Thinking Thin

There are four common thin-client protocols:

A distinction should be made between these protocols and remote framebuffer protocols like VNC. VNC provides remote control of a desktop that is still local, while thin-client protocols provide remote desktops.

X is nothing if not modular. Modularity is a good thing, but seeing how all the pieces of X fit together can be daunting for a new user. Below is a summary of the main modules and their interactions that will make the rest of this article accessible to readers with no previous X networking experience.

All access to the physical display is through the X server. This is a source of confusion for new users, because the display is intuitively client-side. But, the display is the service to which it provides access, hence the name. The clients for an X server are X applications that use it to display their output. We will see relationships later in this article where the X server acts as a client to other services.

The display manager (DM) is the heart of the terminal server. X servers and DMs have a dual client-server/server-client relationship. An X server can, as a client, initiate a connection to a DM on UDP port 177. The DM will then connect to the X server as a client on TCP port 6000 to display a graphical login screen to the user. A client can have multiple displays (windows or virtual terminals), in which case, the second display would be on port 6001 and so on. The protocol for this communication is XDMCP. If the X server and the DM are on the same system, they communicate using a UNIX socket.

The X server and DM are about displays and pixels. Neither has any concept of a window or a widget. These are handled by the window manager (WM).

Although the window manager provides the fundamental functionality and the major aspects of the look and feel, that is not enough to consider it a fully usable system. The desktop environment (DE) completes the user interface with utilities, such as control panels and toolbars, and basic applications, such as calculators and text editors.

There is often one additional component used: an X font server. The name of this server is xfs. In relation to xfs, an X server is a client that connects to an xfs server on TCP port 7100. X servers also can be configured to retrieve fonts from a filesystem folder.

The main decisions to make when deploying thin clients and a terminal server are the DM and the DE. The X server is built in to the thin client, and the DE will have a default WM that there is usually no reason to change. There are two dominant DEs in use today: GNOME (GNU Network Object Model Environment) and KDE (K Desktop Environment). Both have extensive features, and they are about equal in market share. GNOME is written in C and uses the GTK+ libraries. KDE is written in C++ and uses the Qt libraries. Both GNOME and KDE have their own WMs, named Metacity and KWin, respectively. They each also provide their own DMs, GDM and KDM, one of which is normally used in place of the standard XDM provided with X.