java中的可靠UDP

Question

I am working on my assignment to make UDP reliable using java. How can i add Timeout and re-transmission to handle data-grams that are discarded and add Sequence numbers so the client can verify that a reply is for the appropriate request ??

this is client code

import java.net.*;
import java.io.*;
public class EchoClient {
// UDP port to which service is bound
    public static final int SERVICE_PORT = 7;


// Max size of packet
public static final int BUFSIZE = 256;
public static void main(String args[]){
                 if (args.length != 1)
                 {
                         System.err.println ("Syntax - java EchoClient hostname");
                         return;
                  }
String hostname = args[0];
// Get an InetAddress for the specified hostname
    InetAddress addr = null;
        try
        {
            // Resolve the hostname to an InetAddr
            addr = InetAddress.getByName(hostname);
        }
        catch (UnknownHostException uhe)
        {
            System.err.println ("Unable to resolve host");
            return;
        }

        try
        {
            // Bind to any free port
            DatagramSocket socket = new DatagramSocket();

            // Set a timeout value of two seconds
            socket.setSoTimeout (2 * 1000);

            for (int i = 1 ; i <= 10; i++)
            {
                // Copy some data to our packet
                String message = "Packet number " + i ;
                char[] cArray = message.toCharArray();                  
                byte[] sendbuf = new byte[cArray.length];
                for (int offset = 0; offset < cArray.length ; offset++)
                {
                    sendbuf[offset] = (byte) cArray[offset];
                }

                // Create a packet to send to the UDP server
                DatagramPacket sendPacket = new DatagramPacket(sendbuf, cArray.length, addr, SERVICE_PORT);

                System.out.println ("Sending packet to " + hostname);

                // Send the packet
                socket.send (sendPacket);

                System.out.print ("Waiting for packet.... ");

                // Create a small packet for receiving UDP packets
                byte[] recbuf = new byte[BUFSIZE];
                DatagramPacket receivePacket = new DatagramPacket(recbuf, BUFSIZE);

                // Declare a timeout flag
                boolean timeout = false;

                // Catch any InterruptedIOException that is thrown
                // while waiting to receive a UDP packet
                try
                {
                    socket.receive (receivePacket);
                }
                catch (InterruptedIOException ioe)
                {
                    timeout = true;
                }

                if (!timeout)
                {
                    System.out.println ("packet received!");
                    System.out.println ("Details : " + receivePacket.getAddress() );

                    // Obtain a byte input stream to read the UDP packet
                    ByteArrayInputStream bin = new ByteArrayInputStream (
                      receivePacket.getData(), 0, receivePacket.getLength() );

                    // Connect a reader for easier access
                    BufferedReader reader = new BufferedReader ( 
                      new InputStreamReader ( bin ) );

                    // Loop indefinitely
                    for (;;)
                    {
                        String line = reader.readLine();

                        // Check for end of data
                        if (line == null)
                            break;
                        else
                            System.out.println (line);
                    }               
                }
                else
                {
                    System.out.println ("packet lost!");
                }

                // Sleep for a second, to allow user to see packet
                try
                {
                    Thread.sleep(1000);
                }catch (InterruptedException ie) {}
            }
        }
        catch (IOException ioe)
        {
            System.err.println ("Socket error " + ioe);
        }
    }
}

Answer 1

What you can do is adding import TCP headers like sequence number, windows into the UDP message body to make it more like TCP. Here is the a solution that might help you.