Request for Comments #97
NIC 5740
John T. Melvin
Richard W. Watson
15 February 1971


1 Introduction

       This paper describes a first cut at a proposed Telnet protocol.
   _Telnet_ is a process which runs at a _user's_ _site_ and allows him
   to utilize a typewriter-like terminal to gain interactive service
   from a remote _server_ _site over the ARPA Network.  This paper was
   motivated by our need to set specifications for a protocol which
   would allow online access to the Network Information Center (NIC).
   The Online System running at the Network Information Center we will
   refer to as NLS(NIC).  On thinking about the problem of setting
   specifications for access to the NIC, we have tried to generalize our
   ideas so that they would apply to other systems with characteristics
   similar to ours.  We realize that there are other terminal hardware-
   software disciplines which might find it difficult to conform to all
   the requirements stated here and, therefore, the final Telnet
   protocol will differ from the one stated in this NWG/RFC.  One
   conclusion that we may all have to come to is that connection with
   the network may force us toward a more standard way of handling
   terminals and their character streams in our monitors and terminal
   control hardware.  In the meantime, we hope that this paper and
   others on the same subject that may be in process, coupled with a
   survey of hardware-software requirements at each site by a NWG
   subgroup, can result in an initial standard network Telnet protocol
   being agreed upon quickly, as it is important to get users onto the
   network as soon as possible so that interactive network usage can
   indicate further directions for network protocol evolution.  Next we
   outline some design problems, then propose some conventions to solve
   these problems for access to systems such as the NLS(NIC) and
   indicate some problems needing further study.  The proposed
   conventions for access to the NLS(NIC) are summarized in Appendix A.

2 Some Design Problems

2A. Basic Assumption

        The function of the Telnet process is to make a terminal at a
   user site appear over the network as logically equivalent to a
   terminal "directly" connected to the server site.  There are a number
   of implications of this basic function.

i) The user should be able to cause generation of all codes which a server system terminal can generate. With respect to the Network Information Center and some other sites it would seem a reasonable requirement to have keying conventions so that the user can generate all 128 ASCII character codes as input to the network. Other sites with different character codes may require a Telnet process to provide those codes to the network.

ii) The user should be able to escape back to his local system or escape from the server process to the server system.

      iii) The Telnets  of line-at-a-time systems should be able to work
      with character-at-a-time systems and line-at-a-time systems and
      Telnets  of character-at-a-time systems should be able to work
      with line-at-a-time and character-at-a-time systems.
   2B   Echo Control
        We use the term echo control rather than the terms half duplex
   or full duplex because the Telnet connection is in reality full
   duplex with respect to network transmissions.  Three terminal cases
   need to be considered.
      Case 1 - Character-at-a-time serving site echoed
      Case 2 - Character-at-a-time user site echoed
      Case 3 - Line-at-a-time user site echoed

Some serving sites may be able to operate with all three cases and some convention is required to set the mode. Strictly speaking, what characters are echoed for what keys struck is of no concern to the serving site, although one would like to try to minimize differences in typescript as it appears to the user.

   2C   Format Control Characters
        The format control characters of horizontal tab (HT), vertical
   tab (VT), form feed (FF), line feed (LF), and carriage return (CR),
   need to be handled in a consistent way for Cases 2 and 3 above.  With
   Case 1 above, the situation is simplified.
   2D   Network Message Boundaries
        The NCP to NCP protocol was specified with the goal of having
   the network message boundaries being invisible to the user processes.
   It would be good if this goal could be maintained, but it may be
   difficult with some line-at-a-time systems.
   2E   An Implementation Convention
        ConVentions to solve the above problems are most simply
   established if we assume that the character stream received from a
   Telnet process by the server site is entered into that point in the
   server monitor where character input from "direct1y" connected
   terminals is entered and output from the server process is entered
   into the monitor point where normal character output is entered.  The
   server NCP receives its input at the point where normal monitor
   character output is obtained.  In other words, the server process
   would obtain its input from the server monitor character buffers and
   send its output to these buffers rather than obtainIng input directly
   from NCP buffers or outputting to NCP buffers.
        The Telnet process, on the other hand, would obtain and send
   character streams directly from or to its local NCP.
        Other situations exist where the user processes at both ends
   communicate directly with the NCP.  Therefore, we would recommend
   that both modes of connection (user process-monitor-NCP, or user
   process-NCP) be available for communication between the NCP and a
   user process.  These modes would be set under program control by the
   user process.  The initial network convention during the login
   procedure and until changed by the server process would be to obtain
   characters from and send characters to the monitor.  The server NCP
   communicates with the monitor also.  The scheme is illustrated in
   Figure 1.
        The motivation for such flexibility may be clearer from the
   discussion below.

