// If i = 5, we are not connected. If it is a SYN then assign
   // a temporary conection  to it for processing
   if (i == 5)
   {
      if (tcp->flags & FLG_SYN)
      {
         // Find first unused connection (one with IP = 0) 
         for (j=0; j < 5; j++)
         {
            if (conxn[j].ipaddr == 0)
            {
               nr = j;
               // Initialize new connection
               conxn[nr].state = STATE_LISTEN;
               break;
            }
         }
      
         // If all connections are used then drop msg
         if (j == 5) return;
         
         if (debug)
         {
            serial_send("TCP: New connection ");
            memset(text, 0, 10);
            itoa((UINT)nr, text, 10);
            serial_send(text);
			   serial_send("\r");
         }
      }
   }


   // By now we should have a connection number in range of 0-4
   // Do a check to avoid any chance of exceeding size of struct
   if (nr > 4)
   {
      if (debug) serial_send("TCP: Error in assigning conxn number\r");
      return;
   }

   // Eventually put in protection against wrapping sequence
   // numbers, for now make the client start over if his
   // sequence number is close to wrapping
   if (tcp->sequence > 0xFFFFFF00L) 
   {
      if (debug) serial_send("TCP: Rcvd a high sequence number\r");
		conxn[nr].ipaddr = 0;			
		tcp_send(FLG_RST, 20, NO_CONNECTION);
		return;		
   }
           
   // Handle messages whose action is mostly independent of state
   // such as RST, SYN, and segment with no ACK.  That way the
	// state machine below does not need to worry about it.
   if (tcp->flags & FLG_RST)
   {
      // An RST does not depend on state at all.  And it does
      // not count as data so do not send an ACK here.  Close
      // connection
 		if (debug) serial_send("TCP: Rcvd a reset\r");
      conxn[nr].ipaddr = 0;
      return;
   }
	
	else if (tcp->flags & FLG_SYN)
	{
	   // A SYN segment only makes sense if connection is in LISTEN 
	   if ((conxn[nr].state != STATE_LISTEN) &&
          (conxn[nr].state != STATE_CLOSED))
		{
			if (debug) serial_send("TCP: Error, rcvd bogus SYN\r");
			conxn[nr].ipaddr = 0;			
		  	tcp_send(FLG_RST, 20, NO_CONNECTION);
		  	return;		
		}
	}
	
	else if ((tcp->flags & FLG_ACK) == 0)
	{
		// Incoming segments except SYN or RST must have ACK bit set
	 	// See TCP/IP Illustrated, Vol 2, Page 965
      // Drop segment but do not send a reset
		if (debug) serial_send("TCP: Error, rcvd segment has no ACK\r");
		return;
	}
	   
   // Compute length of header including options, and from that
   // compute length of actual data
   header_len =  (tcp->flags & 0xF000) >> 10;
   data_len = len - header_len;


         
   // Handle TCP state machine for this connection
   switch (conxn[nr].state)
   {
      case STATE_CLOSED:
      case STATE_LISTEN:
            
      // If incoming segment contains SYN and no ACK, then handle 
      if ((tcp->flags & FLG_SYN) && ((tcp->flags & FLG_ACK) == 0))
      {
         // Capture his starting sequence number and generate
         // my starting sequence number
         // Fill in connection information
         conxn[nr].ipaddr = ip->source_ipaddr;
         conxn[nr].port = tcp->source_port;
         conxn[nr].state = STATE_LISTEN;
         conxn[nr].his_sequence = 1 + tcp->sequence;
         conxn[nr].his_ack = tcp->ack_number;
         
         // Use system clock for initial sequence number
         EA = 0;
         conxn[nr].my_sequence = initial_sequence_nr;
         initial_sequence_nr += 64000L;
         EA = 1;                  
                  
         // Send header options with the next message
         // Since timestamps are optional and we do not use
         // them, do not have to send them 
         // After sending the SYN ACK the client browser will
         // blast me with 2 messages, an ACK, and a HTTP GET
         tcp_send(FLG_SYN | FLG_ACK, 28, nr);
         
         // My SYN flag increments my sequence number
         // My sequence number is always updated to point to 
         // the next byte to be sent.  So the incoming ack
         // number should equal my sequence number  
         conxn[nr].my_sequence++;
      
         conxn[nr].state = STATE_SYN_RCVD;
         if (debug) serial_send("TCP: Entered SYN RCVD state\r");
      }
		else 
      {
         // Sender is out of sync so send reset
         conxn[nr].ipaddr = 0;
         tcp_send(FLG_RST, 20, NO_CONNECTION);   
      } 
		break;


      case STATE_SYN_RCVD:
      // He may already be sending me data - should process it
		conxn[nr].his_sequence += data_len;
      conxn[nr].his_ack = tcp->ack_number;
            
      if (tcp->flags & FLG_FIN)
		{
		   // His FIN counts as a byte of data
         conxn[nr].his_sequence++;
         tcp_send(FLG_ACK, 20, nr);
         conxn[nr].state = STATE_CLOSE_WAIT;
         if (debug) serial_send("TCP: Entered CLOSE_WAIT state\r");
               
        	// At this point we would normally wait for the	application
         // to close.  For now, send FIN right away.
         tcp_send(FLG_FIN | FLG_ACK, 20, nr);
         conxn[nr].my_sequence++;   // For my FIN
         conxn[nr].state = STATE_LAST_ACK;
         if (debug) serial_send("TCP: Entered LAST ACK state\r");
      }

