186 lines
6.9 KiB
Plaintext
186 lines
6.9 KiB
Plaintext
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X.25 support within isdn4linux
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==============================
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This is experimental code. Use it completely at your own risk.
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As new versions appear, the stuff described here might suddenly change
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or become invalid without notice.
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Keep in mind:
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You are using an experimental kernel (2.1.x series) with an experimental
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X.25 protocol implementation and experimental X.25-on-top-of-isdn extensions.
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Thus, be prepared to face problems related therefrom.
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- If you connect to an X.25 neighbour not operated by yourself, ASK the
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other side first. Be prepared that bugs in the protocol implementation
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might result in problems.
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- This implementation has never wiped out my whole hard disk yet. But as
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this is experimental code, don't blame me if that happened to you
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Backing up important data will never harm.
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- Monitor your isdn connections while using this software. This should
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prevent you from undesired phone bills in case of driver problems.
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How to configure the kernel
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===========================
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The ITU-T (former CCITT) X.25 network protocol layer has been implemented
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in the Linux source tree since version 2.1.16. The isdn subsystem might be
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useful to run X.25 on top of ISDN. If you want to try it, select
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"CCITT X.25 Packet Layer"
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from the networking options as well as
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"ISDN Support" and "X.25 PLP on Top of ISDN"
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from the ISDN subsystem options when you configure your kernel for
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compilation. You currently also need to enable
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"Prompt for development and/or incomplete code/drivers" from the
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"Code maturity level options" menu. For the x25trace utility to work
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you also need to enable "Packet socket".
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For local testing it is also recommended to enable the isdnloop driver
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from the isdn subsystem's configuration menu.
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For testing, it is recommended that all isdn drivers and the X.25 PLP
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protocol are compiled as loadable modules. Like this, you can recover
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from certain errors by simply unloading and reloading the modules.
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What's it for? How to use it?
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=============================
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X.25 on top of isdn might be useful with two different scenarios:
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- You might want to access a public X.25 data network from your Linux box.
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You can use i4l if you were physically connected to the X.25 switch
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by an ISDN line (leased line as well as dial up connection should work,
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but connecting to x.25 network switches is currently untested. Testing
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needs to be done by somebody with access to such a switch.)
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- Or you might want to operate certain ISDN teleservices on your linux
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box. A lot of those teleservices run on top of the ISO-8208
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(DTE-DTE mode) network layer protocol. ISO-8208 is essentially the
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same as ITU-T X.25.
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Popular candidates of such teleservices are EUROfile transfer or any
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teleservice applying ITU-T recommendation T.90.
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To use the X.25 protocol on top of isdn, just create an isdn network
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interface as usual, configure your own and/or peer's ISDN numbers,
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and choose x25iface encapsulation by
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isdnctrl encap <iface-name> x25iface.
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Once encap is set like this, the device can be used by the X.25 packet layer.
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All the stuff needed for X.25 is implemented inside the isdn link
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level (mainly isdn_net.c and some new source files). Thus, it should
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work with every existing HL driver. I was able to successfully open X.25
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connections on top of the isdnloop driver and the hisax driver.
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"x25iface"-encapsulation bypasses demand dialing. Dialing will be
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initiated when the upper (X.25 packet) layer requests the lapb datalink to
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be established. But hangup timeout is still active. The connection
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will not automatically be re-established by the isdn_net module
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itself when new data arrives after the hangup timeout. But
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the X.25 network code will re-establish the datalink connection
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(resulting in re-dialing and an X.25 protocol reset) when new data is
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to be transmitted. (This currently does not work properly with the
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isdnloop driver, see "known problems" below). It is recommended to
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use sufficiently large hangup-timeouts as most application probably
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can't deal with such N-reset events.
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In order to set up a conforming protocol stack you also need to
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specify the proper l2_prot parameter:
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To operate in ISO-8208 X.25 DTE-DTE mode, use
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isdnctrl l2_prot <iface-name> x75i
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To access an X.25 network switch via isdn (your linux box is the DTE), use
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isdnctrl l2_prot <iface-name> x25dte
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To mimic an X.25 network switch (DCE side of the connection), use
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isdnctrl l2_prot <iface-name> x25dce
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However, x25dte or x25dce is currently not supported by any real HL
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level driver. The main difference between x75 and x25dte/dce is that
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x25d[tc]e uses fixed lap_b addresses. With x75i, the side which
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initiates the isdn connection uses the DTE's lap_b address while the
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called side used the DCE's lap_b address. Thus, l2_prot x75i might
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probably work if you access a public X.25 network as long as the
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corresponding isdn connection is set up by you. However, I've never
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tested this.
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How to set up a test installation?
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==================================
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To test X.25 on top of isdn, you need to get
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- a recent version of the "isdnctrl" program that supports setting the new
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X.25 specific parameters.
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- the x25-utils-2.1.x package from ftp.pspt.fi/pub/ham/linux/ax25
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or any mirror site (i.e. ftp://ftp.gwdg.de/pub/linux/misc/ax25/).
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- an application program that uses linux AF_X25 sockets (some are
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contained in the x25-util package).
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Before compiling the user level utilities make sure that the compiler/
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preprocessor will fetch the proper kernel header files of this kernel
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source tree. Either make /usr/include/linux a symbolic link pointing to
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this kernel's include/linux directory or set the appropriate compiler flags.
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When all drivers and interfaces are loaded and configured you need to
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ifconfig the network interfaces up and add x25-routes to them. Use
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the usual ifconfig tool.
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ifconfig <iface-name> up
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But a special x25route tool (distributed with the x25-util package)
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is needed to set up X.25 routes. I.e.
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x25route add 01 <iface-name>
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will cause all x.25 connections to the destination x.25-address
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"01" to be routed to your created isdn network interface.
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There are currently no real X.25 applications available. However, for
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tests, the x25-utils package contains a modified version of telnet
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and telnetd that uses X.25 sockets instead of tcp/ip sockets. You can
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use those for your first tests. Furthermore, you might check
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ftp://ftp.hamburg.pop.de/pub/LOCAL/linux/i4l-eft/ which contains some
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experimental implementation ("eftp4linux") of the EUROfile transfer
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protocol.
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The eftp4linux test releases also contains an "ix25test" script that
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simplifies the task of configuring a set of isdn network interfaces
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and x.25 routes for testing x.25 on top of isdn. On the same site,
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you might also find a list of known problems and maybe patches and
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hints to work around some of them.
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The x25-utility package also contains an x25trace tool that can be
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used to monitor X.25 packets received by the network interfaces.
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The /proc/net/x25* files also contain useful information.
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- Henner
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