What is XDMCP protocol and what is its main function ?

XDMCPX Display Manager Control Protocol is a network protocol.

X Display Manager Control Protocol uses UDPport177. An X server requests that a display manager start a session by sending a Query packet. If the display manager allows access for that X server, it responds by sending a willing packet back to the X server. The XDMCP protocol mandates that the X server starts autonomously and connects to the display manager. In the X Window System paradigm, the server runs on the computer providing the display and input devices. A server can connect, using the XDMCP protocol, to a display manager running on another computer, requesting it to start the session.