GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc857

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.

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

Postel & 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 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.

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, 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.

Postel & Reynolds [Page 5]

/data/webs/external/dokuwiki/data/pages/rfc/rfc857.txt · Last modified: 1991/10/17 17:40 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki