rfc764.txt

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RFC 764, IEN 148                                               June 1980
                                           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.




Postel                                                         [Page 11]

                                                                        
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.








[Page 12]                                                         Postel

                                                                        
RFC 764, IEN 148                                               June 1980
                                           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.









Postel                                                         [Page 13]

                                                                        
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]