PPP

Name

PPP -- Point to Point Protocol.

Description

Point to Point Protocol (PPP) is a protocol suite intended for use over simple links which transport packets between two peers, such as an RS232 link (or pair of virtual COM ports).

Unlike SLIP, it includes facilities for link control (e.g. management of a dial up link), user authentication, negotiation of IP addresses, and the multiplexing of several protocols across a common link.

PPP is designed to be self configuring. Most of the options have standard default values, and peers negotiate anything which differs from the default.

XRPi currently implements the most common PPP sub-protocols:

Link Control Protocol (LCP)
Password Authentication Protocol (PAP)
IP Control Protocol (IPCP)

Most sysops will have no need to remember these protocol names and acronyms. However, for those who wish to experiment, each protocol has its own set of commands.

PPP has largely replaced SLIP for dial-up Internet access, and the interconnection of peers via RS232.

If you want to know more about the internal workings of PPP, it is specified in RFC1661.

Configuring XRPi for PPP

There are three ways in which PPP may be used:

RS232 (and virtual COM port) links.
Dial-in.
Dial-out.

The first case requires an INTERFACE to be configured to use PPP, but the other two cases temporarily establish a PPP link on a MODEM interface. Each case is described in more detail below:

Using PPP on Wire / Virtual COM Links

In XROUTER.CFG, define an interface with TYPE=ASYNC and PROTOCOL=PPP. For example:

	INTERFACE=1
		TYPE=ASYNC
		COM=/dev/ttyUSB1
		SPEED=19200             ; Adjust as necessary
		PROTOCOL=PPP
		MTU=1600
	ENDINTERFACE

Then "bind" a port to that interface thus:

	PORT=1
		ID=PPP link to Win98    ; Text to identify port
		INTERFACENUM=1          ; Bind to PPP interface
		IPADDRESS=192.168.0.4   ; Optional (see below)
	ENDPORT

The use of IPADDRESS is optional. If the keyword is omitted, XRPi's "global" IP address will be used on that port. If the address is specified as 0.0.0.0, it signifies that a "dynamic" IP address should be obtained from the peer, otherwise the specified address will be used.

The port is now ready for PPP using the standard PPP defaults, but if it requires any further configuration you may include the relevant PPP configuration commands in the file BOOTCMDS.SYS, which is read by XRPi at boot-up, after it has finished reading XROUTER.CFG.

Using PPP on Dial-in links

The INTERFACE and PORT should be configured for PROTOCOL=MODEM as described in the manual section "PSTN Modem Support".

When a logged-in caller issues the command XLINK PPP, XRPi automatically re-configures the port to support PPP. It then looks for the file "PPPHOST.n", where n is the port number, and if found, the PPP configuration commands within the file are executed. The file may optionally contain NAT configuration commands if required. The PPPHOST file is optional. If not present, PPP will be initialised to the standard defaults.

When the hardware link disconnects (e.g. user says goodbye or hangs up, link times out, or sysop kills it), the port is automatically re-configured back to its original MODEM protocol.

Using PPP on Dial-out links

The INTERFACE and PORT should be configured for PROTOCOL=MODEM as described in the manual section "PSTN Modem Support". The local IP address follows the same rules as the wire-link case above, but may be overridden temporarily by IPCP commands.

The dialler script (see DUN section) must contain the command "MODE PPP" plus any required PPP configuration commands. Here is an extract from a typical connection script:

   ; Link will use PPP
   mode ppp
   ;
   ; Configure my authentication username and password
   ppp pap user gb7pzt bsfjflavs
   ;
   ; Set inactivity timeout to 2 minutes
   ppp idle 120
   ;
   ; Enable ppp state logging
   ppp log 1
   ;
   ; Set my (static) IP address for the duration of this call
   ppp ipcp local address 194.123.110.4
   ;
   ; Now dial the host
   send ATDT07979654234
   ;
   etc...

The PPP configuration will persist until the call terminates, then the port will revert back to MODEM protocol to await the next incoming or outgoing call.

PPP Configuration Commands

All PPP configuration commands start with "PPP", and are described in the sysop command reference section. When used from the command line, or with a BOOTCMDS.SYS file, the first argument must be a port number. However, PPP commands used within PPPHOST and dialler scripts *do not* include a port number, because XRPi knows which port is executing the script.

For example:

at the command line: PPP 3 IDLE 300
in a dialler script: PPP IDLE 300

IP Routing with PPP

When a PPP link opens, a route to the peer is automatically added to XRPi's IP routing table. If the peer indicates that it knows the addresses of a primary and secondary DNS, those are added to Xrouter's DNS list.

PPP Logging

PPP system activity can be recorded in file PPPLOG.TXT for diagnostic purposes. The amount of detail is controlled by the number specified in the "PPP <port> LOG n" command as follows:

	0       No logging
	1       PPP start / stop / timeout events
	2       As 1, plus layer up / down events
	3       As 2, plus layer start / stop events
	4       as 3, plus option accept / reject events
	5       as 4, plus hexdump of configuration packets

You are advised against using the higher levels other than on a temporary basis because they can create very large logfiles.

If logging is in use, you should regularly remove the logfiles to prevent them growing too large.

See also

BOOTCMDS.SYS(8) -- Bootup Configuration Commands File.
DIAL(2) -- Dial a PSTN connection.
DUN(2) -- Dial Up Networking Configuration Commands.
DUN(9) -- Dial-Up Networking.
PPP(2) -- PPP Configuration Commands.
PPPHOST.n(8) -- PPP Configuration File(s)
PSTN(9) -- PSTN Modem Support.
SCRIPTS(8) -- Dialer Scripts.
XLINK(1) -- Establish a Temporary SLIP / PPP Interlink.
XROUTER.CFG(8) -- Main Configuration File.