rfc1116.txt

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      SLC_CANTCHANGE, then this is a function that is supported, but the
      value for this function, specified in the third octet, cannot be
      changed.

      If the SLC_LEVELBITS in the second octet are equal to
      SLC_NOSUPPORT, then this particular function is not supported and
      should be disabled by the other side.

      If this is a response to a previous request to change a special
      character, and we are agreeing to the change, then the SLC_ACK bit
      must be set in the second octet.

      If the SLC_FLUSHIN bit is set in the second octet, then whenever
      this function is sent, a Telnet "sync" should be sent at the same
      time to flush the input stream.

      If the SLC_FLUSHOUT bit is set in the second octet, then whenever
      this function is sent, output data should be flushed.

      Only the client may send an octet triplet with the first octet
      equal to zero.  In this case, the SLC_LEVELBITS may only be set to
      SLC_DEFAULT or SLC_VALUE, and the third octet does not matter.
      When the server receives 0 SLC_DEFAULT 0, it should switch to its
      system default special character settings, and send all those
      special characters to the client.  When the server receives 0
      SLC_VALUE 0, it should just send its current special character
      settings.  Note that if the server does not support some of the
      editing functions, they should be sent as XXX SLC_DEFAULT 0,
      rather than as XXX SLC_NOSUPPORT 0, so that the client may choose
      to use its own values for those functions, rather than have to
      disable those functions even if it supports them.

      If any of the octets in the list of octet triplets is equal to
      IAC, it must be sent as a double IAC.

   When a connection is established, it is the responsibility of the
   client to either request the remote default values for the special
   characters, or to send across what all the special characters should
   be set to.



Telnet Linemode Working Group                                   [Page 6]

RFC 1116                 Telnet Linemode Option              August 1989


   The function values can be put into two groups; functions that are to
   be translated to their Telnet equivalents before being sent across
   the Telnet connection, and functions that are to be recognized and
   processed locally.

   First, we have those characters that are to be mapped into their
   Telnet equivalents:

      SLC_SYNCH Synch.  See RFC 854, "TELNET PROTOCOL SPECIFICATION",
                for a complete description.

      SLC_BRK   Break.  See RFC 854, "TELNET PROTOCOL SPECIFICATION",
                for a complete description.

      SLC_IP    Interrupt Process.  See RFC 854, "TELNET PROTOCOL
                SPECIFICATION", for a complete description.

      SLC_AO    Abort Output.  See RFC 854, "TELNET PROTOCOL
                SPECIFICATION", for a complete description.

      SLC_AYT   Are You There.  See RFC 854, "TELNET PROTOCOL
                SPECIFICATION", for a complete description.

      SLC_EOR   End of Record.  See RFC 885, "TELNET END OF RECORD
                OPTION" for a complete description.

      SLC_ABORT Abort.  See section 2.5 for a complete description.

      SLC_EOF   End of File.  See section 2.5 for a complete
                description.

      SLC_SUSP  Suspend.  See section 2.5 for a complete description.

   Next, we have the locally interpreted functions:

      SLC_EC     Erase Character.  This is the character that is
                 typed to erase one character from the input
                 stream.  See RFC 854, "TELNET PROTOCOL
                 SPECIFICATION", for a complete description.

      SLC_EL     Erase Line.  This is the character that is typed
                 to erase the entire contents of the current line
                 of input.  See RFC 854, "TELNET PROTOCOL
                 SPECIFICATION", for a complete description.

      SLC_EW     Erase Word.  This is the character that is typed
                 to erase one word from the input stream.  When
                 backing up in the input stream, a word is defined



Telnet Linemode Working Group                                   [Page 7]

RFC 1116                 Telnet Linemode Option              August 1989


                 to be (optionally) whitespace (tab or space
                 characters), and a string of characters up to, but not
                 including, whitespace or line delimiters.

      SLC_RP     Reprint Line.  This is the character that is typed
                 to cause the current line of input to be reprinted,
                 leaving the cursor at the end of the line.

      SLC_LNEXT  Literal Next.  This is the character that is typed
                 to indicate that the next character is to be taken
                 literally, no character processing should be done
                 with it, and if it is a special character that
                 would normally get mapped into a Telnet option,
                 that mapping should not be done.

      SLC_XON    Start Output.  This is the character that is sent
                 to resume output to the users terminal.

      SLC_XOFF   Stop Output.  This is the character that is sent
                 to stop output to the users terminal.

      SLC_FORW1  Forwarding character.  This is a character that
                 should cause all data currently being buffered,
                 and this character, to be sent immediately.

      SLC_FORW2  Forwarding character.  This is another character
                 that is to be treated in the same manner as
                 SLC_FORW1.

2.5 New control characters

   IAC ABORT

      Abort.  Similar to "IAC IP", but means only to abort or terminate
      the process to which the NVT is connected.  (The Telnet spec says
      IP may "suspend, interrupt, abort or terminate" the process.)  If
      a system does not have two methods of interrupting a process, then
      ABORT and IP should have the same effect.

