RFC0726: Remote Controlled Transmission and Echoing Telnet option
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NWG/RFC# 726 JBP DHC 8-MAR-77 08:29 39237
Remote Controlled Transmission & Echoing Telnet Option
Network Working Group Jon Postel & Dave Crocker
Request for Comments: 726 SRI-ARC UC Irvine
NIC: 39237 8 March 1977
Remote Controlled Transmssion and Echoing Telnet Option
1
1. Command name and code: 2
RCTE 7 2a
2. Command meanings: 3
IAC WILL RCTE 3a
The sender of this command REQUESTS or AGREES to use
the RCTE option, and will send instructions for
controlling the other side's terminal printer. 3a1
IAC WON'T RCTE 3b
The sender of this option REFUSES to send instructions
for controlling the other side's terminal printer. 3b1
IAC DO RCTE 3c
The sender REQUEST or AGREES to have the other side
(sender of WILL RCTE) issue commands which will control
his (sender of the DO) output to the terminal printer. 3c1
IAC DON'T RCTE 3d
The sender of this command REFUSES to allow the other
side to control his (sender of DON'T) terminal printer. 3d1
IAC SB RCTE <cmd> [BC1 BC2] [TC1 TC2] IAC SE 3e
where: 3e1
<cmd> is one 8-bit byte having the following flags
(bits are counted from the right): 3e1a
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Remote Controlled Transmission & Echoing Telnet Option
Bit Meaning 3e1b
0 0 = Ignore all other bits in this byte and
repeat the last <cmd> that was sent. Equals
a 'continue what you have been doing'.
1 = Perform actions as indicated by other bits
in this byte. 3e1c
1 0 = Print (echo) break character
1 = Skip (don't echo) break character 3e1d
2 0 = Print (echo) text up to break character
1 = Skip (don't echo) text up to break character 3e1e
3 0 = Continue using same classes of break
characters.
1 = The two 8-bit bytes following this byte
contain flags for the new break classes. 3e1f
4 0 = Continue using same classes of transmit
characters.
1 = Reset transmit classes according to the two
bytes following 1) the break classes bytes,
if the break classes are also being reset,
or 2) this byte, if the break classes are
NOT also being reset. 3e1g
Value (decimal) of the <cmd> byte and its meaning: 3e1h
0 = Continue what you have been doing 3e1i
Even numbers greater than zero (i.e. numbers with the
right most bit off) are in error and should be
interpreted as equal to zero. When the <cmd> is an
even number greater than zero, classes bytes TC1 &
TC2 and/or BC1 & BC2 must not be sent. 3e1j
1 = Print (echo) up to AND INCLUDING break character 3e1k
3 = Print up to break character and SKIP (don't echo)
break character 3e1l
5 = Skip text (don't echo) up to break character, but
PRINT break character 3e1m
7 = Skip up to and including break character 3e1n
Add one of the previous non-zero values to one of the
following values, to get the total decimal value for
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Remote Controlled Transmission & Echoing Telnet Option
the byte (Note that classes may not be reset without
also resetting the printing action; so an odd number
is guaranteed): 3e1o
8 = Set break classes (using the next two bytes [BC1
BC2]) 3e1p
16 = Set transmission classes (using the next two
bytes [TC1 TC2]) 3e1q
24 = Set break classes (using the next two bytes [BC1
BC2]) and the transmission classes (using the two
bytes after that [TC1 TC2]). 3e1r
Sub-commands (IAC SB RCTE...) are only sent by the
controlling host and, in addition to other functions,
functionally replace the Go-Ahead (IAC GA) Telnet
feature. RCTE also functionally replaces the Echo (IAC
ECHO) Telnet option. That is the Suppress Go-Ahead
option should be in force and the Echo option should
not be in force while the RCTE option is in use. The
echo mode on terminating use of the RCTE option should
be the default state, that is DON'T ECHO, WON'T ECHO. 3e2
Classes for break and transmission (the right-most bit
of the second byte (TC2 or BC2) represents class 1; the
left-most bit of the first byte (TC1 or BC1) represents
the currently undefined class 16: 3e3
1: Upper-Case Letter (A-Z) 3e3a
2: Lower-case Letters (a-z) 3e3b
3: Numbers (0-9) 3e3c
4: Format Effectors (<BS> <CR> <LF> <FF> <HT> <VT>) 3e3d
The sequence <cr><lf> counts as one character when
processed as the Telnet end of line, and is a
single break character when class 4 is set. The
sequence <cr><nul> counts as one character and
is a break character if and only if <cr> is a
break character (i.e. class 4 is set).
