X Display Manager Control Protocol (XDMCP) is used for communicating between user's local X server and a remote X display manager. The primary purpose of the protocol is to provide a uniform mechanism for offering X login service independent of any particular host or implementation.
In XDMCP the local X server asks a remote display manager to initiate connection and to negotiate a session which can be then used for controlling graphical user interfaces of programs on the remote machine. XDMCP can be considered as graphical telnet protocol and it shares the same security problem of unencrypted authentication. For this reason X traffic should be run through a secure channel such as ssh tunnel if there is a possibility of someone intercepting the traffic (see x11-forwarding).
