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diff --git a/doc/rfc/rfc1184.txt b/doc/rfc/rfc1184.txt new file mode 100644 index 0000000..9e337df --- /dev/null +++ b/doc/rfc/rfc1184.txt @@ -0,0 +1,1291 @@ + + + + + + +Network Working Group Telnet Working Group +Request for Comments: 1184 D. Borman, Editor +Obsoletes: RFC 1116 Cray Research, Inc. + October 1990 + + + Telnet Linemode Option + +Status of this Memo + + This memo describes a Draft Standard for the Internet community, and + requests discussion and suggestions for improvements. This RFC + specifies an IAB standards track protocol for the Internet community. + Please refer to the current edition of the "IAB Official Protocol + Standards" for the standardization state and status of this protocol. + Distribution of this memo is unlimited. + +Changes from RFC1116: + + Two new mode bits have been added, SOFT_TAB and LIT_ECHO. These + bits allow the server to give the client some advise on how to + echo tabs and non-printable characters. + + Several new special character mappings have been added for cursor + motion when visual editing is supported. These are: Move cursor + one character left/right (SLC_MCL/SLC_MCR), move cursor one word + left/right (SLC_MCWL/SLC_MCWR), move cursor to begining/end of + line (SLC_MCBOL/SLC_MCEOL), enter insert/overstrike mode + (SLC_INSRT/SLC_OVER), erase one character/word to the right + (SLC_ECR/SLC_EWR), and erase to the beginning/end of the line + (SLC_EBOL/SLC_EEOL). + +Overview + + Linemode Telnet is a way of doing terminal character processing on + the client side of a Telnet connection. While in Linemode with + editing enabled for the local side, network traffic is reduced to a + couple of packets per command line, rather than a couple of packets + per character typed. This is very useful for long delay networks, + because the user has local response time while typing the command + line, and only incurs the network delays after the command is typed. + It is also useful to reduce costs on networks that charge on a per + packet basis. Please send comments to the telnet-ietf@cray.com + mailing list. + + + + + + + +Telnet Working Group [Page 1] + +RFC 1184 Telnet Linemode Option October 1990 + + +Table of Contents + + 1. Command Names and Codes 2 + 2. Command Meanings 3 + 2.1 The LINEMODE function 3 + 2.2 LINEMODE suboption MODE 4 + 2.3 LINEMODE suboption FORWARDMASK 5 + 2.4 LINEMODE suboption SLC, Set Local Characters 6 + 2.5 New control characters 10 + 3. Default Specification 11 + 4. Motivation 11 + 5. Implementation Rules 13 + 5.1 User Interface 13 + 5.2 End of line terminators 14 + 5.3 Output processing 14 + 5.4 A terminal driver in Telnet? 14 + 5.5 Setting of Local Characters 14 + 5.6 FORWARDMASK and SLC_FORW1 and SLC_FORW2 15 + 5.7 Valid and invalid modes and values 16 + 5.8 Flushing input and output 16 + 5.9 State diagram for SLC 18 + 5.10 Example of a connection 19 + 6. Other Telnet options and RFCs 22 + 7. Security Considerations 23 + 8. Author's Address 23 + +1. Command Names and Codes + + LINEMODE 34 + MODE 1 + EDIT 1 + TRAPSIG 2 + MODE_ACK 4 + SOFT_TAB 8 + LIT_ECHO 16 + FORWARDMASK 2 + SLC 3 + SLC_SYNCH 1 + SLC_BRK 2 + SLC_IP 3 + SLC_AO 4 + SLC_AYT 5 + SLC_EOR 6 + SLC_ABORT 7 + SLC_EOF 8 + SLC_SUSP 9 + SLC_EC 10 + SLC_EL 11 + + + +Telnet Working Group [Page 2] + +RFC 1184 Telnet Linemode Option October 1990 + + + SLC_EW 12 + SLC_RP 13 + SLC_LNEXT 14 + SLC_XON 15 + SLC_XOFF 16 + SLC_FORW1 17 + SLC_FORW2 18 + SLC_MCL 19 + SLC_MCR 20 + SLC_MCWL 21 + SLC_MCWR 22 + SLC_MCBOL 23 + SLC_MCEOL 24 + SLC_INSRT 25 + SLC_OVER 26 + SLC_ECR 27 + SLC_EWR 28 + SLC_EBOL 29 + SLC_EEOL 30 + + SLC_DEFAULT 3 + SLC_VALUE 2 + SLC_CANTCHANGE 1 + SLC_NOSUPPORT 0 + SLC_LEVELBITS 3 + + SLC_ACK 128 + SLC_FLUSHIN 64 + SLC_FLUSHOUT 32 + EOF 236 + SUSP 237 + ABORT 238 + +2. Command Meanings + +2.1 The LINEMODE function + + IAC WILL LINEMODE + + The sender of this command REQUESTS permission to begin sub- + negotiation of the editing/signaling status. This should only be + sent by the client side of the connection. + + IAC WONT LINEMODE + + The sender of this command DEMANDS that sub-negotiation of the + editing/signaling status not be allowed. + + + + +Telnet Working Group [Page 3] + +RFC 1184 Telnet Linemode Option October 1990 + + + IAC DO LINEMODE + + The sender of this command REQUESTS that the remote side begin + sub-negotiation of the editing/signaling status. This should only + be sent by the server side of the connection. + + IAC DONT LINEMODE + + The sender of this command DEMANDS that the remote side not begin + sub-negotiation of the editing/signaling status. + +2.2 LINEMODE suboption MODE + + IAC SB LINEMODE MODE mask IAC SE + + The sender of this command CONFIRMS, or REQUESTS permission for, a + switch to the mode defined by "mask". + + The "mask" is a bit mask of various modes that the connection can be + in. Under normal operation, the server side of the connection will + initiate mode changes, and the client will confirm the mode changes. + The currently defined modes are: + + EDIT When set, the client side of the connection should + process all input lines, performing any editing functions, + and only send completed lines to the remote side. When + unset, client side should not process any input from the + user, and the server side should take care of all + character processing that needs to be done. + + TRAPSIG When set, the client side should translate appropriate + interrupts/signals to their Telnet equivalent. (These + would be IP, BRK, AYT, ABORT, EOF, and SUSP) When unset, + the client should pass interrupts/signals as their normal + ASCII values. + + FLOW Logically, this belongs in the "mask". However, this + would overlap the Telnet TOGGLE-FLOW-CONTROL option, so + the Telnet TOGGLE-FLOW-CONTROL option is used instead. + When DO/WILL LINEMODE is negotiated, DO/WILL TOGGLE- + FLOW-CONTROL should also be negotiated. See RFC 1080, + "Telnet Remote Flow Control", for correct usage. + + ECHO Logically, this belongs in the "mask". However, this + would overlap the Telnet ECHO option, so the Telnet ECHO + option is used instead. The client side should never + negotiate "WILL ECHO". When the server has negotiated + "WILL ECHO", the client should not echo data typed by the + + + +Telnet Working Group [Page 4] + +RFC 1184 Telnet Linemode Option October 1990 + + + user back to the user. When the server has negotiated + "WONT ECHO", the the client is responsible for echoing + data typed by the user back to the user. See RFC 857, + "Telnet ECHO OPTION" for a complete discussion on the use + of the Telnet ECHO option. + + SOFT_TAB When set, the client side should expand the Horizontal + Tab (HT) code, USASCII 9, into the appropriate number of + spaces to move the printer to the next horizontal tab + stop. When unset, the client side should allow the + Horizontal Tab code to pass through un-modified. + + LIT_ECHO When set, if the client side is echoing a non-printable + character that the user has typed to the users screen, + the character should be echoed as the literal character. + If the LIT_ECHO bit is not set, then the client side may + echo the character in any manner that it desires. (Many + systems echo unprintable characters as two character + sequences, for example, they will echo "^A" for an + ASCII 1 value.) + + When the client side of a connection receives a MODE command, it MUST + agree with at least the state of the EDIT and TRAPSIG bits. If a + MODE command is received with a mode mask that is currently in use + (ignoring the MODE_ACK bit), the MODE command is ignored. If a MODE + command is received that is different from the current mode mask, + then a reply is sent with either the new mode mask and the MODE_ACK + bit set, or a subset of the new mode mask. The only exception is + that if the server receives a MODE with either the EDIT or TRAPSIG + bits not set, it may set the EDIT and TRAPSIG bits in the response, + and if the client receives a MODE with the EDIT or TRAPSIG bits set, + it may not clear them in the response. + + When a MODE command is received with the MODE_ACK bit set, and the + mode is different that what the current mode is, the client will + ignore the new mode, and the server will switch to the new mode. + This ensures that both sides of the connection will resolve to the + same mode. In all cases, a response is never generated to a MODE + command that has the MODE_ACK bit set. + +2.3 LINEMODE suboption FORWARDMASK + + IAC SB LINEMODE DO FORWARDMASK mask0 mask1 ... mask31 IAC SE + + The sender of this command request that the other side send any + buffered data when any of the ASCII characters defined by the bit + mask are received. Only the side of the connection that sent DO + LINEMODE (the server side) may negotiate this. The mask is up to + + + +Telnet Working Group [Page 5] + +RFC 1184 Telnet Linemode Option October 1990 + + + 32 octets long. Each octet represents 8 ASCII character codes. + The high order bit of mask0 corresponds to an ASCII code of 0. + The low order bit of mask0 corresponds to an ASCII code of 7. The + high order bit of mask1 corresponds to an ASCII code of 8. The + low order bit of mask1 corresponds to an ASCII code of 15, and so + on. The mask list may be terminated before the end of the list, in + which case all the rest of the mask octets are assumed to be reset + (equal to zero). When the server side is in DONT TRANSMIT-BINARY + mode, then only the first 16 octets of the mask (ASCII codes 0 + through 127) are used. If any individual octet of the mask is + equal to IAC, it must be sent as a double IAC. + + IAC SB LINEMODE DONT FORWARDMASK IAC SE + + The sender of this command requests that the other side stop using + the forward mask to determine when to send buffered data. + + IAC SB LINEMODE WILL FORWARDMASK IAC SE + + This command is sent in response to a DO FORWARDMASK command. It + indicates that the forward mask will be used to determine when to + send buffered data. + + IAC SB LINEMODE WONT FORWARDMASK IAC SE + + This command is sent in response to a DO FORWARDMASK command. It + indicates that the forward mask will not be used to determine when + to send buffered data. + +2.4 LINEMODE suboption SLC, Set Local Characters + + The SLC suboption uses a list of octet triplets. The first octet + specifies the function, the second octet specifies modifiers to the + function, and the third octet specifies the ASCII character for the + function. + + IAC SB LINEMODE SLC <list of octet triplets> IAC SE + + The sender of this command REQUESTS that the list of octet + triplets be used to set the local character to be used to send to + perform the