💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc1116.txt captured on 2022-07-16 at 23:04:09.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
Network Working Group Internet Engineering Task Force
Request for Comments: 1116 Telnet Linemode Working Group
D. Borman, Editor
Cray Research, Inc.
August 1989
Telnet Linemode Option
Status of this Memo
This RFC describes a proposed elective standard for the Internet
community. Hosts on the Internet that support Linemode within the
Telnet protocol are expected to adopt and implement this standard.
Distribution of this memo is unlimited.
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.
Table of Contents
1. Command Names and Codes 2
2. Command Meanings 3
2.1 The LINEMODE function 3
2.2 LINEMODE suboption MODE 3
2.3 LINEMODE suboption FORWARDMASK 4
2.4 LINEMODE suboption SLC, Set Local Characters 5
2.5 New control characters 8
3. Default Specification 9
4. Motivation 9
5. Implementation Rules 11
5.1 User Interface 11
5.2 End of line terminators 12
5.3 Output processing 12
5.4 A terminal driver in Telnet? 12
5.5 Setting of Local Characters 12
5.6 FORWARDMASK and SLC_FORW1 and SLC_FORW2 13
5.7 Valid and invalid modes and values. 14
5.8 Flushing input and output 14
Telnet Linemode Working Group [Page 1]
RFC 1116 Telnet Linemode Option August 1989
5.9 State diagram for SLC 16
5.10 Example of a connection 17
6. Other Telnet options and RFCs 20
1. Command Names and Codes
LINEMODE 34
MODE 1
EDIT 1
TRAPSIG 2
MODE_ACK 4
FORWARDMASK 2
SLC 3
SLC_SYNCH 1
SLC_BRK 2
SLC_IP 3
SLC_AO 4
SLC_AYT 5
SLC_EOR 6
SLC_ABORT 7
SLC_EOF 8
SLC_SUSP 9
SLC_EC 10
SLC_EL 11
SLC_EW 12
SLC_RP 13
SLC_LNEXT 14
SLC_XON 15
SLC_XOFF 16
SLC_FORW1 17
SLC_FORW2 18
SLC_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
Telnet Linemode Working Group [Page 2]
RFC 1116 Telnet Linemode Option August 1989
2. Command Meanings
2.1 The LINEMODE function
IAC WILL LINEMODE
The sender of this command REQUESTS permission to begin sub-
negotiation of the editing/signaling status. This should only be
sent by the client side of the connection.
IAC WONT LINEMODE
The sender of this command DEMANDS that sub-negotiation of the
editing/signaling status not be allowed.
IAC DO LINEMODE
The sender of this command REQUESTS that the remote side begin
subnegotiation 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
subnegotiation 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.)
Telnet Linemode Working Group [Page 3]
RFC 1116 Telnet Linemode Option August 1989
When unset, the client should pass interrupts/signals
as their normal ASCII values.
FLOW Logically, this belongs in the "mask". However,
this would overlap the Telnet TOGGLE-FLOW-CONTROL
option, so the Telnet TOGGLE-FLOW-CONTROL option is
used instead. When DO/WILL LINEMODE is negotiated,
DO/WILL TOGGLE-FLOW-CONTROL should also be negotiated.
See RFC 1080, "Telnet Remote Flow Control", for
correct usage.
ECHO Logically, this belongs in the "mask". However,
this would overlap the Telnet ECHO option, so the
Telnet ECHO option is used instead. The client side
should never negotiate "WILL ECHO". When the server
has negotiated "WILL ECHO", the client should not
echo data typed by the user back to the user. When
the server has negotiated "WONT ECHO", the 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.
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
Telnet Linemode Working Group [Page 4]
RFC 1116 Telnet Linemode Option August 1989
mask are received. Only the side of the connection that sent DO
LINEMODE (the server side) may negotiate this. The mask is up to
32 octets long. Each octet represents 8 ASCII character codes.
The high order bit of mask0 corresponds to an ASCII code of 0.
The low order bit of mask0 corresponds to an ASCII code of 7. The
high order bit of mask1 corresponds to an ASCII code of 8. The
low order bit of mask1 corresponds to an ASCII code of 15, and so
on. The mask list may be terminated before the end of the list,
in which case all the rest of the mask octets are assumed to be
reset (equal to zero). When the server side is in DONT TRANSMIT-
BINARY mode, then only the first 16 octets of the mask (ASCII
codes 0 through 127) are used. If any individual octet of the
mask is equal to IAC, it must be sent as a double IAC.