3 Proposed Telnet Conventions

   3A   The server site is to assume initially that echoing is performed
   by a user site process until explict1y commanded otherwise.  If the
   user site can send character-at-a-time, then after connection and
   login have been established, tne user could switch Lo server-site-
   echo by command to the server site and then command (invisible to the
   server site) his local Telnet to change its echo mode also.
   3B   The server process is to assume it will receive the same
   character set which terminals "directly" connected to it can
   generate.  (We recommend at least 128 character ASCII.)  The user's
   Telnet may have to recognize two-character sequences to enable
   generation of both upper and lower-case codes and the control codes.
   We recommend that the user be able to set either upper or lower case
   as the default case for single case terminals and be able to specify
   a case shift character.  The user should also be able to specify a
   character to indicate that the next character struck is to be
   converted to the appropriate control character code.  This latter
   convention enables control codes directly generated at the terminal
   to be recognized by the user's system thus enabling escape to the
   user system.  Creating a convention allowing all control codes to
   enter the network and allowing output of the network to feed into the
   server monitor before entering the server process, gives a simple
   mechanism for generating an escape to     many existing systems.
   (The problem is more complicated than this for some systems and we
   discuss it further below.)
   3C   We recommend that network standards be established for the
   meaning of local echoes of HT, VT, and FF or a convention to be
   established for sending the meaning of these characters to the server
   process.  The NLS(NIC), for example, needs to keep track of the
   position of the print head and in the absence of such conventions
   will convert these character codes to spaces and line feeds.  This
   means that the appearance of the page on output may differ from the
   appearance on input.  It would be helpful to the user if his page on
   output could be formatted as it appeared on input.
   3D   LF characters would be handled as if they were generated by
   hitting the line feed key on a terminal "directly" connected to the
   server system.
   3E   The carriage return (CR) character can be the source of
   considerable difficulty.  For example, on input, different systems
   and the same system at different times, can echo and transmit
   different codes to the terminal and the user process.  Some monitor
   systems echo nothing, just a CR, or a CRLF.  Some systems transmit a
   CR, CRLF, or end of line code (EOL) to the user process.  The user
   process may control the echo or add to it.  Given the combinations
   which can exist at each end of the network connection and with
   respect to each other, confusion can exist unless we assume the
   definition of 2A and the implementation convention of 2E.  These
   assumptions imply that when a CR is struck, a CR gets sent over the
   network.  If the user monitor system or terminal control hardware
   converts a CR to a CRLF or EOL, then the Telnet program must convert
   it back to a CR.  When the CR reaches the server monitor it will
   handle it properly for the server process.
        When echoing is handled by the server system, the proper code or
   codes will be echoed.  The user Telnet on receiving a CRLF can pad it
   with the proper nulls to handle carriage movement timing for a
   particular terminal.
        When echoing is handled by the user system it would be ideal if
   the user's Telnet or system used the same echo convention as the

server system would. This means that either the Telnet must have a table of echo conventions for the various systems to which it can connect, or that it can obtain this information from the server system or process, or vice versa.