5: Non-format Effector Control Characters including
<DEL> and <ESC> 3e3e
6: . , ; : ? ! 3e3f
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Remote Controlled Transmission & Echoing Telnet Option
7: { [ ( < > ) ] } 3e3g
8: ' " / \ % @ $ & # + - * = ^ _ | ~ 3e3h
9: <Space> 3e3i
And Telnet commands (IAC . . .) sent by the user are
always to have the effect of a break character. That
is, every instance of an IAC is to be treated as a
break character, except the sequence IAC IAC. 3e3j
The representation to be displayed when printing is
called for is the obvious one for the visible
characters (classes 1, 2, 3, 6, 7, and 8). Space (class
9) is represented by a blank space. The format
effectors (class 4) by their format effect. The
non-format effector controls (class 5) print nothing
(no space). 3e4
Initially no break classes or transmission classes are
in effect. 3e5
Please note that if all the bits are set in a Telnet
subcommand argument byte such as TC2 or BC2 then that
byte must be preceeded by an <IAC> flag byte. This is
the common convention of doubling the escape character
to use its value as data. 3e6
Sub-commands (IAC SB RCTE...) are refered to as "break
reset commands". 3e7
3. Default: 4
WON'T RCTE -- DON'T RCTE 4a
Neither host asserts special control over the other
host's terminal printer. 4a1
4. Motivation for the option: 5
RFC's 1, 5 and 51 discuss Network and process efficiency
and smoothness. 5a
RFC 357, by John Davidson, introduces the problem of
echoing delay that occurs when a remote user accesses a
full-duplex host, thru a satellite link. In order to save
the many thousands of miles of transit time for each
echoed character, while still permitting full server
responsiveness and clean terminal output, an echo control
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Remote Controlled Transmission & Echoing Telnet Option
similar to that used by some time-sharing systems is
suggested for the entire Network. 5b
In effect, the option described in this document
involves making a using host carefully regulate the
local terminal printer according to explicit
instructions from the remote (serving) host. 5b1
An important additional issue is efficient Network
transmission. Implementation of the Davidson Echoing
Scheme will eliminate almost all server-to-user echoing. 5c
The option described in this document also requests
using hosts to buffer a terminal's input to the serving
host until it forms a useful unit (with "useful unit"
delimited by break or transmission characters as
described below). Therefore, fewer messages are sent on
the user-to-server path. 5c1
N.B.: This option is only intended for use with
full-duplex hosts. The Go-Ahead Telnet feature is
completely adequate for half-duplex server hosts. Also,
RCTE should be used in place of the ECHO Telnet option.
