Network Working Group J. Postel
Request for Comments: 857 J. Reynolds
ISI
Obsoletes: NIC 15390 May 1983
TELNET ECHO OPTION
This RFC specifies a standard for the ARPA Internet community. Hosts on
the ARPA Internet are expected to adopt and implement this standard.
IAC WILL ECHO
The sender of this command REQUESTS to begin, or confirms that it
will now begin, echoing data characters it receives over the
TELNET connection back to the sender of the data characters.
IAC WON'T ECHO
The sender of this command DEMANDS to stop, or refuses to start,
echoing the data characters it receives over the TELNET connection
back to the sender of the data characters.
IAC DO ECHO
The sender of this command REQUESTS that the receiver of this
command begin echoing, or confirms that the receiver of this
command is expected to echo, data characters it receives over the
TELNET connection back to the sender.
IAC DON'T ECHO
The sender of this command DEMANDS the receiver of this command
stop, or not start, echoing data characters it receives over the
TELNET connection.
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RFC 857 May 1983
The NVT has a printer and a keyboard which are nominally
interconnected so that "echoes" need never traverse the network; that
is to say, the NVT nominally operates in a mode where characters
typed on the keyboard are (by some means) locally turned around and
printed on the printer. In highly interactive situations it is
appropriate for the remote process (command language interpreter,
etc.) to which the characters are being sent to control the way they
are echoed on the printer. In order to support such interactive
situations, it is necessary that there be a TELNET option to allow
the parties at the two ends of the TELNET connection to agree that
characters typed on an NVT keyboard are to be echoed by the party at
the other end of the TELNET connection.
When the echoing option is in effect, the party at the end performing
the echoing is expected to transmit (echo) data characters it
receives back to the sender of the data characters. The option does
not require that the characters echoed be exactly the characters
received (for example, a number of systems echo the ASCII ESC
character with something other than the ESC character). When the
echoing option is not in effect, the receiver of data characters
should not echo them back to the sender; this, of course, does not
prevent the receiver from responding to data characters received.
The normal TELNET connection is two way. That is, data flows in each
direction on the connection independently; and neither, either, or
both directions may be operating simultaneously in echo mode. There
are five reasonable modes of operation for echoing on a connection
pair:
<----------------
Process 1 Process 2
---------------->
Neither end echoes
<----------------
\
Process 1 / Process 2
---------------->
One end echoes for itself
Postel & Reynolds [Page 2]
RFC 857 May 1983
<----------------
\
Process 1 / Process 2
---------------->
One end echoes for the other
<----------------
\ /
Process 1 / \ Process 2
---------------->
Both ends echo for themselves
<----------------
\ /
Process 1 / \ Process 2
---------------->
One end echoes for both ends
This option provides the capability to decide on whether or not
either end will echo for the other. It does not, however, provide
any control over whether or not an end echoes for itself; this
decision must be left to the sole discretion of the systems at each
end (although they may use information regarding the state of
"remote" echoing negotiations in making this decision).
It should be noted that if BOTH hosts enter the mode of echoing
characters transmitted by the other host, then any character
transmitted in either direction will be "echoed" back and forth
indefinitely. Therefore, care should be taken in each implementation
that if one site is echoing, echoing is not permitted to be turned on
at the other.
As discussed in the TELNET Protocol Specification, both parties to a
full-duplex TELNET connection initially assume each direction of the
connection is being operated in the default mode which is non-echo
(non-echo is not using this option, and the same as DON'T ECHO, WON'T
ECHO).
If either party desires himself to echo characters to the other party
or for the other party to echo characters to him, that party gives
the appropriate command (WILL ECHO or DO ECHO) and waits (and hopes)
for acceptance of the option. If the request to operate the
connection in echo mode is refused, then the connection continues to
operate in non-echo mode. If the request to operate the connection
in echo mode is accepted, the connection is operated in echo mode.
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RFC 857 May 1983
After a connection has been changed to echo mode, either party may
demand that it revert to non-echo mode by giving the appropriate
DON'T ECHO or WON'T ECHO command (which the other party must confirm
thereby allowing the connection to operate in non-echo mode). Just
as each direction of the TELNET connection may be put in remote
echoing mode independently, each direction of the TELNET connection
must be removed from remote echoing mode separately.
Implementations of the echo option, as implementations of all other
TELNET options, must follow the loop preventing rules given in the
General Considerations section of the TELNET Protocol Specification.
Also, so that switches between echo and non-echo mode can be made
with minimal confusion (momentary double echoing, etc.), switches in
mode of operation should be made at times precisely coordinated with
the reception and transmission of echo requests and demands. For
instance, if one party responds to a DO ECHO with a WILL ECHO, all
data characters received after the DO ECHO should be echoed and the
WILL ECHO should immediately precede the first of the echoed
characters.
The echoing option alone will normally not be sufficient to effect
what is commonly understood to be remote computer echoing of
characters typed on a terminal keyboard--the SUPPRESS-GO AHEAD option
will normally have to be invoked in conjunction with the ECHO option
to effect character-at-a-time remote echoing.
The following is a description of a possible implementation for a
simple user system called "UHOST".
A possible implementation could be that for each user terminal, the
UHOST would keep three state bits: whether the terminal echoes for
itself (UHOST ECHO always) or not (ECHO mode possible), whether the
(human) user prefers to operate in ECHO mode or in non-ECHO mode, and
whether the connection from this terminal to the server is in ECHO or
non-ECHO mode. We will call these three bits P(hysical), D(esired),
and A(ctual).
When a terminal dials up the UHOST the P-bit is set appropriately,
the D-bit is set equal to it, and the A-bit is set to non-ECHO. The
P-bit and D-bit may be manually reset by direct commands if the user
so desires. For example, a user in Hawaii on a "full-duplex"
terminal, would choose not to operate in ECHO mode, regardless of the
preference of a mainland server. He should direct the UHOST to
change his D-bit from ECHO to non-ECHO.
When a connection is opened from the UHOST terminal to a server, the
Postel & Reynolds [Page 4]
RFC 857 May 1983
UHOST would send the server a DO ECHO command if the MIN (with
non-ECHO less than ECHO) of the P- and D-bits is different from the
A-bit. If a WON'T ECHO or WILL ECHO arrives from the server, the
UHOST will set the A-bit to the MIN of the received request, the
P-bit, and the D-bit. If this changes the state of the A-bit, the
UHOST will send off the appropriate acknowledgment; if it does not,
then the UHOST will send off the appropriate refusal if not changing
meant that it had to deny the request (i.e., the MIN of the P-and
D-bits was less than the received A-request).
If while a connection is open, the UHOST terminal user changes either
the P-bit or D-bit, the UHOST will repeat the above tests and send
off a DO ECHO or DON'T ECHO, if necessary. When the connection is
closed, the UHOST would reset the A-bit to indicate UHOST echoing.
While the UHOST's implementation would not involve DO ECHO or DON'T
ECHO commands being sent to the server except when the connection is
opened or the user explicitly changes his echoing mode, bigger hosts
might invoke such mode switches quite frequently. For instance,
while a line-at-a-time system were running, the server might attempt
to put the user in local echo mode by sending the WON'T ECHO command
to the user; but while a character-at-a-time system were running, the
server might attempt to invoke remote echoing for the user by sending
the WILL ECHO command to the user. Furthermore, while the UHOST will
never send a WILL ECHO command and will only send a WON'T ECHO to
refuse a server sent DO ECHO command, a server host might often send
the WILL and WON'T ECHO commands.
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