RFC1116

Network Working Group Internet Engineering Task Force Request for Comments: 1116 Telnet Linemode Working Group

                                                      D. Borman, Editor
                                                    Cray Research, Inc.
                                                            August 1989

                        Telnet Linemode Option

Status of this Memo

  This RFC describes a proposed elective standard for the Internet
  community.  Hosts on the Internet that support Linemode within the
  Telnet protocol are expected to adopt and implement this standard.
  Distribution of this memo is unlimited.

Overview

  Linemode Telnet is a way of doing terminal character processing on
  the client side of a Telnet connection.  While in Linemode with
  editing enabled for the local side, network traffic is reduced to a
  couple of packets per command line, rather than a couple of packets
  per character typed.  This is very useful for long delay networks,
  because the user has local response time while typing the command
  line, and only incurs the network delays after the command is typed.
  It is also useful to reduce costs on networks that charge on a per
  packet basis.

Table of Contents

  1.   Command Names and Codes                                        2
  2.   Command Meanings                                               3
  2.1  The LINEMODE function                                          3
  2.2  LINEMODE suboption MODE                                        3
  2.3  LINEMODE suboption FORWARDMASK                                 4
  2.4  LINEMODE suboption SLC, Set Local Characters                   5
  2.5  New control characters                                         8
  3.   Default Specification                                          9
  4.   Motivation                                                     9
  5.   Implementation Rules                                          11
  5.1  User Interface                                                11
  5.2  End of line terminators                                       12
  5.3  Output processing                                             12
  5.4  A terminal driver in Telnet?                                  12
  5.5  Setting of Local Characters                                   12
  5.6  FORWARDMASK and SLC_FORW1 and SLC_FORW2                       13
  5.7  Valid and invalid modes and values.                           14
  5.8  Flushing input and output                                     14

Telnet Linemode Working Group [Page 1]

RFC 1116 Telnet Linemode Option August 1989

  5.9  State diagram for SLC                                         16
  5.10 Example of a connection                                       17
  6.   Other Telnet options and RFCs                                 20

1. Command Names and Codes

      LINEMODE        34
          MODE             1
              EDIT             1
              TRAPSIG          2
              MODE_ACK         4
          FORWARDMASK      2
          SLC              3
              SLC_SYNCH        1
              SLC_BRK          2
              SLC_IP           3
              SLC_AO           4
              SLC_AYT          5
              SLC_EOR          6
              SLC_ABORT        7
              SLC_EOF          8
              SLC_SUSP         9
              SLC_EC          10
              SLC_EL          11
              SLC_EW          12
              SLC_RP          13
              SLC_LNEXT       14
              SLC_XON         15
              SLC_XOFF        16
              SLC_FORW1       17
              SLC_FORW2       18

              SLC_DEFAULT      3
              SLC_VALUE        2
              SLC_CANTCHANGE   1
              SLC_NOSUPPORT    0
              SLC_LEVELBITS    3

              SLC_ACK        128
              SLC_FLUSHIN     64
              SLC_FLUSHOUT    32
      EOF            236
      SUSP           237
      ABORT          238

Telnet Linemode Working Group [Page 2]

RFC 1116 Telnet Linemode Option August 1989

2. Command Meanings

2.1 The LINEMODE function

  IAC WILL LINEMODE

     The sender of this command REQUESTS permission to begin sub-
     negotiation of the editing/signaling status.  This should only be
     sent by the client side of the connection.

  IAC WONT LINEMODE

     The sender of this command DEMANDS that sub-negotiation of the
     editing/signaling status not be allowed.

  IAC DO LINEMODE

     The sender of this command REQUESTS that the remote side begin
     subnegotiation of the editing/signaling status.  This should only
     be sent by the server side of the connection.

  IAC DONT LINEMODE

     The sender of this command DEMANDS that the remote side not begin
     subnegotiation of the editing/signaling status.

2.2 LINEMODE suboption MODE

  IAC SB LINEMODE MODE mask IAC SE

     The sender of this command CONFIRMS, or REQUESTS permission for, a
     switch to the mode defined by "mask".

  The "mask" is a bit mask of various modes that the connection can be
  in.  Under normal operation, the server side of the connection will
  initiate mode changes, and the client will confirm the mode changes.
  The currently defined modes are:

     EDIT    When set, the client side of the connection should
             process all input lines, performing any editing
             functions, and only send completed lines to the remote
             side.  When unset, client side should not process any
             input from the user, and the server side should take
             care of all character processing that needs to be done.

