• UDP Sockets:
  • Are very versatile
  • But there are times when applications programmers need to develop specializations
• Examples:
  • To handle the requirements of a firewall
  • To handle encryption

• Extension:
  • Override some methods in the existing class and inherit others
• Decorator Pattern:
  • The decorator implements the same interface as the decorated
  • The decorator delegates to methods in the decorated class
• Simple Delegation:
  • The decorator delegates some method calls but doesn't implement the same interface as the decorated

• The Type of Application:
  • "Similar" datagrams are transmitted over time
  • When a later datagram becomes available, earlier datagrams "expire"
• Requirements:
  • We are willing to "lose" some packets
  • We we would like to increase the reliability of UDP somewhat
• One Solution:
  • Re-send any datagrams that have not yet expired if they are "lost"

• Some Issues:
  • Java uses a DatagramSocket to both send and receive but our sender and receiver will need different capabilities
  • Many methods in the DatagramSocket class are inappropriate for this application
• Conclusion:
  • Use delegation

• Alternative T1:
  1. Start a Timer
  2. Call receive()
  3. Call send() in the Timer "callback"
• Evaluation:
  Since receive() blocks the method above will block

• Alternative T2:
  1. Call receive() in an ackThread
  2. Start a Timer
  3. Call send() in the Timer "callback"
• Evaluation:
  The method above will return before the actual sending occurs

• Alternative T3:
  1. Call receive() in an ackThread
  2. Call send()
  3. Start a Timer
  4. Call send() for re-sends in the Timer "callback"
• Evaluation:
  Difficult to coordinate the timeout and the Timer

• Alternative T4:
  1. Call setSoTimeout()
  2. Call receive() in an ackThread
  3. Call send()
  4. Call send() for re-sends in the catch for the SocketTimeoutException
• Evaluation:
  Looks good

• Options:
  • Use one thread for all datagrams
  • Use a different thread for each datagram
• Evaluation:
  • It's instructive to use a different thread for each datagram
  • It's instructive to use a thread pool (i.e., a ExecutorService )

javaexamples/esdsocket/ESDSendingSocket.java

import java.io.*;
import java.net.*;
import java.util.concurrent.*;

/**
 * A socket that is used to send an expiring sequence of datagrams.
 * That is, a socket that is used to send a sequence of datagrams,
 * each of which expires when the next becomes available.
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class ESDSendingSocket implements Runnable
{
    private  volatile boolean      isExecuting, keepWaitingForAcknowledgement;
    private  byte                  lastPacketNumber, packetNumber;
    private  DatagramPacket        lastPacket;
    private  DatagramSocket        ds;
    private  final ExecutorService threadPool;    
    private  Object                ackLock;    


    public  static final int  PAYLOAD_LENGTH          =  254;
    private static final int  DEFAULT_RECEIVE_TIMEOUT = 1000;


    /**
     * Default Constructor
     */
    public ESDSendingSocket() throws SocketException
    {
       ds = new DatagramSocket();
       ds.setSoTimeout(DEFAULT_RECEIVE_TIMEOUT);
        
       keepWaitingForAcknowledgement = false;

       lastPacketNumber = 1;
       packetNumber     = 1;

       ackLock = new Object();

       threadPool   = Executors.newCachedThreadPool();
    }

    /**
     * Close this socket
     */
    public void close()
    {
       // Stop the ACK thread
       keepWaitingForAcknowledgement = false;

       // Block until the ACK thread stops
       synchronized(ackLock)
       {
          // Shutdown the thread pool
          threadPool.shutdown();       
       }

       // Close the DatagramSocket
       ds.close();
    }

    /**
     * Wait for acknowledgements in another
     * thread of execution (required by Runnable)
     */
    public void run()
    {
       byte                ackNumber;
       byte[]              esdData;
       DatagramPacket      ack;

       // Setup
       ackNumber = 0;       
       esdData   = new byte[PAYLOAD_LENGTH+1];
       ack       = new DatagramPacket(esdData, esdData.length);

       synchronized(ackLock)
       {
          while (keepWaitingForAcknowledgement) 
          {
             try 
             {
                // Receive the ACK
                ds.receive(ack);

