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.
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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
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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.
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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
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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
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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,
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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.
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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.
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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
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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.
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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
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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.
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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
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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
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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.
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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
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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.
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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
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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
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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
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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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