readme.x25

《嵌入式系统设计与实例开发实验教材二源码》Linux内核移植与编译实验

  
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