SLIP - Serial Line IP

Protocol Details

SLIP is a very simple protocol which encapsulates Internet Protocol (IP) datagrams for transmission over serial (e.g. RS232) lines. It is defined in RFC 1055.

The SLIP protocol specifies the following special characters:

	Name  Hex   Dec  Purpose
	---------------------------
	FEND  0xC0  192  Frame End
	FESC  0xDB  219  Frame Escape

The FEND characters mark the start and end of the frame containing the encapsulated datagram as follows:

	.------.-------------.------.
	| FEND | IP Datagram | FEND |
	'------'-------------'------'

In order to ensure that the FEND character only occurs at the start and end of the frame, FENDs which occur within the unencapsulated datagram are "escaped" to the two byte sequence FESC 220. Likewise FESC is escaped to the sequence FESC 221.

It is permissible for two datagrams to share a FEND:

	.------.-------------.------.-------------.------.
	| FEND | IP Datagram | FEND | IP Datagram | FEND |
	'------'-------------'------'-------------'------'

SLIP was largely replaced by PPP long ago, but the beauty of it is its simplicity. It is so easy to configure, and only requires a pair of COM ports and a 3 core cable.

Temporary SLIP

A dial-in MODEM connection may be switched into SLIP mode for the remainder of the call using the "XLINK SLIP" command, thus emulating an old-fashioned dial-up ISP. This may possibly be of use for controlling remote sites that have telephone lines but no Internet connection. See the manual entry for PSTN for more details.

Configuring a SLIP Link

SLIP can be used to link XRPi's with each other and with other IP systems (e.g. NOS) via real or virtual COM ports. A typical configuration in XROUTER.CFG would be as follows:

	INTERFACE=13
		TYPE=ASYNC        <-- Serial RS232
		COM=/dev/ttyUSB13 <-- RS232 device	
		PROTOCOL=SLIP     <-- Use SLIP
		SPEED=38400       <-- Baud rate
		FLOW=0            <-- No flow control
		MTU=1500          <-- Allows largest IP
	ENDINTERFACE

	PORT=3
		ID=SLIP Link to BBS
		INTERFACENUM=13
	ENDPORT

Serial Line Parameters

Serial lines used for SLIP must run at 8 data bits. Flow control must be hardware or none, as XON/XOFF flow control would interfere with the protocol.

If flow control is used, the cable must contain at least 5 cores, namely TXD, RXD, RTS, CTS and GND. If flow control is not used, only TXD, RXD and GND are required.

In all cases, a NULL MODEM is required. In the case of "real" RS232 this could be an actual null modem device, or a cable that is wired such that the TXDs at each end go to the RXDs at the other end, and the RTSs at each end go to the CTSs at the other. "Virtual" COM port pairs include this functionality as standard.

IP Configuration

Unless overridden with a port IPADDRESS statement, the SLIP link uses XRPi's "core" IP address, i.e. the one specified by the global IPADDRESS. This is usually a 44-net address.

Remember to set up an IP ROUTE entry for the neighbour system via this PORT number, e.g. if the neighbour's IP address is 44.131.91.2, the following entry routes traffic to it via port 3 using datagram mode:

	IP ROUTE ADD 44.131.91.2  * 3  d

Note that "virtual circuit" (v) and "netrom" (n) routing modes can not be used here.

Note: A SLIP link thus created does not involve Linux's IP stack in any way, therefore there is no restriction on the protocols that can be carried within the IP datagrams.