That is the Suppress Go-Ahead option should be in force
and the Echo option should not be in force while the RCTE
option is in use. 5d
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Remote Controlled Transmission & Echoing Telnet Option
5. Explicit description of control mechanism: 6
User Terminal Printing Action & Control Procedure 6a
Negotiate the use of the RCTE option. Once the option
is in force the user Telnet follows the following
procedure. 6a1
1) Read an item from the network. 6a2
If the item is data, then print it and go to 1. 6a2a
If the item is a command, then set the classes and go
to 2. 6a2b
2) If the terminal input buffer is empty, then go to 3,
else go to 4. 6a3
3) Wait for an item to appear either from the terminal
or from the network. 6a4
If an item appears from the terminal, then go to 4. 6a4a
If a data item appears from the network, then print
it and go to 3. 6a4b
If a command appears from the network, then an error
has occured. 6a4c
4) Read an item from the terminal input buffer. 6a5
If the item is not a break, then print/skip it and go
to 2. 6a5a
If the item is a break, then print/skip it and go to
1. 6a5b
Note: Output from the server host may occur at any
time, such "spontaneous output" is printed in step 3. 6a6
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Remote Controlled Transmission & Echoing Telnet Option
Explanation: 6b
Both Hosts agree to use the RCTE option. After that,
the using host (IAC DO RCTE) merely acts upon the
controlling (serving) host's commands and does not
issue any RCTE commands unless and until it (using
host) decides to stop allowing use of the option (by
sending IAC DON'T RCTE). 6b1
1) The using host is synchronized with the server by
initially and when ever it returns to step 1 suspending
terminal echo printing until it receives a command from
the server. 6b2
The server may send either output to the terminal
printer or a command, and usually sends a both. 6b3
The server may send output to the terminal printer
either in response to user input or spontaneously. In
the former case, the output is processed in step 1. In
the latter case, the output is processed in step 3. 6b4
Server sends an RCTE command. The command may redefine
break and transmission classes, action to be performed
on break characters, and action to be performed on
text. Each of these independent functions is controlled
by separate bits in the <cmd> byte. 6b5
A transmission character is one which RECOMMENDS that
the using host transmit all text accumulated up to
and including its occurrence. (For network
efficiency, using hosts are DISCOURAGED (but not
prohibited) from sending before the occurrence of a
transmission character, as defined at the moment the
character is typed). 6b5a
If the transmission classes bit (bit 4) is on, the
two bytes following the two break classes bytes (or
immediately following the <cmd> byte, if the break
classes bit is not on) will indicate what classes
are to be enabled.
If the bit is OFF, the transmission classes remain
unchanged. When the RCTE option is first initiated,
NO CLASSES are in effect. That is, no character
will be considered a transmission character. (As if
both TC1 and TC2 are zero.)
A break character REQUIRES that the using host
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Remote Controlled Transmission & Echoing Telnet Option
transmit all text accumulated up to and including its
occurrence and also causes the using host to stop its
print/discard action upon the user's input text,
until directed to do otherwise by another IAC SB RCTE
<cmd> IAC SE command from the serving host. Break
characters therefore define printing units. "Break
character" as used in this document does NOT mean
Telnet Break character. 6b5b
If the break classes bit (bit 3) is on, the two
bytes following <cmd> will indicate what classes
are to be enabled. There are currently nine (9)
classes defined, with room for expansion.
If the bit is OFF, the break classes remain
unchanged. When the RCTE option is initiated, NO
CLASSES are to be in effect. That is, no
transmission will take place in the user to server
direction until the first break reset command is
received by the user from the server.
The list of character classes, used to define break
and transmission classes are listed at the end of
this document, in the Tables Section. 6b5c
Because break characters are special, the
print/discard action that should be performed upon
them is not always the same as should be performed
upon the rest of the input text. 6b5d
For example, while typing a filename to TENEX, I
want the text of the filename to be printed
(echoed); but I do not want the <escape> (if I use
the name completion feature) to be printed.
If bit 1 is ON the break character is NOT to be
printed.
