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RFC 854 - Telnet Protocol Specification


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Network Working Group                                          J. Postel
Request for Comments: 854                                    J. Reynolds
                                                                     ISI
Obsoletes: NIC 18639                                            May 1983

                     TELNET PROTOCOL SPECIFICATION

This RFC specifies a standard for the ARPA Internet community.  Hosts on
the ARPA Internet are expected to adopt and implement this standard.

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.

   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,

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      applicable even in terminal-to-terminal or process-to-process
      communications, the "user" host is the host which initiated the
      communication.

   2.  The principle of negotiated options takes cognizance of the fact
   that many hosts 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 are various "options" that will be sanctioned and may 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, 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.
      This non-response is essential to prevent endless loops in the
      negotiation.  It is required that a response be sent to requests
      for a change of mode -- even if the mode is not changed.

      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

RFC 854                                                         May 1983

      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 host may wish to
      buffer data, after requesting an 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 detailed 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" might include fields for minimum allowable, maximum
   allowable and desired line lengths.  The important concept is that

RFC 854                                                         May 1983

   such expanded negotiations should never begin until some prior
   (standard) negotiation has established that both parties are capable
   of parsing the expanded syntax.

   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.

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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      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
         that a "line" is terminated (i.e., when the "New Line" key is
         typed by the user).  When this occurs, the attached (local)

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         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.

      Note that the symmetry of the TELNET model requires that there is
      an NVT at each end of the TELNET connection, at least
      conceptually.

   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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      below.  These standard representations have standard, but not
      required, meanings (with the exception that the Interrupt Process
      (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 a
      local user is obliged to provide the same function to 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 server systems which provide this
         function, that there may be buffers external to the system (in

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         the 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) to the user system.

      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
      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.

RFC 854                                                         May 1983

      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.

         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.

         If TCP indicates more Urgent data after the DM is found, it can
         only be because of a subsequent Synch.  TELNET should continue
         the special handling of the data stream until another 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
      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.

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      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      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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         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 vertical 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.

         Note that "CR LF" or "CR NUL" is required in both directions

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         (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, nonetheless, 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.

      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.

RFC 854                                                         May 1983

      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 68 (104 octal) 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 124 (174 octal) 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.

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

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   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
                                 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.

RFC 854                                                         May 1983

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 its 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.

 

User Contributions:

PFudd
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Jul 19, 2005 @ 4:04 am
It's a convoluted RFC, hard to read.It boils down to this:Characters 0-254 are normal, character 255 is the IAC.  If you want to send a character 255 to the user, send two of them.If you want to want to send a command (other than will/won't/do/don't/sb/se), send (for example) char 255 + char 241 (NOP), char 255 + char 246 (Are you there), etc.There's a way to send out-of-band signals with tcp/ip; this is the 'Urgent data' flag, and this paper says you should use it as a SYNCH command, which ends with a 255+242 (Data mark).See "TELNET option specifications" for details about the options.
M0les
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Jan 5, 2006 @ 6:18 pm
What to do outside the rules?(From the perspective of a client implementation between a bi-directional data stream and some terminal implementation/emulation)a)What if a client implementation receives an IAC followed by a byte that is not a valid command? Solution 1 is to ignore the IAC and the invalid command byte (I think this is the best approach). Option two is to assume the sender is trying to send hte IAC and the bad command byte as data for the client to receive directly and pass them both on. I think this approach is flawed as the data stream is going to be corrupted anyway by other parts of the protocol anyway (e.g. valid command sequences the sender unintentionally inserted into the input stream, intending them to be data bytes for the end client).b)What if a CR is recevied that is not followed by either an LF or a NUL? My opinion is that the implementation should pass these on to the end-client without interpretation (unless explicitly asked-to by the client, e.g. translate local CRLF, LF or CR line termination into CRLF network line termination).
Jack
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Jan 10, 2006 @ 1:01 am
Most confusing! I think some examples of protocol negotiation are in order...

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