1  
2X.25 support within isdn4linux
3==============================
4
5This is alpha/beta test code. Use it completely at your own risk.
6As new versions appear, the stuff described here might suddenly change
7or become invalid without notice.
8
9Keep in mind:
10
11You are using several new parts of the 2.2.x kernel series which
12have not been tested in a large scale. Therefore, you might encounter
13more bugs as usual.
14
15- If you connect to an X.25 neighbour not operated by yourself, ASK the
16  other side first. Be prepared that bugs in the protocol implementation
17  might result in problems.
18
19- This implementation has never wiped out my whole hard disk yet. But as
20  this is experimental code, don't blame me if that happened to you.
21  Backing up important data will never harm.
22
23- Monitor your isdn connections while using this software. This should
24  prevent you from undesired phone bills in case of driver problems.
25  
26 
27
28
29How to configure the kernel
30===========================
31 
32The ITU-T (former CCITT) X.25 network protocol layer has been implemented
33in the Linux source tree since version 2.1.16. The isdn subsystem might be 
34useful to run X.25 on top of ISDN. If you want to try it, select
35
36   "CCITT X.25 Packet Layer"
37
38from the networking options as well as
39
40   "ISDN Support" and "X.25 PLP on Top of ISDN"
41
42from the ISDN subsystem options when you configure your kernel for
43compilation. You currently also need to enable
44"Prompt for development and/or incomplete code/drivers" from the
45"Code maturity level options" menu. For the x25trace utility to work
46you also need to enable "Packet socket".
47
48For local testing it is also recommended to enable the isdnloop driver
49from the isdn subsystem's configuration menu.
50
51For testing, it is recommended that all isdn drivers and the X.25 PLP
52protocol are compiled as loadable modules. Like this, you can recover
53from certain errors by simply unloading and reloading the modules.
54
55
56
57What's it for? How to use it?
58=============================
59
60X.25 on top of isdn might be useful with two different scenarios:
61
62- You might want to access a public X.25 data network from your Linux box.
63  You can use i4l if you were physically connected to the X.25 switch
64  by an ISDN B-channel (leased line as well as dial up connection should
65  work).
66
67  This corresponds to ITU-T recommendation X.31 Case A (circuit-mode
68  access to PSPDN [packet switched public data network]).
69
70  NOTE: X.31 also covers a Case B (access to PSPDN via virtual
71  circuit / packet mode service). The latter mode (which in theory
72  also allows using the D-channel) is not supported by isdn4linux.
73  It should however be possible to establish such packet mode connections
74  with certain active isdn cards provided that the firmware supports X.31
75  and the driver exports this functionality to the user. Currently, 
76  the AVM B1 driver is the only driver which does so. (It should be
77  possible to access D-channel X.31 with active AVM cards using the
78  CAPI interface of the AVM-B1 driver).
79
80- Or you might want to operate certain ISDN teleservices on your linux
81  box. A lot of those teleservices run on top of the ISO-8208
82  (DTE-DTE mode) network layer protocol. ISO-8208 is essentially the
83  same as ITU-T X.25.
84
85  Popular candidates of such teleservices are EUROfile transfer or any
86  teleservice applying ITU-T recommendation T.90.
87
88To use the X.25 protocol on top of isdn, just create an isdn network
89interface as usual, configure your own and/or peer's ISDN numbers,
90and choose x25iface encapsulation by
91
92   isdnctrl encap <iface-name> x25iface.
93
94Once encap is set like this, the device can be used by the X.25 packet layer.
95
96All the stuff needed for X.25 is implemented inside the isdn link
97level (mainly isdn_net.c and some new source files). Thus, it should
98work with every existing HL driver. I was able to successfully open X.25
99connections on top of the isdnloop driver and the hisax driver.
100"x25iface"-encapsulation bypasses demand dialing. Dialing will be
101initiated when the upper (X.25 packet) layer requests the lapb datalink to
102be established. But hangup timeout is still active. Whenever a hangup
103occurs, all existing X.25 connections on that link will be cleared
104It is recommended to use sufficiently large hangup-timeouts for the
105isdn interfaces.
106
107
108In order to set up a conforming protocol stack you also need to
109specify the proper l2_prot parameter:
110
111To operate in ISO-8208  X.25 DTE-DTE mode, use
112
113   isdnctrl l2_prot <iface-name> x75i
114
115To access an X.25 network switch via isdn (your linux box is the DTE), use
116
117   isdnctrl l2_prot <iface-name> x25dte
118
119To mimic an X.25 network switch (DCE side of the connection), use
120
121   isdnctrl l2_prot <iface-name> x25dce
122
123However, x25dte or x25dce is currently not supported by any real HL
124level driver. The main difference between x75i and x25dte/dce is that
125x25d[tc]e uses fixed lap_b addresses. With x75i, the side which
126initiates the isdn connection uses the DTE's lap_b address while the
127called side used the DCE's lap_b address. Thus, l2_prot x75i might
128probably work if you access a public X.25 network as long as the
129corresponding isdn connection is set up by you. At least one test
130was successful to connect via isdn4linux to an X.25 switch using this
131trick. At the switch side, a terminal adapter X.21 was used to connect
132it to the isdn.
133
134
135How to set up a test installation?
136==================================
137
138To test X.25 on top of isdn, you need to get
139
140- a recent version of the "isdnctrl" program that supports setting the new
141  X.25 specific parameters.
142
143- the x25-utils-2.X package from 
144  ftp://ftp.hes.iki.fi/pub/ham/linux/ax25/x25utils-*
145  (don't confuse the x25-utils with the ax25-utils)
146
147- an application program that uses linux PF_X25 sockets (some are
148  contained in the x25-util package).
149
150Before compiling the user level utilities make sure that the compiler/
151preprocessor will fetch the proper kernel header files of this kernel
152source tree. Either make /usr/include/linux a symbolic link pointing to 
153this kernel's include/linux directory or set the appropriate compiler flags.
154
155When all drivers and interfaces are loaded and configured you need to
156ifconfig the network interfaces up and add X.25-routes to them. Use
157the usual ifconfig tool.
158
159ifconfig <iface-name> up
160
161But a special x25route tool (distributed with the x25-util package)
162is needed to set up X.25 routes. I.e. 
163
164x25route add 01 <iface-name>
165
166will cause all x.25 connections to the destination X.25-address
167"01" to be routed to your created isdn network interface.
168
169There are currently no real X.25 applications available. However, for
170tests, the x25-utils package contains a modified version of telnet
171and telnetd that uses X.25 sockets instead of tcp/ip sockets. You can
172use those for your first tests. Furthermore, you might check
173ftp://ftp.hamburg.pop.de/pub/LOCAL/linux/i4l-eft/ which contains some
174alpha-test implementation ("eftp4linux") of the EUROfile transfer
175protocol.
176
177The scripts distributed with the eftp4linux test releases might also
178provide useful examples for setting up X.25 on top of isdn.
179
180The x25-utility package also contains an x25trace tool that can be
181used to monitor X.25 packets received by the network interfaces.
182The /proc/net/x25* files also contain useful information. 
183
184- Henner
185