Error : An existing connection was forcibly closed by the remote host in FTP In C#?

When I am trying to do multiple upload i am getting the above error In FTP i have two classes one is for FTPdatatransfer and other is FTPcommand And in my main mage i am doing something like this

FTPCommand ftpcommand = new FTPCommand(ServerAddress, UserName, Password);
ftpcommand.Login();
ftpcommand.sendCommand("CWD ", "/home/praveena/RMS");  
ftpcommand.sendCommand("PASV");              

ftpcommand.sendCommand("STOR ","FTP.pdf");                 
ftpcommand.sendCommand("STOR ","linux.pdf");
ftpcommand.sendCommand("QUIT");

One upload is working fine while other gives above error

FTPCommand.cs

public void sendCommand(String command, params string[] strfilename)
    {


        string ServerAddress = "172.24.18.240";
        string UserName = "praveena";
        string Password = "praveena";


        if (command == "STOR ") //Uploading to Server
        {

            FTPDataTransfer ftpdatatransfer = new FTPDataTransfer(ServerAddress, UserName, Password, 10000, iport);               
            foreach (string dir in strfilename)
            {
                command = "STOR " + strfilename[0];
            }

            Send(command);                
            ftpdatatransfer.Upload(strfilename[0]);                
            this.readResponse();

        }
        else if (command == "MKD ")
        {
            command = "MKD " + strfilename[0];
            Send(command);
            this.readResponse();
        }

        else if (command == "CWD ")
        {
            command = "CWD " + strfilename[0];
            Send(command);
            this.readResponse();
        }

        else if (command == "NLST ") //Listing Files from Server.
        {
            FTPDataTransfer ftpdatatransfer = new FTPDataTransfer(ServerAddress, UserName, Password, 10000, iport);
            foreach (string dir in strfilename)
            {
                command = "NLST " + "*";
            }
            Send(command);
            ftpdatatransfer.GetFilelist();
            this.readResponse();

        }
        else
        {

            Send(command);
            this.readResponse();
            if (command == "PASV")
            {
                iport = GetPort();
            }


        }


    }



    public void Send(string command)
    {
        Byte[] cmdBytes = Encoding.ASCII.GetBytes((command + "\r\n").ToCharArray());
        clientSocket.Send(cmdBytes, cmdBytes.Length, 0);
    }


    public int GetPort()
    {
        int index1 = result.IndexOf('(');
        int index2 = result.IndexOf(')');

        string ipData = this.result.Substring(index1 + 1, index2 - index1 - 1);

        int[] parts = new int[6];

        int len = ipData.Length;
        int partCount = 0;
        string buf = "";

        for (int i = 0; i < len && partCount <= 6; i++)
        {
            char ch = char.Parse(ipData.Substring(i, 1));

            if (char.IsDigit(ch))
                buf += ch;

            else if (ch != ',')
                throw new WinFTPClient.FTPCommand.FtpException("Malformed PASV result: " + result);

            if (ch == ',' || i + 1 == len)
            {
                try
                {
                    parts[partCount++] = int.Parse(buf);
                    buf = "";
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                catch (Exception ex)
                {
                    throw new WinFTPClient.FTPCommand.FtpException("Malformed PASV result (not supported?): " + this.result, ex);
                }
            }
        }


        string ipAddress = parts[0] + "." + parts[1] + "." + parts[2] + "." + parts[3];

        int port = (parts[4] << 8) + parts[5];          

        return port;
    }

FTPdatatransfer.cs

public void Upload(string strfilename)
    {


        Thread thread = new Thread(new ThreadStart(() => UploadFile(strfilename)));
        thread.Start();




    }

    public void UploadFile(string strfilename)
    {


        Socket cSocket = createDataSocket();

         //open stream to read file

        FileStream input = new FileStream(strfilename, FileMode.Open);
        while ((bytes = input.Read(buffer, 0, buffer.Length)) > 0)
        {
            cSocket.Send(buffer, bytes, 0);
        }

        input.Close();
        if (cSocket.Connected)
        {
            cSocket.Close();
        }

       // this.readResponse();

        MessageBox.Show("File Uploaded successfully");

    }

 public Socket createDataSocket()
    {


        IPAddress ipAddress = IPAddress.Parse("172.24.18.240");  
        Socket socket = null;
        IPEndPoint ep = null;


        try
        {
            socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            ep = new IPEndPoint(Dns.GetHostEntry(ipAddress).AddressList[0], port);
            socket.Connect(ep);
        }
        catch (Exception ex)
        {
            // doubtfull....
            if (socket != null && socket.Connected) socket.Close();

            throw new WinFTPClient.FTPCommand.FtpException("Can't connect to remote server", ex);
        }

        return socket;
    }

Most likely the FTP server is limiting the number of simultaneous uploads. There isn't a way to force this from the client side.