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rfc:rfc139

Network Working Group T. O'Sullivan Request for Comments: 139 Raytheon NIC: 6717 7 May 1971

                   Discussion of TELNET Protocol
 The attached discussion is an extension of RFC 137, NIC #6717, and is
 presented to provide useful background to designers and implementers
 to help them interpret the proposed Protocol and evaluate it in
 preparation for further discussion at the Atlantic City meetings.
 While the views in the discussion represent those of various TELNET
 committee members, they should not be interpreted as being the agreed
 view of committee.  They are the author's understanding of some of
 the arguments and background to the PROTOCOL proposed in the TELNET
 PROTOCOL recommendations.
  • See Footnotes to attached discussion for changes to RFC 137.

Discussion of TELNET PROTOCOL

 The use of a standard, network-wide, intermediate representation of
 terminal code between sites eliminates the need for using and serving
 sites to keep information about the characteristics of each other's
 terminals and terminal handling conventions, but only if the user,
 the using site, and the serving site assume certain responsibilities.
    1. The serving site must specify how the intermediate code will be
       mapped by it into the terminal codes that are expected at that
       site.
    2. The user must be familiar with that mapping.
    3. The using site must provide some means for the user to enter
       all of the intermediate codes, and as a convenience, special
       control signals, as well as specify for the user how the
       signals from the serving site will be presented at the user
       terminal.
 Other schemes were considered but rejected.  For example, a proposal
 that the using site be responsible to transmit to and from the code
 expected by the serving site was rejected since it required that the
 using site keep tables of all serving site codes and provide mapping
 for each case.  The information would require constant maintenance as
 new hosts were added to the network.

O'Sullivan [Page 1] RFC 139 Discussion of TELNET Protocol 7 May 1971

 Since it is not known how the current or future sites will specify
 the mapping between the network-wide standard code (7 bit ASCII in an
 8 bit field) and the codes expected from their own terminals, it
 seems necessary to permit the user to cause every one of the 128
 ASCII codes, plus (for full user power) selected control signals
 (either of a TELNET control nature, or of a special terminal nature
 such as break or attention).
 There was strong feeling about the importance of the user/system
 interface at the using site, but equally strong feeling that this
 problem is one of local implementation and should reflect the using
 site installation philosophy rather than the subject to network-wide
 standards.  Some topics of consideration in this area are:
    1. How to represent special graphics, not available at the using
       site, at the user's terminal.
    2. Treatment of upper/lower case problem on TTY 33 and 35.
       a. Representing lower-case output.
       b. Providing users with shift and shift lock signals.
    3. Incorporating editing capability in TELNET.
    4. Extending user options in Network mode not available to local
       users,
       e.g., hold output
             kill print
    5. Permit users to specify how keyboard input is to be translated,
       e.g., let a character from the terminal cause a specified
       string to be sent by the user's TELNET.
 In early discussions, there was pressure to get a simple statement of
 protocol out early to permit early use of selected systems.  The
 counter pressure to provide a richer set of protocol in the first
 release was also present.  Work started in the direction of the
 latter, but the complexities introduced were not necessary for early
 use of the network.  The proposed solution to the TELNET protocol
 problem seems to provide a mechanism for a minimum implementation (to
 be discussed later) while providing a basis for developing richer
 sets of protocol for present and future use in terminal applications,
 process-process communications, and use by other conventions to pass
 data or control information.