   IAC SUSP

      Suspend the execution of the current process attached to the NVT
      in such a way that another process will take over control of the
      NVT, and the suspended process can be resumed at a later time.  If
      the receiving system does not support this functionality, it
      should be ignored.





Telnet Linemode Working Group                                   [Page 8]

RFC 1116                 Telnet Linemode Option              August 1989


   IAC EOF

      End Of File.  The recipient should notify the process connected to
      the NVT that an end of file has been reached.  This is intended
      for systems that support the ability for the user to type in an
      EOF character at the keyboard.

3.  Default Specification

   The default specification for this option is:

      WONT LINEMODE
      DONT LINEMODE

   meaning there will not be any subnegotiation of the mode of the
   connection.

   If WILL LINEMODE is negotiated, the defaults are:

      IAC SB LINEMODE MODE 0 IAC SE
      IAC SB LINEMODE WONT FORWARDMASK IAC SE

   If DO LINEMODE is negotiated, the defaults are:

      IAC SB LINEMODE MODE 0 IAC SE
      IAC SB LINEMODE DONT FORWARDMASK IAC SE

   Character values for SLC default to SLC_NOSUPPORT.

4.  Motivation

   With increasing Telnet usage, it has become apparent that the ability
   to do command line processing on the local machine and send completed
   lines to the remote machine is a feature necesary in several
   environments.  First, in the case of a connection over long delay
   equipment, it is very frustrating to the user to have the echoing of
   his data take several seconds.  Second, some supercomputers, due to
   their nature, are not good at handling and processing single
   character input.  For these machines, it is better to have the front
   end computer do the character processing, and leave the
   supercomputer's cycles available for doing vectorized number
   crunching.

   There have been attempts to make local line editing work within the
   existing Telnet specs.  Indeed, the 4.3 BSD tape includes a version
   of Telnet that attempts to do this through recognition of the state
   of the ECHO and SUPRESS-GO-AHEAD options; other implementations do
   this recognition purely through the ECHO option.



Telnet Linemode Working Group                                   [Page 9]

RFC 1116                 Telnet Linemode Option              August 1989


   There are problems with both of these methods.  Using just the ECHO
   provides no mechanism to have ECHO to the user turned off, and leave
   local character processing on, for example, when a user is typing a
   password.

   The usage of the SUPRESS-GO-AHEAD comes from reading into RFC 858,
   where it states:

      "In many TELNET implementations it will be desirable to couple the
      SUPRESS-GO-AHEAD option to the echo option so that when the echo
      option is in effect, the SUPPRESS-GO-AHEAD option is in effect
      simultaneously: both of these options will normally have to be in
      effect simultaneously to effect what it commonly understood to be
      character at a time echoing by the remote computer."

   The reverse reading of this is that without the ECHO option or the
   SUPPRESS-GO-AHEAD option, you are in line at a time mode, implying
   local line editing.  This has the obvious problem that that is not
   what the SUPPRESS-GO-AHEAD option is supposed to mean.

   Other shortcomings are that the Telnet specification is not rich
   enough to handle all of the special characters that some of the
   current operating systems support.  For example, the ECHO/SGA
   implementation supports two ways of interrupting a process, by
   borrowing the BRK option for the second interrupt.  Some
   implementations have taken the EOR option to send an End-Of-File.
   Obviously, this is using things for which they were not intended, and
   the correct solution would be to define new options.

   Another problem is that some implementations of line mode buffer up
   the input until the end of the line, and then send the whole line
   across, editing characters and all.  No local editing of the line has
   been done.

   After examining several implementations, it has become clear that the
   correct thing to do is to implement new options to enhance the
   current Telnet specification so that it can support local line
   editing in a reasonable, reliable, and consistent manner.

   There are three states that are of interest:

      1)  Local line editing and local signal trapping

      2)  Remote line editing, local signal trapping

      3)  Remote line editing, remote signal trapping

   The case of local line editing and remote signal trapping is not a



Telnet Linemode Working Group                                  [Page 10]

RFC 1116                 Telnet Linemode Option              August 1989


   very interesting case, because you don't recognize the signals, and
   cannot send them to the remote side for it to recognize until the
   line has been completed.  Also, special signals usually will have an
   effect on the line editing function, and if they are not being
   trapped locally the desired action will not happen.

   Local line editing means that all normal command line character
   processing, like "Erase Character" and "Erase Line", happen on the
   local system, and only when "CR LF" (or some other special character)
   is encountered is the edited data sent to the remote system.

   Signal trapping means, for example, that if the user types the
   character associated with the IP function, then the "IAC IP" function
   is sent to the remote side instead of the character typed.  Remote
   signal trapping means, for example, that if the user types the
   character associated with the IP function, then the "IAC IP" function
   is not sent to the remote side, but rather the actual character typed
   is sent to the remote side.

5.  Implementation Rules

   It is expected that any implementation that supports the Telnet
   LINEMODE option will support all of this specification.