1
2 X.25 support within isdn4linux
3 ==============================
4
5 This is alpha/beta test code. Use it completely at your own risk.
6 As new versions appear, the stuff described here might suddenly change
7 or become invalid without notice.
8
9 Keep in mind:
10
11 You are using several new parts of the 2.2.x kernel series which
12 have not been tested in a large scale. Therefore, you might encounter
13 more 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
29 How to configure the kernel
30 ===========================
31
32 The ITU-T (former CCITT) X.25 network protocol layer has been implemented
33 in the Linux source tree since version 2.1.16. The isdn subsystem might be
34 useful to run X.25 on top of ISDN. If you want to try it, select
35
36 "CCITT X.25 Packet Layer"
37
38 from the networking options as well as
39
40 "ISDN Support" and "X.25 PLP on Top of ISDN"
41
42 from the ISDN subsystem options when you configure your kernel for
43 compilation. 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
46 you also need to enable "Packet socket".
47
48 For local testing it is also recommended to enable the isdnloop driver
49 from the isdn subsystem's configuration menu.
50
51 For testing, it is recommended that all isdn drivers and the X.25 PLP
52 protocol are compiled as loadable modules. Like this, you can recover
53 from certain errors by simply unloading and reloading the modules.
54
55
56
57 What's it for? How to use it?
58 =============================
59
60 X.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
88 To use the X.25 protocol on top of isdn, just create an isdn network
89 interface as usual, configure your own and/or peer's ISDN numbers,
90 and choose x25iface encapsulation by
91
92 isdnctrl encap <iface-name> x25iface.
93
94 Once encap is set like this, the device can be used by the X.25 packet layer.
95
96 All the stuff needed for X.25 is implemented inside the isdn link
97 level (mainly isdn_net.c and some new source files). Thus, it should
98 work with every existing HL driver. I was able to successfully open X.25
99 connections on top of the isdnloop driver and the hisax driver.
100 "x25iface"-encapsulation bypasses demand dialing. Dialing will be
101 initiated when the upper (X.25 packet) layer requests the lapb datalink to
102 be established. But hangup timeout is still active. Whenever a hangup
103 occurs, all existing X.25 connections on that link will be cleared
104 It is recommended to use sufficiently large hangup-timeouts for the
105 isdn interfaces.
106
107
108 In order to set up a conforming protocol stack you also need to
109 specify the proper l2_prot parameter:
110
111 To operate in ISO-8208 X.25 DTE-DTE mode, use
112
113 isdnctrl l2_prot <iface-name> x75i
114
115 To access an X.25 network switch via isdn (your linux box is the DTE), use
116
117 isdnctrl l2_prot <iface-name> x25dte
118
119 To mimic an X.25 network switch (DCE side of the connection), use
120
121 isdnctrl l2_prot <iface-name> x25dce
122
123 However, x25dte or x25dce is currently not supported by any real HL
124 level driver. The main difference between x75i and x25dte/dce is that
125 x25d[tc]e uses fixed lap_b addresses. With x75i, the side which
126 initiates the isdn connection uses the DTE's lap_b address while the
127 called side used the DCE's lap_b address. Thus, l2_prot x75i might
128 probably work if you access a public X.25 network as long as the
129 corresponding isdn connection is set up by you. At least one test
130 was successful to connect via isdn4linux to an X.25 switch using this
131 trick. At the switch side, a terminal adapter X.21 was used to connect
132 it to the isdn.
133
134
135 How to set up a test installation?
136 ==================================
137
138 To 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
150 Before compiling the user level utilities make sure that the compiler/
151 preprocessor will fetch the proper kernel header files of this kernel
152 source tree. Either make /usr/include/linux a symbolic link pointing to
153 this kernel's include/linux directory or set the appropriate compiler flags.
154
155 When all drivers and interfaces are loaded and configured you need to
156 ifconfig the network interfaces up and add X.25-routes to them. Use
157 the usual ifconfig tool.
158
159 ifconfig <iface-name> up
160
161 But a special x25route tool (distributed with the x25-util package)
162 is needed to set up X.25 routes. I.e.
163
164 x25route add 01 <iface-name>
165
166 will cause all x.25 connections to the destination X.25-address
167 "01" to be routed to your created isdn network interface.
168
169 There are currently no real X.25 applications available. However, for
170 tests, the x25-utils package contains a modified version of telnet
171 and telnetd that uses X.25 sockets instead of tcp/ip sockets. You can
172 use those for your first tests. Furthermore, you might check
173 ftp://ftp.hamburg.pop.de/pub/LOCAL/linux/i4l-eft/ which contains some
174 alpha-test implementation ("eftp4linux") of the EUROfile transfer
175 protocol.
176
177 The scripts distributed with the eftp4linux test releases might also
178 provide useful examples for setting up X.25 on top of isdn.
179
180 The x25-utility package also contains an x25trace tool that can be
181 used to monitor X.25 packets received by the network interfaces.
182 The /proc/net/x25* files also contain useful information.
183
184 - Henner
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