X display manager (program type)#X Display Manager Control Protocol
XDMCP (X Display Manager Control Protocol) is a network protocol that is used for communication between an X server and an X Display Manager. The querying the X server usually runs on an X terminal, which a registration service should be provided.
The X Display Manager acts as a server in XDMCP. He is waiting on UDP port 177 for requests of type QUERY (single request) and BROADCAST_QUERY ( a broadcast request that was sent directly to any X server, but to an entire subnet goes ). If it receives such a request, it may respond with a WILLING packet. This contains information about the server, the client but not the obligation to establish the connection.
The display manager is usually a list of accessible X server available (using a " graphical Choosers "). This can be a list of predefined hosts with their respective network address or a list that was obtained by a BROADCAST_QUERY inquiry be. In most cases also the host of the display manager is even listed because this is a XDMCP server.
If the user selects a host from the list, the X server on the local computer connects to the X display manager of the remote computer.