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rfc:ien:ien148

IEN 148 J. Postel RFC 764 ISI

                                                             June 1980
                   TELNET PROTOCOL SPECIFICATION

INTRODUCTION

 The purpose of the TELNET Protocol is to provide a fairly general,
 bi-directional, eight-bit byte oriented communications facility.  Its
 primary goal is to allow a standard method of interfacing terminal
 devices and terminal-oriented processes to each other.  It is
 envisioned that the protocol may also be used for terminal-terminal
 communication ("linking") and process-process communication
 (distributed computation).

GENERAL CONSIDERATIONS

 A TELNET connection is a Transmission Control Protocol (TCP)
 connection used to transmit data with interspersed TELNET control
 information.  TCP and the connection establishment procedure are
 documentented in the ARPA Internet Protocol Handbook.
 The TELNET Protocol is built upon three main ideas:  first, the
 concept of a "Network Virtual Terminal"; second, the principle of
 negotiated options; and third, a symmetric view of terminals and
 processes.
 1.  When a TELNET connection is first established, each end is
 assumed to originate and terminate at a "Network Virtual Terminal",
 or NVT.  An NVT is an imaginary device which provides a standard,
 network-wide, intermediate representation of a canonical terminal.
 This eliminates the need for "server" and "user" Hosts* to keep
 information about the characteristics of each other's terminals and
 terminal handling conventions.  All Hosts, both user and server, map
 their local device characteristics and conventions so as to appear to
 be dealing with an NVT over the network, and each can assume a
 similar mapping by the other party.  The NVT is intended to strike a
 balance between being overly restricted (not providing Hosts a rich
 enough vocabulary for mapping into their local character sets), and
 being overly inclusive (penalizing users with modest terminals).
  • NOTE: The "user" Host is the Host to which the physical terminal

is normally attached, and the "server" host is the Host which is

    normally providing some service.  As an alternate point of view,
    applicable even in terminal-to-terminal or process-to-process
    communications, the "user" Host is the Host which initiated the
    communication.

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June 1980 RFC 764, IEN 148 Telnet Protocol Specification

 2.  The principle of negotiated options takes cognizance of the fact
 that many sites will wish to provide additional services over and
 above those available within an NVT, and many users will have
 sophisticated terminals and would like to have elegant, rather than
 minimal, services.  Independent of, but structured within, the TELNET
 Protocol various "options" will be sanctioned which can be used with
 the "DO, DON'T, WILL, WON'T" structure (discussed below) to allow a
 user and server to agree to use a more elaborate (or perhaps just
 different) set of conventions for their TELNET connection.  Such
 options could include changing the character set, the echo mode, the
 line width, the page length, etc.
 The basic strategy for setting up the use of options is to have
 either party (or both) initiate a request that some option take
 effect.  The other party may then either accept or reject the
 request.  If the request is accepted the option immediately takes
 effect; if it is rejected the associated aspect of the connection
 remains as specified for an NVT.  Clearly, a party may always refuse
 a request to enable, and must never refuse a request to disable, some
 option since all parties must be prepared to support the NVT.
 The syntax of option negotiation has been set up so that if both
 parties request an option simultaneously, each will see the other's
 request as the positive acknowledgment of its own.
 3.  The symmetry of the negotiation syntax can potentially lead to
 nonterminating acknowledgment loops -- each party seeing the incoming
 commands not as acknowledgments but as new requests which must be
 acknowledged.  To prevent such loops, the following rules prevail:
    a.  Parties may only request a change in option status; i.e., a
        party may not send out a "request" merely to announce what
        mode it is in.
    b.  If a party receives what appears to be a request to enter some
        mode it is already in, the request should not be acknowledged.
    c.  Whenever one party sends an option command to a second party,
        whether as a request or an acknowledgment, and use of the
        option will have any effect on the processing of the data
        being sent from the first party to the second, then the
        command must be inserted in the data stream at the point where
        it is desired that it take effect.  (It should be noted that
        some time will elapse between the transmission of a request
        and the receipt of an acknowledgment, which may be negative.
        Thus, a site may wish to buffer data, after requesting an

