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      indicates that any special signal has already occurred and the <br>
      recipient can return to normal processing of the data stream. <br>
         The Synch is sent via the TCP send operation with the Urgent <br>
         flag set and the DM as the last (or only) data octet. <br>
      When several Synchs are sent in rapid succession, the Urgent <br>
      notifications may be merged.  It is not possible to count Urgents <br>
      since the number received will be less than or equal the number <br>
      sent.  When in normal mode, a DM is a no operation; when in urgent <br>
      mode, it signals the end of the urgent processing. <br>
         If TCP indicates the end of Urgent data before the DM is found, <br>
         TELNET should continue the special handling of the data stream <br>

         until the DM is found. <br>
         If TCP indicates more Urgent data after the DM is found, it can <br>
         only be because of a subsequent Synch.  TELNET should continue <br>
         the special handling of the data stream until another DM is <br>
         found. <br>
      "Interesting" signals are defined to be:  the TELNET standard <br>
      representations of IP, AO, and AYT (but not EC or EL); the local <br>
      analogs of these standard representations (if any); all other <br>
      TELNET commands; other site-defined signals which can be acted on <br>
      without delaying the scan of the data stream. <br>
      Since one effect of the SYNCH mechanism is the discarding of <br>
      essentially all characters (except TELNET commands) between the <br>
      sender of the Synch and its recipient, this mechanism is specified <br>
      as the standard way to clear the data path when that is desired. <br>
      For example, if a user at a terminal causes an AO to be <br>
      transmitted, the server which receives the AO (if it provides that <br>
      function at all) should return a Synch to the user. <br>
      Finally, just as the TCP Urgent notification is needed at the <br>
      TELNET level as an out-of-band signal, so other protocols which <br>
      make use of TELNET may require a TELNET command which can be <br>
      viewed as an out-of-band signal at a different level. <br>
Postel & Reynolds                                               [Page 9] <br>

  <br>
RFC 854                                                         May 1983 <br>
      By convention the sequence [IP, Synch] is to be used as such a <br>
      signal.  For example, suppose that some other protocol, which uses <br>
      TELNET, defines the character string STOP analogously to the <br>
      TELNET command AO.  Imagine that a user of this protocol wishes a <br>
      server to process the STOP string, but the connection is blocked <br>
      because the server is processing other commands.  The user should <br>
      instruct his system to: <br>
         1. Send the TELNET IP character; <br>
         2. Send the TELNET SYNC sequence, that is: <br>
            Send the Data Mark (DM) as the only character <br>
            in a TCP urgent mode send operation. <br>
         3. Send the character string STOP; and <br>
         4. Send the other protocol's analog of the TELNET DM, if any. <br>
      The user (or process acting on his behalf) must transmit the <br>
      TELNET SYNCH sequence of step 2 above to ensure that the TELNET IP <br>
      gets through to the server's TELNET interpreter. <br>
         The Urgent should wake up the TELNET process; the IP should <br>
         wake up the next higher level process. <br>
   THE NVT PRINTER AND KEYBOARD <br>
      The NVT printer has an unspecified carriage width and page length <br>

      and can produce representations of all 95 USASCII graphics (codes <br>
      32 through 126).  Of the 33 USASCII control codes (0 through 31 <br>
      and 127), and the 128 uncovered codes (128 through 255), the <br>
      following have specified meaning to the NVT printer: <br>
         NAME                  CODE         MEANING <br>
         NULL (NUL)              0      No Operation <br>
         Line Feed (LF)         10      Moves the printer to the <br>
                                        next print line, keeping the <br>
                                        same horizontal position. <br>
         Carriage Return (CR)   13      Moves the printer to the left <br>
                                        margin of the current line. <br>
Postel & Reynolds                                              [Page 10] <br>
  <br>
RFC 854                                                         May 1983 <br>
         In addition, the following codes shall have defined, but not <br>
         required, effects on the NVT printer.  Neither end of a TELNET <br>
         connection may assume that the other party will take, or will <br>
         have taken, any particular action upon receipt or transmission <br>
         of these: <br>
         BELL (BEL)              7      Produces an audible or <br>
                                        visible signal (which does <br>
                                        NOT move the print head). <br>

         Back Space (BS)         8      Moves the print head one <br>
                                        character position towards <br>
                                        the left margin. <br>
         Horizontal Tab (HT)     9      Moves the printer to the <br>
                                        next horizontal tab stop. <br>
                                        It remains unspecified how <br>
                                        either party determines or <br>
                                        establishes where such tab <br>
                                        stops are located. <br>
         Vertical Tab (VT)       11     Moves the printer to the <br>
                                        next vertical tab stop.  It <br>
                                        remains unspecified how <br>
                                        either party determines or <br>
                                        establishes where such tab <br>
                                        stops are located. <br>
         Form Feed (FF)          12     Moves the printer to the top <br>
                                        of the next page, keeping <br>
                                        the same horizontal position. <br>
      All remaining codes do not cause the NVT printer to take any <br>
      action. <br>
      The sequence "CR LF", as defined, will cause the NVT to be <br>
      positioned at the left margin of the next print line (as would, <br>