A separate bit (bit 2) signals whether or not the
text itself should be printed (echoed) to the
terminal. If bit 2 = 0, then the text IS to be
printed. 6b5e
Yet another bit (bit 0 - right-most bit) signals
whether or not any of the other bits of the command
should be checked. If this bit is OFF, then the
command should be interpreted to mean "continue
whatever echoing strategy you have been following,
using the same break and transmission classes." 6b5f
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Remote Controlled Transmission & Echoing Telnet Option
2) The user Telnet now checks the terminal input
buffer, if it contains data it is processed in step 4,
otherwise the user Telnet waits in step 3 for further
developments. 6b6
3) The user Telnet waits until either the human user
enters some data in which case Telnet proceeeds to step
4, or an item is received from the network. If the item
from the network is data it is spontaneous output and
is printed, Telnet then continues to wait. If the item
from the network is a command then an error has
occured. In this case the user Telnet may attempt to
resynchronize the use of RCTE as indicated below. 6b7
4) Items from the terminal are processed with printing
controlled by the settings of the latest break reset
command. When a break character is processed, the cycle
of control is complete and action re-commences at step
1. 6b8
Input from the terminal is (hopefully) buffered into
units ending with a transmission or break character;
and echoing of input text is suspended after the
occurrence of a break character and until receipt of a
break reset command from the serving host. The most
recent break reset command determines the break
actions. 6b9
In summary, what is required is that for every break
character sent in the user to server direction there be
a break reset command sent in the server to user
direction. The user host initially has no knowledge of
which characters are break characters and so starts in
a state that assumes that there are no break characters
and also that no echoing is to be provided. The server
host is expected to send a break reset command to
establish the break classes and the echoing mode before
it receives any data from the user. 6b10
Synchronization and Resynchronization: 6c
The serving and using hosts must carefully synchronize
break reset commands with the transmission of break
characters. Except at the beginning of an interaction,
the serving host may only send a break reset command in
response to the Using host's having sent a break
character as defined at that time. This should
establish a one-to-one correspondence between them. (A
<cmd> value of zero, in this context, is interpreted as
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Remote Controlled Transmission & Echoing Telnet Option
a break classes reset to the same class(es) as before.)
The break reset command may be preceded by terminal
output. 6c1
The re-synchronization of the break characters and the
break reset commands is done via the exchange of the
Telnet signal Abort Output (AO) in the server to user
direction and the SYNCH in the user to server
direction. 6c2
Suppose the server wants to resynchronize the break
characters and the break reset commands. 6c3
a. The server should be sure all output to the
terminal has been printed by using, for example, the
Timing Mark Option. 6c3a
b. The server sends the AO signal. 6c3b
c. The user receives the AO signal. The user flushes
all user to server data wheather it has been echoed
or not. The user sends a SYNCH to the server. [The
SYNCH consists of the Telnet Data Mark (DM) and the
host-to-host interrupt (INS).] The user now enters
the initial state at step 1. 6c3c
d. The server receives the SYNCH and flushes any
data preceeding the DM (as always). The server now
sends a break reset command. (Actually the break
reset command could be sent at any time following the
AO.) 6c3d
Suppose the user wants to resynchronize the break
characters and the break reset commands. 6c4
a. The user should discard all user to server data
wheather it has been echoed or not. 6c4a
b. The user sends the AO signal. The user now enters
the algorithm at step 1. 6c4b
c. The server receives the AO signal. The server
discards all data buffered but not yet sent to the
user. The server sends a SYNCH to the user. The
server sends a break reset command to the user. 6c4c
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Remote Controlled Transmission & Echoing Telnet Option
Notes and Comments: 6d
Even-numbered commands, greater than zero, are in
error, since they will have the low-order bit off. The
command should be interpreted as equal to zero, which
means that any classes reset bytes ([TC1 TC2] [BC1
BC2]) will be in error. (The IAC SE, at the end of the
command, eliminates any parsing problems due to this
error.) 6d1
Serving hosts will generally instruct using hosts not
to echo break characters, even though it might be
alright to echo most break characters. For example,
<cr> is usually a safe character to echo but <esc> is
not. TENEX Exec is willing to accept either, during
filename specification. Therefore, the using host must
be instructed not to echo any break characters. 6d2
This is generally a tolerable problem, since the
serving host has to send an RCTE command at this
point, anyhow. Adding an echo for the break character
to the message will not cause any extra network
traffic. 6d2a
The RCTE Option entails a rather large overhead. In a
true character-at-a-time situation, this overhead is
not justified. But on the average, it should result in
significant savings, both in network traffic and host
wake-ups. 6d3
Buffering Problems and Transmission vs. Printing
Constraints: 6d4
There are NO mandatory transmission constraints. The
using host is allowed to send a character a time,
though this would be a waste of RCTE. The
transmission classes commands are GUIDELINES, so
deviating from them, as when the user's buffer gets
full, is allowed. 6d4a
Additionally, the using host may send a break class
character, without knowing that it is one (as with
type-ahead). 6d4b
If the user implementation is clever it may send
the user entered data to the server before it is
actually needed. This type ahead data may contain
break characters.
