoming data to a particular process running on the computer

• Ports are 16 bit numbers

• Need IP Address (32-bits) and port.

• 0-1023 Restricted out of 65,535.

Networking Classes in Java

• TCP: URL, URLConnection, Socket, ServerSocket

• UDP: DatagramPacket, DatagramSocket, MulticastSocket

• Socket and ServerSocket form two ends of communicaiton link.

import java.io.*;import java.net.*;

public class EchoClient { public static void main(String[] args) throws IOException {

Socket echoSocket = null; PrintWriter out = null; BufferedReader in = null;

try { echoSocket = new Socket("campione", 7); out = new PrintWriter(echoSocket.getOutputStream(), true); in = new BufferedReader(newInputStreamReader(echoSocket.getInputStream())); } catch (UnknownHostException e) { System.err.println("Don't know about host: taranis."); System.exit(1); } catch (IOException e) { System.err.println("Couldn't get I/O for the connection to:taranis."); System.exit(1); }

BufferedReader stdIn = new BufferedReader(newInputStreamReader(System.in));

String userInput;

while ((userInput = stdIn.readLine()) != null) { out.println(userInput); System.out.println("echo: " + in.readLine());}

out.close();in.close();stdIn.close();echoSocket.close();

}}

Procedure

• Open a socket

• Open an input and output stream to the socket

• Read from and write to the stream according to the server’s protocol

• close streams

• close socket