Telnet SUPDUP option
A Telnet option code that lets two hosts agree to switch a connection over to the SUPDUP display protocol.
Crispin, M.R., "Telnet SUPDUP option", RFC 736, DOI 10.17487/RFC0736, October 1977, <https://www.rfc-editor.org/info/rfc736>.
@misc{rfc736,
series = {Request for Comments},
number = 736,
howpublished = {RFC 736},
publisher = {RFC Editor},
doi = {10.17487/RFC0736},
url = {https://www.rfc-editor.org/info/rfc736},
author = {M.R. Crispin},
title = {{Telnet SUPDUP option}},
pagetotal = 1,
year = 1977,
month = oct,
} RFC text
NWG/RFC# 736 MRC 31-OCT-77 23:28 42213
Telnet SUPDUP Option
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.
NWG/RFC# 736 MRC 31-OCT-77 23:28 42213
Telnet SUPDUP Option
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.
AI ELI5
On this page
§1The SUPDUP Option
RFC 736, by Mark Crispin, defines a TELNET option named SUPDUP with code 21. The option is a negotiation hook for the SUPDUP display protocol. The text is only a short option definition: a name and code, the meaning of the commands, and a default.
This gives the fancy screen language a name tag, number 21, so computers can ask for it.
§2Command Meanings
IAC WILL SUPDUP requests permission to, or confirms it will, use the SUPDUP display protocol. IAC WON'T SUPDUP refuses to use it. IAC DO SUPDUP requests that the receiver use it, or grants the receiver permission to use it.
The fourth command, a demand that the receiver not use the protocol, is printed in the RFC as IAC DON'T with the option name missing, which is an evident omission in the text.
One side asks 'can I use it?', the other says yes or no, and either can ask the other to start.
§3Default
The default is WON'T SUPDUP and DON'T SUPDUP, meaning the SUPDUP display protocol is not in use. Until both sides agree through these commands, the connection stays an ordinary Telnet connection.
Unless both say yes, nothing changes and you keep talking the plain way.