O'Sullivan [Page 2] RFC 139 Discussion of TELNET Protocol 7 May 1971

 The understanding that ASCII be used as a network-wide code has been
 established for some time.  Its use in TELNET provided a problem with
 respect to the limitation of a maximum character set of 128.  Some
 systems provide for more than this number in their operation, and
 therefor, as serving sites cannot map on a one for one basis.
 Each such serving site could probably provide a reasonably useful
 character set, including all system control signals, by mapping 128
 of its codes and just not provide a network user access to the other
 codes.  However, any character left out might later be used in a
 major application at that site as a special control signal.  This
 could result in denying network users the facility offered by that
 application.  Serving sites are, therefor, encouraged to provide a
 full mapping between the ASCII code and the code used on the serving
 system.
 The ASCII code for ESC (known to some as ALT MODE) has been selected
 as an escape [1].  For each serving site character not mapped on a
 one for one basis, the serving site can specify an escape character
 or string of escape characters (preferably a printable graphic) to
 represent it.  Thus, the user could enter the full set of serving
 site code from any network terminal operating through the Network
 Virtual Terminal (NVT) ASCII convention.  The serving site, in
 generating output directed at the user's terminal, would be expected
 to map out such a character and transmit the appropriate ESC
 character or string of ESC characters.
    Example: A serving site, whose normal code is EBCDIC, has
    specified that cent ([5]) has not been mapped on a one for one
    basis and that to transmit the character, users must enter ESC
    followed by C.  At a using site, the TELNET implementers have
    decided to try to print out all ESC characters using \ to indicate
    ESC.  On receipt of the representation for cent, the user would
    see \C on his print-out.
 The representation of the end of a physical line at a terminal is
 implemented differently on network HOSTS.  For example, some use a
 return (or new line) key, the terminal hardware both returns the
 carriage or printer to start of line and feeds the paper to the next
 line.  In other implementations, the user hits carriage return and
 the hardware returns carriage while the software returns to the
 terminal a line feed.  The network-wide representation will be
 carriage return followed by line feed.  It represents the physical
 formatting that is being attempted, and is to be interpreted and
 appropriately translated by both using site and serving site.

O'Sullivan [Page 3] RFC 139 Discussion of TELNET Protocol 7 May 1971

    Example:  A Multics user is working, through the network, on some
    serving site HOST.  In the course of the session, the user has
    numerous occasions to hit New Line on his Mod 37 TTY.  Each time
    the Multics system is awakened by a New Line interrupt, the line
    of buffered characters is passed to TELNET where it is scanned for
    special characters.  If none is found, carriage return followed by
    line feed is inserted where New Line was entered, and the line is
    turned over to the NCP for transmission.  When the TELNET finds
    the carriage return line feed sequence in the data stream coming
    from the serving site, the two characters are replaced with New
    Line code and sent to the terminal.
 The decision to have the assumed condition for echo be that the using
 site will provide any echo necessary for its terminals was taken
 because of the difficulties faced by some installations that cannot
 turn off their echo or that have terminals that print locally as a
 result of key strokes.  Serving sites could take the position "let
 the user turn my echo off", but this seems an unnecessary burden on
 the user.  In addition, some serving sites may choose not to supply
 any echo service, in which case the no echo assumption will supply a
 network-wide condition, while other assumptions would give a mixed
 starting connection. [2]
 The convention of using "I ECHO", "YOU ECHO" seems to fill both the
 requirements for dynamic echo control and for a minimum
 implementation of TELNET Protocol. [3]  An agreed-upon exchange to
 pass echo control (i.e., two sites exchange the I ECHO/YOU ECHO
 codes) results in passing the control from one site to the other.
    Example:  A serving site is exchanging control information with
    the USER in an area where the serving system asks for pass word
    and wants to suppress the printing of the pass word at the using
    site's user terminal. (In this case, the using site has the
    ability to control the print capability at the user's terminal.)
    Using site has been echoing to the user's terminal.
       Serving Site to Using Site (--->)
          I ECHO
       Using Site to Serving Site (<---)
          YOU ECHO
  1. –>Pass word:
       <--- (User enters password at terminal)

