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rfc:rfc97

NETWORK WORKING GROUP NIC 5740 Request for Comments #97 John T. Melvin

                                                     Richard W. Watson
                                                               SRI-ARC
                                                      15 February 1971
             A FIRST CUT AT A PROPOSED TELNET PROTOCOL

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.

Melvin & Watson [Page 1] RFC 97 Proposed Telnet Protocol February 1971

    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.

Melvin & Watson [Page 2] RFC 97 Proposed Telnet Protocol February 1971

 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

Melvin & Watson [Page 3] RFC 97 Proposed Telnet Protocol February 1971

 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

Melvin & Watson [Page 4] RFC 97 Proposed Telnet Protocol February 1971

 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
 network.
      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

Melvin & Watson [Page 5] RFC 97 Proposed Telnet Protocol February 1971

 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
 study.
      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.

Melvin & Watson [Page 6] RFC 97 Proposed Telnet Protocol February 1971

APPENDIX A

 NETWORK CONNECTION PROTOCOL TO SRI-NETWORK INFORMATION CENTER
 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

Melvin & Watson [Page 7] RFC 97 Proposed Telnet Protocol February 1971

                  .
                  .
                  .
      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

Melvin & Watson [Page 8] RFC 97 Proposed Telnet Protocol February 1971

 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
 otherwise.
    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
          output
               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)

Melvin & Watson [Page 9] RFC 97 Proposed Telnet Protocol February 1971

               presently ASCII code '37
               carriage return followed by line feed on echo and
          output
               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.
 5 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
    NLS(NIC).
         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                        |

Melvin & Watson [Page 10] RFC 97 Proposed Telnet Protocol February 1971

       +----+                      |
       |    |                      |
       |    |        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   ]

Melvin & Watson [Page 11]

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