        For an initial Telnet protocol this is probably not necessary.
   The user system can default and echo a CRLF on each CR received.
   This default should be satisfactory for all the situations we are
   familiar with and for the NIC.
   3F   For communication from character- and line-at-a-time systems,
   the Telnet process may need to recognize a character (user
   assignable) which we call end of stream (EOS).  This character is to
   have the function defined in the following discussion.  The important
   point is to distinguish end-of-stream as a network function and end-
   of-line as a user or server system function.  Consider line-at-a-time
   systems first.  We have not had much experience with line-at-a-time
   systems, so what follows will need further study and clarification.
   As we understand it, line-at-a-time systems recognize a character
   such as CR or a break signal as the code to wake up the user process
   and cause transmission to it of the line of text.  From the point of
   view of NLS(NIC) it is important that the user be able to enter lines
   of text each terminated by a CR where appropriate and at other times
   to be able to enter text not terminated with a CR.  (A statement for
   NLS(NIC) is a string of text of "arbitrary" length and need not have
   CRs in it; on output the line is folded for the user at his (user
   definable) page boundary.)
        As an example of what is required, consider the case where the
   user's system recognizes CR as end-of-line.  In this case the Telnet
   would be awakened when a CR is received.  We would recommend that in
   this case the CR code be literally entered into the Telnet output
   buffer.  If a CR is preceded by an EOS character, then the CR should
   not be placed in the Telnet output buffer.  Transmission through the
   network can take place either when an EOS is received or
   automatically when the Telnet output buffer fills.  Transmission to
   character-at-a-time systems from line-at-a-time systems could require
   the awkward striking of three keys to get one character through the
        Now consider transmission for a character-at-a-time system to a
   server line-at-a-time system.  A similar problem to the one to be
   described also exists between line-at-a-time systems.  Given the
   definition of an EOS character different from CR, a line can be
   buffered up until the EOS is received and then sent without the EOS.
   How is the serving system to know that a line has been sent?  One way
   would be for the serving NCP to recognize message boundaries.  This
   convention would violate a design goal.  Another way would be for the

user Telnet to request its NCI to send an INS command. The sending of INS type of control commands might introduce race conditions in the network and should be investigated before their use with a Telnet process is established. Since some of the line-at-a-time systems we know have special hardware that recognizes the end-of-line signal, we need some way to be compatible with this hardware using software control signals. We leave this problem for further NWG subgroup study.

   3G   We now come back to the problem of interrupting or escaping in
   the remote server system.  In systems which do not lock out the input
   keyboard when output is going on, the mechanisms and conventions
   outlined above would seem adequate unless a special break signal is
   the escape signal.  This latter case requires more study.  In systems
   which allow no input while output is occurring, one may have to live
   with the consequences of such a terminal discipline and be prepared
   to wait until output stops before an escape code can be sent.  If the
   keyboard is locked and an escape break signal can be sent to the
   user's system, it can prevent output from going to the terminal, but
   must be prepared to continue receiving it from the server site until
   the user can inform his Telnet process to send an interrupt or escape
   signal to the server site.  Again this is a problem for further
        The Online System of the Network Information Center operates on
   a character-at-a-time monitor system and the conventions established
   in this paper are adequate for access to it.  These conventions are
   summarized in Appendix A.



   1    Initial Connection Protocol
       Connection establishment to NIC is identical to that presented in
   Section II of NWG/RFC 80 NIC (5608,); it is reproduced here:
   Telnet contacts NIC

NIC <- user site

RTS <us> <l> <p>

NIC logger is socket 1

user site <- NIC

STR <l> <us> CLS <l> <us>

             if accepted
        CLS <l> <us>
             if rejected
   assuming NIC accepts

user site <- NIC

        STR <ss+l> <us>

RTS <ss> <us+l> <q>

NIC receives text thru local socket ss from remote socket us+l via link q

assuming user site accepts

NIC <- user site

             STR <us> <ss+l>

RTS <us+l> <ss> <r>

NIC sends text to remote socket us thru local socket ss+l via link r


user site <- NIC

             ALL <q> <space>


NIC <- user site

             ALL <r> <space>
   2    Connection Breaking Protocol
        A CLS trade is made between the NCPs for each of the two
   connections as per Document #1 NIC (5143,).
           We may decide to put a time-out into the NIC connections such
      that no interaction for some (as yet unspecified) "reasonable"
      length of time will result in a CLS-out of the connections being
      initiated by NIC.
   3    Third Level Protocol
        The first 8 bits received by NIC thru socket ss should be the
   message data type designating that an 8-bit ASCII stream follows, as
   per NWG/RFC #63, NIC (4963,).
         I.e., the first 8 bits are 00000001
        The first 8 bits received by Telnet thru socket us will also
   indicate a message data type of l.  Each network message should have
   an integral multiple of 8 bits.  If a network standard is established
   different from the suggestion of NWG/RFC #63, NIC (4963,), then we
   would change this protocol to conform.

