Network Working Group                                         Jon PostelRequest for Comments: 318                                       UCLA-NMCNIC: 9348                                                  April 3, 1972References: 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 (213) 825-2368 or by   mail.                              Jon Postel                              SPADE Group                              3804 Boelter Hall                              UCLA                              Los Angeles, California 90024Postel                                                          [Page 1]RFC 318                     Telnet Protocol                   April 1972I.  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 1972The 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 1972TELNET 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           EBCDICUSER 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 1972STANDARD 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.