IAC SB LINEMODE DONT FORWARDMASK IAC SE
The sender of this command requests that the other side stop using
the forward mask to determine when to send buffered data.
IAC SB LINEMODE WILL FORWARDMASK IAC SE
This command is sent in response to a DO FORWARDMASK command. It
indicates that the forward mask will be used to determine when to
send buffered data.
IAC SB LINEMODE WONT FORWARDMASK IAC SE
This command is sent in response to a DO FORWARDMASK command. It
indicates that the forward mask will not be used to determine when
to send buffered data.
2.4 LINEMODE suboption SLC, Set Local Characters
The SLC suboption uses a list of octet triplets. The first octet
specifies the function, the second octet specifies modifiers to the
function, and the third octet specifies the ASCII character for the
function.
IAC SB LINEMODE SLC <list of octet triplets> IAC SE
The sender of this command REQUESTS that the list of octet
triplets be used to set the local character to be used to send to
perform the specified function.
There are four levels that a function may be set to.
SLC_NOSUPPORT is the lowest, SLC_CANTCHANGE is the next higher
level, SLC_VALUE is above that, and SLC_DEFAULT is the highest
level.
Telnet Linemode Working Group [Page 5]
RFC 1116 Telnet Linemode Option August 1989
If the SLC_LEVELBITS in the second octet are equal to SLC_DEFAULT,
then this particular function should use the system default on the
other side of the connection.
If the SLC_LEVELBITS in the second octet are equal to SLC_VALUE,
then this function is supported, and the current value is
specified by the third octet.
If the SLC_LEVELBITS in the second octet are equal to
SLC_CANTCHANGE, then this is a function that is supported, but the
value for this function, specified in the third octet, cannot be
changed.
If the SLC_LEVELBITS in the second octet are equal to
SLC_NOSUPPORT, then this particular function is not supported and
should be disabled by the other side.
If this is a response to a previous request to change a special
character, and we are agreeing to the change, then the SLC_ACK bit
must be set in the second octet.
If the SLC_FLUSHIN bit is set in the second octet, then whenever
this function is sent, a Telnet "sync" should be sent at the same
time to flush the input stream.
If the SLC_FLUSHOUT bit is set in the second octet, then whenever
this function is sent, output data should be flushed.
Only the client may send an octet triplet with the first octet
equal to zero. In this case, the SLC_LEVELBITS may only be set to
SLC_DEFAULT or SLC_VALUE, and the third octet does not matter.
When the server receives 0 SLC_DEFAULT 0, it should switch to its
system default special character settings, and send all those
special characters to the client. When the server receives 0
SLC_VALUE 0, it should just send its current special character
settings. Note that if the server does not support some of the
editing functions, they should be sent as XXX SLC_DEFAULT 0,
rather than as XXX SLC_NOSUPPORT 0, so that the client may choose
to use its own values for those functions, rather than have to
disable those functions even if it supports them.
If any of the octets in the list of octet triplets is equal to
IAC, it must be sent as a double IAC.
When a connection is established, it is the responsibility of the
client to either request the remote default values for the special
characters, or to send across what all the special characters should
be set to.
Telnet Linemode Working Group [Page 6]
RFC 1116 Telnet Linemode Option August 1989
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. When
backing up in the input stream, a word is defined
Telnet Linemode Working Group [Page 7]
RFC 1116 Telnet Linemode Option August 1989
to be (optionally) whitespace (tab or space
characters), and a string of characters up to, but not
including, whitespace or line delimiters.
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.
2.5 New control characters
IAC ABORT
Abort. Similar to "IAC IP", but means only to abort or terminate
the process to which the NVT is connected. (The Telnet spec says
IP may "suspend, interrupt, abort or terminate" the process.) If
a system does not have two methods of interrupting a process, then
ABORT and IP should have the same effect.
IAC SUSP
Suspend the execution of the current process attached to the NVT
in such a way that another process will take over control of the
NVT, and the suspended process can be resumed at a later time. If
the receiving system does not support this functionality, it
should be ignored.
Telnet Linemode Working Group [Page 8]
RFC 1116 Telnet Linemode Option August 1989
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 necesary in several
environments. First, in the case of a connection over long delay
equipment, it is very frustrating to the user to have the echoing of
his data take several seconds. Second, some supercomputers, due to
their nature, are not good at handling and processing single
character input. For these machines, it is better to have the front
end computer do the character processing, and leave the
supercomputer's cycles available for doing vectorized number
crunching.
