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rfc: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.

VanBokkelen [Page 1] 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.

VanBokkelen [Page 2] 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

VanBokkelen [Page 3] 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

VanBokkelen [Page 4] 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

VanBokkelen [Page 5] 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.

VanBokkelen [Page 6] 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: jbvb@ftp.com

VanBokkelen [Page 7]

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