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Remote Controlled Transmission & Echoing Telnet Option
Assume that only space is a break character (that
is the last break reset command specified print up
to and including the break characters and set the
break classes to class 9). Suppose the user had
typed "abc<space>def<esc>ghi<cr>". The user side
RCTE could send it all to the server, but it could
print only "abc<space>", and would have to buffer
"def<esc>ghi<cr>" at least until a break reset
command was received from the server. That break
reset command could change the break ckasses
requiring rescanning the buffered string.
For example suppose the break reset command set the
break characters to class 5 and the action to print
up to but not including the break character. The
user RCTE could then print "def" and discard the
<esc>, but whould have to continue to buffer the
"ghi<cr>".
The problem with buffering occurs when printing on
the user's terminal must be suspended, after the user
has typed a currently valid break character and until
a break reset command is received from the serving
host. During this time, the user may be typing
merrily along. The text being typed may be SENT, but
may not yet be PRINTED. 6d4c
The more common problem of filling the transmission
buffer, while awaiting a host to host allocate from
the serving host, may also occur, but this problem is
well known to implementors and in no way special to
RCTE. 6d4d
In any case, when the buffer does fill and further
text typed by the user will be lost, the user should
be notified (perhaps by ringing the terminal bell). 6d4e
Text should be buffered by the using host until the
user types a character which belongs to the
transmission class in force at the moment the character
is typed. 6d5
Transmission class reset commands may be sent by the
serving host at any time. If they are frequently sent
separate from break class reset commands, it will
probably be better to exit from RCTE and enter regular
character at a time transmission. 6d6
It is not immediately clear what the using host should
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Remote Controlled Transmission & Echoing Telnet Option
do with currently buffered text, when a transmission
classes reset command is received. The buffering is
according to the previous transmission classes scheme. 6d7
The using host clearly should not simply wait until a
transmission character (according to the new scheme)
is typed. 6d7a
Either the buffered text should be rescanned, under
the new scheme; 6d7b
Or the buffered text should simply be sent as a
group. This is the simpler approach, and probably
quite adequate. 6d7c
It is possible to define NO BREAK CHARACTERS except
Telnet commands (IAC ...). This seems undesirable and
should not be done. 6d8
If this situation were to occur the using host should
send a Telnet command to allow the server to know
when he may reset the break classes, but the
mechanism is awkward and this case should be avoided. 6d8a
6. Sample Interaction: 7
"S:" is sent from serving (WILL RCTE) host to using host.
"U:" is sent from using (DO RCTE) host to serving host.
"T:" is entered by the terminal user.
"P:" is printed on the terminal.
Text surrounded by square brackets ([]) is commentary.
Text surrounded by angle brackets (<>) is to be taken as
a single unit. E.g., carriage return is <cr>, and the
decimal value 27 is represented <27>. 7a
The following interaction shows a logon to a Tenex,
initiation of the DED editor, insertion of some text and
the return to the Exec level. 7b
An attempt has been made to give some flavor of the
asynchrony of network I/O and the user's terminal
input. Many other possible combinations, using the same
set of actions listed below, could be devised. The
actual order of events will depend upon network and
hosts' load and the user's typing speed. 7b1
We assume that the user's Telnet is also in an "insert
linefeed" mode. That is, whenever the user types carriage
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Remote Controlled Transmission & Echoing Telnet Option
return <cr> the user Telnet sends both carriage return
and linefeed <cr><lf> (the Telnet end of line signal).
