RFC1091

Network Working Group J. VanBokkelen Request for Comments: 1091 FTP Software, Inc. Obsoletes: RFC 930 February 1989

                     Telnet Terminal-Type Option

Status of This Memo

  This RFC specifies a standard for the Internet community.  Hosts on
  the Internet that exchange terminal type information within the
  Telnet protocol are expected to adopt and implement this standard.

  This standard supersedes RFC 930.  A change is made to permit cycling
  through a list of possible terminal types and selecting the most
  appropriate.

  Distribution of this memo is unlimited.

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.

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RFC 1091 Telnet Terminal-Type Option February 1989

     IAC SB TERMINAL-TYPE SEND IAC SE

        Server requests client to transmit his (the client's) next
        terminal type, and switch emulation modes (if more than one
        terminal type is supported).  The code for SEND is 1. (See
        below.)

     IAC SB TERMINAL-TYPE IS ... IAC SE

        Client is stating the name of his current (or only) 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

  On most machines with bit-mapped displays (e.g., PCs and graphics
  workstations) a client terminal emulation program is used to simulate
  a conventional ASCII terminal.  Most of these programs have multiple
  emulation modes, frequently with widely varying characteristics.
  Likewise, modern host system software and applications can deal with
  a variety of terminal types.  What is needed is a means for the
  client to present a list of available terminal emulation modes to the
  server, from which the server can select the one it prefers (for
  arbitrary reasons).  There is also need for a mechanism to change
  emulation modes during the course of a session, perhaps according to
  the needs of applications programs.

  Existing terminal-type passing mechanisms within Telnet were not
  designed with multiple emulation modes in mind.  While multiple names
  are allowed, they are assumed to be synonyms.  Emulation mode changes
  are not defined, and the list of modes can only be scanned once.

  This document defines a simple extension to the existing mechanisms,
  which meets both of the above criteria.  It makes one assumption
  about the behaviour of implementations coded to the previous standard
  in order to obtain full backwards-compatibility.

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RFC 1091 Telnet Terminal-Type Option February 1989

5. Description of the Option

  Willingness to exchange terminal-type information is agreed upon via
  conventional Telnet option negotiation.  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 (the server) is free to request type information.
  Only the server may send requests (IAC SB TERMINAL-TYPE SEND IAC SE)
  and only the client 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
  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 [4].

  The transmission of terminal type information by the Telnet client in
  response to a query from the Telnet server implies that the client
  must simultaneously change emulation mode, unless the terminal type
  sent is a synonym of the preceding terminal type, or there are other
  prerequisites for entering the new regime (e.g., having agreed upon
  the Telnet binary option).  The receipt of such information by the
  Telnet server does not imply any immediate 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 special functions not available to
  a standard Network Virtual Terminal.

  Note that this specification is deliberately asymmetric.  It is
  assumed that server operating systems and applications in general
  cannot change terminal types at arbitrary points in a session.  Thus,
  the client may only send a new type (and potentially change emulation
  modes) when the server requests that it do so.

6. Implementation Issues

  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

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RFC 1091 Telnet Terminal-Type Option February 1989

  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, either a particular terminal may be known by more than
  one name, for example a specific type and a more generic type, or the
  client may be a workstation with integrated display capable of
  emulating more than one kind of terminal.  In such cases, the sender
  of the TERMINAL-TYPE IS command should reply to successive TERMINAL-
  TYPE SEND commands with the various names.  In this way, a telnet
  server that does not understand the first response can prompt for
  alternatives.  If different terminal emulations are supported by the
  client, the mode of the emulator must be changed to match the last
  type sent, unless the particular emulation has other Telnet options
  (e.g., BINARY) as prerequisites (in which case, the emulation will
  switch to the last type sent when the prerequisite is fulfilled).
  When types are synonyms, they should be sent in order from most to
  least specific.

  When the server (the receiver of the TERMINAL-TYPE IS) receives the
  same response two consecutive times, this indicates the end of the
  list of available types.  Similarly, the client should indicate it
  has sent all available names by repeating the last one sent.  If an
  additional request is received, this indicates that the server (the
  sender of the IS) wishes to return to the top of the list of
  available types (probably to select the least of N evils).

  Server implementations conforming to the previous standard will cease
  sending TERMINAL-TYPE SEND commands after receiving the same response
  two consecutive times, which will work according to the old standard.
  It is assumed that client implementations conforming to the previous
  standard will send the last type on the list in response to a third
  query (as well as the second).  New-style servers must recognize this
  and not send more queries.

