Installing and Administering PPP

70 Chapter 4
Common pppd Options
SLIP Framing Option
SLIP Framing Option
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