There have been attempts to make local line editing work within the
existing Telnet specs. Indeed, the 4.3 BSD tape includes a version
of Telnet that attempts to do this through recognition of the state
of the ECHO and SUPRESS-GO-AHEAD options; other implementations do
this recognition purely through the ECHO option.
Telnet Linemode Working Group [Page 9]
RFC 1116 Telnet Linemode Option August 1989
There are problems with both of these methods. Using just the ECHO
provides no mechanism to have ECHO to the user turned off, and leave
local character processing on, for example, when a user is typing a
password.
The usage of the SUPRESS-GO-AHEAD comes from reading into RFC 858,
where it states:
"In many TELNET implementations it will be desirable to couple the
SUPRESS-GO-AHEAD option to the echo option so that when the echo
option is in effect, the SUPPRESS-GO-AHEAD option is in effect
simultaneously: both of these options will normally have to be in
effect simultaneously to effect what it commonly understood to be
character at a time echoing by the remote computer."
The reverse reading of this is that without the ECHO option or the
SUPPRESS-GO-AHEAD option, you are in line at a time mode, implying
local line editing. This has the obvious problem that that is not
what the SUPPRESS-GO-AHEAD option is supposed to mean.
Other shortcomings are that the Telnet specification is not rich
enough to handle all of the special characters that some of the
current operating systems support. For example, the ECHO/SGA
implementation supports two ways of interrupting a process, by
borrowing the BRK option for the second interrupt. Some
implementations have taken the EOR option to send an End-Of-File.
Obviously, this is using things for which they were not intended, and
the correct solution would be to define new options.
Another problem is that some implementations of line mode buffer up
the input until the end of the line, and then send the whole line
across, editing characters and all. No local editing of the line has
been done.
After examining several implementations, it has become clear that the
correct thing to do is to implement new options to enhance the
current Telnet specification so that it can support local line
editing in a reasonable, reliable, and consistent manner.
There are three states that are of interest:
1) Local line editing and local signal trapping
2) Remote line editing, local signal trapping
3) Remote line editing, remote signal trapping
The case of local line editing and remote signal trapping is not a
Telnet Linemode Working Group [Page 10]
RFC 1116 Telnet Linemode Option August 1989
very interesting case, because you don't recognize the signals, and
cannot send them to the remote side for it to recognize until the
line has been completed. Also, special signals usually will have an
effect on the line editing function, and if they are not being
trapped locally the desired action will not happen.
Local line editing means that all normal command line character
processing, like "Erase Character" and "Erase Line", happen on the
local system, and only when "CR LF" (or some other special character)
is encountered is the edited data sent to the remote system.
Signal trapping means, for example, that if the user types the
character associated with the IP function, then the "IAC IP" function
is sent to the remote side instead of the character typed. Remote
signal trapping means, for example, that if the user types the
character associated with the IP function, then the "IAC IP" function
is not sent to the remote side, but rather the actual character typed
is sent to the remote side.
5. Implementation Rules
It is expected that any implementation that supports the Telnet
LINEMODE option will support all of this specification.
5.1 User Interface
Normally, the entire user interface is left up to the implementor.
However, there is functionality that the user should be able to
specify on the client side of the connection. During a Telnet
session, the client side should allow some mechanism for the user to
give commands to the local Telnet process. These commands should at
least allow the user to:
1) Change the mode of the connection. The user should be able
to attempt to turn EDIT, FLOW, TRAPSIG, and ECHO on and off.
The server may refuse to change the state of the EDIT and
TRAPSIG bits.
2) Import or export SLC. The user should be able to tell the
local Telnet process whether he wants to use the local or
the current or default remote definitions of the special
characters.
3) Manual sending of options. The user should be able to tell
the local Telnet process to explicitly send any of the Telnet
options (like IP, ABORT, AYT, etc.).
Telnet Linemode Working Group [Page 11]
RFC 1116 Telnet Linemode Option August 1989
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. Specifically, it should
send "CR LF" when it wants the "newline" function, "CR NUL" when it
wants just a carriage return, and "LF" when it wants just a linefeed.
5.4 A terminal driver in Telnet?
Conforming implementations need not do all the line editing
themselves. There is nothing wrong with letting the system terminal
driver handle the line editing, and have it hand to the Telnet
application the completed and edited line, which is then sent to the
remote system.
