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rfc:rfc137

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

+——————–+—–+—–+———-+—–+——————

←— level 1 —→
←—————– level 2 ——————>
     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

+ 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

+ 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]

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