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rfc:rfc158

Network Working Group T. O'Sullivan Request for Comments: 158 Raytheon NIC: 6768 19 May 1971

                          TELNET Protocol
 This request for comment is being circulated by the TELNET committee
 to solicit comments, evaluation, and requests for modification of the
 proposed protocol presented here.  Unless comments are received by
 the writer within two weeks of the date of this RFC, they will not be
 considered in the preparation of official TELNET PROTOCOL document.
 The proposed document is the result of the work of the committee.  It
 represents a TELNET protocol felt to be adequate for initial
 implementation.
 Readers are referenced to the following previous releases of
 information:
    1. Conventions for Using an IBM 2741 Terminal or a User Console
       for Access to Network Server HOSTS
          Joel Winett, RFC 110 (NIC #5809)
    2. Response to RFD 110
          Wayne Hathaway, RFC 135 (NIC #6712)
    3. Level III Server Protocol for the Lincoln Laboratory 360/67
       HOST
          Joel Winett, RFC 109 (NIC #5808)
    4. First Cut at a Proposed TELNET Protocol
          J. Melvin, D. Watson, RFC 97 (NIC #5740)
    5. ASCII Format for Network Interchange
          V. Cerf, RFC 20 (NIC #4722)
    6. Discussion of TELNET PROTOCOL
          Tom O' Sullivan, RFC 139 (NIC 6717)
       (Although relevant to the obsoleted RFC 137 (NIC 6714) many of
       the examples still hold.  A replacement discussion document RFC
       159 (NIC 6769) will be forthcoming in the near future).

T. O'Sullivan [Page 1] RFC 158 TELNET PROTOCOL 19 May 1971

                               TELNET PROTOCOL
                             A Proposed Document
                    T. O'Sullivan for the TELNET Committee
                   Will Crowther                 BBN
                   Bob Long                      SDC
                   John Melvin                   SRI-ARC
                   Bob Metcalfe                  Harvard
                   Ed Meyer                      MAC
                   Tom O'Sullivan (Chairman)     Raytheon
                   Joel Winett                   MIT-LL

T. O'Sullivan [Page 2] RFC 158 TELNET PROTOCOL 19 May 1971

Telnet Protocol

 TELNET is a third-level protocol, the function of which is to make a
 terminal (or process) at a using site appear to the system or a
 process at a serving site as logically equivalent to a terminal
 "directly" connected to the serving site.  In performing this
 function, the protocol attempts to minimize the amount of information
 each HOST must keep about the characteristics of other HOSTS.
 Definitions
 Protocol Levels (see Figure 1)
    Level 1
       HOST-IMP protocol specified by BBN in NIC 5735, Specifications
       for the Interconnection of a HOST and an IMP (BBN Report 1822)
    Level 2
       HOST-HOST protocol performed by NCPs as described in Document
       Number 1 (NIC 5413) and subsequent amendments, see RFC 107 (NIC
       #5806)
          One view of the NCP's function is that it takes information
          from the net and routes it to receiving processes via
          mechanisms internal to each HOST; conversely, processes use
          the NCP, via internal system calls, to have information
          routed to other processes in the net (via the other
          processes' NCPs).
    Level 3 (see Figure 2)
       Level 3 is, by definition, the place to which and from which
       the NCP communicates internally in its own host.
          This level may be equivalent to the user process level in
          some systems, but this may not be the case in all systems.
          In using sites, the TELNET process operates at this level.
          In serving sites, the TELNET server operates at this level.
 Initial Connection Protocol (ICP)
    An agreed-upon sequence of level 3 exchanges between two processes
    which is, in general, used to synchronize the connection dialogue
    between the processes, e.g., RFC 80 (NIC #5608) #1, as revised by
    subsequent information.

T. O'Sullivan [Page 3] RFC 158 TELNET PROTOCOL 19 May 1971

 Serving Site
    The HOST into which the TELNET process is directing the user's
    keyboard input and from which the TELNET process is receiving
    control information and data effecting the user's terminal.  At
    the serving site, a TELNET server is executing.
 Using Site
    The HOST in which the TELNET process is executing.
 Sending Site
    The HOST transmitting data, could be either using site or serving
    site.
 Receiving Site
    Converse of sending site.
 User
    The person or process "driving" the TELNET process.
 In providing services the TELNET protocol will use established
 network conventions, specifically the Network Control Program, and
 Initial Connection Protocol referenced in the above definitions,
 using a byte size of 8 bits on the permanent connection.
 The TELNET protocol provides for a Network Virtual Terminal (NVT)
 through which users may transmit and receive data over connections
 between the using site and the serving site.
 The code of the NVT will be full 7 bit ASCII.  The seven-bit code
 will be transmitted in eight-bit bytes, the high order bit set to
 zero.
 It will be the responsibility of the using site to provide its users
 with a means of producing all 128 ASCII codes, as well as a selected
 set of special TELNET control signals (see Figure 3).
 The ASCII character ESC will be employed by the user as an escape
 signal indicating that the next character(s) has special meaning.
 The meaning assigned to escape code will be serving site defined and
 therefore may not be consistant across the network.

