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+  
+X.25 support within isdn4linux
+==============================
+
+This is alpha/beta test code. Use it completely at your own risk.
+As new versions appear, the stuff described here might suddenly change
+or become invalid without notice.
+
+Keep in mind:
+
+You are using several new parts of the 2.2.x kernel series which
+have not been tested in a large scale. Therefore, you might encounter
+more bugs as usual.
+
+- If you connect to an X.25 neighbour not operated by yourself, ASK the
+  other side first. Be prepared that bugs in the protocol implementation
+  might result in problems.
+
+- This implementation has never wiped out my whole hard disk yet. But as
+  this is experimental code, don't blame me if that happened to you.
+  Backing up important data will never harm.
+
+- Monitor your isdn connections while using this software. This should
+  prevent you from undesired phone bills in case of driver problems.
+  
+ 
+
+
+How to configure the kernel
+===========================
+ 
+The ITU-T (former CCITT) X.25 network protocol layer has been implemented
+in the Linux source tree since version 2.1.16. The isdn subsystem might be 
+useful to run X.25 on top of ISDN. If you want to try it, select
+
+   "CCITT X.25 Packet Layer"
+
+from the networking options as well as
+
+   "ISDN Support" and "X.25 PLP on Top of ISDN"
+
+from the ISDN subsystem options when you configure your kernel for
+compilation. You currently also need to enable
+"Prompt for development and/or incomplete code/drivers" from the
+"Code maturity level options" menu. For the x25trace utility to work
+you also need to enable "Packet socket".
+
+For local testing it is also recommended to enable the isdnloop driver
+from the isdn subsystem's configuration menu.
+
+For testing, it is recommended that all isdn drivers and the X.25 PLP
+protocol are compiled as loadable modules. Like this, you can recover
+from certain errors by simply unloading and reloading the modules.
+
+
+
+What's it for? How to use it?
+=============================
+
+X.25 on top of isdn might be useful with two different scenarios:
+
+- You might want to access a public X.25 data network from your Linux box.
+  You can use i4l if you were physically connected to the X.25 switch
+  by an ISDN B-channel (leased line as well as dial up connection should
+  work).
+
+  This corresponds to ITU-T recommendation X.31 Case A (circuit-mode
+  access to PSPDN [packet switched public data network]).
+
+  NOTE: X.31 also covers a Case B (access to PSPDN via virtual
+  circuit / packet mode service). The latter mode (which in theory
+  also allows using the D-channel) is not supported by isdn4linux.
+  It should however be possible to establish such packet mode connections
+  with certain active isdn cards provided that the firmware supports X.31
+  and the driver exports this functionality to the user. Currently, 
+  the AVM B1 driver is the only driver which does so. (It should be
+  possible to access D-channel X.31 with active AVM cards using the
+  CAPI interface of the AVM-B1 driver).
+
+- Or you might want to operate certain ISDN teleservices on your linux
+  box. A lot of those teleservices run on top of the ISO-8208
+  (DTE-DTE mode) network layer protocol. ISO-8208 is essentially the
+  same as ITU-T X.25.
+
+  Popular candidates of such teleservices are EUROfile transfer or any
+  teleservice applying ITU-T recommendation T.90.
+
+To use the X.25 protocol on top of isdn, just create an isdn network
+interface as usual, configure your own and/or peer's ISDN numbers,
+and choose x25iface encapsulation by
+
+   isdnctrl encap <iface-name> x25iface.
+
+Once encap is set like this, the device can be used by the X.25 packet layer.
+
+All the stuff needed for X.25 is implemented inside the isdn link
+level (mainly isdn_net.c and some new source files). Thus, it should
+work with every existing HL driver. I was able to successfully open X.25
+connections on top of the isdnloop driver and the hisax driver.
+"x25iface"-encapsulation bypasses demand dialing. Dialing will be
+initiated when the upper (X.25 packet) layer requests the lapb datalink to
+be established. But hangup timeout is still active. Whenever a hangup
+occurs, all existing X.25 connections on that link will be cleared
+It is recommended to use sufficiently large hangup-timeouts for the
+isdn interfaces.
+
+
+In order to set up a conforming protocol stack you also need to
+specify the proper l2_prot parameter:
+
+To operate in ISO-8208  X.25 DTE-DTE mode, use
+
+   isdnctrl l2_prot <iface-name> x75i
+
+To access an X.25 network switch via isdn (your linux box is the DTE), use
+
+   isdnctrl l2_prot <iface-name> x25dte
+
+To mimic an X.25 network switch (DCE side of the connection), use
+
+   isdnctrl l2_prot <iface-name> x25dce
+
+However, x25dte or x25dce is currently not supported by any real HL
+level driver. The main difference between x75i and x25dte/dce is that
+x25d[tc]e uses fixed lap_b addresses. With x75i, the side which
+initiates the isdn connection uses the DTE's lap_b address while the
+called side used the DCE's lap_b address. Thus, l2_prot x75i might
+probably work if you access a public X.25 network as long as the
+corresponding isdn connection is set up by you. At least one test
+was successful to connect via isdn4linux to an X.25 switch using this
+trick. At the switch side, a terminal adapter X.21 was used to connect
+it to the isdn.
+
+
+How to set up a test installation?
+==================================
+
+To test X.25 on top of isdn, you need to get
+
+- a recent version of the "isdnctrl" program that supports setting the new
+  X.25 specific parameters.
+
+- the x25-utils-2.X package from 
+  ftp://ftp.hes.iki.fi/pub/ham/linux/ax25/x25utils-*
+  (don't confuse the x25-utils with the ax25-utils)
+
+- an application program that uses linux PF_X25 sockets (some are
+  contained in the x25-util package).
+
+Before compiling the user level utilities make sure that the compiler/
+preprocessor will fetch the proper kernel header files of this kernel
+source tree. Either make /usr/include/linux a symbolic link pointing to 
+this kernel's include/linux directory or set the appropriate compiler flags.
+
+When all drivers and interfaces are loaded and configured you need to
+ifconfig the network interfaces up and add X.25-routes to them. Use
+the usual ifconfig tool.
+
+ifconfig <iface-name> up
+
+But a special x25route tool (distributed with the x25-util package)
+is needed to set up X.25 routes. I.e. 
+
+x25route add 01 <iface-name>
+
+will cause all x.25 connections to the destination X.25-address
+"01" to be routed to your created isdn network interface.
+
+There are currently no real X.25 applications available. However, for
+tests, the x25-utils package contains a modified version of telnet
+and telnetd that uses X.25 sockets instead of tcp/ip sockets. You can
+use those for your first tests. Furthermore, you might check
+ftp://ftp.hamburg.pop.de/pub/LOCAL/linux/i4l-eft/ which contains some
+alpha-test implementation ("eftp4linux") of the EUROfile transfer
+protocol.
+
+The scripts distributed with the eftp4linux test releases might also
+provide useful examples for setting up X.25 on top of isdn.
+
+The x25-utility package also contains an x25trace tool that can be
+used to monitor X.25 packets received by the network interfaces.
+The /proc/net/x25* files also contain useful information. 
+
+- Henner