O'Sullivan [Page 4] RFC 139 Discussion of TELNET Protocol 7 May 1971

  1. –> (No echo sent)
  1. –> YOU ECHO
       <--- I ECHO
    After the exchange, the original normal condition is re-
    established.  If the using site did not have dynamic echo control
    installed in its TELNET implementation, the serving site would
    have signaled I ECHO several times, received no response, and
    assumed that the using site could not comply proceeding to call
    for the pass word without the normal protection of inhibiting
    print.
 TELNET control signals are of two types: one that results in
 transmission of signals down the network to a receiving site; the
 other intended for the user/process site only.  The latter type will
 be discussed later.  So far, we have discussed the former type,
 specifically dealing with echo control.
 The use of ESC should not be considered a TELNET-wide standard, but a
 convention limited to the 7 bit ASCII mode of transmission.  Other
 conventions, to be incorporated later, may include binary
 transmission, EBCDIC, etc.  Presumably, each will have its own
 convention for an escape character to extend its code set.
 Since it is expected that conventions other than ASCII will be
 implemented under TELNET, a code to indicate a DATA TYPE representing
 each set of conventions will be employed.  The control code X'AO' has
 been selected to represent the ASCII convention in TELNET.  Since a
 number of applications may wish to transmit transparently (i.e., 8
 bit binary data), X'Al' is being reserved for that purpose.  The
 TELNET control code X'A2' is reserved for an expected set of EBCDIC
 conventions.  The DATA TYPE is expected as the first byte of data
 over a TELNET connection.  Minimum implementations will be aided by
 providing a default.  That is, if the first byte over a connection
 has the high order bit set as zero, then the transmission has begun
 in ASCII mode.
 Each set of conventions, i.e., each DATA TYPE will be expected to
 have a convention for that DATA TYPE to signal that it is returning
 to control mode.  This return may be for the purpose of making use of
 an existing control codes or to change data type.  X'88' is used [4].
    Example:  At the using site, a terminal has a special device on it
    (e.g., plotter, laboratory instrument, control box, etc.) that is
    controlled by binary code in 8 bit bytes.  The terminal uses a
    special "enter" code that routes signals to the device and cuts

O'Sullivan [Page 5] RFC 139 Discussion of TELNET Protocol 7 May 1971

    off printing at the terminal until a special "leave" signal is
    received from the driving process.  The driving process in this
    case is at a remote serving site.  It is assumed in this example
    that a DLE convention is used for transparent transmission, a
    single DLE signal representing return to control.  Normal
    transmission has been in ASCII.
    Driving Process (at Serving Site) to Using Site) ---->
       X'88'X'A1'
    Using Site to Serving Site <----
       X'88'X'88'
  1. ———→
       ENTER code...8 bit binary bytes...
    Using Site TELNET to Terminal |
                                  |
                                  V
       Enter code...8 bit binary bytes...
    Terminal
       Turn printer off, feed transparently to special device, look
       for LEAVE signal
  1. ———–>
       8 bit binary bytes...LEAVE signal...single DLE
       X'A0'
    <-----------
       X'88'X'88
  1. ———–>
    Message
     |
     |
     V
       8 bit binary data...LEAVE signal MESSAGE

O'Sullivan [Page 6] RFC 139 Discussion of TELNET Protocol 7 May 1971

    _Terminal_
    During this sequence of exchanges - at the terminal, feed binary
    data to special device until LEAVE signal is sensed, strip off
    LEAVE signal, turn on printer and block data path to special
    device, print MESSAGE at terminal.
 There is a special control signal on some terminals that has no
 corresponding bit pattern in ASCII, but is transmitted by a special
 electrical signal.  This control signal is ATTN on a 2741 and BREAK
 on a teletype.  The ASCII DATA TYPE in TELNET will use the code X'81'
 to represent BREAK.  (There is a corresponding control signal for use
 from serving sites to using sites for reverse break, and it is
 assigned the code X'82').
 Some systems treat the break as an extra code available for use in
 conjunction with the data stream.  For example, one system uses break
 as a special editing code meaning "delete the current line to this
 point".  In these cases, the code may simply be inserted in the data
 stream with no special additional action by the user.
 Other systems use BREAK or ATTN in a special interrupt fashion, to
 mean stop processing the application and give me the supervisor, or
 cancel the present job, etc.  (Other systems use normal characters
 for this purpose, such as "Control C".)  In these cases, because of
 differences in the ways both serving and using sites operate, it is
 necessary to take a route in addition to the normal TELNET data
 stream to signal that the special control signal is imbedded in the
 data stream.
    _Examples-Problem_
    The PDP-10 normally will, when it fills its input buffer, continue
    to accept characters from a terminal examining each to see if it
    is a control character, then act on it if it is or throw it away
    if it is not.
    Since the TELNET server at the serving site is at the mercy of the
    NCP with respect to controlling the bunching, and therefor,
    arrival at the TELNET of bursts of characters, TELNET
    implementations might be expected to choke off flow to the buffers
    until they are ready to accept characters without throwing them
    away.
 Under this condition, the serving process might be outputting to the
 using terminal, the input buffers fill up, and a control C get stuck
 in the data stream that has been choked off.