When space character occurs at the end of a line in the
example description it is shown explicitly by <sp> to
avoid confusion. Other uses of the space character are
not so marked to avoid destroying the readability of the
example. 7c
A Telnet connection has already been opened, but the
TENEX prompt has not yet been issued. The hosts first
discuss using the RCTE option: 7d
S: <IAC><WILL><RCTE> 7d1
U: <IAC><DO><RCTE> 7d2
S: TENEX 1.31.18, TENEX EXEC 1.50.2<cr><lf>@
<IAC><SB><RCTE><11><1><24><IAC><SE> 7d3
[Print the herald and echo input text up to a break
character, but do not echo the break character.
Classes 4 (Format Effectors), 5 (Non-format Effector
Controls and <DEL>), and 9 (<sp>) act as break
characters.] 7d3a
P: TENEX 1.31.18, TENEX EXEC 1.50.2<cr><lf>@ 7d4
T: LOGIN ARPA<cr> 7d5
P: LOGIN 7d6
U: LOGIN<sp> 7d7
U: ARPA<cr><lf> 7d8
S: <sp><IAC><SB><RCTE><0><IAC>SE> 7d9
P: <sp>ARPA 7d10
S: <cr><lf>(PASSWORD): <IAC><SB><RCTE><7><IAC><SE> 7d11
P: <cr><lf>(PASSWORD):<sp> 7d12
T: WASHINGTON 1000<cr> 7d13
[The password "WASHINGTON" is not echoed. Printing of
"1000<cr>" is withheld] 7d13a
U: WASHINGTON<sp> 7d14
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Remote Controlled Transmission & Echoing Telnet Option
U: 1000<cr><lf> 7d15
S: <sp><IAC><SB><RCTE><3><IAC><SE> 7d16
S: <cr><lf>JOB 17 ON TTY41 7-JUN-73 14:13<cr><lf>@
<IAC><SB><RCTE><0><IAC><SE> 7d17
P: <sp>1000 7d18
[Printing is slow at this point; so the account
number is not printed as soon as the server's command
for it is received.] 7d18a
P: <cr><lf>JOB 17 ON TTY41 7-JUN-73 14:13<cr><lf>@ 7d19
T: DED<esc><cr> 7d20
P: DED 7d21
U: DED<esc> 7d22
S: .SAV;1<IAC><SB><RCTE><0><IAC><SE> 7d23
P: .SAV;1 7d24
U: <cr><lf> 7d25
S: <cr><lf><lf>DED 3/14/73 DRO,KRK<cr><lf>:
<IAC><SB><RCTE><15><1><IAC><255><IAC><SE> 7d26
[The program is started and the DED prompt ":" is
sent. At the command level, DED responds to every
character. The server sets the break classes to all
classes.] 7d26a
P: <cr><lf><lf>DED 3/14/73 DRO,KRK<cr><lf>: 7d27
T: IThis is a test line.<cr>This is another test
line.<^Z>Q 7d28
["I" means Insert Text. The text follows, terminated
by a Control-Z. The "Q" instructs DED to Quit.] 7d28a
U: I 7d29
U: This is a test line.<cr><lf> 7d30
S: I<cr><lf>*<IAC><SB><RCTE><11><0><24><IAC><SE> 7d31
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Remote Controlled Transmission & Echoing Telnet Option
[DED prompts the user, during text input, with an
asterisk at the beginning of every line. The server
sets the break classes to classes 4 and 5, the format
effectors and the non-format effector controls.] 7d31a
P: I<cr><lf>*This is a test line. 7d32
S: <cr><lf>*<IAC><SB><RCTE><0><IAC><SE> 7d33
P: <cr><lf>*This is another test line. 7d34
U: This is another test line.<^Z> 7d35
U: Q 7d36
[Note that the "Q" will not immediately be printed on
the terminal, since it must wait for authorization.] 7d36a
S: ^Z<cr><lf>:<IAC><SB><RCTE><15><1><IAC><255><IAC><SE> 7d37
[The returned "^Z" is two characters, not the ASCII
Control-Z or <sub>.] 7d37a
S: Q<cr><lf>@<IAC><SB><RCTE><11><1><24><IAC><SE> 7d38
P: Q<cr><lf>@ 7d39
And the user is returned to the Exec level. 7d40
[page 16]