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RFC 1184 - Telnet Linemode Option


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Network Working Group                               Telnet Working Group
Request for Comments: 1184                             D. Borman, Editor
Obsoletes: RFC 1116                                  Cray Research, Inc.
                                                            October 1990

                         Telnet Linemode Option

Status of this Memo

   This memo describes a Draft Standard for the Internet community, and
   requests discussion and suggestions for improvements.  This RFC
   specifies an IAB standards track protocol for the Internet community.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Changes from RFC1116:

      Two new mode bits have been added, SOFT_TAB and LIT_ECHO.  These
      bits allow the server to give the client some advise on how to
      echo tabs and non-printable characters.

      Several new special character mappings have been added for cursor
      motion when visual editing is supported.  These are: Move cursor
      one character left/right (SLC_MCL/SLC_MCR), move cursor one word
      left/right (SLC_MCWL/SLC_MCWR), move cursor to begining/end of
      line (SLC_MCBOL/SLC_MCEOL), enter insert/overstrike mode
      (SLC_INSRT/SLC_OVER), erase one character/word to the right
      (SLC_ECR/SLC_EWR), and erase to the beginning/end of the line
      (SLC_EBOL/SLC_EEOL).

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.  Please send comments to the telnet-ietf@cray.com
   mailing list.

Table of Contents

   1.   Command Names and Codes                                        2
   2.   Command Meanings                                               3
   2.1  The LINEMODE function                                          3
   2.2  LINEMODE suboption MODE                                        4
   2.3  LINEMODE suboption FORWARDMASK                                 5
   2.4  LINEMODE suboption SLC, Set Local Characters                   6
   2.5  New control characters                                        10
   3.   Default Specification                                         11
   4.   Motivation                                                    11
   5.   Implementation Rules                                          13
   5.1  User Interface                                                13
   5.2  End of line terminators                                       14
   5.3  Output processing                                             14
   5.4  A terminal driver in Telnet?                                  14
   5.5  Setting of Local Characters                                   14
   5.6  FORWARDMASK and SLC_FORW1 and SLC_FORW2                       15
   5.7  Valid and invalid modes and values                            16
   5.8  Flushing input and output                                     16
   5.9  State diagram for SLC                                         18
   5.10 Example of a connection                                       19
   6.   Other Telnet options and RFCs                                 22
   7.   Security Considerations                                       23
   8.   Author's Address                                              23

1.  Command Names and Codes

       LINEMODE        34
           MODE             1
               EDIT             1
               TRAPSIG          2
               MODE_ACK         4
               SOFT_TAB         8
               LIT_ECHO        16
           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_MCL         19
               SLC_MCR         20
               SLC_MCWL        21
               SLC_MCWR        22
               SLC_MCBOL       23
               SLC_MCEOL       24
               SLC_INSRT       25
               SLC_OVER        26
               SLC_ECR         27
               SLC_EWR         28
               SLC_EBOL        29
               SLC_EEOL        30

               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

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
      sub-negotiation 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
      sub-negotiation 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) 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 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.

      SOFT_TAB When set, the client side should expand the Horizontal
               Tab (HT) code, USASCII 9, into the appropriate number of
               spaces to move the printer to the next horizontal tab
               stop.  When unset, the client side should allow the
               Horizontal Tab code to pass through un-modified.

      LIT_ECHO When set, if the client side is echoing a non-printable
               character that the user has typed to the users screen,
               the character should be echoed as the literal character.
               If the LIT_ECHO bit is not set, then the client side may
               echo the character in any manner that it desires.  (Many
               systems echo unprintable characters as two character
               sequences, for example, they will echo "^A" for an
               ASCII 1 value.)

   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
      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.

      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.

   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.

      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.

      SLC_MCL   Move cursor one character left.  When visual editing is
                supported, this is the character that, when typed, will
                move the cursor one character to the left in the
                display.

