Programming Assignment: UDP Ping Lab

You are given the complete code for the Ping server in Java below. Your job is to write the Ping client in C.

Server Code

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 buffered 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) );
   }
}

Packet Loss

UDP provides applications with an unreliable transport service, because messages may get lost in the network due to router queue overflows or other reasons. In contrast, TCP provides applications with a reliable transport service and takes care of any lost packets by retransmitting them until they are successfully received. Applications using UDP for communication must therefore implement any reliability they need separately in the application level (each application can implement a different policy, according to its specific needs).

Because packet loss is rare or even non-existent in typical campus networks, the server in this lab injects artificial loss to simulate the effects of network packet loss. The server has a parameter LOSS_RATE that determines which percentage of packets should be lost.

The server also has another parameter AVERAGE_DELAY that is used to simulate transmission delay from sending a packet across the Internet. You should set AVERAGE_DELAY to a positive value when testing your client and server on the same machine, or when machines are close by on the network. You can set AVERAGE_DELAY to 0 to find out the true round trip times of your packets.

Compiling and Running Server

To compile the server, do the following:

 javac PingServer.java 

 java PingServer port 

 java -classpath . PingServer port 

You should write the client so that it sends 10 ping requests to the server, separated by approximately one second. Each message contains a payload of data that includes the keyword PING, a sequence number, and a timestamp. After sending each packet, the client waits up to one second to receive a reply. If one seconds goes by without a reply from the server, then the client assumes that its packet or the server's reply packet has been lost in the network.

You should write the client so that it starts with the following command:

 PingClient host port 

Message Format

The ping messages in this lab are formatted in a simple way. Each message contains a sequence of characters terminated by a carriage return character (r) and a line feed character (n). The message contains the following string:

 PING sequence_number time CRLF 

Client Output

Your client should produce output modeled after the actual ping command. To see the ping command output, type ping . For example, ping www.google.com, or ping bart. Similar to the output you see from this command, your output should be in the following format:

 
PING received from machine_name: seq#=X time=Y ms

 
--- ping statistics ---
X packets transmitted, Y received, Z% packet loss
rtt min/avg/max = MIN AVG MAX ms

Turn-in