The LBX Mini-HOWTO

The LBX Mini-HOWTO
Paul D. Smith, [email protected]
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
wait.

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
course.

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!)

On LOCAL:

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
information.

You should consult the Remote X Apps Mini-HOWTO
<http://www.xs4all.nl/~zweije/xauth.html> if you're not familiar
with remote X access permission setup.

On REMOTE:

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

$ 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
<http://www.xs4all.nl/~zweije/xauth.html> for more information on
this.

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 <http://www.xs4all.nl/~zweije/xauth.html> 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
LBX.

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 <ftp://ftp.x.org/pub/R6.3/xc/doc/>.

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 <[email protected]>

Current Maintainer: Zachary Vonler <[email protected]>

dxpc <http://ccwf.cc.utexas.edu/~zvonler/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
separately.

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 ftp.x.org
<ftp://ftp.x.org/contrib/utilities/>.

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:

<http://ccwf.cc.utexas.edu/~zvonler/dxpc/>

11.2. Ssh (Secure Shell)

Ken Chase <[email protected]> notes that ssh
<http://www.cs.hut.fi/ssh/> can 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.

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.