Question

I’m trying to code a C# UDP server. It receives a specific ID from the client, and return the song associated with it. The client is a PHP webpage, and stocks the bytes received into a file. Right now I’m doing some tests, trying to simply start a fake lecture of the song (just a javascript alert) when the transfer is at 2048 bytes. But I have plenty of bugs… The PHP page seems to finish the transfer into the file BEFORE having received all the data… The server continue to send bytes but the file is complete, with the good weight and all…

I know I don’t have a very good english, so if you don’t undersood something, just ask !

Here is the C# code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;  
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Threading;
using System.IO;
using System.Net.Sockets;
using System.Net;
using System.Data.SQLite;

namespace cdCollector
{
public partial class Streaming : Form
{
    private static List<IPAddress> clients_ = new List<IPAddress>();

    public Streaming()
    {
        InitializeComponent();
        listen();
    }

    public class ThreadClient
    {
        private static UdpClient socket_;
        private static IPEndPoint ipepClient_;
        private static int noChanson_;
        private static SQLiteConnection connexion_;

        public void setSocket(ref UdpClient socket) { socket_ = socket; }
        public void setIpepClient(ref IPEndPoint ipepClient) { ipepClient_ = ipepClient; }
        public void setNoChanson(int noChanson) { noChanson_ = noChanson; }
        public void setConnexion(ref SQLiteConnection connexion) { connexion_ = connexion; }

        public static void send()
        {
            try
            {
                while (Thread.CurrentThread.IsAlive)
                {
                    Chanson uneChanson;
                    FileStream stream;
                    byte[] buffer = new byte[1024];
                    int read;

                    uneChanson = new Chanson(noChanson_);
                    uneChanson.load(ref connexion_);

                    stream = new FileStream("C:\\Users\\Julie\\Documents\\toune.flac", FileMode.Open, FileAccess.Read);

                    socket_.Send(Encoding.ASCII.GetBytes(stream.Length.ToString()), stream.Length.ToString().Length, ipepClient_);

                    while ((read = stream.Read(buffer, 0, buffer.Length)) > 0)
                        socket_.Send(buffer, buffer.Length, ipepClient_);

                    Console.WriteLine("finished");
                }
            }
            catch (ThreadAbortException tae)
            { }
            catch (Exception)
            {
                Thread.CurrentThread.Abort();
            }
        }
    }

    public static void listen()
    {
        byte[] data = new byte[1024];

        IPEndPoint ipepServer = new IPEndPoint(IPAddress.Any, 7575); // IP du serveur
        IPEndPoint ipepClient = new IPEndPoint(IPAddress.Any, 0); // IP du client;
        UdpClient socket = new UdpClient(ipepServer); // socket serveur
        int noChanson;
        SQLiteConnection connexion = new SQLiteConnection("Data Source=" + Application.StartupPath + "\\cdCollector.db");
        SQLiteCommand command = new SQLiteCommand(connexion);
        SQLiteDataReader dr;
        Thread thread;

        connexion.Open();

        while (true)
        {
            try
            {
                Console.WriteLine("Waiting for a client...");

                data = socket.Receive(ref ipepClient);

                Console.WriteLine("Message received from {0}:", ipepClient.ToString());
                Console.WriteLine(Encoding.ASCII.GetString(data, 0, data.Length));



                command.CommandText = "SELECT KeyLocale FROM AssocKeys WHERE NomTable = 'Chanson' AND KeyWeb = "
                                        + int.Parse(Encoding.ASCII.GetString(data, 0, data.Length));

                dr = command.ExecuteReader();


                if (dr.HasRows)
                {
                    dr.Read();

                    noChanson = dr.GetInt32(0);

                    dr.Close();

                    ThreadClient client = new ThreadClient();
                    client.setConnexion(ref connexion);
                    client.setIpepClient(ref ipepClient);
                    client.setNoChanson(noChanson);
                    client.setSocket(ref socket);

                    thread = new Thread(new ThreadStart(ThreadClient.send));
                    thread.Start();
                }
                else
                    socket.Send(Encoding.ASCII.GetBytes("Erreur: Chanson introuvable"), ("Erreur: Chanson introuvable").Length, ipepClient);



            }
            catch (SocketException se)
            {
                Console.WriteLine("Erreur Socket:" + se.Message);
            }
            catch (Exception ex)
            {
                Console.WriteLine("Erreur: " + ex.Message);
            }

        }

        connexion.Close();
    }

}   

}

And the PHP code:

<?php
 session_start();
$address="192.168.2.2";
$read = false;
$port = 7575;
$length = 0;
$started = false;

if (isset($port) and
($socket=socket_create(AF_INET, SOCK_DGRAM, SOL_UDP)) and
(socket_connect($socket, $address, $port)))
{
    $text =  "Connection successful on IP $address, port $port <br />";

    $from = '';
    $port = 0;
    $length = 0;

    socket_send( $socket, $_GET['no'], 1024, MSG_EOR );
    socket_recvfrom( $socket, $buf, 1024, 12, $from, $port);

    $lengthTotal = $buf;
    echo "Taille prévue du fichier: " . $lengthTotal . "<br />";

    if( file_exists( "temp" . $_SESSION['ID_Membre'] . ".flac" ) )
        unlink("temp" . $_SESSION['ID_Membre'] . ".flac");

    $file = fopen("temp" . $_SESSION['ID_Membre'] . ".flac", 'a');
    $buf = null;

    while( $length < $lengthTotal )
    {
        $length += socket_recvfrom( $socket, $buf, 1024, 12, $from, $port );
        if( $length > 2048 && !$started )
        {
            ?>
            <script type="text/javascript">
            <!--
                alert("Lecture...");
            //->
            </script>
            <?php
            $started = true;
        }

        fputs($file, $buf);

        flush();
    }

    echo "<br />" . $length . "<br />";
    fclose($file);
}
else
        $text="Unable to connect<pre>".socket_strerror(socket_last_error())."</pre>";

echo $text;
?>

Thanks a lot !

Answer 1

UDP is an inherently unreliable transport. You will need to implement acknowledgements, timeouts, retransmissions and sequence numbers on top of UDP in order to guarantee transmission of all of your data in the expected order, unless your client application can live with dropped packets. I would advise you to consider using TCP sockets instead if you need guaranteed transmission of data between server and client and don’t want to have to implement all of this stuff yourself (which might need to include client-side buffering to rearrange out-of-order datagrams). If you want reliability on top of UDP, I would advise you to read a good textbook on the subject (e.g. “Unix Network Programming” by W. Richard Stevens etc.).

Pointers on TCP:

You should take a look at System.Net.Sockets.TcpClient and System.Net.Sockets.TcpListener for the C# side of things and consult the PHP documentation for info on the PHP side of things.

Using TCP sockets isn’t really that much different except you’ll be using send and recv (or C#/PHP equivalents) instead of send_to and recv_from. Setting up the server side of things is a little more complicated since you need to bind and listen etc. but there are plenty of resources, e.g.:

http://www.switchonthecode.com/tutorials/csharp-tutorial-simple-threaded-tcp-server