     TRAPSIG When set, the client side should translate appropriate
             interrupts/signals to their Telnet equivalent.
             (These would be IP, BRK, AYT, ABORT, EOF, and SUSP.)

Telnet Linemode Working Group [Page 3]

RFC 1116 Telnet Linemode Option August 1989

             When unset, the client should pass interrupts/signals
             as their normal ASCII values.

     FLOW    Logically, this belongs in the "mask".  However,
             this would overlap the Telnet TOGGLE-FLOW-CONTROL
             option, so the Telnet TOGGLE-FLOW-CONTROL option is
             used instead.  When DO/WILL LINEMODE is negotiated,
             DO/WILL TOGGLE-FLOW-CONTROL should also be negotiated.
             See RFC 1080, "Telnet Remote Flow Control", for
             correct usage.

     ECHO    Logically, this belongs in the "mask".  However,
             this would overlap the Telnet ECHO option, so the
             Telnet ECHO option is used instead.  The client side
             should never negotiate "WILL ECHO".  When the server
             has negotiated "WILL ECHO", the client should not
             echo data typed by the user back to the user.  When
             the server has negotiated "WONT ECHO", the client is
             responsible for echoing data typed by the user back
             to the user.  See RFC 857, "Telnet ECHO OPTION" for
             a complete discussion on the use of the Telnet ECHO
             option.

  When the client side of a connection receives a MODE command, it MUST
  agree with at least the state of the EDIT and TRAPSIG bits.  If a
  MODE command is received with a mode mask that is currently in use
  (ignoring the MODE_ACK bit), the MODE command is ignored.  If a MODE
  command is received that is different from the current mode mask,
  then a reply is sent with either the new mode mask and the MODE_ACK
  bit set, or a subset of the new mode mask.  The only exception is
  that if the server receives a MODE with either the EDIT or TRAPSIG
  bits not set, it may set the EDIT and TRAPSIG bits in the response,
  and if the client receives a MODE with the EDIT or TRAPSIG bits set,
  it may not clear them in the response.

  When a MODE command is received with the MODE_ACK bit set, and the
  mode is different that what the current mode is, the client will
  ignore the new mode, and the server will switch to the new mode.
  This ensures that both sides of the connection will resolve to the
  same mode.  In all cases, a response is never generated to a MODE
  command that has the MODE_ACK bit set.

2.3 LINEMODE suboption FORWARDMASK

  IAC SB LINEMODE DO FORWARDMASK mask0 mask1 ... mask31 IAC SE

     The sender of this command request that the other side send any
     buffered data when any of the ASCII characters defined by the bit

Telnet Linemode Working Group [Page 4]

RFC 1116 Telnet Linemode Option August 1989

     mask are received.  Only the side of the connection that sent DO
     LINEMODE (the server side) may negotiate this.  The mask is up to
     32 octets long.  Each octet represents 8 ASCII character codes.
     The high order bit of mask0 corresponds to an ASCII code of 0.
     The low order bit of mask0 corresponds to an ASCII code of 7.  The
     high order bit of mask1 corresponds to an ASCII code of 8.  The
     low order bit of mask1 corresponds to an ASCII code of 15, and so
     on.  The mask list may be terminated before the end of the list,
     in which case all the rest of the mask octets are assumed to be
     reset (equal to zero).  When the server side is in DONT TRANSMIT-
     BINARY mode, then only the first 16 octets of the mask (ASCII
     codes 0 through 127) are used.  If any individual octet of the
     mask is equal to IAC, it must be sent as a double IAC.

  IAC SB LINEMODE DONT FORWARDMASK IAC SE

     The sender of this command requests that the other side stop using
     the forward mask to determine when to send buffered data.

  IAC SB LINEMODE WILL FORWARDMASK IAC SE

     This command is sent in response to a DO FORWARDMASK command.  It
     indicates that the forward mask will be used to determine when to
     send buffered data.

  IAC SB LINEMODE WONT FORWARDMASK IAC SE

     This command is sent in response to a DO FORWARDMASK command.  It
     indicates that the forward mask will not be used to determine when
     to send buffered data.

2.4 LINEMODE suboption SLC, Set Local Characters

  The SLC suboption uses a list of octet triplets.  The first octet
  specifies the function, the second octet specifies modifiers to the
  function, and the third octet specifies the ASCII character for the
  function.

