Telnet Echo Option(1)

　　Network Working Group J. Postel

　　Request for Comments: 857 J. Reynolds

　　ISI

　　Obsoletes: NIC 15390 May 1983

　　TELNETECHO OPTION

　　This RFC specifies a standard for the ARPA Internet community. Hosts on　the ARPA Internet are expected to adopt and implement this standard.

　　1. Command Name and Code

　　ECHO 1

　　2. Command Meanings

　　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.

　　3. Default

　　WON'T ECHO

　　DON'T ECHO

　　No echoing is done over the TELNET connection.

　　4. Motivation for the Option

　　RFC857May 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.

　　5. Description of the Option

　　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

---------------->　　Process 1 Process 2

　　---------------->

　　Neither end echoes

　　<----------------

\

Process 1 / Process 2

---------------->　　\

　　Process 1 / Process 2

　　---------------->

　　One end echoes for itself

　　RFC857May 1983

　　<----------------

\

Process 1 / Process 2

---------------->　　\

　　Process 1 / Process 2

　　---------------->

　　One end echoes for the other

　　<----------------

\ /

Process 1 / \ Process 2

---------------->　　\ /

　　Process 1 / \ Process 2

　　---------------->

　　Both ends echo for themselves

　　<----------------

\ /

Process 1 / \ Process 2

---------------->　　\ /

　　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).