O'Sullivan [Page 7] RFC 139 Discussion of TELNET Protocol 7 May 1971

 A similar problem could occur with the Multics or some IBM system as
 a server.  The user at a using site gets into an output loop at the
 serving site and wants to break the process without having to release
 his TELNET connection.  The buffers clog the connection, transmission
 is choked off, and the control C break, or other user control signal
 gets stuck in the pipeline.
 _Example - Solution_
 The user at the using site knows he is entering a special control
 signal (break, ATTN, control C, etc.) and follows it with an X'80'.
 (The local instructions at using sites for accomplishing this may
 differ from site to site.)
    Using Site TELNET to Serving Site
       Insert X'80' in Data Stream
    Using Site TELNET to Using Site NCP
       Send an INS
    Sending Site NCP to TELNET Server
       Look out, here she come
    Serving Site TELNET
       Does its special thing until it sees X'80' then resumes
       normal handling
 Thus, depending on the server's local implementation to provide
 adequate service, a special handling of the data stream can be
 invoked whenever an INS is received in order to get the special
 character.  When it sees X'80', it recognizes it as a SYNC character
 and knowing that the special character has been passed on, strips the
 X'80' from the data stream and returns to normal mode.
 If the X'80' arrives before the INS, a counting scheme can keep the
 activity appropriate to the serving site conditions.
 This approach to handling selected special characters or signals
 relieves the using TELNET processes from having to recognize the
 special serving site characters, as well as from having to know how
 the serving site wants to handle them.  At the same time, the

O'Sullivan [Page 8] RFC 139 Discussion of TELNET Protocol 7 May 1971

 procedure requires only a minimum level of user understanding of the
 serving site.  This seems appropriate, since the TELNET ASCII
 conventions are providing a Network Virtual Terminal, not a Network
 Virtual User.
 The ability of the user to cause the using site TELNET to send any
 combination of ASCII characters in a string, and only that
 combination, is viewed as important to the user utility of the TELNET
 ASCII conventions.  Because of this, some user sites may find it
 necessary to provide special local TELNET control signalling from the
 user to the using site.
    _Examples_
    A user on a line at a time system (Multics, System 360, GECOS,
    etc.)  is working through the Network on a serving site that
    operates a character at a time.  The application is a debugging
    aid that permits the user to type in a memory location = to which
    it will respond with n where n represents the current contents of
    that location.  The serving site process does not expect to see
    the location = followed by a carriage return line feed sequence.
    The user at the using site should be able to type in the location,
    follow it with a signal to suppress the end of a line convention,
    followed by a new line or return, and expect the location number =
    to be transmitted immediately without an end of line sequence.
    In another case, a using site has decided that it is convenient to
    accumulate four characters at a time and transmit them to the
    serving site, unless an end of line is observed, in which case the
    end of line sequence is sent preceded by whatever number of
    characters have been accumulated, (presumably three or less).  In
    the same debugging application, the address is such that the end
    does not correspond with the four character buffer demarcation.
    The user should have the ability to enter a code for "transmit
    immediately" in place of the Carriage Return in order to preserve
    neat formatting, and expect the address to be sent to the serving
    site.
    TELNET controls have been discussed and those introduced to date
    are probably sufficient for an early implementation of TELNET
    ASCII convention.  There will be a need to establish a mechanism
    for the controlled assignment (on request by Network Sites), and
    announcement of DATA TYPE and CONTROL codes.
    It should be noted that some controls are network-wide TELNET
    controls, while others are specific to the ASCII Data Type.  It
    should be further recognized that some local control messages do
    not require a corresponding network-wide code.