5.5 Setting of Local Characters
When this RFC was being developed, the original thought was that both
sides of the connection would use their own defaults for the special
characters, even if they were not the same on both sides of the
connection. If this scheme is used, though, the view that the user
has is that the local special characters are being used, and the
remote character settings don't matter. It was decided that the
client side of the connection should be in control of the character
settings.
When LINEMODE is negotiated, the client must either export the local
character settings to the server, or send a request (SLC 0
SLC_DEFAULT 0) to import the servers special characters. The usual
action would be that a client running on a full fledged computer
would export the special characters, and a client running where there
are no local defaults (like on some terminal servers) would import
the special characters.
When an SLC command is received, the action taken should be:
1) Ignore it if it is the same as the current settings.
Telnet Linemode Working Group [Page 12]
RFC 1116 Telnet Linemode Option August 1989
2) If the SLC_LEVELBITS are the same as the current level bits,
but the value is different and the SLC_ACK bit is set, no
reply is generated. On the server side, the command is
ignored, and on the client side, a switch is made to the new
value. This is so that if a request to change the same
character is generated by both the server and the client,
they will both settle on the clients requested value.
3) If we agree with the new setting, we switch to it and reply
with the same value, but also set the SLC_ACK bit.
4) If we don't agree, we send a response with what we think
the value should be. The SLC_ACK bit is NOT set in this
case. You may only disagree with a value by sending a
different value at a lower level.
If the remote system doesn't support some of the line editing
characters, but the front end does, then the front end may use the
local definitions for those characters when in line mode. In this
case, the server should send "SLC xxx SLC_DEFAULT 0" in response to a
"SLC 0 SLC_DEFAULT 0" request, and just ack whatever value the client
requests to set the function to.
The SLC_FORW2 character should only be used if SLC_FORW1 is already
in use.
5.6 FORWARDMASK and SLC_FORW1 and SLC_FORW2
To help ease the amount of work needed to implement the client side,
two methods of setting forwarding characters are provided. The
SLC_FORW1 and SLC_FORW2 allow for the setting of two additional
characters on which to forward buffered input data. Since many
terminal drivers have the ability to set one or more line delimiters,
it is fairly easy to support these without having to implement
through the local terminal driver, rather than putting a terminal
driver into Telnet. If the local terminal driver has functionality
that maps easily into the FORWARDMASK, then it can also be easily
supported. If the local terminal driver does not support that, then
it would require more work to support FORWARDMASK.
Also note that the client side is required to forward data when it
sees one of SLC_FORW1, SLC_FORW2, or FORWARDMASK characters, or when
any normal line termination or special signal is encountered. The
client side is also free to forward on other characters that it
chooses. For example, if the server side sent a FORWARDMASK that
asked for data to be forwarded on the first 20 control characters
(ASCII codes 1 through 024), and the client side cannot have its
local terminal driver forward on just the first 20 control
Telnet Linemode Working Group [Page 13]
RFC 1116 Telnet Linemode Option August 1989
characters, but it can have the local terminal driver forward on any
control character (ASCII codes 1 through 039), then the client side
could validly accept the FORWARDMASK, and forward on any control
character. When in EDIT mode, care should be taken to not forward at
random times, since once that data is forwarded, no more editing on
the forwarded part of the line can be done. The only time (other
than the normal times) that data should be forwarded when in EDIT
mode would be if a single input line is too long to handle locally.
5.7 Valid and invalid modes and values
At no time should "DO LINEMODE" be negotiated in both directions of
the Telnet connection. The side that is the "DO LINEMODE" is
considered to be the server side, and the side that is "WILL
LINEMODE" is the client side.
At no time should "SB LINEMODE DO/DONT FORWARDMASK", be sent unless
"DO LINEMODE" has been previously negotiated. At no time should "SB
LINEMODE WILL/WONT FORWARDMASK", be sent unless "WILL LINEMODE" has
been previously negotiated.
If an ABORT, EOF or SUSP, is received and the system does not support
that functionality, it may just be ignored.