specified function. + + There are four levels that a function may be set to. + SLC_NOSUPPORT is the lowest, SLC_CANTCHANGE is the next higher + level, SLC_VALUE is above that, and SLC_DEFAULT is the highest + level. + + If the SLC_LEVELBITS in the second octet are equal to SLC_DEFAULT, + + + +Telnet Working Group [Page 6] + +RFC 1184 Telnet Linemode Option October 1990 + + + then this particular function should use the system default on the + other side of the connection. + + If the SLC_LEVELBITS in the second octet are equal to SLC_VALUE, + then this function is supported, and the current value is + specified by the third octet. + + If the SLC_LEVELBITS in the second octet are equal to + SLC_CANTCHANGE, then this is a function that is supported, but the + value for this function, specified in the third octet, cannot be + changed. + + If the SLC_LEVELBITS in the second octet are equal to + SLC_NOSUPPORT, then this particular function is not supported and + should be disabled by the other side. + + If this is a response to a previous request to change a special + character, and we are agreeing to the change, then the SLC_ACK bit + must be set in the second octet. + + If the SLC_FLUSHIN bit is set in the second octet, then whenever + this function is sent, a Telnet "sync" should be sent at the same + time to flush the input stream. + + If the SLC_FLUSHOUT bit is set in the second octet, then whenever + this function is sent, output data should be flushed. + + Only the client may send an octet triplet with the first octet + equal to zero. In this case, the SLC_LEVELBITS may only be set to + SLC_DEFAULT or SLC_VALUE, and the third octet does not matter. + When the server receives 0 SLC_DEFAULT 0, it should switch to its + system default special character settings, and send all those + special characters to the client. When the server receives 0 + SLC_VALUE 0, it should just send its current special character + settings. Note that if the server does not support some of the + editing functions, they should be sent as XXX SLC_DEFAULT 0, + rather than as XXX SLC_NOSUPPORT 0, so that the client may choose + to use its own values for those functions, rather than have to + disable those functions even if it supports them. + + If any of the octets in the list of octet triplets is equal to + IAC, it must be sent as a double IAC. + + When a connection is established, it is the responsibility of the + client to either request the remote default values for the special + characters, or to send across what all the special characters should + be set to. + + + + +Telnet Working Group [Page 7] + +RFC 1184 Telnet Linemode Option October 1990 + + + The function values can be put into two groups, functions that are to + be translated to their Telnet equivalents before being sent across + the Telnet connection, and functions that are to be recognized and + processed locally. + + First, we have those characters that are to be mapped into their + Telnet equivalents: + + SLC_SYNCH Synch. See RFC 854, "TELNET PROTOCOL SPECIFICATION", + for a complete description. + + SLC_BRK Break. See RFC 854, "TELNET PROTOCOL SPECIFICATION", + for a complete description. + + SLC_IP Interrupt Process. See RFC 854, "TELNET PROTOCOL + SPECIFICATION", for a complete description. + + SLC_AO Abort Output. See RFC 854, "TELNET PROTOCOL + SPECIFICATION", for a complete description. + + SLC_AYT Are You There. See RFC 854, "TELNET PROTOCOL + SPECIFICATION", for a complete description. + + SLC_EOR End of Record. See RFC 885, "TELNET END OF RECORD + OPTION" for a complete description. + + SLC_ABORT Abort. See section 2.5 for a complete description. + + SLC_EOF End of File. See section 2.5 for a complete + description. + + SLC_SUSP Suspend. See section 2.5 for a complete description. + + Next, we have the locally interpreted functions. + + SLC_EC Erase Character. This is the character that is typed to + erase one character from the input stream. See RFC 854, + "TELNET PROTOCOL SPECIFICATION", for a complete + description. + + SLC_EL Erase Line. This is the character that is typed to + erase the entire contents of the current line of input. + See RFC 854, "TELNET PROTOCOL SPECIFICATION", for a + complete description. + + SLC_EW Erase Word. This is the character that is typed to + erase one word from the input stream. + + + + +Telnet Working Group [Page 8] + +RFC 1184 Telnet Linemode Option October 1990 + + + SLC_RP Reprint Line. This is the character that is typed to + cause the current line of input to be reprinted, leaving + the cursor at the end of the line. + + SLC_LNEXT Literal Next. This is the character that is typed to + indicate that the next character is to be taken + literally, no character processing should be done with + it, and if it is a special character that would normally + get mapped into a Telnet option, that mapping should + not be done. + + SLC_XON Start Output. This is the character that is sent to + resume output to the users terminal. + + SLC_XOFF Stop Output. This is the character that is sent to stop + output to the users terminal. + + SLC_FORW1 Forwarding character. This is a character that should + cause all data currently being buffered, and this + character, to be sent immediately. + + SLC_FORW2 Forwarding character. This is another character that is + to be treated in the same manner