FtpWebRequest只能上传12,484,608 字节的问题

用.net 2.0的FtpWebRequest类来上传文件，ftp server是serv-u 6.0版，传输小于12,484,608 字节的文件是成功的（上传正确），可是大于这个大小的文件就断了，错误提示：..."放弃了一个已建立的连接",意思就是ftp连接关闭了。不知道这个问题在哪里，怎么解决，请帮我。代码如下：private void Upload(string filename)
        {
            FileInfo fileInf = new FileInfo(filename);
            string uri = "ftp://" + ftpServerIP + "/" + fileInf.Name;
            FtpWebRequest reqFTP;            // Create FtpWebRequest object from the Uri provided
            reqFTP = (FtpWebRequest)FtpWebRequest.Create(new Uri("ftp://" + ftpServerIP + "/" + fileInf.Name));            // Provide the WebPermission Credintials
            reqFTP.Credentials = new NetworkCredential(ftpUserID, ftpPassword);            // By default KeepAlive is true, where the control connection is not closed
            // after a command is executed.
            reqFTP.KeepAlive = false;
         //   reqFTP.KeepAlive = true;            // Specify the command to be executed.
            reqFTP.Method = WebRequestMethods.Ftp.UploadFile;            // Specify the data transfer type.
            reqFTP.UseBinary = true;            // Notify the server about the size of the uploaded file
            reqFTP.ContentLength = fileInf.Length;            // The buffer size is set to 2kb
         //   int buffLength = 2048;
            int buffLength = 1024;
            byte[] buff = new byte[buffLength];
            int contentLen;            // Opens a file stream (System.IO.FileStream) to read the file to be uploaded
            FileStream fs = fileInf.OpenRead();
           
            try
            {
                // Stream to which the file to be upload is written
                Stream strm = reqFTP.GetRequestStream();
                
                // Read from the file stream 2kb at a time
                contentLen = fs.Read(buff, 0, buffLength);                // Till Stream content ends
                  while (contentLen != 0)
            //    int count = 0;
             //   while ((count = strm.Read(buff, 0, 1024)) > 0)
                {
                    // Write Content from the file stream to the FTP Upload Stream
                    strm.Write(buff, 0, contentLen);
             //       strm.Write(buff, 0, count);
                   contentLen = fs.Read(buff, 0, buffLength);
                }                // Close the file stream and the Request Stream
                strm.Close();
                fs.Close();
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message, "Upload Error");
            }
        }