Unlike
PPP, SLIP cannot perform Internet Protocol Control Protocol (IPCP)
negotiations. Therefore a SLIP connection cannot be assigned an
address by a peer at connection time. Normally, PPP can use the
tilde (~) on the pppd command line
for this purpose. To obtain a similar "feature," a remote side
SLIP connection must provide the IP address during the connection
process and a new local address must be obtained using the Systems
file `\A` chat script feature.
The Login script can tell
the peer its address textually, just before invoking pppd,
if you include something like this in the script:
#!/bin/sh
localaddr='calculate'
peeraddr='calculate'
echo my address is $localaddr and your address is $peeraddr
exec pppd $localaddr:$peeraddr slip idle 120 ... |
The incoming peer is responsible for parsing the "my address
is" line, and doing something sensible with the addresses it finds
there.
SLIP Data Compression |
 |
If you
specify vjcomp and SLIP as pppd
options, pppd will always use
RFC 1144 'VJ' TCP header compression with the
default 16 slots. This is useful for talking to 'CSLIP',
a SLIP protocol that allows compression. By default, pppd
running with the SLIP option does not use VJ header compression,
although it will start sending VJ-compressed packets if it first
receives VJ-compressed messages from its peer. Header compression
can be completely disabled with the novjcomp option. SLIP's
default maximum receive unit (MRU) and maximum transmission unit
(MTU) values are 1006.
Using SLIP to Dial a Cisco Terminal Server |
|
You can use PPP in its SLIP framing mode to dial into a Cisco
systems terminal server. Since the SLIP protocol supports no option
negotiations, Cisco terminal servers print the incoming system's
IP address in text on the serial line, before beginning the SLIP
protocol. pppd parses when the
'\A' token is inserted in the 'expect'
phase of the Systems file chat
script. The daemon passes the assigned address to the UNIX end of
the point-to-point networking interface. See Systems.ex
for an example use of this facility.
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