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Network Working Group                               Telnet Working GroupRequest for Comments: 1184                             D. Borman, EditorObsoletes: RFC1116                                  Cray Research, Inc.                                                            October 1990Telnet Linemode OptionStatus 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 fromRFC1116:      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 1990Table 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                                              231.  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          11Telnet 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          2382.  Command Meanings2.1 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.  SeeRFC 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 theTelnet 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.  SeeRFC 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 toTelnet 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.  SeeRFC 854, "TELNET PROTOCOL SPECIFICATION",                for a complete description.      SLC_BRK   Break.  SeeRFC 854, "TELNET PROTOCOL SPECIFICATION",                for a complete description.      SLC_IP    Interrupt Process.  SeeRFC 854, "TELNET PROTOCOL                SPECIFICATION", for a complete description.      SLC_AO    Abort Output.  SeeRFC 854, "TELNET PROTOCOL                SPECIFICATION", for a complete description.      SLC_AYT   Are You There.  SeeRFC 854, "TELNET PROTOCOL                SPECIFICATION", for a complete description.      SLC_EOR   End of Record.  SeeRFC 885, "TELNET END OF RECORD                OPTION" for a complete description.      SLC_ABORT Abort.  Seesection 2.5 for a complete description.      SLC_EOF   End of File.  Seesection 2.5 for a complete                description.      SLC_SUSP  Suspend.  Seesection 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.  SeeRFC 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.                SeeRFC 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 editingTelnet 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 frontTelnet 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 intoRFC 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 theTelnet 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 computerTelnet 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 WILLTelnet 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.  SeeRFC854, "TELNET PROTOCOL SPECIFICATION", for a complete description of   the Synch signal.Telnet Working Group                                           [Page 17]

RFC 1184                 Telnet Linemode Option             October 19905.9 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: ACKTelnet 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 response5.10 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 3Telnet 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|TRAPSIGTelnet 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 setTelnet 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 FORWARDMASK6.  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 1990RFC 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.COMTelnet Working Group                                           [Page 23]

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