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Network Working Group                                          J. PostelRequest for Comments: 857                                    J. Reynolds                                                                     ISIObsoletes: NIC 15390                                            May 1983TELNET ECHO OPTIONThis RFC specifies a standard for the ARPA Internet community.  Hosts onthe ARPA Internet are expected to adopt and implement this standard.1. Command Name and Code   ECHO       12. 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 OptionPostel & Reynolds                                               [Page 1]

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.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                ---------------->                 Neither end echoes                <----------------                   \      Process 1    /              Process 2                ---------------->             One end echoes for itselfPostel & 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.Postel & Reynolds                                               [Page 3]

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.6. A Sample Implementation of the Option   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, thePostel & 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.Postel & Reynolds                                               [Page 5]