ATCP  [protocol]
GMCP's predecessor. Out-of-band data with a looser payload, largely superseded,
and still negotiated by servers that never removed it.

Measured adoption
  0 of 886 listed games were observed offering it (0.0%)
  /games?protocol=ATCP

  The games not counted here are not games without the protocol. A game is
  counted when we observed its server offering the option in a handshake; the
  rest are servers that did not offer it to us and servers whose handshake we
  have not read, and we cannot tell you which.

  By codebase, of the games we identified
    AresMUSH             0 of 18 offered it
    CircleMUD            0 of 47 offered it
    CobraMUSH            0 of 2 offered it
    CoffeeMUD            0 of 32 offered it
    DikuMUD              0 of 35 offered it
    Evennia              0 of 11 offered it
    FluffOS              0 of 28 offered it
    MUCK                 0 of 24 offered it
    PennMUSH             0 of 34 offered it
    RhostMUSH            0 of 40 offered it
    ROM                  0 of 55 offered it
    SMAUG                0 of 20 offered it
    tbaMUD               0 of 5 offered it
    TinyMUSH             0 of 9 offered it

ATCP — the Achaea Telnet Client Protocol — is telnet option 200, and it is where
the idea of sending structured data alongside MUD text was first widely
deployed. A server sends a module name and a payload; the client routes it.

Its payload format is looser than GMCP [/reference/protocols/gmcp]'s JSON, which
is essentially why GMCP replaced it. Clients that support ATCP now generally
document it as deprecated and point you at GMCP instead.

WHY IT IS STILL HERE

Because nothing breaks by leaving it on. A server that implemented ATCP in 2008
and added GMCP in 2014 usually still negotiates both, and a client that supports
both will take whichever it is offered.

For a new implementation there is no reason to choose it.

WHAT WE MEASURE

Servers offering telnet option 200 in a handshake we observed. A low figure here
is expected and is about age rather than about anything else.

See also
  GMCP — /reference/protocols/gmcp
  MSDP — /reference/protocols/msdp
  Mudlet — /reference/clients/mudlet