RFC 854                                                         May 1983


         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
             BS ...

      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.