Network Working Group Marvin Solomon
Request for Comments: 930 Edward Wimmers
Supersedes: RFC 884 University of Wisconsin - Madison
January 1985
TELNET TERMINAL TYPE OPTION
Status of This Memo
This RFC specifies a standard for the ARPA Internet community. Hosts
on the ARPA Internet that exchange terminal type information within
the Telnet protocol are expected to adopt and implement this
standard. Distribution of this memo is unlimited.
This standard supersedes RFC 884. The only change is to specify that
the TERMINAL-TYPE IS sub-negotiation should be sent only in response
to the TERMINAL-TYPE SEND sub-negotiation. See below for further
explanation.
1. Command Name and Code
TERMINAL-TYPE 24
2. Command Meanings
IAC WILL TERMINAL-TYPE
Sender is willing to send terminal type information in a
subsequent sub-negotiation
IAC WON'T TERMINAL-TYPE
Sender refuses to send terminal type information
IAC DO TERMINAL-TYPE
Sender is willing to receive terminal type information in a
subsequent sub-negotiation
IAC DON'T TERMINAL-TYPE
Sender refuses to accept terminal type information
IAC SB TERMINAL-TYPE SEND IAC SE
Sender requests receiver to transmit his (the receiver's) terminal
type. The code for SEND is 1. (See below.)
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Telnet Terminal Type Option
IAC SB TERMINAL-TYPE IS ... IAC SE
Sender is stating the name of his terminal type. The code for IS
is 0. (See below.)
3. Default
WON'T TERMINAL-TYPE
Terminal type information will not be exchanged.
DON'T TERMINAL-TYPE
Terminal type information will not be exchanged.
4. Motivation for the Option
This option allows a telnet server to determine the type of terminal
connected to a user telnet program. The transmission of such
information does not immediately imply any change of processing.
However, the information may be passed to a process, which may alter
the data it sends to suit the particular characteristics of the
terminal. For example, some operating systems have a terminal driver
that accepts a code indicating the type of terminal being driven.
Using the TERMINAL TYPE and BINARY options, a telnet server program
on such a system could arrange to have terminals driven as if they
were directly connected, including such special functions as cursor
addressing, multiple colors, etc., not included in the Network
Virtual Terminal specification. This option fits into the normal
structure of TELNET options by deferring the actual transfer of
status information to the SB command.
5. Description of the Option
WILL and DO are used only to obtain and grant permission for future
discussion. The actual exchange of status information occurs within
option subcommands (IAC SB TERMINAL-TYPE...).
Once the two hosts have exchanged a WILL and a DO, the sender of the
DO TERMINAL-TYPE is free to request type information. Only the
sender of the DO may send requests (IAC SB TERMINAL-TYPE SEND IAC SE)
and only the sender of the WILL may transmit actual type information
(within an IAC SB TERMINAL-TYPE IS ... IAC SE command). Terminal
type information may not be sent spontaneously, but only in response
to a request.
The terminal type information is an NVT ASCII string. Within this
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Telnet Terminal Type Option
string, upper and lower case are considered equivalent. The complete
list of valid terminal type names can be found in the latest
"Assigned Numbers" RFC.
The following is an example of use of the option:
Host1: IAC DO TERMINAL-TYPE
Host2: IAC WILL TERMINAL-TYPE
(Host1 is now free to request status information at any time.)
Host1: IAC SB TERMINAL-TYPE SEND IAC SE
Host2: IAC SB TERMINAL-TYPE IS IBM-3278-2 IAC SE
6. Implementation Suggestions
The "terminal type" information may be any NVT ASCII string
meaningful to both ends of the negotiation. The list of terminal
type names in "Assigned Numbers" is intended to minimize confusion
caused by alternative "spellings" of the terminal type. For example,
confusion would arise if one party were to call a terminal
"IBM3278-2" while the other called it "IBM-3278/2". There is no
negative acknowledgement for a terminal type that is not understood,
but certain other options (such as switching to BINARY mode) may be
refused if a valid terminal type name has not been specified. In
some cases, a particular terminal may be known by more than one name,
for example a specific type and a more generic type. In such cases,
the sender of the TERMINAL-TYPE IS command should reply to successive
TERMINAL-TYPE SEND commands with the various names, from most to
least specific. In this way, a telnet server that does not
understand the first response can prompt for alternatives. However,
it should cease sending TERMINAL-TYPE SEND commands after receiving
the same response two consecutive times. Similarly, a sender should
indicate it has sent all available names by repeating the last one
sent. Note that TERMINAL-TYPE IS must only be sent in response to a
request (TERMINAL-TYPE SEND), because a host that sent TERMINAL-TYPE
IS and then received TERMINAL-TYPE SEND couldn't determine whether
the other host was requesting a second option or the TERMINAL-TYPE
SEND and the TERMINAL-TYPE IS crossed in midstream.
The type "UNKNOWN" should be used if the type of the terminal is
unknown or unlikely to be recognized by the other party.
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The complete and up-to-date list of terminal type names will be
maintained in the "Assigned Numbers". The maximum length of a
terminal type name is 40 characters.
