In computing, the X Window System (commonly: X11, or X) is a network-transparent windowing system for bitmap displays. This article details the protocols and technical structure of X11.
Client–server model and network transparency
X uses a client–server model. An X server program runs on a computer with a graphical display and communicates with various client programs. The X server acts as a go-between for the user and the client programs, accepting requests on TCP port 6000 for graphical output (windows) from the client programs and displaying them to the user (display), and receiving user input (keyboard, mouse) and transmitting it to the client programs.
In X, the server runs on the user's computer, while the clients may run on remote machines. This terminology reverses the common notion of client–server systems, where the client normally runs on the user's local computer and the server runs on the remote computer. The X Window terminology takes the perspective that the X Window program is at the centre of all activity, i.e. the X Window program accepts and responds to requests from applications, and from the user's mouse and keyboard input. Therefore, applications (on remote computers) are viewed as clients of the X Window server program.
The communication protocol between server and client runs network-transparently: the client and server may run on the same machine or on different ones, possibly with different architectures and operating systems. A client and server can communicate securely over the Internet by tunneling the connection over an encrypted connection.
Design principles
Bob Scheifler and Jim Gettys set out the early principles of X as follows (as listed in Scheifler/Gettys 1996): Do not add new functionality unless an implementor cannot complete a real application without it. It is as important to decide what a system is not as to decide what it is. Do not serve all the world's needs; rather, make the system extensible so that additional needs can be met in an upwardly compatible fashion. The only thing worse than generalizing from one example is generalizing from no examples at all. If a problem is not completely understood, it is probably best to provide no solution at all. If you can get 90 percent of the desired effect for 10 percent of the work, use the simpler solution. (See also Worse is better.) Isolate complexity as much as possible. Provide mechanism rather than policy. In particular, place user interface policy in the clients' hands.
The first principle was modified during the design of X11 to: Do not add new functionality unless you know of some real application that will require it.
X has largely kept to these principles since. The X.Org Foundation develops the reference implementation with a view to extension and improvement of the implementation, whilst keeping it almost entirely compatible with the original 1987 protocol.
Core protocol
Communication between server and clients is done by exchanging packets over a network channel. The client establishes the connection, sending the first packet. The server answers by sending back a packet stating the acceptance or refusal of the connection, or with a request for a further authentication. If the connection is accepted, the acceptance packet contains data for the client to use in the subsequent interaction with the server.
After connection is established, the client and the server exchange four different types of packets over the channel:
Request: the client requests information from the server or requests it to perform an action. Reply: the server responds to a request. Not all requests generate replies. Event: the server sends an event to the client, e.g., keyboard or mouse input, or a window being moved, resized or exposed. Error: the server sends an error packet if a request is invalid. Since requests are queued, error packets generated by a request may not be sent immediately.
The X server provides a set of basic services. The client programs realize more complex functionalities by interacting with the server.
Windows
What other graphical user interfaces usually call a window is a top-level window in the X Window System. The term window is also used for windows that lie within another window, that is, the subwindows of a parent window. Graphical elements such as buttons, menus, icons, etc. are all realized using windows.