                // Get the packet number being ACKed
                esdData = ack.getData();
                ackNumber = (byte)((-1) * (int)(esdData[PAYLOAD_LENGTH]));

                // If it's the right ACK, stop waiting
                if (ackNumber == lastPacketNumber) 
                {
                   keepWaitingForAcknowledgement = false;
                }
             }
             catch (SocketTimeoutException ste)
             {
                try
                {
                   // Re-send the packet
                   ds.send(lastPacket);
                }
                catch (IOException ioe)
                {
                   // Try to receive() again
                }
             }
             catch (IOException ioe) 
             {
                // Try again
             }
          }
       }
       isExecuting = false;       
    }

    /**
     * Send a packet
     *
     * @param p    The packet to send
     */
    public void send(DatagramPacket p) throws IOException
    {
       byte[]           data, esdData;
       DatagramPacket   packet;


       // Construct a new array that is long enough
       // to handle the data and the packet number
       data = p.getData();
       esdData = new byte[PAYLOAD_LENGTH+1];

       // Copy the appropriate bytes into the new array
       System.arraycopy(data, p.getOffset(), 
                        esdData, 0, data.length);

       // Insert the packet number
       esdData[PAYLOAD_LENGTH] = packetNumber;


       // Construct the DatagramPacket
       packet = new DatagramPacket(esdData, esdData.length,
                                   p.getAddress(), p.getPort());

       // Stop the previous ACK thread (if there is one)
       if (isExecuting)
       {
          keepWaitingForAcknowledgement = false;

          // Block until the ACK thread stops
          synchronized(ackLock)
          {
             // Setup the next ACK thread
             keepWaitingForAcknowledgement = true;
             isExecuting = true;       
          }
       }

       // Create and start another ACK thread
       keepWaitingForAcknowledgement = true;
       isExecuting = true;       
       threadPool.execute(this);       

       // Send the packet for the first time
       lastPacket       = packet;
       lastPacketNumber = packetNumber;
       ds.send(packet);

       // Increment the packet number for the next send
       packetNumber++;
       if (packetNumber < 0) packetNumber = 1;
    }
}
        

• Alternative R1:
  1. Call receive()
  2. Call send() after receive() returns
• Evaluation:
  From the caller's perspective, there is a slight delay after receive() stops blocking

• Alternative R2:
  1. Call receive()
  2. Call send() in an ackThread after receive() returns
• Evaluation:
  Seems more complicated than is needed

javaexamples/esdsocket/ESDReceivingSocket.java

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

/**
 * A socket that is used to receive an expiring sequence of datagrams.
 * That is, a socket that is used to receive a sequence of
 * datagrams, each of which expires when the next becomes available.
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class ESDReceivingSocket
{
    private DatagramSocket         ds;
    private int                    PAYLOAD_LENGTH;


    /**
     * Explicit Value Constructor
     *
     * @param port   The port to use
     */
    public ESDReceivingSocket(int port) throws SocketException
    {
        ds = new DatagramSocket(port);
        PAYLOAD_LENGTH = ESDSendingSocket.PAYLOAD_LENGTH;
    }

    /**
     * Close this socket
     */
    public void close()
    {
       ds.close();
    }

    /**
     * Receive a packet
     *
     * @param p   The DatagramPacket to fill
     */
    public void receive(DatagramPacket p) throws IOException
    {
        byte[]              data, esdData;
        DatagramPacket      packet;


        esdData = new byte[PAYLOAD_LENGTH+1];
        packet = new DatagramPacket(esdData, esdData.length);
        ds.receive(packet);
        esdData = packet.getData();

        // Copy the appropriate bytes into the new array
        data = new byte[PAYLOAD_LENGTH];
        System.arraycopy(esdData, p.getOffset(), 
                         data, 0, PAYLOAD_LENGTH);

        // Fill the datagram packet to be returned
        p.setAddress(packet.getAddress());
        p.setData(data, packet.getOffset(), PAYLOAD_LENGTH);
        p.setPort(packet.getPort());

        // An acknowledgement is required so negate 
        // the packet number and send it back
        esdData[esdData.length-1]=(byte)
                                  ((-1)*(int)(esdData[PAYLOAD_LENGTH]));

        ds.send(packet);
    }
}