		// Make sure he is ACKing my SYN
		else if (tcp->ack_number == conxn[nr].my_sequence)
      {
         conxn[nr].state = STATE_ESTABLISHED;
         if (debug) serial_send("TCP: Entered ESTABLISHED state\r");
         // If sender sent data ignore it and he will resend
         // Do not send response because we received no
         // data... wait for client to send something to me 
      }
      break;


      case STATE_ESTABLISHED:
      conxn[nr].his_ack = tcp->ack_number;
           
      if (tcp->flags & FLG_FIN)
		{
		   // His FIN counts as a byte of data
         conxn[nr].his_sequence++;
         tcp_send(FLG_ACK, 20, nr);
         conxn[nr].state = STATE_CLOSE_WAIT;
         if (debug) serial_send("TCP: Entered CLOSE_WAIT state\r");
               
        	// At this point we would normally wait for the	application
         // to close.  For now, send FIN immediately.
         tcp_send(FLG_FIN | FLG_ACK, 20, nr);
         conxn[nr].my_sequence++;   // For my FIN
         conxn[nr].state = STATE_LAST_ACK;
         if (debug) serial_send("TCP: Entered LAST ACK state\r");
      }
		else if (data_len != 0)
      {
			// Received normal TCP segment from sender with data
      	// Send an ACK immediately and pass the data on to
			// the application
			conxn[nr].his_sequence += data_len;
         tcp_send(FLG_ACK, 20, nr); 		// Send ACK
      	  
      	      	      	 									
			// Send pointer to start of TCP payload
			// http_server increments my sequence number when 
         // sending so don't worry about it here
         result = http_server(inbuf, header_len, nr, 0);
								
			// Start timer to close conxn if no activity
			conxn[nr].inactivity = INACTIVITY_TIME;
		}
	   break;


      case STATE_CLOSE_WAIT:
      // With this code, should not get here
      if (debug) serial_send("TCP: Oops! Rcvd unexpected message\r");
      
      break;

      
      case STATE_LAST_ACK:
      conxn[nr].his_ack = tcp->ack_number;
            
      // If he ACK's my FIN then close
      if (tcp->ack_number == conxn[nr].my_sequence)
      {
         conxn[nr].state = STATE_CLOSED;
         conxn[nr].ipaddr = 0;  // Free up struct area
         just_closed = TRUE;
      }
      break;

      
      case STATE_FIN_WAIT_1:
      // He may still be sending me data - should process it
		conxn[nr].his_sequence += data_len;
      conxn[nr].his_ack = tcp->ack_number;
                  
      if (tcp->flags & FLG_FIN)
      {
         // His FIN counts as a byte of data
         conxn[nr].his_sequence++;
         tcp_send(FLG_ACK, 20, nr);
         
         // If he has ACK'd my FIN then we can close connection
         if (tcp->ack_number == conxn[nr].my_sequence)
			{
         	conxn[nr].state = STATE_TIME_WAIT;
         	if (debug) serial_send("TCP: Entered TIME_WAIT state\r");
               
         	conxn[nr].state = STATE_CLOSED;
         	conxn[nr].ipaddr = 0;  // Free up connection
         	just_closed = TRUE;
      	}
			else
			{
				// He has not ACK'd my FIN.  This happens when there is a simultaneous
				// close - I got his FIN but he has not yet ACK'd my FIN
				conxn[nr].state = STATE_CLOSING;
				if (debug) serial_send("TCP: Entered CLOSING state\r");
			}
		}
      else if (tcp->ack_number == conxn[nr].my_sequence)
      {
         // He has ACK'd my FIN but has not sent a FIN yet himself
         conxn[nr].state = STATE_FIN_WAIT_2;
         if (debug) serial_send("TCP: Entered FIN_WAIT_2 state\r");
      }
      break;

      
      case STATE_FIN_WAIT_2:
      // He may still be sending me data - should process it
		conxn[nr].his_sequence += data_len;
      conxn[nr].his_ack = tcp->ack_number;
      
      if (tcp->flags & FLG_FIN)
      {
         conxn[nr].his_sequence++; // For his FIN flag
         tcp_send(FLG_ACK, 20, nr);
         conxn[nr].state = STATE_TIME_WAIT;
         if (debug) serial_send("TCP: Entered TIME_WAIT state\r");
         conxn[nr].state = STATE_CLOSED;
         conxn[nr].ipaddr = 0;  // Free up struct area
         just_closed = TRUE;
      }
      break;
            
            
      case STATE_TIME_WAIT:
      // With this code, should not get here
      if (debug) serial_send("TCP: Oops! In TIME_WAIT state\r");
      break;

      
      case STATE_CLOSING:
      // Simultaneous close has happened. I have received his FIN
      // but he has not yet ACK'd my FIN.  Waiting for ACK.
		// Will not receive data in this state
		conxn[nr].his_ack = tcp->ack_number;
      		
		if (tcp->ack_number == conxn[nr].my_sequence)
      {
		   conxn[nr].state = STATE_TIME_WAIT;
         if (debug) serial_send("TCP: Entered TIME_WAIT state\r");
         
         // Do not send any response to his ACK
         conxn[nr].state = STATE_CLOSED;
         conxn[nr].ipaddr = 0;  // Free up struct area
         just_closed = TRUE;
      }
      break;

      
      default:
      if (debug) serial_send("TCP: Error, no handler\r");
      break;
   }
   
   // This is for debug, to see when conxn closes
   if (just_closed)
   {
      just_closed = FALSE;
      if (debug)
      {
         serial_send("TCP: Closed connection ");
         memset(text, 0, 10);
         itoa((UINT)nr, text, 10);
		   serial_send(text);
		   serial_send("\r");
      }
   }
}