as SLC_FORW1. + + SLC_MCL Move cursor one character left. When visual editing is + supported, this is the character that, when typed, will + move the cursor one character to the left in the + display. + + SLC_MCR Move cursor one character right. When visual editing is + supported, this is the character that, when typed, will + move the cursor one character to the right in the + display. + + SLC_MCWL Move cursor one word left. When visual editing is + supported, this is the character that, when typed, will + move the cursor one word to the left in the display. + + SLC_MCWR Move cursor one word right. When visual editing is + supported, this is the character that, when typed, will + move the cursor one word to the right in the display. + + SLC_MCBOL Move cursor to the begining of the line. When visual + editing is supported, this is the character that, when + typed, will move the cursor to the begining of the line + that is being edited. + + SLC_MCEOL Move cursor to the end of the line. When visual editing + + + +Telnet Working Group [Page 9] + +RFC 1184 Telnet Linemode Option October 1990 + + + is supported, this is the character that, when typed, + will move the cursor to the end of the line that is + being edited. + + SLC_INSRT Enter insert mode. When visual editing is supported, + after this character is typed, all normal characters + that are subsequently typed will be inserted into the + display. + + SLC_OVER Enter overstrike mode. When visual editing is + supported, after this character is typed, all normal + charactersthat are subsequently typed will overwrite + any characters in the current display. If the + SLC_INSRT and SLC_OVER variables are set to the same + value, then that value is to act as a toggle between + overstrike and insert mode. + + SLC_ECR Erase character to the right. When visual editing is + supported, this is the character that, when typed, will + erase one character to the right of the cursor. + + SLC_EWR Erase word to the right. When visual editing is + supported, this is the character that, when typed, + will erase one word to the right of the cursor. + + SLC_EBOL Erase to the begining of the line. When visual editing + is supported, this is the character that, when typed, + will erase all the characters to the left of the cursor. + + SLC_EEOL Erase to the end of the line. When visual editing is + supported, this is the character that, when typed, will + erase all characters to the right of the cursor. + + For SLC_EEOL, SLC_EWR, and SLC_ECR, if a system has a cursor that is + not diplayed between characters, but is positioned over a character, + that character is assumed to be to the right of the cursor. Thus, + the SLC_ECR will erase the character that is under the current cursor + position. + +2.5 New control characters + + IAC ABORT + + Abort. Similar to "IAC IP", but means only to abort or terminate + the process to which the NVT is connected. (The Telnet spec says + IP may "suspend, interrupt, abort or terminate" the process.) If a + system does not have two methods of interrupting a process, then + ABORT and IP should have the same effect. + + + +Telnet Working Group [Page 10] + +RFC 1184 Telnet Linemode Option October 1990 + + + IAC SUSP + + Suspend the execution of the current process attached to the NVT + in such a way that another process will take over control of the + NVT, and the suspended process can be resumed at a later time. If + the receiving system does not support this functionality, it + should be ignored. + + IAC EOF + + End Of File. The recipient should notify the process connected to + the NVT that an end of file has been reached. This is intended + for systems that support the ability for the user to type in an + EOF character at the keyboard. + +3. Default Specification + + The default specification for this option is + + WONT LINEMODE + + DONT LINEMODE + + meaning there will not be any subnegotiation of the mode of the + connection. + + If WILL LINEMODE is negotiated, the defaults are: + + IAC SB LINEMODE MODE 0 IAC SE + IAC SB LINEMODE WONT FORWARDMASK IAC SE + + If DO LINEMODE is negotiated, the defaults are: + + IAC SB LINEMODE MODE 0 IAC SE + IAC SB LINEMODE DONT FORWARDMASK IAC SE + + Character values for SLC default to SLC_NOSUPPORT. + +4. Motivation + + With increasing Telnet usage, it has become apparent that the ability + to do command line processing on the local machine and send completed + lines to the remote machine is a feature necessary in several + environments. First, in the case of a connection over long delay + equipment, it is very frustrating to the user to have the echoing of + his data take several seconds. Second, some supercomputers, due to + their nature, are not good at handling and processing single + character input. For these machines, it is better to have the front + + + +Telnet Working Group [Page 11] + +RFC 1184 Telnet Linemode Option October 1990 + + + end computer do the character processing, and leave the + supercomputer's cycles available for doing vectorized number + crunching. + + There have been attempts to make local line editing work within the + existing Telnet specs. Indeed, the 4.3 BSD tape includes a version + of Telnet that attempts to do this through recognition of the state + of the ECHO and SUPRESS-GO-AHEAD options; other implementations do + this