Network Working Group Mark Crispin Request for Comments 736 SU-AI NIC 42213 31 October 1977 TELNET SUPDUP Option 1. Command name and code. SUPDUP 21 2. Command meanings. IAC WILL SUPDUP The sender of this command REQUESTS permission to, or confirms that it will, use the SUPDUP display protocol IAC WON'T SUPDUP The sender of this command REFUSES to use the SUPDUP protocol. IAC DO SUPDUP The sender of this command REQUESTS that the receiver use, or grants the receiver permission to use, the SUPDUP protocol. IAC DON'T The sender of this command DEMANDS that the receiver not use the SUPDUP protocol. 3. Default. WON'T SUPDUP DON'T SUPDUP i.e., the SUPDUP display protocol is not in use.
4. Motivation for the option. Since the publication of RFC 734, I have been requested to design an option to the TELNET protocol to provide for SUPDUP service. This option allows a host to provide SUPDUP service on the normal TELNET socket (27 octal) instead of 137 (octal) which is the normal SUPDUP ICP socket. 5. Description of the option. A user TELNET program which wishes to use the SUPDUP display protocol instead of the NVT terminal service should send an IAC DO SUPDUP. If the server is willing to use the SUPDUP display protocol, it should respond with IAC WILL SUPDUP; otherwise it should refuse with IAC WONT SUPDUP. For hosts which normally provide SUPDUP terminal services, the server can send IAC WILL SUPDUP upon ICP which the user may then accept or refuse. If the SUPDUP option is in effect, no further TELNET negotiations are allowed. They are meaningless, since SUPDUP has its own facilities to perform the functions that are needed. Hence, octal 377 will become an ordinary transmitted character (in this case an invalid %TD code) instead of an IAC. Following the mutual acceptance of the SUPDUP option, the SUPDUP negotiation proceeds as described in RFC 734.