NIC will have NCP-generated interrupts disabled, i.e.,

INR will be ignored

INS will not be sent to the remote host

   4    NLS(NIC) Character Conventions of Interest to Telnet
        Echoing can either be under control of NIS(NIC) or under control
   of the user site.  When we refer to echoing below, we mean under
   control of NLS(NIC).  When echoing is handled by the user site we
   would expect the user to set the NLS(NIC) output conventions to
   conform to the echoing conventions at his site.  NLS(NIC) assumes
   echoing is handled by the user site unless explicitly commanded

Format affecting control characters

         horizontal tab
                 spaces to next (user definable) stop on both echoing
            and output.

if during literal input, enters file as ASCII '11.

         form feed
                 carriage return and (user definable) appropriate number
            of line feeds on echo and output.

If during literal input, enters file as ASCII '14

         vertical tab
                 carriage return and (user definable) appropriate number
            of line feeds on echo and output

if during literal input, enters file as ASCII '13

         carriage return
                 carriage return followed by line feed on echo and

if during literal input, enters file as EOL (see below)

         line feed

line feed on echo and output

enters file as ASCII '12 on literal input

EOL (end of line)

presently ASCII code '37

                 carriage return followed by line feed on echo and

if during literal input, enters file as ASCII '37

                 If the user's system automatically appends a LF to a CR
            before sending it to Telnet or converts CR to some EOL code
            not ASCII '37, we would expect Telnet to send NLS(NIC) just
            a CR or ASCII '37.  If we receive CRLF, then on output we
            will send CRLFLF.
  1. NLS(NIC) Interrupt Attention Convention
       A (user definable) ASCII code in the text input stream is used to
   abort the executing process and return control to the main NLS(NIC)
   command processor.

This code is presently DEL (ASCII '177).

           Escape to the NIC monitor:  No escape is required as all
      operations needed for use of the NIC can be performed within
           Character Set:  We strongly recommend that the Telnet process
      be able to generate by some set of keying conventions all 128
      ASCII codes.  Use of NLS(NIC) will probably feel most comfortable
      from a device with upper and lower case graphics, although we can
      provide service to single case devices.  We can provide a useful
      service if the full ASCII set cannot be sent, but would like to
      minimize the special cases we have to handle.  Sites which cannot
      provide the full ASCII set should contact us.
         +----+                      |
         |    |        Server        |
         |    |        Program       |
         |    |                      |
         +----+                      |
          ^ |                        |
          | v                        |
         +----+        Terminal      |
         |    |        control       |
         |    |        software      |    SERVER
         |    |        and           |     SITE
         +----+        possibly      |
          ^ |          hardware      |
          | v                        |
         +----+                      |
         |    |                      |
         |    |        NCP           |
         |    |                      |
         +----+                      |
          ^ |                        |
          | v                        |
          . .
          . .                                            Figure 1 -
          . .
          . .                                        Telnet Connection
          ^ |
          | v
         +----+                      |
         |    |                      |
         |    |        NCP           |
         |    |                      |
         +----+                      |
          ^ |                        |
          | v                        |
         +----+                      |
         |    |                      |
         |    |        Telnet        |
         |    |                      |
         +----+                      |
          ^ |                        |    USER
          | v                        |    SITE
         +----+        Terminal      |
         |    |        control       |
         |    |        hardware-     |
         |    |        software      |
         +----+                      |
          ^ |                        |
          | v                        |
         +----+                      |
         |    |        User          |
          \   |        terminal      |
           \--+                      |
       [ This RFC was put into machine readable form for entry ]
       [   into the online RFC archives by Tony Hansen 08/08   ]