recognition purely through the ECHO option. + + There are problems with both of these methods. Using just the ECHO + provides no mechanism to have ECHO to the user turned off, and leave + local character processing on, for example, when a user is typing a + password. + + The usage of the SUPRESS-GO-AHEAD comes from reading into RFC 858, + where it states: + + "In many TELNET implementations it will be desirable to couple the + SUPRESS-GO-AHEAD option to the echo option so that when the echo + option is in effect, the SUPPRESS-GO-AHEAD option is in effect + simultaneously: both of these options will normally have to be in + effect simultaneously to effect what it commonly understood to be + character at a time echoing by the remote computer." + + The reverse reading of this is that without the ECHO option or the + SUPPRESS-GO-AHEAD option, you are in line at a time mode, implying + local line editing. This has the obvious problem that that is not + what the SUPPRESS-GO-AHEAD option is supposed to mean. + + Other shortcomings are that the Telnet specification is not rich + enough to handle all of the special characters that some of the + current operating systems support. For example, the ECHO/SGA + implementation supports two ways of interrupting a process, by + borrowing the BRK option for the second interrupt. Some + implementations have taken the EOR option to send an End-Of-File. + Obviously, this is using things for which they were not intended, and + the correct solution would be to define new options. + + Another problem is that some implementations of line mode buffer up + the input until the end of the line, and then send the whole line + across, editing characters and all. No local editing of the line has + been done. + + After examining several implementations, it has become clear that the + correct thing to do is to implement new options to enhance the + current Telnet specification so that it can support local line + editing in a reasonable, reliable, and consistent manner. + + + +Telnet Working Group [Page 12] + +RFC 1184 Telnet Linemode Option October 1990 + + + There are three states that are of interest. + + 1) Local line editing and local signal trapping + + 2) Remote line editing, local signal trapping + + 3) Remote line editing, remote signal trapping + + The case of local line editing and remote signal trapping is not a + very interesting case, because you don't recognize the signals, and + cannot send them to the remote side for it to recognize until the + line has been completed. Also, special signals usually will have an + effect on the line editing function, and if they are not being + trapped locally the desired action will not happen. + + Local line editing means that all normal command line character + processing, like "Erase Character" and "Erase Line", happen on the + local system, and only when "CR LF" (or some other special character) + is encountered is the edited data sent to the remote system. + + Signal trapping means, for example, that if the user types the + character associated with the IP function, then the "IAC IP" function + is sent to the remote side instead of the character typed. Remote + signal trapping means, for example, that if the user types the + character associated with the IP function, then the "IAC IP" function + is not sent to the remote side, but rather the actual character typed + is sent to the remote side. + +5. Implementation Rules + + It is expected that any implementation that supports the Telnet + LINEMODE option will support all of this specification. + +5.1 User Interface + + Normally, the entire user interface is left up to the implementor. + However, there is functionality that the user should be able to + specify on the client side of the connection. During a Telnet + session, the client side should allow some mechanism for the user to + give commands to the local Telnet process. These commands should at + least allow the user to: + + 1) Change the mode of the connection. The user should be able to + attempt to turn EDIT, FLOW, TRAPSIG, and ECHO on and off. The + server may refuse to change the state of the EDIT and TRAPSIG + bits. + + 2) Import or export SLC. The user should be able to tell the + + + +Telnet Working Group [Page 13] + +RFC 1184 Telnet Linemode Option October 1990 + + + local Telnet process whether he wants to use the local or + the current or default remote definitions of the special + characters. + + 3) Manual sending of options. The user should be able to tell + the local Telnet process to explicitly send any of the Telnet + options (like IP, ABORT, AYT, etc). + +5.2 End of line terminators. + + When LINEMODE is turned on, and when in EDIT mode, when any normal + line terminator on the client side operating system is typed, the + line should be transmitted with "CR LF" as the line terminator. When + EDIT mode is turned off, a carriage return should be sent as "CR + NUL", a line feed should be sent as LF, and any other key that cannot + be mapped into an ASCII character, but means the line is complete + (like a DOIT or ENTER key), should be sent as "CR LF". + +5.3 Output processing + + Regardless of what mode has been negotiated, the server side is + responsible for doing all output processing. Specificly, it should + send "CR LF" when it wants the "newline" function, "CR