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RFC 764, IEN 148 June 1980

                                         Telnet Protocol Specification
        option, until it learns whether the request is accepted or
        rejected, in order to hide the "uncertainty period" from the
        user.)
 Option requests are likely to flurry back and forth when a TELNET
 connection is first established, as each party attempts to get the
 best possible service from the other party.  Beyond that, however,
 options can be used to dynamically modify the characteristics of the
 connection to suit changing local conditions.  For example, the NVT,
 as will be explained later, uses a transmission discipline well
 suited to the many "line at a time" applications such as BASIC, but
 poorly suited to the many "character at a time" applications such as
 NLS.  A server might elect to devote the extra processor overhead
 required for a "character at a time" discipline when it was suitable
 for the local process and would negotiate an appropriate option.
 However, rather than then being permanently burdened with the extra
 processing overhead, it could switch (i.e., negotiate) back to NVT
 when the more "taut" control was no longer necessary.
 It is possible for requests initiated by processes to stimulate a
 nonterminating request loop if the process responds to a rejection by
 merely re-requesting the option.  To prevent such loops from
 occurring, rejected requests should not be repeated until something
 changes.  Operationally, this can mean the process is running a
 different program, or the user has given another command, or whatever
 makes sense in the context of the given process and the given option.
 A good rule of thumb is that a re-request should only occur as a
 result of subsequent information from the other end of the connection
 or when demanded by local human intervention.
 Option designers should not feel constrained by the somewhat limited
 syntax available for option negotiation.  The intent of the simple
 syntax is to make it easy to have options--since it is
 correspondingly easy to profess ignorance about them.  If some
 particular option requires a richer negotiation structure than
 possible within "DO, DON'T, WILL, WON'T", the proper tack is to use
 "DO, DON'T, WILL, WON'T" to establish that both parties understand
 the option, and once this is accomplished a more exotic syntax can be
 used freely.  For example, a party might send a request to alter
 (establish) line length.  If it is accepted, then a different syntax
 can be used for actually negotiating the line length--such a
 "sub-negotiation" perhaps including fields for minimum allowable,
 maximum allowable and desired line lengths.  The important concept is
 that such expanded negotiations should never begin until some prior
 (standard) negotiation has established that both parties are capable
 of parsing the expanded syntax.

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June 1980 RFC 764, IEN 148 Telnet Protocol Specification

 In summary, WILL XXX is sent, by either party, to indicate that
 party's desire (offer) to begin performing option XXX, DO XXX and
 DON'T XXX being its positive and negative acknowledgments; similarly,
 DO XXX is sent to indicate a desire (request) that the other party
 (i.e., the recipient of the DO) begin performing option XXX, WILL XXX
 and WON'T XXX being the positive and negative acknowledgments.  Since
 the NVT is what is left when no options are enabled, the DON'T and
 WON'T responses are guaranteed to leave the connection in a state
 which both ends can handle.  Thus, all Hosts may implement their
 TELNET processes to be totally unaware of options that are not
 supported, simply returning a rejection to (i.e., refusing) any
 option request that cannot be understood.
 As much as possible, the TELNET protocol has been made server-user
 symmetrical so that it easily and naturally covers the user-user
 (linking) and server-server (cooperating processes) cases.  It is
 hoped, but not absolutely required, that options will further this
 intent.  In any case, it is explicitly acknowledged that symmetry is
 an operating principle rather than an ironclad rule.
 A companion document, "TELNET Option Specifications," should be
 consulted for information about the procedure for establishing new
 options.  That document, as well as descriptions of all currently
 defined options, is contained in the TELNET section of the ARPA
 Internet Protocol Handbook.