  IAC SB LINEMODE SLC <list of octet triplets> IAC SE

     The sender of this command REQUESTS that the list of octet
     triplets be used to set the local character to be used to send to
     perform the specified function.

     There are four levels that a function may be set to.
     SLC_NOSUPPORT is the lowest, SLC_CANTCHANGE is the next higher
     level, SLC_VALUE is above that, and SLC_DEFAULT is the highest
     level.

Telnet Linemode Working Group [Page 5]

RFC 1116 Telnet Linemode Option August 1989

     If the SLC_LEVELBITS in the second octet are equal to SLC_DEFAULT,
     then this particular function should use the system default on the
     other side of the connection.

     If the SLC_LEVELBITS in the second octet are equal to SLC_VALUE,
     then this function is supported, and the current value is
     specified by the third octet.

     If the SLC_LEVELBITS in the second octet are equal to
     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.

5.1 User Interface

  Normally, the entire user interface is left up to the implementor.
  However, there is functionality that the user should be able to
  specify on the client side of the connection.  During a Telnet
  session, the client side should allow some mechanism for the user to
  give commands to the local Telnet process.  These commands should at
  least allow the user to:

     1)  Change the mode of the connection.  The user should be able
         to attempt to turn EDIT, FLOW, TRAPSIG, and ECHO on and off.
         The server may refuse to change the state of the EDIT and
         TRAPSIG bits.

     2)  Import or export SLC.  The user should be able to tell the
         local Telnet process whether he wants to use the local or
         the current or default remote definitions of the special
         characters.

     3)  Manual sending of options.  The user should be able to tell
         the local Telnet process to explicitly send any of the Telnet
         options (like IP, ABORT, AYT, etc.).

Telnet Linemode Working Group [Page 11]

RFC 1116 Telnet Linemode Option August 1989

5.2 End of line terminators

  When LINEMODE is turned on, and when in EDIT mode, when any normal
  line terminator on the client side operating system is typed, the
  line should be transmitted with "CR LF" as the line terminator.  When
  EDIT mode is turned off, a carriage return should be sent as "CR
  NUL", a line feed should be sent as LF, and any other key that cannot
  be mapped into an ASCII character, but means the line is complete
  (like a DOIT or ENTER key), should be sent as "CR LF".

5.3 Output processing

  Regardless of what mode has been negotiated, the server side is
  responsible for doing all output processing.  Specifically, it should
  send "CR LF" when it wants the "newline" function, "CR NUL" when it
  wants just a carriage return, and "LF" when it wants just a linefeed.

5.4 A terminal driver in Telnet?

  Conforming implementations need not do all the line editing
  themselves.  There is nothing wrong with letting the system terminal
  driver handle the line editing, and have it hand to the Telnet
  application the completed and edited line, which is then sent to the
  remote system.

5.5 Setting of Local Characters

  When this RFC was being developed, the original thought was that both
  sides of the connection would use their own defaults for the special
  characters, even if they were not the same on both sides of the
  connection.  If this scheme is used, though, the view that the user
  has is that the local special characters are being used, and the
  remote character settings don't matter.  It was decided that the
  client side of the connection should be in control of the character
  settings.

  When LINEMODE is negotiated, the client must either export the local
  character settings to the server, or send a request (SLC 0
  SLC_DEFAULT 0) to import the servers special characters.  The usual
  action would be that a client running on a full fledged computer
  would export the special characters, and a client running where there
  are no local defaults (like on some terminal servers) would import
  the special characters.

  When an SLC command is received, the action taken should be:

     1)  Ignore it if it is the same as the current settings.

Telnet Linemode Working Group [Page 12]

RFC 1116 Telnet Linemode Option August 1989

     2)  If the SLC_LEVELBITS are the same as the current level bits,
         but the value is different and the SLC_ACK bit is set, no
         reply is generated.  On the server side, the command is
         ignored, and on the client side, a switch is made to the new
         value.  This is so that if a request to change the same
         character is generated by both the server and the client,
         they will both settle on the clients requested value.

     3)  If we agree with the new setting, we switch to it and reply
         with the same value, but also set the SLC_ACK bit.

     4)  If we don't agree, we send a response with what we think
         the value should be.  The SLC_ACK bit is NOT set in this
         case.  You may only disagree with a value by sending a
         different value at a lower level.

