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  The LBX Mini-HOWTO
  Paul D. Smith,
  v1.04, 11 December 1997

  LBX (Low Bandwidth X) is an X server extension which performs compres�
  sion on the X protocol.  It is meant to be used in conjunction with X
  applications and an X server which are separated by a slow network
  connection, to improve display and response time.

  Table of Contents

  1. Introduction

  2. What's The Status Of LBX?

  3. Who Can Benefit From LBX?

  4. Who Doesn't Need LBX?

  5. How Does LBX Work?

  6. What Do I Need To Use LBX?

  7. What Don't I Need To Use LBX?

  8. How Do I Start LBX?

  9. Problems

  10. Documentation

  11. Alternatives

     11.1 dxpc - The Differential X Protocol Compressor
        11.1.1 Advantages
        11.1.2 Disadvantages
        11.1.3 Where Can I Get dxpc?
     11.2 Ssh (Secure Shell)
     11.3 Which Is Better?


  1.  Introduction

  Low-Bandwidth X (LBX) attempts to recognize that in this day and age,
  not everyone will be a fast LAN hop or two away from the system that
  they are running their applications on.

  The X protocol can generate an extraordinary amount of traffic,
  especially for simple-seeming things such as creating new windows.  As
  anyone who has tried to use X over a dial-in modem at 28.8 or even
  higher can attest, creating new X windows can involve an excruciating

  LBX is fundamentally a compression and caching scheme designed to
  minimize the amount of X traffic generated between two systems.

  2.  What's The Status Of LBX?

  As of the X Consortium's release of X11R6.3 in December, 1996, LBX is
  a full extension to the X protocol.  For XFree86 folks, that's XFree86
  version 3.3.

  3.  Who Can Benefit From LBX?

  If you use a modem to dial into a service provider, then run X
  applications on remote machines with their DISPLAYs set to your local
  machine (or vice versa), LBX will speed up that connection.  Also if
  you set DISPLAYs from systems across WANs (other countries, for
  example) or other slow links, LBX can help.

  4.  Who Doesn't Need LBX?

  LBX is useless, of course, if you're only running applications
  locally, or if you're not running X at all.

  Also, if you're running on a fast LAN, LBX won't be much help.  Some
  people say "if LBX cuts down on network traffic, wouldn't it be good
  to use even on fast LANs?"  It might be, if your goal is to reduce
  network traffic.  But if your goal is to get better response time LBX
  probably isn't what you want.  Although it does introduce caching and
  compression, that comes at a cost on both ends (extra memory for
  caching, and extra CPU for decompression).  If your link is fairly
  speedy LBX will probably result in an overall slowdown.

  5.  How Does LBX Work?

  LBX works by introducing a proxy server at the client side, which
  performs caching and compression.  The X server knows that the client
  is using a proxy server, and decompresses accordingly.

  Here's a normal setup for remote X clients.  In our discussion, LOCAL
  is always the workstation sitting in front of you, whose monitor
  you're looking at, and REMOTE is the remote workstation, where the
  actual application is running.

            REMOTE                               LOCAL
        +-----+                                             +-----+
        | APP |-\          Network            +----------+  |     |\
        +-----+  \--------------------------->| X SERVER |=>|     ||
        +-----+  /       (X Protocol)         +----------+  +-----+\
        | APP |-/                                          /_____//

  When using LBX, a proxy server (lbxproxy) is introduced on the remote
  side, and the applications talk to that process instead of directly to
  the LOCAL server.  That process then performs the caching and
  compression of X requests and forwards them.  It looks like this:

       REMOTE                                         LOCAL
   +-----+  +-------+           Network            +----------+  |     |\
   | APP |->| PROXY |----------------------------->| X SERVER |=>|     ||
   +-----+  +-------+       (LBX/X Protocol)       +----------+  +-----+\
   +-----+   /                                                  /_____//
   | APP |--/

  Details on exactly what caching and compression LBX does is beyond the
  scope of this document.

  6.  What Do I Need To Use LBX?

  You need an X server on your LOCAL system which has the LBX extension
  compiled in.  Unless you explicitly told it not to when building it,
  X11R6.3 servers automatically enable LBX.  Also, all XFree86 3.3
  servers have LBX enabled by default.

  You can use the xdpyinfo command to see if your server has the LBX
  extension: run xdpyinfo and look at the list just under "number of
  extensions"; you should see "LBX" listed there.

  Next, you need to get an lbxproxy program compiled for the REMOTE
  system.  This is the tricky part.  If the remote system is not the
  same type as your local system, the lbxproxy on your local system will
  do you no good, of course.

  There is unfortunately no "broken out" distribution of lbxproxy, so
  you will have to either (a) get and build most, if not all, of X11R6.3
  for the remote system, or (b) find someplace to get a pre-compiled
  lbxproxy binary for your system.  The latter is much simpler of

  The lbxproxy is simply a single executable.  There are no
  configuration files, resource files, etc. associated with it.

  7.  What Don't I Need To Use LBX?

  The REMOTE system does not need a new X server (as always, the REMOTE
  system doesn't need any X server running).

  The application you want to run does not need to be linked with any
  special version of X, or any special libraries; I regularly use
  commercial X11R5 apps over LBX with no trouble.

