/*===================================================================
    *  class: Ping
    *===================================================================*/
   #region Ping
   public class Ping
   {
      /*_______________________________________________________________*/
      // methods: Construction
      #region 
      // Constructor
      public Ping()
      {
         // create command field
         pingCommand = new byte[8];
         pingCommand[0] = 8;        // Type
         pingCommand[1] = 0;        // Subtype
         pingCommand[2] = 0;        // Checksum
         pingCommand[3] = 0;  
         pingCommand[4] = 1;        // Identifier
         pingCommand[5] = 0;
         pingCommand[6] = 0;        // Sequence number
         pingCommand[7] = 0;
         // create result field
         pingResult = new byte[pingCommand.Length + 20]; // 20 byte = returned IP Header
      }
      #endregion

      /*_______________________________________________________________*/
      // methods: Open, Close, Send
      #region 
      // Open
      public void Open()
      {
         // create socket and connect
         socket = new Socket(AddressFamily.InterNetwork, SocketType.Raw, ProtocolType.Icmp);
         isOpen = true;
         
         // create read complete event
         readComplete = new ManualResetEvent(false);
      }
      // Close
      public void Close()
      {
         // clear open flag
         isOpen = false;
         // close socket
         socket.Close();
         // close event
         readComplete.Close();
      }
      // Send
      public TimeSpan Send(string address, TimeSpan timeout)
      {
         // empty result buffer
         while (socket.Available > 0) 
            socket.Receive(pingResult, Math.Min(socket.Available, pingResult.Length), SocketFlags.None);
         
         // reset event
         readComplete.Reset();
         // save start time
         DateTime timeSend = DateTime.Now;
         // complete ping command
         pingCommand[6] = lastSequenceNr++;
         SetChecksum(pingCommand);
         // send ping command, start receive command
         int iSend = socket.SendTo(pingCommand, new IPEndPoint(IPAddress.Parse(address), 0));
         socket.BeginReceive(pingResult, 0, pingResult.Length, SocketFlags.None, new AsyncCallback(CallBack), null);
         // wait until data received or timeout
         if (readComplete.WaitOne(timeout, false))
         {
            // check result
            if ((pingResult[20] == 0) &&
               (pingCommand[4] == pingResult[24]) && (pingCommand[5] == pingResult[25]) &&
               (pingCommand[6] == pingResult[26]) && (pingCommand[7] == pingResult[27]))
               // return time delay
               return DateTime.Now.Subtract(timeSend);
         }
         // return timeout
         return TimeSpan.MaxValue; 
      }
      #endregion
   
      /*_______________________________________________________________*/
      // methods: CallBack
      #region 
      protected void CallBack(IAsyncResult result)
      {
         if (isOpen)
         {
            socket.EndReceive(result);
            readComplete.Set();
         }
      }
      #endregion

      /*_______________________________________________________________*/
      // methods: SetChecksum
      #region 
      protected void SetChecksum(byte[] tel)
      {
         // reset old checksum
         tel[2] = 0;
         tel[3] = 0;
         // calculate new checksum
         uint cs = 0;
         for (int i = 0; i < pingCommand.Length; i = i+2)
            cs += BitConverter.ToUInt16(pingCommand, i);
         cs = ~((cs & 0xffffu) + (cs >> 16));
         // set new checksum
         tel[2] = (byte) cs;
         tel[3] = (byte) (cs >> 8);
      }
      #endregion

      /*...............................................................*/
      // fields
      // socket and event
      protected Socket socket;
      protected bool isOpen;
      protected ManualResetEvent readComplete;
      protected byte lastSequenceNr = 0;
      // ping command and result fields
      protected byte[] pingCommand;
      protected byte[] pingResult;
   }
   #endregion