T. O'Sullivan [Page 4] RFC 158 TELNET PROTOCOL 19 May 1971

 It will be the responsibility of the serving site to specify for
 users how the NVT code will be used to represent the codes normally
 generated by a local terminal.  The serving sites specification of
 this representation is expected, where reasonable, to map on a one-
 for-one basis for ASCII graphics and controls that are provided
 through local terminals.  The serving site will also specify how the
 escape conventions will be interpreted by the system.
 The end of a line will be represented in the NVT as carriage return
 (X'0D') followed by line feed. (X'0A')
 The protocol assumes that initially the serving site will not provide
 any echo to the using site.
 Each TELNET control signal for which code must be sent over the
 connection will be represented in the NVT by an eight-bit code, with
 the high order bit set to one.  Following are the special codes
 established to date. (U) indicates that in most implementations the
 user would be expected to have the ability to signal the TELNET
 process from his terminal to initiate the code.
 Code X'A0'
    Source:  Both Sites (U)
    Meaning: A DATA TYPE[1] signal indicating that code will be
             transmitted by NVT, i.e., using the seven-bit ASCII
             conventions.
 Code X'80'
    Source:  Using Site (U)
    Meaning: Order using site NCP to send an INS and insert X'80' in
             data stream.
 Code X'81'
    Source:  Using Site (U)
    Meaning: Break or Attention, and reverse break.
 Code X'82'
    Source:  Both Sites
    Meaning: No op
 Code X'83'
    Source:  Both Sites
    Meaning: Don't Echo

T. O'Sullivan [Page 5] RFC 158 TELNET PROTOCOL 19 May 1971

 Code X'84'
    Source:  Both Sites
    Meaning: You Echo
 Code X'85'
    Source:  Serving Site
    Meaning: Hide your input [2]
 Some special TELNET control signals are required to permit the user
 on some systems to send control information to the using site TELNET
 process[3].  These do not require a corresponding control code for
 transmission.  The local TELNET control signals are:
    1. Transmit all data to this point.
    2. Suppress transmission of end of line, send all other data.
 Data is to be forwarded to the NCP for transmission as convenient,
 but at least at the end of line, end of line suppression, and
 transmit signals.  If the normal line length of the sending site is
 greater than the allocation given by the receiving site, the sending
 sites NCP, TELNET process, or TELNET server must be prepared to send
 line segments in convenient lengths until the full line has been
 sent.
 A minimum implementation for TELNET for both using site and serving
 site follows:
    Using Site
       1.)   Provide User (human or process) with ability to cause all
             128 ASCII codes to be transmitted in the required 8 bit
             field to the serving site.
       2.)   Ignore (and strip) all TELNET control characters received
             from the serving site.
       3.)   Provide echo or local print capability to local user
             terminals.
       4.)   Provide for CR-LF end of line convention.
       5.)   Implement local TELNET controls (See discussion above of
             local TELNET control signals) for transmit or suppress
             end of line.

T. O'Sullivan [Page 6] RFC 158 TELNET PROTOCOL 19 May 1971

    Serving Site
       1.)   Provide (and announce) one for one mapping between ASCII
             and Serving Site character and control set (or if Serving
             Site set greater than 128, a sub set.)
       2.)   Ignore (and strip) all TELNET control characters received
             from the Using Site.
       3.)   Assume Using Site will provide local terminal echo or
             print capability.
       4.)   Provide for CR-LF end of line convention.

This document will be revised as necessary to provide conventions for data types in addition to the NVT ASCII type.

←—— 32 ——→←8→←8→←- 16 –>←8→

+——————–+—–+—–+———-+—–+——————

leader x size count x

+——————–+—–+—–+———-+—–+——————

←— level 1 —→
←—————– level 2 ——————>

T. O'Sullivan [Page 7] RFC 158 TELNET PROTOCOL 19 May 1971

     USING HOST                                       Serving HOST

———————–+ +———————-

                      |                      |
  \                   |                      |                  /

Sub- \ —————–| +-+ +-+ |—————–/ Sys- \ | |I| |I| | NCP / tems+—> ←–>|M|—NETWORK–|M|←–> ^ /

   |  \     NCP       |  |P|            |P|  |  +-----|-----/
   |   \              |  +-+            +-+  |  |     v    /
   |    \             |                      |  | TELNET  /  USER

TELNET )___| |–|Protocol( PROCESS

   |      )           |                      |  |Server <--->Sub
   |    /             |                      |  |    ^^    \Systems
   |   /     TTY      |                      |  +----||-----\ETC

User +—> HANDLER ←–> Local | TTY vv \ Pro- / | Terminals | Handles \ cesses/—————–| |—————–\

   /                  |                      |                  \
                      |                      |

———————–+ +———————-

←–> TELNET path path

        Figure 2. Current and Candidate Future TELNET Paths

T. O'Sullivan [Page 8] RFC 158 TELNET PROTOCOL 19 May 1971

+—————————+—-+—-+—-+—-+—-+—-+—-+—-+

\ b8 → 0 0 0 0 0 0 0 0
\ b7 → 0 0 0 0 1 1 1 1
\ b6 → 0 0 1 1 0 0 1 1
B \ b5 → 0 1 0 1 0 1 0 1