  The type "UNKNOWN" should be used if the type of the terminal is
  unknown or unlikely to be recognized by the other party.

  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.

7. User Interfaces

  Telnet clients and servers conforming to this specification should

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RFC 1091 Telnet Terminal-Type Option February 1989

  provide the following functions in their user interfaces:

  Clients supporting multiple emulation modes should allow the user to
  specify which of the modes is preferred (which name is sent first),
  prior to connection establishment.  The order of the names sent
  cannot be changed after the negotiation has begun.  This initial mode
  will also become the default with servers which do not support
  TERMINAL TYPE.

  Servers should store the current terminal type name and the list of
  available names in a manner such that they are accessible to both the
  user (via a keyboard command) and any applications which need the
  information.  In addition, there should be a corresponding mechanism
  to request a change of terminal types, by initiating a series of
  SEND/IS sub-negotiations.

8. Examples

  In this example, the server finds the first type acceptable.

     Server: IAC DO TERMINAL-TYPE

     Client: IAC WILL TERMINAL-TYPE

        (Server may now request a terminal type at any time.)

     Server: IAC SB TERMINAL-TYPE SEND IAC SE

     Client: IAC SB TERMINAL-TYPE IS IBM-3278-2 IAC SE

  In this example, the server requests additional terminal types, and
  accepts the second (and last on the client's list) type sent (RFC 930
  compatible):

     Server: IAC DO TERMINAL-TYPE

     Client: IAC WILL TERMINAL-TYPE

        (Server may now request a terminal type at any time.)

     Server: IAC SB TERMINAL-TYPE SEND IAC SE

     Client: IAC SB TERMINAL-TYPE IS ZENITH-H19 IAC SE

     Server: IAC SB TERMINAL-TYPE SEND IAC SE

     Client: IAC SB TERMINAL-TYPE IS UNKNOWN IAC SE

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RFC 1091 Telnet Terminal-Type Option February 1989

     Server: IAC SB TERMINAL-TYPE SEND IAC SE

     Client: IAC SB TERMINAL-TYPE IS UNKNOWN IAC SE

  In this example, the server requests additional terminal types, and
  proceeds beyond the end-of-list, to select the first type offered by
  the client (new-type client and server):

     Server: IAC DO TERMINAL-TYPE

     Client: IAC WILL TERMINAL-TYPE

        (Server may now request a terminal type at any time.)

     Server: IAC SB TERMINAL-TYPE SEND IAC SE

     Client: IAC SB TERMINAL-TYPE IS DEC-VT220 IAC SE

     Server: IAC SB TERMINAL-TYPE SEND IAC SE

     Client: IAC SB TERMINAL-TYPE IS DEC-VT100 IAC SE

     Server: IAC SB TERMINAL-TYPE SEND IAC SE

     Client: IAC SB TERMINAL-TYPE IS DEC-VT52 IAC SE

     Server: IAC SB TERMINAL-TYPE SEND IAC SE

     Client: IAC SB TERMINAL-TYPE IS DEC-VT52 IAC SE

     Server: IAC SB TERMINAL-TYPE SEND IAC SE

     Client: IAC SB TERMINAL-TYPE IS DEC-VT220 IAC SE

9. References:

    [1]  Postel, J., and J. Reynolds, "Telnet Protocol Specification",
         RFC 854, USC Information Sciences Institute, May 1983.

    [2]  Postel, J., and J. Reynolds, "Telnet Option Specification",
         RFC 855, USC Information Sciences Institute, May 1983.

    [3]  Solomon, M., and E. Wimmers, "Telnet Terminal Type Option",
         RFC 930, University of Wisconsin - Madison, January 1985.

    [4]  Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1010,
         USC Information Sciences Institute, May 1987.

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RFC 1091 Telnet Terminal-Type Option February 1989

Reviser's note:

  I owe much of this text to RFCs 884 and 930, by Marvin Solomon and
  Edward Wimmers of the University of Wisconsin - Madison, and I owe
  the idea of the extension to discussions on the "tn3270" mailing list
  in the Summer of 1987.

Author's Address

  James VanBokkelen
  FTP Software, Inc.
  26 Princess Street
  Wakefield, MA 01880-3004

  Phone: (617) 246-0900

  Email: [email protected]

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