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