+ T | b | b | b | b |\COL→| | | | | | | | | \ S| 4 | 3 | 2 | 1 | \ | | | | | | | | | \ | | | | | |\ | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |

\  | | | | | | | | | v \  |    |    |    |    |    |    |    |    |
 \ | v | v | v | v |ROW \ |    |    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 0 | 0 | 0 |   0  |NUL |DLE | SP | 0  | @  | P  | \  | p  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 0 | 0 | 1 |   1  |SOH |DC1 | !  | 1  | A  | Q  | a  | q  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 0 | 1 | 0 |   2  |STX |DC2 | "  | 2  | B  | R  | b  | r  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 0 | 1 | 1 |   3  |ETX |DC3 | #  | 3  | C  | S  | c  | s  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 1 | 0 | 0 |   4  |EOT |DC4 | $  | 4  | D  | T  | d  | t  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 1 | 0 | 1 |   5  |ENO |NAC | %  | 5  | E  | U  | e  | u  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 1 | 1 | 0 |   6  |ACK |SYN | &  | 6  | F  | V  | f  | v  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 1 | 1 | 1 |   7  |BEL |ETB | '  | 7  | G  | W  | g  | w  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 0 | 0 | 0 |   8  | BS |CAN | (  | 8  | H  | X  | h  | x  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 0 | 0 | 1 |   9  | HT | EM | )  | 9  | I  | Y  | i  | y  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 0 | 1 | 0 |  10  | LF |SUB | *  | :  | J  | Z  | j  | z  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 0 | 1 | 1 |  11  | VT |ESC | +  | ;  | K  | [  | k  | {  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 1 | 0 | 0 |  12  | FF | FS | ,  | <  | L  | \  | l  | |  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 1 | 0 | 1 |  13  | CR | GS | -  | =  | M  | ]  | m  | }  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 1 | 1 | 0 |  14  | S0 | RS | .  | >  | N  | ^  | n  | ~  |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 1 | 1 | 1 |  15  | S1 | US | /  | ?  | O  | _  | o  |DEL |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
       Code Structure      8   7   6   5   4   3   2   1
                      --- --- --- --- --- --- --- --- --- ---

T. O'Sullivan [Page 9] RFC 158 TELNET PROTOCOL 19 May 1971

+—————————+—-+—-+—-+—-+—-+—-+—-+—-+

\ b8 → 1 1 1 1 1 1 1 1
\ b7 → 0 0 0 0 1 1 1 1
\ b6 → 0 0 1 1 0 0 1 1
B \ b5 → 0 1 0 1 0 1 0 1

+ T | b | b | b | b |\COL→| | | | | | | | | \ S| 4 | 3 | 2 | 1 | \ | | | | | | | | | \ | | | | | |\ | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 |

\  | | | | | | | | | v \  |    |    |    |    |    |    |    |    |
 \ | v | v | v | v |ROW \ |    |    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 0 | 0 | 0 |   0  |'80'|    |'A0'|    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 0 | 0 | 1 |   1  |'81'|    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 0 | 1 | 0 |   2  |'82'|    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 0 | 1 | 1 |   3  |'83'|    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 1 | 0 | 0 |   4  |'84'|    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 1 | 0 | 1 |   5  |'85'|    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 1 | 1 | 0 |   6  |    |    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 0 | 1 | 1 | 1 |   7  |    |    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 0 | 0 | 0 |   8  |    |    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 0 | 0 | 1 |   9  |    |    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 0 | 1 | 0 |  10  |    |    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 0 | 1 | 1 |  11  |    |    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 1 | 0 | 0 |  12  |    |    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 1 | 0 | 1 |  13  |    |    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 1 | 1 | 0 |  14  |    |    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+
   | 1 | 1 | 1 | 1 |  15  |    |    |    |    |    |    |    |    |
   +---+---+---+---+------+----+----+----+----+----+----+----+----+

'XX' = HEX designation for codes assigned to TELNET Control Signals.

          Figure 3. Official Network Virtual Terminal Code

T. O'Sullivan [Page 10] RFC 158 TELNET PROTOCOL 19 May 1971

Endnotes

 [1] A one-byte DATA TYPE signal is sent as the first byte of data
 over a connection.  A default is employed if the first byte over a
 connection has the high order bit set to zero, and it is assumed that
 the seven-bit ASCII NVT convention will be employed.  Most
 implementations and applications may expect the DATA TYPES to be
 symmetrical at any point in time.  (i.e. both using a serving site
 using the same DATA TYPE.)  Other data types for which codes are
 currently assigned are:
    X'A1'  Transparency
    X'A2'  EBCDIC
    X'A3'  Special String to TELNET (I'll use your code)
    X'A4'  End Special String to TELNET (I'll use my code)
 [2] i.e. suppress printing of password.
 [3] In some cases, for prolonged [periods of special treatment, local
 implementation may dictate permitting the user to set a "mode" to
 prevail until explicitly discarded.
        [This RFC was put into machine readable form for entry]
         [into the online RFC archives by Lorrie Shiota 2/02]

T. O'Sullivan [Page 11]

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