stun - Serial Tunnel
====================

 Application to control serial ports on the jn5139 over a tcp/ip stream or udp
packet stream. After an initial configuration packet, data on the uart is sent
to the client and data sent from the client is sent to over the serial line.
This creates a serial over tcp/udp tunnel.

 Two operation modes are supported:
  - raw mode, data is simply forwarded as is.
  - frame mode, data from the serial port has to comply to the framing protocol
                described below and is sent to the tcp client with a inter-frame
                timestamp and sequence number.

TCP operation
-------------

 Configuration has to be done after connecting to the tcp server. Baudrate, data
format and  flow control make up the modeline which configures the serial port.
This modeline is sent as a ascii line.  For example if the jennic modules
address is 3ffe::15:86:00:00:0b:67:c3, and your network is properly configured
you can access the modules serial port by opening a tcp connection on port 4404.
We use telnet here as an example:

> telnet 3ffe::15:86:00:00:0b:67:c3 4404
  115200 8N1
  ...

 This opens up UART0 on the jennic module, selects the baudrate to be 115200
bauds in 8bit, no parity, 1 stop bit and no flowcontrol. After that all data
seen on the serial port will be sent to the pc through the telnet connection and
vice-versa.

 The modeline has to comply to this format:
     (UART) [FRAMING](BAUDRATE) (DATABITS)(PARITY)(STOPBITS) [FLOWCONTROL]
 where anything in [] is optional, and anything in () is mandatory:

 - UART, either UART0 or UART1, selects the serial port to use for that
         connection.
 - FRAMING, either R for raw mode or F for framing serial protocol, defaults to
            raw mode.
 - BAUDRATE, baudrate of the serial port, one of 9600, 19200, 38400, 57600,
             115200, 230400, 460800, 921600, 1843200
 - DATABITS, either 7 or 8
 - PARITY, N for no-parity, E for even parity, O for odd parity
 - STOPBITS, 1 for 1 stop bit, 2 for 2 stop bits (or 1.5 bits if 7 databits are
             selected)
 - FLOWCONTROL, RTSCTS if rts/cts flow control is to be enabled. DTR/DTS and
                XON/XOFF flowcontrol is not supported.

 After the modeline has been sent, the server relays data either in RAW mode,
where all data is sent as is vice versa between the client and the serial port.
Or in FRAMING mode, where data sent to the client as frames with according to
this struct:
    typedef struct st_data_t {
        uint8_t  len;        /* length of the complete frame   */
        uint16_t delta_t;    /* time difference to last packet */
        uint16_t seq;        /* sequence number of the frame   */
        uint8_t  data[];     /* raw data */
    } st_data_t;

 The framing protocol on the serial line has to adhere to the following struct:
    typedef struct ser_frame_t {
        uint8_t len;         /* length of the frame */
        uint8_t data[];      /* data buffer         */
    } ser_frame_t;
 So framing is implemented as length-prefixed frames with a hardware reset line
to get the uart slave into sync with the master. In framing mode of operation,
framing is implemented by syncing the slave with a hardware reset line and
length-prefixed messages. In high-speed operation flow control is mandatory.

 RTS/CTS flowcontrol works by asserting (pulling/checking high whenever no
buffer space is available). The RTS is pulled by the Jennic module when there is
no space left for receiving. Data is only sent by the jennic module when the CTS
line is high. No timeout is implemented, so the module can lock-up when the CTS
line is driven wrongly.


== TODO ==
 - implement using a different uart port without recompilation (currently only
   uart0 is usable)
 - implement uart flow control (see JN-AN-1069 802.15.4 Wireless UART with Flow
   Control source code, for automatic and manual flowcontrol)
 - implement other busses like i2c, spi, dio
 - generalize the command handler (i.e. similar to the telnet server)
 - rfc2217 compliance?!?