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 [email protected] mailing list.

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

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

Command Meanings

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.

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.

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.

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.

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.

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.

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.

Implementation Rules

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

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).

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".

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.

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.

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.

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.

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.

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.

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

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: [email protected]

IETF Telnet WG Mailing List: [email protected]