  If the remote system doesn't support some of the line editing
  characters, but the front end does, then the front end may use the
  local definitions for those characters when in line mode.  In this
  case, the server should send "SLC xxx SLC_DEFAULT 0" in response to a
  "SLC 0 SLC_DEFAULT 0" request, and just ack whatever value the client
  requests to set the function to.

  The SLC_FORW2 character should only be used if SLC_FORW1 is already
  in use.

5.6 FORWARDMASK and SLC_FORW1 and SLC_FORW2

  To help ease the amount of work needed to implement the client side,
  two methods of setting forwarding characters are provided.  The
  SLC_FORW1 and SLC_FORW2 allow for the setting of two additional
  characters on which to forward buffered input data.  Since many
  terminal drivers have the ability to set one or more line delimiters,
  it is fairly easy to support these without having to implement
  through the local terminal driver, rather than putting a terminal
  driver into Telnet.  If the local terminal driver has functionality
  that maps easily into the FORWARDMASK, then it can also be easily
  supported.  If the local terminal driver does not support that, then
  it would require more work to support FORWARDMASK.

  Also note that the client side is required to forward data when it
  sees one of SLC_FORW1, SLC_FORW2, or FORWARDMASK characters, or when
  any normal line termination or special signal is encountered.  The
  client side is also free to forward on other characters that it
  chooses.  For example, if the server side sent a FORWARDMASK that
  asked for data to be forwarded on the first 20 control characters
  (ASCII codes 1 through 024), and the client side cannot have its
  local terminal driver forward on just the first 20 control

Telnet Linemode Working Group [Page 13]

RFC 1116 Telnet Linemode Option August 1989

  characters, but it can have the local terminal driver forward on any
  control character (ASCII codes 1 through 039), then the client side
  could validly accept the FORWARDMASK, and forward on any control
  character.  When in EDIT mode, care should be taken to not forward at
  random times, since once that data is forwarded, no more editing on
  the forwarded part of the line can be done.  The only time (other
  than the normal times) that data should be forwarded when in EDIT
  mode would be if a single input line is too long to handle locally.

5.7 Valid and invalid modes and values

  At no time should "DO LINEMODE" be negotiated in both directions of
  the Telnet connection.  The side that is the "DO LINEMODE" is
  considered to be the server side, and the side that is "WILL
  LINEMODE" is the client side.

  At no time should "SB LINEMODE DO/DONT FORWARDMASK", be sent unless
  "DO LINEMODE" has been previously negotiated.  At no time should "SB
  LINEMODE WILL/WONT FORWARDMASK", be sent unless "WILL LINEMODE" has
  been previously negotiated.

  If an ABORT, EOF or SUSP, is received and the system does not support
  that functionality, it may just be ignored.

5.8 Flushing input and output

  When an IP, BRK or ABORT is sent, it is usually desirable to be able
  to flush the input stream, and to flush output to the user until the
  IP, BRK, or ABORT is processed.  The SLC_FLUSHIN and SLC_FLUSHOUT
  bits are used to indicate what action should be done.  These bits are
  advisory only, but should be honored if possible.  The standard
  method for processing the SLC_FLUSHIN is to use the Telnet "Synch"
  signal, and the SLC_FLUSHOUT is processed using the TIMING-MARK
  option.  If both are to be sent, the IAC DM is sent before the DO
  TIMING-MARK.  Thus, the sender would send "IAC XXX IAC DM IAC DO
  TIMING-MARK", where XXX may be IP, BRK or ABORT, or any other special
  character.  The IAC DM is sent as TCP urgent data with the DM as the
  last (or only) data octet; this is used to flush the input stream.
  The "IAC DO TIMING-MARK" is used to tell when to stop flushing
  output; once it is sent, all data is discarded until an "IAC WILL
  TIMING-MARK" or an "IAC WONT TIMING-MARK" is received.

  Since the SLC_FLUSHIN and SLC_FLUSHOUT bit are only advisory, the
  user interface should provide a method so that the user can override
  the sending (or not sending) of the "Synch" and TIMING-MARK, but the
  default action should be to send them according to the SLC_FLUSHIN
  and SLC_FLUSHOUT bits.

