authentication the client expects the server to use. 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, four types of packets are exchanged between client and server 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 informs the client of an event, such as keyboard or mouse input, a window being moved, resized or exposed, etc. 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. Request and reply packets have varying length, while event and error packets have a fixed length of 32 bytes. Request packets are numbered sequentially by the server as soon as it receives them: the first request from a client is numbered 1, the second 2, etc. The least significant 16 bits of the sequential number of a request is included in the reply and error packets generated by the request, if any. They are also included in event packets to indicate the sequential number of the request that the server is currently processing or has just finished processing. Windows What is usually called a window in most graphical user interfaces is called a top-level window in the X Window System. The term window is also used to denote windows that lie within another window, that is, the subwindows of a parent window. Graphical elements such as buttons, menus, icons, etc. can be realized using subwindows. A client can request the creation of a window. More precisely, it can request the creation of a subwindow of an existing window. As a result, the windows created by clients are arranged in a tree (a hierarchy). The root of this tree is the root window, which is a special window created automatically by the server at startup. All other windows are directly or indirectly subwindows of the root window. The top-level windows are the direct subwindows of the root window. Visibly, the root window is as large as the virtual desktop, and lies behind all