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rfc:rfc1184

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.

Telnet Working Group [Page 1] RFC 1184 Telnet Linemode Option October 1990

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

Telnet Working Group [Page 2] RFC 1184 Telnet Linemode Option October 1990

             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.

Telnet Working Group [Page 3] RFC 1184 Telnet Linemode Option October 1990

 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

Telnet Working Group [Page 4] RFC 1184 Telnet Linemode Option October 1990

             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

Telnet Working Group [Page 5] RFC 1184 Telnet Linemode Option October 1990

    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,

Telnet Working Group [Page 6] RFC 1184 Telnet Linemode Option October 1990

    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 Working Group [Page 7] RFC 1184 Telnet Linemode Option October 1990

 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.

Telnet Working Group [Page 8] RFC 1184 Telnet Linemode Option October 1990

    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

Telnet Working Group [Page 9] RFC 1184 Telnet Linemode Option October 1990

              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.

Telnet Working Group [Page 10] RFC 1184 Telnet Linemode Option October 1990

 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

Telnet Working Group [Page 11] RFC 1184 Telnet Linemode Option October 1990

 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.

Telnet Working Group [Page 12] RFC 1184 Telnet Linemode Option October 1990

 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

Telnet Working Group [Page 13] RFC 1184 Telnet Linemode Option October 1990

        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

Telnet Working Group [Page 14] RFC 1184 Telnet Linemode Option October 1990

 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.

Telnet Working Group [Page 15] RFC 1184 Telnet Linemode Option October 1990

 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

Telnet Working Group [Page 16] RFC 1184 Telnet Linemode Option October 1990

 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.

Telnet Working Group [Page 17] RFC 1184 Telnet Linemode Option October 1990

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

Telnet Working Group [Page 18] RFC 1184 Telnet Linemode Option October 1990

         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

Telnet Working Group [Page 19] RFC 1184 Telnet Linemode Option October 1990

                                      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

Telnet Working Group [Page 20] RFC 1184 Telnet Linemode Option October 1990

     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

Telnet Working Group [Page 21] RFC 1184 Telnet Linemode Option October 1990

       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",

Telnet Working Group [Page 22] RFC 1184 Telnet Linemode Option October 1990

     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

Telnet Working Group [Page 23]

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