THE NETWORK VIRTUAL TERMINAL

 The Network Virtual Terminal (NVT) is a bi-directional character
 device.  The NVT has a printer and a keyboard.  The printer responds
 to incoming data and the keyboard produces outgoing data which is
 sent over the TELNET connection and, if "echoes" are desired, to the
 NVT's printer as well.  "Echoes" will not be expected to traverse the
 network (although options exist to enable a "remote" echoing mode of
 operation, no Host is required to implement this option).  The code
 set is seven-bit USASCII in an eight-bit field, except as modified
 herein.  Any code conversion and timing considerations are local
 problems and do not affect the NVT.
 TRANSMISSION OF DATA
    Although a TELNET connection through the network is intrinsically
    full duplex, the NVT is to be viewed as a half-duplex device
    operating in a line-buffered mode.  That is, unless and until

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                                         Telnet Protocol Specification
    options are negotiated to the contrary, the following default
    conditions pertain to the transmission of data over the TELNET
    connection:
       1)  Insofar as the availability of local buffer space permits,
       data should be accumulated in the Host where it is generated
       until a complete line of data is ready for transmission, or
       until some locally-defined explicit signal to transmit occurs.
       This signal could be generated either by a process or by a
       human user.
       The motivation for this rule is the high cost, to some Hosts,
       of processing network input interrupts, coupled with the
       default NVT specification that "echoes" do not traverse the
       network.  Thus, it is reasonable to buffer some amount of data
       at its source.  Many systems take some processing action at the
       end of each input line (even line printers or card punches
       frequently tend to work this way), so the transmission should
       be triggered at the end of a line.  On the other hand, a user
       or process may sometimes find it necessary or desirable to
       provide data which does not terminate at the end of a line;
       therefore implementers are cautioned to provide methods of
       locally signaling that all buffered data should be transmitted
       immediately.
       2)  When a process has completed sending data to an NVT printer
       and has no queued input from the NVT keyboard for further
       processing (i.e., when a process at one end of a TELNET
       connection cannot proceed without input from the other end),
       the process must transmit the TELNET Go Ahead (GA) command.
       This rule is not intended to require that the TELNET GA command
       be sent from a terminal at the end of each line, since server
       Hosts do not normally require a special signal (in addition to
       end-of-line or other locally-defined characters) in order to
       commence processing.  Rather, the TELNET GA is designed to help
       a user's local Host operate a physically half duplex terminal
       which has a "lockable" keyboard such as the IBM 2741.  A
       description of this type of terminal may help to explain the
       proper use of the GA command.
       The terminal-computer connection is always under control of
       either the user or the computer.  Neither can unilaterally
       seize control from the other; rather the controlling end must
       relinguish its control explicitly.  At the terminal end, the
       hardware is constructed so as to relinquish control each time

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June 1980 RFC 764, IEN 148 Telnet Protocol Specification

       that a "line" is terminated (i.e., when the "New Line" key is
       typed by the user).  When this occurs, the attached (local)
       computer processes the input data, decides if output should be
       generated, and if not returns control to the terminal.  If
       output should be generated, control is retained by the computer
       until all output has been transmitted.
       The difficulties of using this type of terminal through the
       network should be obvious.  The "local" computer is no longer
       able to decide whether to retain control after seeing an
       end-of-line signal or not; this decision can only be made by
       the "remote" computer which is processing the data.  Therefore,
       the TELNET GA command provides a mechanism whereby the "remote"
       (server) computer can signal the "local" (user) computer that
       it is time to pass control to the user of the terminal.  It
       should be transmitted at those times, and only at those times,
       when the user should be given control of the terminal.  Note
       that premature transmission of the GA command may result in the
       blocking of output, since the user is likely to assume that the
       transmitting system has paused, and therefore he will fail to
       turn the line around manually.
    The foregoing, of course, does not apply to the user-to-server
    direction of communication.  In this direction, GAs may be sent at
    any time, but need not ever be sent.  Also, if the TELNET
    connection is being used for process-to-process communication, GAs
    need not be sent in either direction.  Finally, for
    terminal-to-terminal communication, GAs may be required in
    neither, one, or both directions.  If a Host plans to support
    terminal-to-terminal communication it is suggested that the Host
    provide the user with a means of manually signaling that it is
    time for a GA to be sent over the TELNET connection; this,
    however, is not a requirement on the implementer of a TELNET
    process.
 STANDARD REPRESENTATION OF CONTROL FUNCTIONS
    As stated in the Introduction to this document, the primary goal
    of the TELNET protocol is the provision of a standard interfacing
    of terminal devices and terminal-oriented processes through the
    network.  Early experiences with this type of interconnection have
    shown that certain functions are implemented by most servers, but
    that the methods of invoking these functions differ widely.  For a
    human user who interacts with several server systems, these
    differences are highly frustrating.  TELNET, therefore, defines a
    standard representation for five of these functions, as described