O'Sullivan [Page 9] RFC 139 Discussion of TELNET Protocol 7 May 1971

    While it is recognized that even a minimum implementation of
    TELNET for a using site is expected to permit the user to send any
    selected ASCII string (and only that string) to the serving site,
    it is not necessary for a serving site to implement a full mapping
    from ASCII to local code, nor is it necessary for either the using
    or serving sites to implement all control codes.
    _Example - Using Site_
    A minimum implementation of the TELNET protocol for the using site
    would permit ignoring (and stripping) any control signals from the
    serving site since they would all either require agreement or
    acknowledgement (e.g., DATA TYPE, ECHO CONTROL, etc.) or can be
    ignored with no particularly harmful results (e.g., reverse
    break).
    _Example - Serving Site_
    A minimum implementation of the TELNET protocol for the serving
    site could provide one for one mapping for the most important 128
    serving system controls and graphic signals, and ignore all
    control signals.
 It would be helpful if a minimally implemented receiving site, when
 it recognizes an incoming control signal for which appropriate
 reaction is not available, could respond with X'87' (The following
 not implemented at this site) and follow it with the code just
 received.
 Whenever an ASCII TELNET connection is lost, it should be assumed
 that the process at the other end of the connection has been quit,
 aborted, failed, etc.  In this way, a minimum using site installation
 can fail to implement the break and break synchronization, and have
 the user rely on the using site local procedure for leaving a running
 local process and returning to the supervisor to break a connection
 to a remote serving site.
    _Example_
    User recognizes that he is caught in an output loop and wishes to
    stop his user process at the serving site.  The serving site
    requires a break, but the using site minimum implementation has
    not made it available.  Even if it had, the INS was not
    implemented and could not be used to unblock the input pipe.
    Locally, the using site convention for leaving a process and
    getting to supervisory level is to hit the attention key on the
    2741 terminal.  The user does this and is passed to the supervisor
    where he signals to release the TELNET connection.  The serving

O'Sullivan [Page 10] RFC 139 Discussion of TELNET Protocol 7 May 1971

    site, seeing that an ASCII TELNET connection has been lost,
    assumes that the user is ended either normally or abnormally.
    Serving site cancels the user's process.  The user tries again by
    re-establishing the connection, logging in again, re-initiating
    the process, etc.
 Other conventions under TELNET may make quite different assumptions
 about lost connections, and some may go as far as dynamic
 establishing and releasing of connections.
 The proposed TELNET ASCII implementation leaves much uncovered, but
 seems to permit early simple implementation with varying levels of
 capability, along with the capacity to expand in several ways to meet
 others needs.
 There is an important open question.  Should a PROTOCOL such as
 TELNET provide the basis for extending a system to perform functions
 that go beyond the normal capacity of the local system.  For example,
 a local system may not provide functions such as Hold Output, Kill
 Print, etc., but it could extend it for network purposes through
 TELNET.  If so, to what extent should such extensions be thought of
 as Network-wide standards as opposed to purely local implementations.

Endnotes

 [1] Please drop the (s) at the end of "character" in paragraph 3,
 page 3, RFC 137, NIC #6714.
 [2] Also make note that the starting assumption in the initial
 exchange between using site and serving site will be that the using
 site will (if necessary) provide echo and the serving site will not.
 [3] Note: Please change RFC #137, NIC #6714, page 4 - Code X'85' to
 read Reserved.
 [4] Please note on page 4 of RFC 137 that the receipt of an X'88'
 should be responded with by the receiver sending a double signal,
 i.e., X'88'X'88' if the new DATA TYPE can be handled.
 [5] Cent sign
        [This RFC was put into machine readable form for entry]
         [into the online RFC archives by Lorrie Shiota, 1/02]

O'Sullivan [Page 11]

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