rfc318.txt

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Network Working Group                                         Jon Postel
Request for Comments: 318                                       UCLA-NMC
NIC: 9348                                                  April 3, 1972
References: RFC 139, 158, and NIC 7104


                            Telnet Protocol


   At the October 1971 Network Working Group Meeting, I promised to
   promptly produce a document which clearly and succinctly specified
   and explained the Official Telnet Protocol.  This document fails to
   meet any part of that promise.  This document was not produced
   promptly.  This document is neither clear nor succinct.  There is NO
   Official Telnet Protocol.

   The following pages present my understanding of the ad hoc Telnet
   protocol.  There are some who have serious questions about this
   protocol.  The proposed changes to the protocol are given in Section
   IV.

   Any comments should be promptly directed to me via the Network
   Information Center (Ident = JBP) or by phone [phone] or by
   mail.

                              Jon Postel
                              SPADE Group
                              3804 Boelter Hall
                              UCLA
                              Los Angeles, California 90024





















Postel                                                          [Page 1]

RFC 318                     Telnet Protocol                   April 1972


I.  DEFINITION OF THE NETWORK VIRTUAL TERMINAL

   The Network Virtual Terminal (NVT) is a bi-directional character
   device.  The characters are represented by 8 bit codes.  The NVT has
   no timing characteristics.  The character codes 0 through 127 are the
   USASCII codes.  (Note all code values are given in decimal.)  The
   codes 128 through 255 are used for special control signals.  The NVT
   is described as having a printer and a keyboard.  The printer
   responds to incoming data and the keyboard produces outgoing data.

The Printer

   The NVT printer has an unspecified carriage width (common values are
   40, 72, 80, 120, 128, 132).  The printer can produce representations
   of all 95 USASCII graphics (codes 32 through 126).  Of the 33 USASCII
   control codes (0 through 31 and 127) the following 8 have specific
   meaning to the NVT printer.

   NAME                  CODE    MEANING

   NULL (NUL)            0       A no operation.

   BELL (BEL)            7       Produces an audible or visible signal.

   Back Space (BS)       8       Backspaces the printer one character
                                 position.

   Horizontal Tab (HT)   9       Moves the printer to next horizontal
                                 tab stop.

   Line Feed (LF)        10      Moves the printer to next line (keeping
                                 the same horizontal position).

   Vertical Tab (VT)     11      Moves the printer to the next vertical
                                 tab stop.

   Form Feed (FF)        12      Moves the printer to the top of the
                                 next page.

   Carriage Return (CR)  13      Moves the printer to the left margin
                                 of the current line.

   The remaining USASCII codes (1 through 6, 14 through 31, and 127) do
   not cause the NVT printer to take any action.







Postel                                                          [Page 2]

RFC 318                     Telnet Protocol                   April 1972


The Keyboard

   The NVT Keyboard has keys or key combinations or key sequences for
   generating all of the 128 USASCII codes.  Note that although there
   are codes which have no effect on the NVT printer, the NVT Keyboard
   is capable of generating these codes.

The End of the Line Convention

   The end of a line of text shall be indicated by the character
   sequence Carriage Return Line Feed (CR, LF).  This convention applies
   to both the sending (Keyboard) and receiving (Printer)  (virtual)
   mechanisms.

Break and Reverse Break

   The Telnet control signals provide a BREAK signal which can be used
   to simulate the use of the break or attention or interrupt button
   found on most terminals.  This signal has no effect on the NVT.  When
   the BREAK Telnet control signal is used from server to user it is
   sometimes called "reverse break".  Such a reverse break has no effect
   on the NVT.

II.  DEFINITION OF TELNET PROTOCOL

   The purpose of Telnet Protocol is to provide a standard method of
   interfacing terminals devices at one site to processes at another
   site.

   The Telnet Protocol is built up from three major substructures, first
   the Initial Connection Protocol (ICP), second the Network Virtual
   Terminal (NVT), and third the Telnet control signals described
   herein.

