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1da177e4 LT |
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 |