MCCP
MUD Client Compression Protocol — compressing the stream of text coming out of a MUD server.
A MUD pushes out vast amounts of repetitive text: room descriptions, combat spam, channel chatter. MCCP is negotiated through Telnet options; if the client agrees, everything the server sends from then on goes through a zlib stream. That kind of text compresses extremely well, which mattered both to players on modems and to servers paying for bandwidth.
Version 1 used Telnet option 85 but had a malformed subnegotiation, so version 2 moved to option 86. The protocol caught on around the turn of the millennium and is supported by most MUD servers and clients still being maintained.