5.8 Flushing input and output
When an IP, BRK or ABORT is sent, it is usually desirable to be able
to flush the input stream, and to flush output to the user until the
IP, BRK, or ABORT is processed. The SLC_FLUSHIN and SLC_FLUSHOUT
bits are used to indicate what action should be done. These bits are
advisory only, but should be honored if possible. The standard
method for processing the SLC_FLUSHIN is to use the Telnet "Synch"
signal, and the SLC_FLUSHOUT is processed using the TIMING-MARK
option. If both are to be sent, the IAC DM is sent before the DO
TIMING-MARK. Thus, the sender would send "IAC XXX IAC DM IAC DO
TIMING-MARK", where XXX may be IP, BRK or ABORT, or any other special
character. The IAC DM is sent as TCP urgent data with the DM as the
last (or only) data octet; this is used to flush the input stream.
The "IAC DO TIMING-MARK" is used to tell when to stop flushing
output; once it is sent, all data is discarded until an "IAC WILL
TIMING-MARK" or an "IAC WONT TIMING-MARK" is received.
Since the SLC_FLUSHIN and SLC_FLUSHOUT bit are only advisory, the
user interface should provide a method so that the user can override
the sending (or not sending) of the "Synch" and TIMING-MARK, but the
default action should be to send them according to the SLC_FLUSHIN
and SLC_FLUSHOUT bits.
Telnet Linemode Working Group [Page 14]
RFC 1116 Telnet Linemode Option August 1989
Whenever an IAC AO is received, a Synch must be returned. Whenever 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 Linemode Working Group [Page 15]
RFC 1116 Telnet Linemode Option August 1989
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
Telnet Linemode Working Group [Page 16]
RFC 1116 Telnet Linemode Option August 1989
Levels: DEFAULT, VALUE, CANT_CHANGE, NOSUPPORT
Flags: ACK
Receive Response
------- --------
f,SLC_DEFAULT,x f,SLC_VALUE,v
f,SLC_CANTCHANGE,v
f,SLC_NOSUPPORT,x
f,SLC_VALUE,v f,SLC_ACK|SLC_VALUE,v
f,SLC_CANTCHANGE,w
f,SLC_NOSUPPORT,x
f,SLC_CANTCHANGE,v f,SLC_ACK|SLC_CANTCHANGE,v
f,SLC_NOSUPPORT,x
f,SLC_NOSUPPORT,x f,SLC_ACK|SLC_NOSUPPORT,x
x,SLC_ACK|x,x no response
5.10 Examples of a connection
In these examples, the symbolic names are used rather than the actual
values, to make them readable. When two or more symbolic names are
joined by a |, it means that the actual value will be the logical
"or" of the values of the symbolic names. In the interest of
clarity, for these examples the leading IAC and IAC SB sequences, and
the trailing IAC SE sequences have been omitted. Also, the SLC_
prefix has been left off where ever it would normally occur.
CLIENT SERVER
------ ------
DO TOGGLE-FLOW-CONTROL
DO LINEMODE
WILL TOGGLE-FLOW-CONTROL
WILL LINEMODE
[ Subnegotiation may now proceed in both directions. The client
sends of the list of special characters. ]
LINEMODE SLC SYNCH DEFAULT 0
IP VALUE|FLUSHIN|FLUSHOUT 3 AO
VALUE 15 AYT DEFAULT 0 ABORT
VALUE|FLUSHIN|FLUSHOUT 28 EOF
VALUE 4 SUSP VALUE|FLUSHIN 26
EC VALUE 127 EL VALUE 21 EW
VALUE 23 RP VALUE 18 LNEXT
VALUE 22 XON VALUE 17 XOFF
VALUE 19
[ Now that linemode is enabled, the server sets the initial
Telnet Linemode Working Group [Page 17]
RFC 1116 Telnet Linemode Option August 1989
mode, and acknowledges the special characters. ]
LINEMODE MODE EDIT
LINEMODE SLC SYNCH NOSUPPORT 0 IP
VALUE|FLUSHIN|FLUSHOUT|ACK 3 AO
NOSUPPORT 0 AYT NOSUPPORT 0 ABORT
VALUE|FLUSHIN|FLUSHOUT|ACK 28 EOF
VALUE|ACK 4 SUSP NOSUPPORT 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 NOSUP-
