import java.io.*; import java.net.*;  public class UDPEchoServer {    private static final int ECHOMAX = 255; // Maximum size of echo datagram    public static void main(String[] args) throws IOException {      int servPort = 5500; // Server port      DatagramSocket socket = new DatagramSocket(servPort);     DatagramPacket packet = new DatagramPacket(new byte[ECHOMAX], ECHOMAX);      while (true) { // Run forever, receiving and echoing datagrams       socket.receive(packet); // Receive packet from client       System.out.println("Handling client at " + packet.getAddress().getHostAddress() + " on port " + packet.getPort());       socket.send(packet); // Send the same packet back to client       packet.setLength(ECHOMAX); // Reset length to avoid shrinking buffer     }     /* NOT REACHED */   } } 

import java.net.*; import java.io.*;  public class UDPEchoClientTimeout {    private static final int TIMEOUT = 3000; // Resend timeout (milliseconds)   private static final int MAXTRIES = 5; // Maximum retransmissions    public static void main(String[] args) throws IOException {     InetAddress serverAddress = InetAddress.getByName("127.0.0.1"); // Server address     int servPort = 5500; // Server port     // Convert the argument String to bytes using the default encoding     byte[] bytesToSend = "Hi, World".getBytes();      DatagramSocket socket = new DatagramSocket();     socket.setSoTimeout(TIMEOUT); // Maximum receive blocking time(milliseconds)     // Sending packet     DatagramPacket sendPacket = new DatagramPacket(bytesToSend, bytesToSend.length, serverAddress, servPort);      DatagramPacket receivePacket = // Receiving packet     new DatagramPacket(new byte[bytesToSend.length], bytesToSend.length);      int tries = 0; // Packets may be lost, so we have to keep trying     boolean receivedResponse = false;     do {       socket.send(sendPacket); // Send the echo string       try {         socket.receive(receivePacket); // Attempt echo reply reception          if (!receivePacket.getAddress().equals(serverAddress)) {// Check                                     // source           throw new IOException("Received packet from an unknown source");         }         receivedResponse = true;       } catch (InterruptedIOException e) { // We did not get anything         tries += 1;         System.out.println("Timed out, " + (MAXTRIES - tries) + " more tries...");       }     } while ((!receivedResponse) && (tries < MAXTRIES));      if (receivedResponse) {       System.out.println("Received: " + new String(receivePacket.getData()));     } else {       System.out.println("No response -- giving up.");     }     socket.close();   }}