PingServer.java

import java.io.*;

import java.net.*;

import java.util.*;

/*

 * Server to process ping requests over UDP.

 */

public class PingServer

{

   private static final double LOSS_RATE = 0.3;

   private static final int AVERAGE_DELAY = 100;  // milliseconds

   public static void main(String[] args) throws Exception

   {

      // Get command line argument.

      if (args.length != 1) {

         System.out.println("Required arguments: port");

         return;

      }

      int port = Integer.parseInt(args[0]);

      // Create random number generator for use in simulating

      // packet loss and network delay.

 Random random = new Random();

      // Create a datagram socket for receiving and sending UDP packets

      // through the port specified on the command line.

      DatagramSocket socket = new DatagramSocket(port);

      // Processing loop.

      while (true) {

         // Create a datagram packet to hold incomming UDP packet.

         DatagramPacket request = new DatagramPacket(new byte[1024], 1024);

         // Block until the host receives a UDP packet.

         socket.receive(request);

        

         // Print the recieved data.

         printData(request);

         // Decide whether to reply, or simulate packet loss.

         if (random.nextDouble() < LOSS_RATE) {

            System.out.println("   Reply not sent.");

            continue;

         }

         // Simulate network delay.

         Thread.sleep((int) (random.nextDouble() * 2 * AVERAGE_DELAY));

         // Send reply.

         InetAddress clientHost = request.getAddress();

         int clientPort = request.getPort();

         byte[] buf = request.getData();

         DatagramPacket reply = new DatagramPacket(buf, buf.length, clientHost, clientPort);

         socket.send(reply);

         System.out.println("   Reply sent.");

      }

   }

   /*

    * Print ping data to the standard output stream.

    */

   private static void printData(DatagramPacket request) throws Exception

   {

      // Obtain references to the packet's array of bytes.

      byte[] buf = request.getData();

      // Wrap the bytes in a byte array input stream,

      // so that you can read the data as a stream of bytes.

      ByteArrayInputStream bais = new ByteArrayInputStream(buf);

      // Wrap the byte array output stream in an input stream reader,

      // so you can read the data as a stream of characters.

      InputStreamReader isr = new InputStreamReader(bais);

      // Wrap the input stream reader in a bufferred reader,

      // so you can read the character data a line at a time.

      // (A line is a sequence of chars terminated by any combination of \r and \n.)

      BufferedReader br = new BufferedReader(isr);

      // The message data is contained in a single line, so read this line.

      String line = br.readLine();

      // Print host address and data received from it.

      System.out.println(

         "Received from " +

         request.getAddress().getHostAddress() +

         ": " +

         new String(line) );

   }

}