The LBX Mini-HOWTO
v1.04, 11 December 1997
LBX (Low Bandwidth X) is an X server extension which performs
compression 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.
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.
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.
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.
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.
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.
| 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:
+-----+ +-------+ 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.
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
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
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
lbxproxy is simply a single executable. There are no
configuration files, resource files, etc. associated with it.
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
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.
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!)
- Start your X server.
- 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 information.
You should consult the
Remote X Apps Mini-HOWTO if you're not familiar with remote X access permission
lbxproxy and tell it to forward to the LOCAL X server,
$ lbxproxy -display LOCAL:0 :1 &
lbxproxy to use display
the REMOTE system; if that system has >1 display already you can use
:2 or whatever instead.
- Set your DISPLAY environment variable to point to the display
lbxproxy is providing, instead of the normal display:
Or, if you use csh or clones:
$ export DISPLAY
% setenv DISPLAY :1
- 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 this.
- 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
LOCAL:0 and have both running at the same time.
Here are some common problems:
lbxproxy exits with an "access denied" error.
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
xclock on REMOTE and have it display on the local
system without using
$ xclock -display LOCAL:0
If that doesn't work, it's
xhost or some other basic X
problem, not LBX.
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)
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
the X Consortium's FTP site.
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?)
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.
- Since it's a completely separate application that does not
require any X internals, it's much simpler to compile and
- 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
- 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.
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:
Ken Chase <firstname.lastname@example.org>
be used for compression. Although its main purpose is to provide
security, it also compresses the data it sends.
Thus, if you run X over a
ssh link you will automatically obtain
some amount of compression.
I don't know. Both LBX and
dxpc are certainly better at raw
ssh. Of course,
ssh provides the added
advantage of security. And of course, there's no reason you can't use
ssh and one of the other two, to get good compression and
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.