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                                         Telnet Protocol Specification
    below.  These standard representations have standard, but not
    required, meanings (with the exception that the IP function may be
    required by other protocols which use TELNET); that is, a system
    which does not provide the function to local users need not
    provide it to network users and may treat the standard
    representation for the function as a No-operation.  On the other
    hand, a system which does provide the function to local is obliged
    to provide the same function as a network user who transmits the
    standard representation for the function.
    Interrupt Process (IP)
       Many systems provide a function which suspends, interrupts,
       aborts, or terminates the operation of a user process.  This
       function is frequently used when a user believes his process is
       in an unending loop, or when an unwanted process has been
       inadvertently activated.  IP is the standard representation for
       invoking this function.  It should be noted by implementers
       that IP may be required by other protocols which use TELNET,
       and therefore should be implemented if these other protocols
       are to be supported.
    Abort Output (AO)
       Many systems provide a function which allows a process, which
       is generating output, to run to completion (or to reach the
       same stopping point it would reach if running to completion)
       but without sending the output to the user's terminal.
       Further, this function typically clears any output already
       produced but not yet actually printed (or displayed) on the
       user's terminal.  AO is the standard representation for
       invoking this function.  For example, some subsystem might
       normally accept a user's command, send a long text string to
       the user's terminal in response, and finally signal readiness
       to accept the next command by sending a "prompt" character
       (preceded by <CR><LF>) to the user's terminal.  If the AO were
       received during the transmission of the text string, a
       reasonable implementation would be to suppress the remainder of
       the text string, but transmit the prompt character and the
       preceding <CR><LF>.  (This is possibly in distinction to the
       action which might be taken if an IP were received; the IP
       might cause suppression of the text string and an exit from the
       subsystem.)
       It should be noted, by systems which provide this function,
       that there may be buffers external to the system (in the

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June 1980 RFC 764, IEN 148 Telnet Protocol Specification

       network and the user's "local" Host) which should be cleared;
       the appropriate way to do this is to transmit the "Synch"
       signal described below.
    Are You There (AYT)
       Many systems provide a function which provides the user with
       some visible (e.g., printable) evidence that the system is
       still up and running.  This function may be invoked by the user
       when the system is unexpectedly "silent" for a long time,
       because of the unanticipated (by the user) length of a
       computation, an unusually heavy system load, etc.  AYT is the
       standard representation for invoking this function.
    Erase Character (EC)
       Many systems provide a function which deletes the last
       preceding undeleted character or "print position"* from the
       stream of data being supplied by the user.  This function is
       typically used to edit keyboard input when typing mistakes are
       made.  EC is the standard representation for invoking this
       function.
  • NOTE: A "print position" may contain several characters

which are the result of overstrikes, or of sequences such as

          <char1> BS <char2>...
    Erase Line (EL)
       Many systems provide a function which deletes all the data in
       the current "line" of input.  This function is typically used
       to edit keyboard input.  EL is the standard representation for
       invoking this function.
 THE TELNET "SYNCH" SIGNAL
    Most time-sharing systems provide mechanisms which allow a
    terminal user to regain control of a "runaway" process; the IP and
    AO functions described above are examples of these mechanisms.
    Such systems, when used locally, have access to all of the signals
    supplied by the user, whether these are normal characters or
    special "out of band" signals such as those supplied by the
    teletype "BREAK" key or the IBM 2741 "ATTN" key.  This is not
    necessarily true when terminals are connected to the system