  You do not need root or other privileged access on the REMOTE system;
  the lbxproxy process runs under your normal access permissions.
  Further, you can run it right from your home directory: it does not
  have to be installed anywhere.

  8.  How Do I Start LBX?

  OK, here it is... after all that it's actually quite simple.  Replace
  LOCAL and REMOTE below with the hostnames of your local workstation
  and remote system, respectively (don't get them mixed up!)


  1. Start your X server.

  2. Tell your X server that the remote system is allowed access.  Using
     the host-list method, type xhost +REMOTE.  If you use xauth you may
     need to do more than this; see the xauth(1) man page for more

     You should consult the Remote X Apps Mini-HOWTO
     <> if you're not familiar
     with remote X access permission setup.


  1. Start lbxproxy and tell it to forward to the LOCAL X server, like

         $ lbxproxy -display LOCAL:0 :1 &

  This tells lbxproxy to use display :1 on the REMOTE system; if that
  system has >1 display already you can use :2 or whatever instead.

  2. Set your DISPLAY environment variable to point to the display that
     lbxproxy is providing, instead of the normal display:

         $ DISPLAY=:1
         $ export DISPLAY

  Or, if you use csh or clones:

         % setenv DISPLAY :1

  3. If you're using xauth you will need to ensure that your cookie is
     available locally.  See the Remote X Apps Mini-HOWTO
     <> for more information on

  4. Start your X applications!

  That's it; all X apps that are started up pointing to :1 will use LBX.
  Of course, there's no reason you couldn't also start X apps pointing
  to LOCAL:0 and have both running at the same time.

  9.  Problems

  Here are some common problems:

     Q) lbxproxy exits with an "access denied" error.

     A) This means the LOCAL system isn't accepting connections from the
        REMOTE system due to permissions errors.  See the Remote X Apps
        Mini-HOWTO <> for details
        on these issues.

        As a simple trouble-shooting measure, try running a simple X app
        like xclock on REMOTE and have it display on the local system
        without using lbxproxy:

            $ xclock -display LOCAL:0

     If that doesn't work, it's xhost or some other basic X problem, not

  10.  Documentation

  The only documentation available in a standard X distribution may be
  the lbxproxy(1) man page.

  If you have access to the X source tree, then very interesting
  information on LBX is available there:

  �  xc/doc/specs/Xext/lbx.mif (Framemaker MIF)

  �  xc/doc/hardcopy/Xext/lbx.PS.Z (Compressed Postscript)

  �  xc/doc/hardcopy/Xext/lbxTOC.html (HTML)

  More detailed discussion of specific LBX algorithms is available here:

  �  xc/doc/specs/Xext/lbxalg.mif (Framemaker MIF)

  �  xc/doc/specs/Xext/lbxalg.PS.Z (Compressed Postscript)

  If you don't have access to the X11 source, you can obtain these files
  from the X Consortium's FTP site <>.

  11.  Alternatives

  If you don't like lbxproxy for some reason: you're not satisfied with
  the performance, it doesn't work for you, you don't want to hassle
  with creating an lbxproxy for the remote host, or you simply are
  interested in trying other options, there is at least one other
  package for X protocol compression (anyone have others?)

  11.1.  dxpc - The Differential X Protocol Compressor

  �  Original Author: Brian Pane <>

  �  Current Maintainer: Zachary Vonler <>

  dxpc <> works in essentially
  the same way as LBX.  However, to avoid having to implement an X
  extension and modify the X server code, dxpc uses two proxies: one
  that runs on the REMOTE host, like lbxproxy, and one that runs on the
  LOCAL host.

  The REMOTE host proxy communicates between the X clients and the LOCAL
  host proxy, and the LOCAL host proxy communicates between the X server
  and the REMOTE host proxy.

  So, to both the X clients and the X server, it looks like X protocol
  as usual.

  11.1.1.  Advantages

  �  Since it's a completely separate application that does not require
     any X internals, it's much simpler to compile and install.

  �  It's maintained separately, so you don't have to wait for the OSF
     to release new X versions for enhancements or fixes.

  �  It provides more and better compression information and statistics
     than lbxproxy.

  11.1.2.  Disadvantages

  �  It is not a standard part of X; you must obtain and build it

  �  It is slightly more complex to set up, since it requires a LOCAL-
     side proxy as well as the REMOTE proxy.

  11.1.3.  Where Can I Get dxpc?

  The source for dxpc is available at

  There is a WWW homepage for dxpc that gives a lot of good information,
  including pointers to the dxpc mailing list, access to the source
  code, and a number of pre-built binaries for various platforms:


  11.2.  Ssh (Secure Shell)

  Ken Chase <> notes that ssh
  <> can be used for compression.  Although its
  main purpose is to provide security, it also compresses the data it

  Thus, if you run X over a ssh link you will automatically obtain some
  amount of compression.

  11.3.  Which Is Better?

  I don't know.  Both LBX and dxpc are certainly better at raw
  compression than ssh.  Of course, ssh provides the added advantage of
  security.  And of course, there's no reason you can't use both ssh and
  one of the other two, to get good compression and security.

  It shouldn't be hard to run some benchmarking against these options
  and get both subjective and statistical measurings of performance.
  But I haven't done this, and I don't know of anyone who has.

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