NUL" when it + wants just a carriage return, and "LF" when it wants just a linefeed. + +5.4 A terminal driver in Telnet? + + Conforming implementations need not do all the line editing + themselves. There is nothing wrong with letting the system terminal + driver handle the line editing, and have it hand to the Telnet + application the completed and edited line, which is then sent to the + remote system. + +5.5 Setting of Local Characters + + When this RFC was being developed, the original thought was that both + sides of the connection would use their own defaults for the special + characters, even if they were not the same on both sides of the + connection. If this scheme is used, though, the view that the user + has is that the local special characters are being used, and the + remote character settings don't matter. It was decided that the + client side of the connection should be in control of the character + settings. + + When LINEMODE is negotiated, the client must either export the local + character settings to the server, or send a request (SLC 0 + SLC_DEFAULT 0) to import the servers special characters. The usual + action would be that a client running on a full fledged computer + + + +Telnet Working Group [Page 14] + +RFC 1184 Telnet Linemode Option October 1990 + + + would export the special characters, and a client running where there + are no local defaults (like on some terminal servers) would import + the special characters. + + When an SLC command is received, the action taken should be: + + 1) Ignore it if it is the same as the current settings. + + 2) If the SLC_LEVELBITS are the same as the current level bits, + but the value is different and the SLC_ACK bit is set, no + reply is generated. On the server side, the command is + ignored, and on the client side, a switch is made to the + new value. This is so that if a request to change the same + character is generated by both the server and the client, + they will both settle on the clients requested value. + + 3) If we agree with the new setting, we switch to it and reply + with the same value, but also set the SLC_ACK bit. + + 4) If we don't agree, we send a response with what we think the + value should be. The SLC_ACK bit is NOT set in this case. + You may only disagree with a value by sending a different + value at a lower level. + + If the remote system doesn't support some of the line editing + characters, but the front end does, then the front end may use the + local definitions for those characters when in line mode. In this + case, the server should send "SLC xxx SLC_DEFAULT 0" in response to a + "SLC 0 SLC_DEFAULT 0" request, and just ack whatever value the client + requests to set the function to. + + The SLC_FORW2 character should only be used if SLC_FORW1 is already + in use. + +5.6 FORWARDMASK and SLC_FORW1 and SLC_FORW2 + + To help ease the amount of work needed to implement the client side, + two methods of setting forwarding characters are provided. The + SLC_FORW1 and SLC_FORW2 allow for the setting of two additional + characters on which to forward buffered input data. Since many + terminal drivers have the ability to set one or more line delimiters, + it is fairly easy to support these without having to implement + through the local terminal driver, rather than putting a terminal + driver into Telnet. If the local terminal driver has functionality + that maps easily into the FORWARDMASK, then it can also be easily + supported. If the local terminal driver does not support that, then + it would require more work to support FORWARDMASK. + + + + +Telnet Working Group [Page 15] + +RFC 1184 Telnet Linemode Option October 1990 + + + Also note that the client side is required to forward data when it + sees one of SLC_FORW1, SLC_FORW2, or FORWARDMASK characters, or when + any normal line termination or special signal is encountered. The + client side is also free to forward on other characters that it + chooses. For example, if the server side sent a FORWARDMASK that + asked for data to be forwarded on the first 20 control characters + (ASCII codes 1 through 024), and the client side cannot have its + local terminal driver forward on just the first 20 control + characters, but it can have the local terminal driver forward on any + control character (ASCII codes 1 through 039), then the client side + could validly accept the FORWARDMASK, and forward on any control + character. When in EDIT mode, care should be taken to not forward at + random times, since once that data is forwarded, no more editing on + the forwarded part of the line can be done. The only time (other + than the normal times) that data should be forwarded when in EDIT + mode would be if a single input line is too long to handle locally. + +5.7 Valid and invalid modes and values + + At no time should "DO LINEMODE" be negotiated in both directions of + the Telnet connection. The side that is the "DO LINEMODE" is + considered to be the server side, and the side that is "WILL + LINEMODE" is the client side. + + At no time should "SB LINEMODE DO/DONT FORWARDMASK", be sent unless + "DO LINEMODE" has been previously negotiated. At no time should "SB + LINEMODE WILL/WONT FORWARDMASK", be sent unless "WILL LINEMODE" has + been previously negotiated. + + If an ABORT, EOF or SUSP, is received and the system