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RFC 764, IEN 148 June 1980

                                         Telnet Protocol Specification
    through the network; the network's flow control mechanisms may
    cause such a signal to be buffered elsewhere, for example in the
    user's Host.
    To counter this problem, the TELNET "Synch" mechanism is
    introduced.  A Synch signal consists of a TCP Urgent notification,
    coupled with the TELNET command DATA MARK.  The Urgent
    notification, which is not subject to the flow control pertaining
    to the TELNET connection, is used to invoke special handling of
    the data stream by the process which receives it.  In this mode,
    the data stream is immediately scanned for "interesting" signals
    as defined below, discarding intervening data.  The TELNET command
    DATA MARK (DM) is the synchronizing mark in the data stream which
    indicates that any special signal has already occurred and the
    recipient can return to normal processing of the data stream.
       The Synch is sent via the TCP send operation with the Urgent
       flag set and the DM as the last (or only) data octet.
    When several Synchs are sent in rapid succession, the Urgent
    notifications may be merged.  It is not possible to count Urgents
    since the number received will be less than or equal the number
    sent.  When in normal mode a DM is a no operation, when in urgent
    mode it signals the end of the urgent processing (this should
    correspond with the end of Urgent pointer indicated by TCP).
       If TCP indicates the end of Urgent data before the DM is found,
       TELNET should continue the special handling of the data stream
       until the DM is found.
    "Interesting" signals are defined to be:  the TELNET standard
    representations of IP, AO, and AYT (but not EC or EL); the local
    analogs of these standard representations (if any); all other
    TELNET commands; other site-defined signals which can be acted on
    without delaying the scan of the data stream.
    Since one effect of the SYNCH mechanism is the discarding of
    essentially all characters (except TELNET commands) between the
    sender of the Synch and its recipient, this mechanism is specified
    as the standard way to clear the data path when that is desired.
    For example, if a user at a terminal causes an AO to be
    transmitted, the server which receives the AO (if it provides that
    function at all) should return a Synch to the user.
    Finally, just as the TCP Urgent notification is needed at the

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June 1980 RFC 764, IEN 148 Telnet Protocol Specification

    TELNET level as an out-of-band signal, so other protocols which
    make use of TELNET may require a TELNET command which can be
    viewed as an out-of-band signal at a different level.
    By convention the sequence [IP, Synch] is to be used as such a
    signal.  For example, suppose that some other protocol, which uses
    TELNET, defines the character string STOP analogously to the
    TELNET command AO.  Imagine that a user of this protocol wishes a
    server to process the STOP string, but the connection is blocked
    because the server is processing other commands.  The user should
    instruct his system to:
       1. Send the TELNET IP character;
       2. Send the TELNET SYNC sequence, that is:
          Send the Data Mark (DM) as the only character
          in a TCP urgent mode send operation.
       3. Send the character string STOP; and
       4. Send the other protocol's analog of the TELNET DM, if any.
    The user (or process acting on his behalf) must transmit the
    TELNET SYNCH sequence of step 2 above to ensure that the TELNET IP
    gets through to the server's TELNET interpreter.
       The Urgent should wake up the TELNET process, the IP should
       wake up the next higher level process.
 THE NVT PRINTER AND KEYBOARD
    The NVT printer has an unspecified carriage width and page length
    and can produce representations of all 95 USASCII graphics (codes
    32 through 126).  Of the 33 USASCII control codes (0 through 31
    and 127), and the 128 uncovered codes (128 through 255), the
    following have specified meaning to the NVT printer:
       NAME                  CODE         MEANING
       NULL (NUL)              0   A no operation
       Line Feed (LF)         10   Moves the printer to the
                                   next print line, keeping the
                                   same horizontal position
       Carriage Return (CR)   13   Moves the printer to the left
                                   margin of the current line.

