    Network Working Group                                   T. C. O'Sullivan
    Request for Comments: 137                                       Raytheon
    NIC 6714                                                   30 April 1971

                                TELNET Protocol

       This is a request for comment and is being distributed in advance of
       the Atlantic City meetings for review and comment prior to or during
       discussions on TELNET in preparation for issuing an official
       document.

       It is also being distributed so that selected installations planning
       to implement early versions of TELNET will have a common basis for
       such implementation.

       The proposed document is the result of the work of the committee.  It
       represents a TELNET protocol felt to be adequate for initial
       implementation.  A few recent suggestions by committee members and
       others have been incorporated where even though not thoroughly
       cleared with all members, the chairman felt that they clarified the
       protocol or would tend to simplify implementation but not
       substantially change the agreed-upon approach.

       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. Level III Server Protocol for the Lincoln Laboratory 360/67 HOST
             Joel Winett, RFC 109 (NIC #5808)

       3. First Cut at a Proposed TELNET Protocol
             J. Melvin, D. Watson, RFC 97 (NIC #5740)

       4. ASCII Format for Network Interchange
             V. Cerf, RFC 20 (NIC# 4722)

       Another RFC will be distributed prior to the Atlantic City Meetings
       containing many of the arguments supporting the proposal.

    O'Sullivan                                                      [Page 1]

    RFC 137                     TELNET Protocol                   April 1971

                                TELNET PROTOCOL

                              A Proposed Document

                    T. O'Sullivan for the TELNET Committee

                   Will Crowther                 BBN
                   Bob Long                      SDC
                   John Melvin                   SRI-ARC
                   Bob Metcalf                   Harvard
                   Ed Meyer                      MAC
                   Tom O'Sullivan (Chairman)     Raytheon
                   Joel Winett                   MIT-LL

    O'Sullivan                                                      [Page 2]

    RFC 137                     TELNET Protocol                   April 1971

       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 dialogue between the
          processes, e.g., RFC 80 (NIC #5608) #1.

    O'Sullivan                                                      [Page 3]

    RFC 137                     TELNET Protocol                   April 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.

       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 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).

    O'Sullivan                                                      [Page 4]

    RFC 137                     TELNET Protocol                   April 1971

       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.

       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
       followed by line feed.

       The protocol assumes that at 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

    O'Sullivan                                                      [Page 5]

    RFC 137                     TELNET Protocol                   April 1971

       Code X'82'

          Source:  Serving System
          Meaning: Reverse Break

       Code X'83'

          Source:  Both Sites
          Meaning: I Echo

       Code X'84'

          Source:  Both Sites
          Meaning: You Echo

       Code X'85'

          Reserved

       Code X'86'

          Reserved

       Code X'87'

          Source:  Both Sites
          Meaning: This site has not implemented the following control code.

       Code X'88'

          Source:  Both Sites
          Meaning: [2]Return to control mode, i.e. next byte will be a
                   control signal, possible a new DATA TYPE.

       Some special TELNET control signals are required to permit the user
       on some systems to send control information to the using site TELNET
       process.  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.

    O'Sullivan                                                      [Page 6]

    RFC 137                     TELNET Protocol                   April 1971

       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 TELNET process or TELNET server will be responsible for
       breaking the line into convenient lengths and turning them over to
       the NCP for transmission.

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

    Footnotes:

       [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.  After initial
       connection, the DATA TYPE may be changed (see code X'88').  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.).

       [2] A site receiving a DATA TYPE signal is to respond with a double
       X'88' if the new DATA TYPE is acceptable.

    O'Sullivan                                                      [Page 7]

    RFC 137                     TELNET Protocol                   April 1971

    |<------- 32 ------->|<-8->|<-8->|<-- 16 -->|<-8->|<---
    +--------------------+-----+-----+----------+-----+------------------
    |       leader       |  x  |size |  count   |  x  | TEXT
    +--------------------+-----+-----+----------+-----+------------------
    |<---- level 1  ---->|
        message leader

    |<------------------  level 2  ------------------>|
                      message preamble

                                                             level 3
                                                      |<- message text..-->

                     Figure 1. Network Message on Link 2-31
               Indicating Portions of Interest to Various Levels

           USING HOST                                       Serving HOST
     -----------------------+                      +----------------------
                            |                      |
        \                   |                      |                  /
         \ -----------------|  +-+            +-+  |-----------------/
          \           NCP   |  |I|            |I|  |   NCP          /
           \       ^      <--->|M|---NETWORK--|M|<--->      ^      /
            \ -----|-----+  |  |P|            |P|  |  +-----|-----/
             \     v     |  |  +-+            +-+  |  |     v    /
    USER      \          |  |                      |  | TELNET  /  USER
    PROCESSES,  ) TELNET |--|                      |--|Protocol(   PROCESS
    Sub     <===>        |  |                      |  |Routing<--->Sub
    Systems,  /    ^     |  |                      |  |    ^^    \Systems
    ETC      /-----|-----+  |                      |  +----||-----\ETC
            / TTY  v      <---> Local     Local  <===> TTY vv      \
           /  Handles       |   Terminals Terminals|   Handles    <===>
          /-----------------|                      |-----------------\
         /                  |                      |                  \
                            |                      |
     -----------------------+                      +----------------------

    <---> Current TELNET paths
    <===> Candidate future TELNET paths

              Figure 2. Current and Candidate Future TELNET Paths

    O'Sullivan                                                      [Page 8]

    RFC 137                     TELNET Protocol                   April 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  |
    | I  +---+---+---+---+------+----+----+----+----+----+----+----+----+
    +  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  |ENQ |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
                            --- --- --- --- --- --- --- --- --- ---

    O'Sullivan                                                      [Page 9]

    RFC 137                     TELNET Protocol                   April 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  |
    | I  +---+---+---+---+------+----+----+----+----+----+----+----+----+
    +  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  |'86'|    |    |    |    |    |    |    |
         +---+---+---+---+------+----+----+----+----+----+----+----+----+
         | 0 | 1 | 1 | 1 |   7  |'87'|    |    |    |    |    |    |    |
         +---+---+---+---+------+----+----+----+----+----+----+----+----+
         | 1 | 0 | 0 | 0 |   8  |'88'|    |    |    |    |    |    |    |
         +---+---+---+---+------+----+----+----+----+----+----+----+----+
         | 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

    O'Sullivan                                                     [Page 10]

    RFC 137                     TELNET Protocol                   April 1971

            [This RFC was put into machine readable form for entry]
             [into the online RFC archives by Sergio Kleiman, 8/01]

    O'Sullivan                                                     [Page 11]