      for example, the sequence "LF CR").  However, many systems and <br>
  <br>
      terminals do not treat CR and LF independently, and will have to <br>
      go to some effort to simulate their effect.  (For example, some <br>
      terminals do not have a CR independent of the LF, but on such <br>
      terminals it may be possible to simulate a CR by backspacing.) <br>
      Therefore, the sequence "CR LF" must be treated as a single "new <br>
      line" character and used whenever their combined action is <br>
      intended; the sequence "CR NUL" must be used where a carriage <br>
      return alone is actually desired; and the CR character must be <br>
      avoided in other contexts.  This rule gives assurance to systems <br>
      which must decide whether to perform a "new line" function or a <br>
      multiple-backspace that the TELNET stream contains a character <br>
      following a CR that will allow a rational decision. <br>
         Note that "CR LF" or "CR NUL" is required in both directions <br>
Postel & Reynolds                                              [Page 11] <br>
  <br>
RFC 854                                                         May 1983 <br>
         (in the default ASCII mode), to preserve the symmetry of the <br>
         NVT model.  Even though it may be known in some situations <br>
         (e.g., with remote echo and suppress go ahead options in <br>
         effect) that characters are not being sent to an actual <br>

         printer, nonetheless, for the sake of consistency, the protocol <br>
         requires that a NUL be inserted following a CR not followed by <br>
         a LF in the data stream.  The converse of this is that a NUL <br>
         received in the data stream after a CR (in the absence of <br>
         options negotiations which explicitly specify otherwise) should <br>
         be stripped out prior to applying the NVT to local character <br>
         set mapping. <br>
      The NVT keyboard has keys, or key combinations, or key sequences, <br>
      for generating all 128 USASCII codes.  Note that although many <br>
      have no effect on the NVT printer, the NVT keyboard is capable of <br>
      generating them. <br>
      In addition to these codes, the NVT keyboard shall be capable of <br>
      generating the following additional codes which, except as noted, <br>
      have defined, but not reguired, meanings.  The actual code <br>
      assignments for these "characters" are in the TELNET Command <br>
      section, because they are viewed as being, in some sense, generic <br>
      and should be available even when the data stream is interpreted <br>
      as being some other character set. <br>
      Synch <br>
         This key allows the user to clear his data path to the other <br>
         party.  The activation of this key causes a DM (see command <br>
         section) to be sent in the data stream and a TCP Urgent <br>

         notification is associated with it.  The pair DM-Urgent is to <br>
         have required meaning as defined previously. <br>
      Break (BRK) <br>
         This code is provided because it is a signal outside the <br>
         USASCII set which is currently given local meaning within many <br>
         systems.  It is intended to indicate that the Break Key or the <br>
         Attention Key was hit.  Note, however, that this is intended to <br>
         provide a 129th code for systems which require it, not as a <br>
         synonym for the IP standard representation. <br>
      Interrupt Process (IP) <br>
         Suspend, interrupt, abort or terminate the process to which the <br>
         NVT is connected.  Also, part of the out-of-band signal for <br>
         other protocols which use TELNET. <br>
Postel & Reynolds                                              [Page 12] <br>
  <br>
RFC 854                                                         May 1983 <br>
      Abort Output (AO) <br>
         Allow the current process to (appear to) run to completion, but <br>
         do not send its output to the user.  Also, send a Synch to the <br>
         user. <br>
      Are You There (AYT) <br>
         Send back to the NVT some visible (i.e., printable) evidence <br>

         that the AYT was received. <br>
      Erase Character (EC) <br>
         The recipient should delete the last preceding undeleted <br>
         character or "print position" from the data stream. <br>
      Erase Line (EL) <br>
         The recipient should delete characters from the data stream <br>
         back to, but not including, the last "CR LF" sequence sent over <br>
         the TELNET connection. <br>
      The spirit of these "extra" keys, and also the printer format <br>
      effectors, is that they should represent a natural extension of <br>
      the mapping that already must be done from "NVT" into "local". <br>
      Just as the NVT data byte 68 (104 octal) should be mapped into <br>
      whatever the local code for "uppercase D" is, so the EC character <br>
      should be mapped into whatever the local "Erase Character" <br>
      function is.  Further, just as the mapping for 124 (174 octal) is <br>
      somewhat arbitrary in an environment that has no "vertical bar" <br>
      character, the EL character may have a somewhat arbitrary mapping <br>
      (or none at all) if there is no local "Erase Line" facility. <br>
      Similarly for format effectors:  if the terminal actually does <br>
      have a "Vertical Tab", then the mapping for VT is obvious, and <br>
      only when the terminal does not have a vertical tab should the <br>
      effect of VT be unpredictable. <br>

TELNET COMMAND STRUCTURE <br>