Telnet Linemode Working Group [Page 14]

RFC 1116 Telnet Linemode Option August 1989

  Whenever an IAC AO is received, a Synch must be returned.  Whenever a
  Synch is being processed, (by the TCP connection going into Urgent
  mode), all data must be discarded (but not Telnet commands!) until an
  IAC DM is found, and the connection goes out of Urgent mode.  See RFC
  854, "TELNET PROTOCOL SPECIFICATION", for a complete description of
  the Synch signal.

Telnet Linemode Working Group [Page 15]

RFC 1116 Telnet Linemode Option August 1989

5.9 State diagram for SLC

  +---------------------------------------------------------------+
  |                                IDLE                           |
  +----------------------+------+------+-------+-------+---------++
   ^      ^     ^        |      | ^    | ^     | ^     |       ^ |
   |      |     |        v      v |    | |     | |     v       | |
   |      |     |    +------+ +---+--+ | |     | | ########### | |
   |      |     |    | Get  | | Send | | |     | | #   Get   # | |
   |      |     |    | SPC0 | | SPC0 | | |     | | # 0,DEF,0 # | |
   |      |     |    +---+--+ +------+ | |     | | ########### | |
   |      |     |        |       ^     | |     | |     |       | |
   |      |     |        v       |     v |     | |     v       | |
   |      |     |       / \      | *********** | | ########### | |
   |      |     |     /     \    | *  Send   * | | # Switch  # | |
   | ********** |Yes/ Same as \  | * 0,VAL,0 * | | # to      # | |
   | * Change * +--<  current? > | *********** | | # default # | |
   | * to new *     \         /  |             v | ########### | |
   | * value  *       \     /    |     ***********     |       | |
   | **********         \ /      |     *  Send   *     v       | |
   |      ^              |No     |     * 0,DEF,0 *  #########  | |
   |      |Yes           v       |     ***********  # Send  #--+ |
   |     / \            / \      |                  # SPC-A #    |
   |   /     \        /     \    |                  #########    |
   | / Is ACK  \ Yes/  Same   \  |                     ^         |
   |< bit set?  ><-<  level as > |                     |         |
   | \         /    \ current?/  |                 ###########   |
   |   \     /        \     /    |                 #   Get   #<--+
   |     \ /            \ /    +-+---+             # 0,VAL,0 #
   |      |No            |No   | Set |             ###########
   |      +--------------+     | ACK |
   |                     v     | bit |      * - Client side only
   |                    / \    +-----+      # - Server side only
   |   +------+       /     \      ^
   |   | Send |  No /  Do we  \ Yes|
   +---| SPC1 |<---<   agree?  >---+
       +------+     \         /
                      \     /
                        \ /

          SPC0    Initial setting for a special character
          SPC1    A changed special character < SPC0
          SPC-A   All current special character settings
          VAL     SLC_VALUE level
          DEF     SLC_DEFAULT level

Telnet Linemode Working Group [Page 16]

RFC 1116 Telnet Linemode Option August 1989

  Levels: DEFAULT, VALUE, CANT_CHANGE, NOSUPPORT
  Flags: ACK

          Receive                 Response
          -------                 --------
          f,SLC_DEFAULT,x         f,SLC_VALUE,v
                                  f,SLC_CANTCHANGE,v
                                  f,SLC_NOSUPPORT,x

          f,SLC_VALUE,v           f,SLC_ACK|SLC_VALUE,v
                                  f,SLC_CANTCHANGE,w
                                  f,SLC_NOSUPPORT,x

          f,SLC_CANTCHANGE,v      f,SLC_ACK|SLC_CANTCHANGE,v
                                  f,SLC_NOSUPPORT,x

          f,SLC_NOSUPPORT,x       f,SLC_ACK|SLC_NOSUPPORT,x

          x,SLC_ACK|x,x           no response

5.10 Examples of a connection

  In these examples, the symbolic names are used rather than the actual
  values, to make them readable.  When two or more symbolic names are
  joined by a |, it means that the actual value will be the logical
  "or" of the values of the symbolic names.  In the interest of
  clarity, for these examples the leading IAC and IAC SB sequences, and
  the trailing IAC SE sequences have been omitted.  Also, the SLC_
  prefix has been left off where ever it would normally occur.

