X Window System Core Protocol - Overview

Overview

Communication between server and clients is done by exchanging packets over a channel. The connection is established by the client (how the client is started is not specified in the protocol). The client also sends the first packet, containing the byte order to be used and information about the version of the protocol and the kind of 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:

1. Request: The client requests information from the server or requests it to perform an action.
2. Reply: The server responds to a request. Not all requests generate replies.
3. Event: The server informs the client of an event, such as keyboard or mouse input, a window being moved, resized or exposed, etc.
4. 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.