   Telnet user and server processes follow the ICP to establish
   connections.  The term "Logger" has been associated with the set of
   processes in the serving system which respond to the ICP and perform
   the initial interactions e.g. obtain a name and password.  The ICP is
   defined and the initial socket number and byte size parameters are
   defined in "Current Network Protocols" (NIC #7104).

   The data transmitted between the user and server programs (and vice
   versa) is treated as a character stream with embedded control
   signals.

   Note that all code values are given in decimal.





Postel                                                          [Page 3]

RFC 318                     Telnet Protocol                   April 1972


TELNET CONTROL SIGNALS

   NAME             CODE    MEANING

   DATA MARK        128     Used to mark a point in the data stream.
                            Used in conjunction with INS.  See SYNCH.

   BREAK            129     User-to-Server:  Has the same meaning to
                            the server as the "Break," "Interrupt," or
                            "Attention" button found on many terminals.

                            Server-to-User:  Has the same meaning to
                            to use as the "reverse break" used with
                            some terminals.

   NOP              130     No Operation.

   NO ECHO          131     User-to-Server:  Asks the server not to
                            return Echos of the transmitted data.

                            Server-to-User:  States that the server is
                            not sending echos of the transmitted data.
                            Sent only as a reply to ECHO or NO ECHO,
                            or to end the hide your input.

   ECHO             132     User-to-Server:  Asks the server to send
                            Echos of the transmitted data.

                            Server-to User:  States that the server is
                            sending echos of the transmitted data.
                            Sent only as a reply to ECHO or NO ECHO.

   Hide your input  133     The intention is that a server will send
                            this signal to a user system which is
                            echoing locally (to the user) when the user
                            is about to type something secret (e.g. a
                            password).  In this case, the user system
                            is to suppress local echoing or overprint
                            the input (or something) until the server
                            sends a NOECHO signal.  In situations where
                            the user system is not echoing locally,
                            this signal must not be sent by the server.

   INS              ---     This is the "Interrupt on Send" signal,
                            defined by the Host-to-Host protocol and
                            implemented by the Network Control Program
                            (NCP).  See SYNCH.




Postel                                                          [Page 4]

RFC 318                     Telnet Protocol                   April 1972


   SYNCH            ---     This is a condition indicated by the
                            combination of the DATA MARK and the INS.

                            User-to-Server:  The Server is to examine
                            the input data stream looking for a DATA
                            MARK signal; if a DATA MARK is found, the
                            server must not process further until an
                            INS is received.  If the server receives an
                            INS, it is required to examine the data
                            stream at once, taking any appropriate
                            action on "break type" characters (e.g.
                            etx, sub, BREAK), up to a DATA MARK signal
                            and thereupon continue its normal processing.
                            The passed over characters may be discarded.

                            Server-to-User:  If the user finds a DATA
                            MARK in the data stream, it must wait for
                            an INS.  If the user receives an INS, it
                            must examine and discard characters up to
                            and including a DATA MARK.

DATA TYPES

   Telnet normally deals in ASCII characters, but there are provisions
   for escaping to other code sets.  If one of these escapes is used, it
   is undefined (here) whether or not the Telnet signals still have
   meaning or even how to return to the ASCII set:  The Telnet signals
   used to indicate a change of code set are:

                      CODE          MEANING

                      160           ASCII - Standard Telnet
                      161           Transparent
                      162           EBCDIC

USER TELNET SIGNALS

   The following signals are to be available to the human user to cause
   the user Telnet to take the indicated action.

   Transmit Now         -  Transmit all data entered and locally
                           buffered now.  Intended to be used with line
                           mode.

   Suppress end-of-line -  Transmit all data entered and locally
                           buffered now, and do not transmit the
                           end-of-line immediately following this signal.




Postel                                                          [Page 5]

RFC 318                     Telnet Protocol                   April 1972


STANDARD TELNET IMPLEMENTATION

   Using Site

   1)  User is able to enter and transmit all ASCII codes

   2)  User is able to cause the Telnet signals BREAK, SYNCH, ECHO and
       NOECHO to be transmitted.

   3)  Provides for the User Telnet signals, (e.g. Transmit Now).

   4)  Implements the CR LF end-of-line convention.

   5)  Provides local echo for local user terminals.