      CLIENT                        SERVER
      ------                        ------
                                    DO TOGGLE-FLOW-CONTROL
                                    DO LINEMODE
      WILL TOGGLE-FLOW-CONTROL
      WILL LINEMODE
      [ Subnegotiation may now proceed in both directions.  The client
        sends of the list of special characters.  ]
      LINEMODE SLC SYNCH DEFAULT 0
      IP VALUE|FLUSHIN|FLUSHOUT 3 AO
      VALUE 15 AYT DEFAULT 0 ABORT
      VALUE|FLUSHIN|FLUSHOUT 28 EOF
      VALUE 4 SUSP VALUE|FLUSHIN 26
      EC VALUE 127 EL VALUE 21 EW
      VALUE 23 RP VALUE 18 LNEXT
      VALUE 22 XON VALUE 17 XOFF
      VALUE 19
      [ Now that linemode is enabled,  the  server  sets  the  initial

Telnet Linemode Working Group [Page 17]

RFC 1116 Telnet Linemode Option August 1989

        mode, and acknowledges the special characters.  ]
                                    LINEMODE MODE EDIT

                                    LINEMODE SLC SYNCH NOSUPPORT 0 IP
                                    VALUE|FLUSHIN|FLUSHOUT|ACK 3 AO
                                    NOSUPPORT 0 AYT NOSUPPORT 0 ABORT
                                    VALUE|FLUSHIN|FLUSHOUT|ACK 28 EOF
                                    VALUE|ACK 4 SUSP NOSUPPORT 0 EC
                                    VALUE|ACK 127 EL VALUE|ACK 21 EW
                                    VALUE|ACK 23 RP VALUE|ACK 18 LNEXT
                                    VALUE|ACK 22 XON VALUE|ACK 17 XOFF
                                    VALUE|ACK 19
      [ The client gets the mode and ack of the special characters,
        and acks the mode and any special characters that the server
        changed.  ]
      LINEMODE MODE EDIT|MODE_ACK

      LINEMODE SLC SYNCH
      NOSUPPORT|ACK 0 AO
      NOSUPPORT|ACK 0 AYT|ACK NOSUP-
      PORT 0 SUSP NOSUPPORT|ACK 0
                                    "Login:"
      "my_account"
      [ Turn off echo to the user.  ]
                                    WILL ECHO
      DO ECHO
                                    "Password:"
      "my_password"
      [ Turn back on echo to the user.  ]
                                    WONT ECHO
      DONT ECHO
      [ User does some stuff, and then runs an application that wants
        to use single character mode, doing its own echoing of
        characters, but keep signal trapping on.  ]
                                    WILL ECHO
      DO ECHO
                                    LINEMODE MODE TRAPSIG
      LINEMODE MODE TRAPSIG|MODE_ACK
      [ Application finishes.  ]
                                    WONT ECHO
      DONT ECHO
                                    LINEMODE MODE EDIT|TRAPSIG
      LINEMODE MODE
      EDIT|TRAPSIG|MODE_ACK
      [ Another application, that wants full control of everything.  ]
                                    WILL ECHO
      DO ECHO
                                    LINEMODE MODE 0

Telnet Linemode Working Group [Page 18]

RFC 1116 Telnet Linemode Option August 1989

      LINEMODE MODE 0|MODE_ACK
      [ Application finishes.  ]
                                    WONT ECHO
      DONT ECHO
                                    LINEMODE MODE EDIT|TRAPSIG
      LINEMODE MODE
      EDIT|TRAPSIG|MODE_ACK
      [ The user changes his erase character to ^H.  ]
                                    LINEMODE SLC EC VALUE 8
      LINEMODE SLC EC VALUE|ACK 8
      [ The user decides to revert to all the original client side
        special characters.  ]
      LINEMODE SLC SYNCH DEFAULT 0
      IP VALUE|FLUSHIN|FLUSHOUT 3 AO
      VALUE 15 AYT DEFAULT 0 ABORT
      VALUE|FLUSHIN|FLUSHOUT 28 EOF
      VALUE 4 SUSP VALUE|FLUSHIN 26
      EC VALUE 127 EL VALUE 21 EW
      VALUE 23 RP VALUE 18 LNEXT
      VALUE 22 XON VALUE 17 XOFF
      VALUE 19
                                    LINEMODE SLC SYNCH NOSUPPORT 0 AO
                                    NOSUPPORT 15 AYT NOSUPPORT 0 SUSP
                                    NOSUPPORT|FLUSHIN 26 EC VALUE|ACK
                                    127 EW VALUE|ACK 23 RP VALUE|ACK
                                    18 LNEXT VALUE|ACK 22 XON
                                    VALUE|ACK 17 XOFF VALUE|ACK 19
      LINEMODE SLC SYNCH
      NOSUPPORT|ACK 0 AO
      NOSUPPORT|ACK 15 AYT
      NOSUPPORT|ACK 0 SUSP
      NOSUPPORT|ACK|FLUSHIN 26
      [ The user decides to import the remote sides default special
        characters.  ]
      LINEMODE SLC 0 DEFAULT 0
                                    LINEMODE SLC IP
                                    VALUE|FLUSHIN|FLUSHOUT 3 ABORT
                                    VALUE|FLUSHIN|FLUSHOUT 28 EOF
                                    VALUE 4 EC VALUE 127 EL VALUE 21
      [ Since these are the same as the current local settings, no
        response is generated.  ]
      [ This next example is what would happen if an editor was fired
        up, that wanted to let the client side do the echoing and
        buffering of characters, but did not want it to do any line
        editing, and only forward the data when got a control
        character.  Note that we have preceded all the the 0377s in the
        forward mask with an IAC.  ]
                                    LINEMODE MODE 0

Telnet Linemode Working Group [Page 19]

RFC 1116 Telnet Linemode Option August 1989

                                    LINEMODE DO FORWARDMASK IAC 0377
                                    IAC 0377 IAC 0377 IAC 0377 0 0 0 0
                                    0 0 0 0 0 0 0 01
      LINEMODE MODE 0
      LINEMODE WILL FORWARDMASK
      [ Application runs to completion, and then things are to be set
        back to what they were before.  ]
                                    LINEMODE MODE EDIT|TRAPSIG
                                    LINEMODE DONT FORWARDMASK
      LINEMODE MODE EDIT|TRAPSIG
      LINEMODE WONT FORWARDMASK

6. Other Telnet options and RFCs

  The following is a list of RFCs for various Telnet options  that
  should be supported along with LINEMODE.

  1.  Postel, J. and J. Reynolds, "TELNET PROTOCOL SPECIFICATION", RFC
      854, USC/Information Sciences Institute, May 1983.

  2.  Postel, J. and J. Reynolds, "TELNET OPTION SPECIFICATIONS", RFC
      855, USC/Information Sciences Institute, May 1983.

  3.  Postel, J. and J. Reynolds, "TELNET BINARY TRANSMISSION", RFC
      856, USC/Information Sciences Institute, May 1983.

  4.  Postel, J. and J. Reynolds, "TELNET ECHO OPTION", RFC 857,
      USC/Information Sciences Institute, May 1983.

  5.  Postel, J. and J. Reynolds, "TELNET SUPRESS GO AHEAD OPTION", RFC
      858, USC/Information Sciences Institute, May 1983.

  6.  Postel, J. and J. Reynolds, "TELNET TIMING MARK OPTION", RFC 860,
      USC/Information Sciences Institute, May 1983.

  7.  VanBokkeln, J., "Telnet Terminal-Type Option", RFC 1091, FTP
      Software, Inc., February 1989.

  8.  Waitzman, D., "Telnet Window Size Option", RFC 1073, BBN STC,
      October 1988.

  9.  Hedrick, C., "Telnet Remote Flow Control Option", RFC 1080,
      Rutgers University, November, 1988.

 10.  Hedrick, C., "Telnet Terminal Speed Option", RFC 1079, Rutgers
      University, December, 1988.

  The following is a list of RFCs that need not be supported for

Telnet Linemode Working Group [Page 20]

RFC 1116 Telnet Linemode Option August 1989

  LINEMODE, but which would enhance any TELNET implementation.

 11.  Postel, J. and J. Reynolds, "TELNET STATUS OPTION", RFC 859,
      USC/Information Sciences Institute, May 1983.

 12.  Postel, J. and J. Reynolds, "TELNET END OF RECORD OPTION", RFC
      885, USC/Information Sciences Institute, December 1983.

 13.  Silverman, S., "OUTPUT MARKING TELNET OPTION", RFC 933, MITRE-
      Washington, January 1985.

 14.  Marcy, G., "Telnet X Display Location Option", RFC 1096, Carnegie
      Mellon University, March 1989.

Author's Address

      Dave Borman
      Cray Research Inc.
      1440 Northland Drive
      Mendota Heights, MN 55120

      Phone: (612) 681-3398

      EMail: [email protected]

Telnet Linemode Working Group [Page 21]