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                                         Telnet Protocol Specification
       In addition, the following codes shall have defined, but not
       required, effects on the NVT printer.  Neither end of a TELNET
       connection may assume that the other party will take, or will
       have taken, any particular action upon receipt or transmission
       of these:
       
       BELL (BEL)              7   Produces an audible or
                                   visible signal (which does
                                   NOT move the print head)
       Back Space (BS)         8   Moves the print head one
                                   character position towards
                                   the left margin.
       Horizontal Tab (HT)     9   Moves the printer to the
                                   next horizontal tab stop.
                                   It remains unspecified how
                                   either party determines or
                                   establishes where such tab
                                   stops are located.
       Vertical Tab (VT)       11  Moves the printer to the
                                   next horizontal tab stop.It
                                   remains unspecified how
                                   either party determines or
                                   establishes where such tab
                                   stops are located.
       Form Feed (FF)          12  Moves the printer to the top
                                   of the next page, keeping
                                   the same horizontal position
    All remaining codes do not cause the NVT printer to take any
    action.
    The sequence "CR LF", as defined, will cause the NVT to be
    positioned at the left margin of the next print line (as would,
    for example, the sequence "LF CR").  However, many systems and
    terminals do not treat CR and LF independently, and will have to
    go to some effort to simulate their effect.  (For example, some
    terminals do not have a CR independent of the LF, but on such
    terminals it may be possible to simulate a CR by backspacing.)
    Therefore, the sequence "CR LF" must be treated as a single "new
    line" character and used whenever their combined action is
    intended; the sequence "CR NUL" must be used where a carriage
    return alone is actually desired; and the CR character must be
    avoided in other contexts.  This rule gives assurance to systems
    which must decide whether to perform a "new line" function or a
    multiple-backspace that the TELNET stream contains a character
    following a CR that will allow a rational decision.

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June 1980 RFC 764, IEN 148 Telnet Protocol Specification

       Note that "CR LF" or "CR NUL" is required in both directions
       (in the default ASCII mode), to preserve the symmetry of the
       NVT model.  Even though it may be known in some situations
       (e.g., with remote echo and suppress go ahead options in
       effect) that characters are not being sent to an actual
       printer, none the less, for the sake of consistency, the
       protocol requires that a NUL be inserted following a CR not
       followed by a LF in the data stream.  The converse of this is
       that a NUL received in the data stream after a CR (in the
       absence of options negotiations which explicitly specify
       otherwise) should be stripped out prior to applying the NVT to
       local character set mapping.
    The NVT keyboard has keys, or key combinations, or key sequences,
    for generating all 128 USASCII codes.  Note that although many
    have no effect on the NVT printer, the NVT keyboard is capable of
    generating them.
    In addition to these codes, the NVT keyboard shall be capable of
    generating the following additional codes which, except as noted,
    have defined, but not reguired, meanings.  The actual code
    assignments for these "characters" are in the TELNET Command
    section, because they are viewed as being, in some sense, generic
    and should be available even when the data stream is interpreted
    as being some other character set.
    Synch
       This key allows the user to clear his data path to the other
       party.  The activation of this key causes a DM (see command
       section) to be sent in the data stream and a TCP Urgent
       notification is associated with it.  The pair DM-Urgent is to
       have required meaning as defined previously.
    Break (BRK)
       This code is provided because it is a signal outside the
       USASCII set which is currently given local meaning within many
       systems.  It is intended to indicate that the Break Key or the
       Attention Key was hit.  Note, however, that this is intended to
       provide a 129th code for systems which require it, not as a
       synonym for the IP standard representation.