does not support + that functionality, it may just be ignored. + +5.8 Flushing input and output + + When an IP, BRK or ABORT is sent, it is usually desirable to be able + to flush the input stream, and to flush output to the user until the + IP, BRK, or ABORT is processed. The SLC_FLUSHIN and SLC_FLUSHOUT + bits are used to indicate what action should be done. These bits are + advisory only, but should be honored if possible. The standard + method for processing the SLC_FLUSHIN is to use the Telnet "Synch" + signal, and the SLC_FLUSHOUT is processed using the TIMING-MARK + option. If both are to be sent, the IAC DM is sent before the DO + TIMING-MARK Thus, the sender would send "IAC XXX IAC DM IAC DO + TIMING-MARK", where XXX may be IP, BRK or ABORT, or any other special + character. The IAC DM is sent as TCP urgent data with the DM as the + last (or only) data octet; this is used to flush the input stream. + The "IAC DO TIMING-MARK" is used to tell when to stop flushing + output; once it is sent, all data is discarded until an "IAC WILL + + + +Telnet Working Group [Page 16] + +RFC 1184 Telnet Linemode Option October 1990 + + + TIMING-MARK" or an "IAC WONT TIMING-MARK" is received. + + Since the SLC_FLUSHIN and SLC_FLUSHOUT bit are only advisory, the + user interface should provide a method so that the user can override + the sending (or not sending) of the "Synch" and TIMING-MARK, but the + default action should be to send them according to the SLC_FLUSHIN + and SLC_FLUSHOUT bits. + + Whenever an IAC AO is received, a Synch must be returned. When ever + a Synch is being processed, (by the TCP connection going into Urgent + mode), all data must be discarded (but not Telnet commands!) until an + IAC DM is found, and the connection goes out of Urgent mode. See RFC + 854, "TELNET PROTOCOL SPECIFICATION", for a complete description of + the Synch signal. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Telnet Working Group [Page 17] + +RFC 1184 Telnet Linemode Option October 1990 + + +5.9 State diagram for SLC + + +---------------------------------------------------------------+ + | IDLE | + +----------------------+------+------+-------+-------+---------++ + ^ ^ ^ | | ^ | ^ | ^ | ^ | + | | | v v | | | | | v | | + | | | +------+ +---+--+ | | | | ########### | | + | | | | Get | | Send | | | | | # Get # | | + | | | | SPC0 | | SPC0 | | | | | # 0,DEF,0 # | | + | | | +---+--+ +------+ | | | | ########### | | + | | | | ^ | | | | | | | + | | | v | v | | | v | | + | | | / \ | *********** | | ########### | | + | | | / \ | * Send * | | # Switch # | | + | ********** |Yes/ Same as \ | * 0,VAL,0 * | | # to # | | + | * Change * +--< current? > | *********** | | # default # | | + | * to new * \ / | v | ########### | | + | * value * \ / | *********** | | | + | ********** \ / | * Send * v | | + | ^ |No | * 0,DEF,0 * ######### | | + | |Yes v | *********** # Send #--+ | + | / \ / \ | # SPC-A # | + | / \ / \ | ######### | + | / Is ACK \ Yes/ Same \ | ^ | + |< bit set? ><-< level as > | | | + | \ / \ current?/ | ########### | + | \ / \ / | # Get #<--+ + | \ / \ / +-+---+ # 0,VAL,0 # + | |No |No | Set | ########### + | +--------------+ | ACK | + | v | bit | * - Client side only + | / \ +-----+ # - Server side only + | +------+ / \ ^ + | | Send | No / Do we \ Yes| + +---| SPC1 |<---< agree? >---+ + +------+ \ / + \ / + \ / + + SPC0 Initial setting for a special character + SPC1 A changed special character < SPC0 + SPC-A All current special character settings + VAL SLC_VALUE level + DEF SLC_DEFAULT level + + Levels: DEFAULT, VALUE, CANT_CHANGE, NOSUPPORT + Flags: ACK + + + +Telnet Working Group [Page 18] + +RFC 1184 Telnet Linemode Option October 1990 + + + Receive Response + ------- -------- + f,SLC_DEFAULT,x f,SLC_VALUE,v + f,SLC_CANTCHANGE,v + f,SLC_NOSUPPORT,x + + f,SLC_VALUE,v f,SLC_ACK|SLC_VALUE,v + f,SLC_CANTCHANGE,w + f,SLC_NOSUPPORT,x + + f,SLC_CANTCHANGE,v f,SLC_ACK|SLC_CANTCHANGE,v + f,SLC_NOSUPPORT,x + + f,SLC_NOSUPPORT,x f,SLC_ACK|SLC_NOSUPPORT,x + + x,SLC_ACK|x,x no response + +5.10 Examples of a connection + + In these examples, the symbolic names are used rather than the actual + values, to make them readable. When two or more symbolic names are + joined by a |, it means that the actual value will be the logical + "or" of the values of the symbolic names. In the interest of + clarity, for these examples the leading IAC and IAC SB sequences, and + the trailing IAC SE sequences have been omitted. Also, the SLC_ + prefix has been left off where ever it would normally occur. + + CLIENT SERVER + DO TOGGLE-FLOW-CONTROL + DO LINEMODE + WILL TOGGLE-FLOW-CONTROL + WILL LINEMODE + [ Subnegotiation may now proceed in both directions. The client + sends of the list of special characters. ] + LINEMODE SLC SYNCH DEFAULT 0 IP + VALUE|FLUSHIN|FLUSHOUT 3 AO + VALUE 15 AYT DEFAULT 0 ABORT + VALUE|FLUSHIN|FLUSHOUT 28 EOF + VALUE 4 SUSP VALUE|FLUSHIN 26 EC + VALUE 127 EL VALUE 21 EW VALUE + 23 RP VALUE 18 LNEXT VALUE 22 + XON VALUE 17 XOFF VALUE 19 + [ Now that linemode is enabled, the server sets the initial mode, + and acknowledges the special characters. ] + LINEMODE MODE EDIT + + LINEMODE SLC SYNCH NOSUPPORT 0 + IP VALUE|FLUSHIN|FLUSHOUT|ACK 3 + + + +Telnet Working Group [Page 19] + +RFC 1184 Telnet Linemode Option October 1990 + + + AO NOSUPPORT 0 AYT NOSUPPORT 0 + ABORT