      SLC_MCR   Move cursor one character right.  When visual editing is
                supported, this is the character that, when typed, will
                move the cursor one character to the right in the
                display.

      SLC_MCWL  Move cursor one word left.  When visual editing is
                supported, this is the character that, when typed, will
                move the cursor one word to the left in the display.

      SLC_MCWR  Move cursor one word right.  When visual editing is
                supported, this is the character that, when typed, will
                move the cursor one word to the right in the display.

      SLC_MCBOL Move cursor to the begining of the line.  When visual
                editing is supported, this is the character that, when
                typed, will move the cursor to the begining of the line
                that is being edited.

      SLC_MCEOL Move cursor to the end of the line.  When visual editing

                is supported, this is the character that, when typed,
                will move the cursor to the end of the line that is
                being edited.

      SLC_INSRT Enter insert mode.  When visual editing is supported,
                after this character is typed, all normal characters
                that are subsequently typed will be inserted into the
                display.

      SLC_OVER  Enter overstrike mode.  When visual editing is
                supported, after this character is typed, all normal
                charactersthat are subsequently typed will overwrite
                any characters in the current display.  If the
                SLC_INSRT and SLC_OVER variables are set to the same
                value, then that value is to act as a toggle between
                overstrike and insert mode.

      SLC_ECR   Erase character to the right.  When visual editing is
                supported, this is the character that, when typed, will
                erase one character to the right of the cursor.

      SLC_EWR   Erase word to the right.  When visual editing is
                supported, this is the character that, when typed,
                will erase one word to the right of the cursor.

      SLC_EBOL  Erase to the begining of the line.  When visual editing
                is supported, this is the character that, when typed,
                will erase all the characters to the left of the cursor.

      SLC_EEOL  Erase to the end of the line.  When visual editing is
                supported, this is the character that, when typed, will
                erase all characters to the right of the cursor.

   For SLC_EEOL, SLC_EWR, and SLC_ECR, if a system has a cursor that is
   not diplayed between characters, but is positioned over a character,
   that character is assumed to be to the right of the cursor.  Thus,
   the SLC_ECR will erase the character that is under the current cursor
   position.

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.

   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 necessary 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.

   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
   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).

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.  Specificly, 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.

      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
   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.

   Whenever an IAC AO is received, a Synch must be returned.  When ever
   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.

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

   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 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 NOSUP-
                                        PORT 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
       NOSUPPORT 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
       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
                                        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 Reynolds, J., "TELNET PROTOCOL SPECIFICATION",
       RFC 854, ISI, May 1983.

   2.  Postel, J. and Reynolds, J., "TELNET OPTION SPECIFICATIONS",
       RFC 855, ISI, May 1983.

   3.  Postel, J. and Reynolds, J., "TELNET BINARY TRANSMISSION",
       RFC 856, ISI, May 1983.

   4.  Postel, J. and Reynolds, J., "TELNET ECHO OPTION", RFC 857,
       ISI, May 1983.

   5.  Postel, J. and Reynolds, J., "TELNET SUPRESS GO AHEAD OPTION",
       RFC 858, ISI, May 1983.

   6.  Postel, J. and Reynolds, J., "TELNET TIMING MARK OPTION",
       RFC 860, ISI, 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
   LINEMODE, but which would enhance any TELNET implementation.

  11.  Postel, J. and Reynolds, J., "TELNET STATUS OPTION", RFC 859,
       ISI, May 1983.

  12.  Postel, J. and Reynolds, J., "TELNET END OF RECORD OPTION",

       RFC 885, ISI, 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.

Security Consideratiions

   Security issues are not discussed in this memo.

Author's Address

   David A. Borman
   Cray Research Inc.
   655F Lone Oak Drive
   Eagan, MN 55123

   Phone: (612) 452-6650
   EMail: dab@CRAY.COM

   IETF Telnet WG Mailing List: telnet-ietf@CRAY.COM

 

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