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                                         Telnet Protocol Specification
    Interrupt Process (IP)
       Suspend, interrupt, abort or terminate the process to which the
       NVT is connected.  Also, part of the out-of-band signal for
       other protocols which use TELNET.
    Abort Output (AO)
       Allow the current process to (appear to) run to completion, but
       do not send its output to the user.  Also, send a Synch to the
       user.
    Are You There (AYT)
       Send back to the NVT some visible (i.e., printable) evidence
       that the AYT was received.
    Erase Character (EC)
       The recipient should delete the last preceding undeleted
       character or "print position" from the data stream.
    Erase Line (EL)
       The recipient should delete characters from the data stream
       back to, but not including, the last "CR LF" sequence sent over
       the TELNET connection.
    The spirit of these "extra" keys, and also the printer format
    effectors, is that they should represent a natural extension of
    the mapping that already must be done from "NVT" into "local".
    Just as the NVT data byte 104 should be mapped into whatever the
    local code for "uppercase D" is, so the EC character should be
    mapped into whatever the local "Erase Character" function is.
    Further, just as the mapping for 174 is somewhat arbitrary in an
    environment that has no "vertical bar" character, the EL character
    may have a somewhat arbitrary mapping (or none at all) if there is
    no local "Erase Line" facility.  Similarly for format effectors:
    if the terminal actually does have a "Vertical tab", then the
    mapping for VT is obvious, and only when the terminal does not
    have a vertical tab should the effect of VT be unpredictable.

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June 1980 RFC 764, IEN 148 Telnet Protocol Specification

TELNET COMMAND STRUCTURE

 All TELNET commands consist of at least a two byte sequence:  the
 "Interpret as Command" (IAC) escape character followed by the code
 for the command.  The commands dealing with option negotiation are
 three byte sequences, the third byte being the code for the option
 referenced.  This format was chosen so that as more comprehensive use
 of the "data space" is made -- by negotiations from the basic NVT, of
 course -- collisions of data bytes with reserved command values will
 be minimized, all such collisions requiring the inconvenience, and
 inefficiency, of "escaping" the data bytes into the stream.  With the
 current set-up, only the IAC need be doubled to be sent as data, and
 the other 255 codes may be passed transparently.
 The following are the defined TELNET commands.  Note that these codes
 and code sequences have the indicated meaning only when immediately
 preceded by an IAC.
    NAME               CODE              MEANING
    SE                  240 End of subnegotiation parameters
    NOP                 241 No operation
    Data Mark           242 The data stream portion of a Synch
                            This should always be accompanied
                            by a TCP Urgent notification.
    Break               243 NVT character BRK
    Interrupt Process   244 The function IP
    Abort output        245 The function AO
    Are You There       246 The function AYT
    Erase character     247 The function EC
    Erase Line          248 The function EL
    Go ahead            249 The GA signal
    SB                  250 Indicates that what follows is
                            subnegotiation of the indicated
                            option
    WILL (option code)  251 Indicates the desire to begin
                            performing, or confirmation that
                            you are now performing, the
                            indicated option
    WON't (option code) 252 Indicates the refusal to perform,
                            or continue performing, the
                            indicated option.
    DO (option code)    253 Indicates the request that the
                            other party perform, or
                            confirmation that you are expecting
                            the other party to perform, the

[Page 14] Postel

RFC 764, IEN 148 June 1980

                                         Telnet Protocol Specification
                            indicated option.
    DON'T (option code) 254 Indicates the demand that the
                            other party stop performing,
                            or confirmation that you are no
                            longer expecting the other party
                            to perform, the indicated option.
    IAC                 255 Data Byte 255.

CONNECTION ESTABLISHMENT

 The TELNET TCP connection is established between the user's port U
 and the server's port L.  The server listens on its well known port L
 for such connections.  Since a TCP connection is full duplex and
 identified by the pair of ports, the server can engage in many
 simultaneous connections involving it's port L and different user
 ports U.
 Port Assignment
    When used for remote user access to service hosts (i.e., remote
    terminal access) this protocol is assigned server port 23 (27
    octal).  That is L=23.

Postel [Page 15]

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