VALUE|FLUSHIN|FLUSHOUT|ACK + 28 EOF VALUE|ACK 4 SUSP NOSUP- + PORT 0 EC VALUE|ACK 127 EL + VALUE|ACK 21 EW VALUE|ACK 23 RP + VALUE|ACK 18 LNEXT VALUE|ACK 22 + XON VALUE|ACK 17 XOFF VALUE|ACK + 19 + [ The client gets the mode and ack of the special characters, and + acks the mode and any special characters that the server + changed. ] + LINEMODE MODE EDIT|MODE_ACK + + LINEMODE SLC SYNCH NOSUPPORT|ACK + 0 AO NOSUPPORT|ACK 0 AYT|ACK + NOSUPPORT 0 SUSP NOSUPPORT|ACK 0 + "Login:" + "my_account" + [ Turn off echo to the user. ] + WILL ECHO + DO ECHO + "Password:" + "my_password" + [ Turn back on echo to the user. ] + WONT ECHO + DONT ECHO + [ User does some stuff, and then runs an application that wants + to use single character mode, doing its own echoing of + characters, but keep signal trapping on. ] + WILL ECHO + DO ECHO + LINEMODE MODE TRAPSIG + LINEMODE MODE TRAPSIG|MODE_ACK + [ Application finishes ] + WONT ECHO + DONT ECHO + LINEMODE MODE EDIT|TRAPSIG + LINEMODE MODE + EDIT|TRAPSIG|MODE_ACK + [ Another application, that wants full control of everything. ] + WILL ECHO + DO ECHO + LINEMODE MODE 0 + LINEMODE MODE 0|MODE_ACK + [ Application finishes ] + WONT ECHO + DONT ECHO + LINEMODE MODE EDIT|TRAPSIG + + + +Telnet Working Group [Page 20] + +RFC 1184 Telnet Linemode Option October 1990 + + + LINEMODE MODE + EDIT|TRAPSIG|MODE_ACK + [ The user changes his erase character to ^H. ] + LINEMODE SLC EC VALUE 8 + LINEMODE SLC EC VALUE|ACK 8 + [ The user decides to revert to all the original client side + special characters. ] + LINEMODE SLC SYNCH DEFAULT 0 IP + VALUE|FLUSHIN|FLUSHOUT 3 AO + VALUE 15 AYT DEFAULT 0 ABORT + VALUE|FLUSHIN|FLUSHOUT 28 EOF + VALUE 4 SUSP VALUE|FLUSHIN 26 EC + VALUE 127 EL VALUE 21 EW VALUE + 23 RP VALUE 18 LNEXT VALUE 22 + XON VALUE 17 XOFF VALUE 19 + LINEMODE SLC SYNCH NOSUPPORT 0 + AO NOSUPPORT 15 AYT NOSUPPORT 0 + SUSP NOSUPPORT|FLUSHIN 26 EC + VALUE|ACK 127 EW VALUE|ACK 23 RP + VALUE|ACK 18 LNEXT VALUE|ACK 22 + XON VALUE|ACK 17 XOFF VALUE|ACK + 19 + LINEMODE SLC SYNCH NOSUPPORT|ACK + 0 AO NOSUPPORT|ACK 15 AYT + NOSUPPORT|ACK 0 SUSP + NOSUPPORT|ACK|FLUSHIN 26 + [ The user decides to import the remote sides default special + characters. ] + LINEMODE SLC 0 DEFAULT 0 + LINEMODE SLC IP + VALUE|FLUSHIN|FLUSHOUT 3 ABORT + VALUE|FLUSHIN|FLUSHOUT 28 EOF + VALUE 4 EC VALUE 127 EL VALUE 21 + [ Since these are the same as the current local settings, no + response is generated. ] + [ This next example is what would happen if an editor was fired + up, that wanted to let the client side do the echoing and + buffering of characters, but did not want it to do any line + editing, and only forward the data when got a control + character. Note that we have preceded all the the 0377s + in the forward mask with an IAC. ] + LINEMODE MODE 0 + LINEMODE DO FORWARDMASK IAC 0377 + IAC 0377 IAC 0377 IAC 0377 0 0 0 + 0 0 0 0 0 0 0 0 01 + LINEMODE MODE 0 + LINEMODE WILL FORWARDMASK + [ Application runs to completion, and then things are to be set + + + +Telnet Working Group [Page 21] + +RFC 1184 Telnet Linemode Option October 1990 + + + back to what they were before. ] + LINEMODE MODE EDIT|TRAPSIG + LINEMODE DONT FORWARDMASK + LINEMODE MODE EDIT|TRAPSIG + LINEMODE WONT FORWARDMASK + +6. Other Telnet options and RFCs + + The following is a list of RFCs for various Telnet options that + should be supported along with LINEMODE. + + 1. Postel, J. and Reynolds, J., "TELNET PROTOCOL SPECIFICATION", + RFC 854, ISI, May 1983. + + 2. Postel, J. and Reynolds, J., "TELNET OPTION SPECIFICATIONS", + RFC 855, ISI, May 1983. + + 3. Postel, J. and Reynolds, J., "TELNET BINARY TRANSMISSION", + RFC 856, ISI, May 1983. + + 4. Postel, J. and Reynolds, J., "TELNET ECHO OPTION", RFC 857, + ISI, May 1983. + + 5. Postel, J. and Reynolds, J., "TELNET SUPRESS GO AHEAD OPTION", + RFC 858, ISI, May 1983. + + 6. Postel, J. and Reynolds, J., "TELNET TIMING MARK OPTION", + RFC 860, ISI, May 1983. + + 7. VanBokkeln, J., "Telnet Terminal-Type Option", RFC 1091, + FTP Software, Inc., February 1989. + + 8. Waitzman, D., "Telnet Window Size Option", RFC 1073, BBN STC, + October 1988. + + 9. Hedrick, C., "Telnet Remote Flow Control Option", RFC 1080, + Rutgers University, November, 1988. + + 10. Hedrick, C., "Telnet Terminal Speed Option", RFC 1079, Rutgers + University, December, 1988. + + The following is a list of RFCs that need not be supported for + LINEMODE, but which would enhance any TELNET implementation. + + 11. Postel, J. and Reynolds, J., "TELNET STATUS OPTION", RFC 859, + ISI, May 1983. + + 12. Postel, J. and Reynolds, J., "TELNET END OF RECORD OPTION", + + + +Telnet Working Group [Page 22] + +RFC 1184 Telnet Linemode Option October 1990 + + + RFC 885, ISI, December 1983. + + 13. Silverman, S., "OUTPUT MARKING TELNET OPTION", RFC 933, + MITRE-Washington, January 1985. + + 14. Marcy, G., "Telnet X Display Location Option", RFC 1096, + Carnegie Mellon University, March 1989. + +Security Consideratiions + + Security issues are not discussed in this memo. + +Author's Address + + David A. Borman + Cray Research Inc. + 655F Lone Oak Drive + Eagan, MN 55123 + + Phone: (612) 452-6650 + EMail: dab@CRAY.COM + + IETF Telnet WG Mailing List: telnet-ietf@CRAY.COM + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Telnet Working Group [Page 23] +
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