SLIP(9) XR32 REFERENCE MANUAL 5/2/2013
NAME
SLIP -- Serial Line IP.
DESCRIPTION
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 |
'------'-------------'------'-------------'------'
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 such as Com0Com include
this functionality as standard.
Configuring a SLIP Link
~~~~~~~~~~~~~~~~~~~~~~~
SLIP can be used to link XR32 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=13 <-- COM port number
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
Unless overridden with a port IPADDRESS statement, the SLIP
link will use XR32'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.
A SLIP link thus created does not involve Windows' IP stack in
any way, therefore there is no restriction on the protocols
that can be carried within the IP datagrams. For example you
may create an AX25 link using AXIP, or tunnel traffic over the
link using IPEncap.
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.
SEE ALSO
IP(1) -- IP Routing / Configuration Commands.
IPROUTE.SYS(8) -- IP Configuration File.
KISS(9) -- KISS Protocol.
XLINK(1) -- Establish a Temporary SLIP / PPP Interlink
XROUTER.CFG(8) -- Main Configuration File.
SLIP(9) END OF DOCUMENT
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