PORT 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
Telnet Linemode Working Group [Page 18]
RFC 1116 Telnet Linemode Option August 1989
LINEMODE MODE 0|MODE_ACK
[ Application finishes. ]
WONT ECHO
DONT ECHO
LINEMODE MODE EDIT|TRAPSIG
LINEMODE MODE
EDIT|TRAPSIG|MODE_ACK
[ The user changes his erase character to ^H. ]
LINEMODE SLC EC VALUE 8
LINEMODE SLC EC VALUE|ACK 8
[ The user decides to revert to all the original client side
special characters. ]
LINEMODE SLC SYNCH DEFAULT 0
IP VALUE|FLUSHIN|FLUSHOUT 3 AO
VALUE 15 AYT DEFAULT 0 ABORT
VALUE|FLUSHIN|FLUSHOUT 28 EOF
VALUE 4 SUSP VALUE|FLUSHIN 26
EC VALUE 127 EL VALUE 21 EW
VALUE 23 RP VALUE 18 LNEXT
VALUE 22 XON VALUE 17 XOFF
VALUE 19
LINEMODE SLC SYNCH NOSUPPORT 0 AO
NOSUPPORT 15 AYT NOSUPPORT 0 SUSP
NOSUPPORT|FLUSHIN 26 EC VALUE|ACK
127 EW VALUE|ACK 23 RP VALUE|ACK
18 LNEXT VALUE|ACK 22 XON
VALUE|ACK 17 XOFF VALUE|ACK 19
LINEMODE SLC SYNCH
NOSUPPORT|ACK 0 AO
NOSUPPORT|ACK 15 AYT
NOSUPPORT|ACK 0 SUSP
NOSUPPORT|ACK|FLUSHIN 26
[ The user decides to import the remote sides default special
characters. ]
LINEMODE SLC 0 DEFAULT 0
LINEMODE SLC IP
VALUE|FLUSHIN|FLUSHOUT 3 ABORT
VALUE|FLUSHIN|FLUSHOUT 28 EOF
VALUE 4 EC VALUE 127 EL VALUE 21
[ Since these are the same as the current local settings, no
response is generated. ]
[ This next example is what would happen if an editor was fired
up, that wanted to let the client side do the echoing and
buffering of characters, but did not want it to do any line
editing, and only forward the data when got a control
character. Note that we have preceded all the the 0377s in the
forward mask with an IAC. ]
LINEMODE MODE 0
Telnet Linemode Working Group [Page 19]
RFC 1116 Telnet Linemode Option August 1989
LINEMODE DO FORWARDMASK IAC 0377
IAC 0377 IAC 0377 IAC 0377 0 0 0 0
0 0 0 0 0 0 0 01
LINEMODE MODE 0
LINEMODE WILL FORWARDMASK
[ Application runs to completion, and then things are to be set
back to what they were before. ]
LINEMODE MODE EDIT|TRAPSIG
LINEMODE DONT FORWARDMASK
LINEMODE MODE EDIT|TRAPSIG
LINEMODE WONT FORWARDMASK
6. Other Telnet options and RFCs
The following is a list of RFCs for various Telnet options that
should be supported along with LINEMODE.
1. Postel, J. and J. Reynolds, "TELNET PROTOCOL SPECIFICATION", RFC
854, USC/Information Sciences Institute, May 1983.
2. Postel, J. and J. Reynolds, "TELNET OPTION SPECIFICATIONS", RFC
855, USC/Information Sciences Institute, May 1983.
3. Postel, J. and J. Reynolds, "TELNET BINARY TRANSMISSION", RFC
856, USC/Information Sciences Institute, May 1983.
4. Postel, J. and J. Reynolds, "TELNET ECHO OPTION", RFC 857,
USC/Information Sciences Institute, May 1983.
5. Postel, J. and J. Reynolds, "TELNET SUPRESS GO AHEAD OPTION", RFC
858, USC/Information Sciences Institute, May 1983.
6. Postel, J. and J. Reynolds, "TELNET TIMING MARK OPTION", RFC 860,
USC/Information Sciences Institute, 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
Telnet Linemode Working Group [Page 20]
RFC 1116 Telnet Linemode Option August 1989
LINEMODE, but which would enhance any TELNET implementation.
11. Postel, J. and J. Reynolds, "TELNET STATUS OPTION", RFC 859,
USC/Information Sciences Institute, May 1983.
12. Postel, J. and J. Reynolds, "TELNET END OF RECORD OPTION", RFC
885, USC/Information Sciences Institute, 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.
Author's Address
Dave Borman
Cray Research Inc.
1440 Northland Drive
Mendota Heights, MN 55120
Phone: (612) 681-3398
EMail: dab@CRAY.COM
Telnet Linemode Working Group [Page 21]