19.4.2 GMCP

General MUD Telnet Protocol (GMCP) is a protocol used to send data from the
game server to the player client. This data transfer happens behind the scenes
and is not seen by the players. This allows the game server to send messages to
the client to update basic character, room, and game information. Character
health is an example of information that is continually passed to the client
from the game.

For full details of this protocol, please see the GMCP page on the Nexus wiki: