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Network Working Group                                      T. O'SullivanRequest for Comments: 139                                       RaytheonNIC: 6717                                                     7 May 1971Discussion of TELNET Protocol   The attached discussion is an extension ofRFC 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 toRFC 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         --->Pass word:         <--- (User enters password at terminal)O'Sullivan                                                      [Page 4]

RFC 139              Discussion of TELNET Protocol            7 May 1971         ---> (No echo sent)         ---> 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 cutsO'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'      ----------->         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      ------------>         8 bit binary bytes...LEAVE signal...single DLE         X'A0'      <-----------         X'88'X'88      ------------>      Message       |       |       V         8 bit binary data...LEAVE signal MESSAGEO'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, theO'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 servingO'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 ofRFC 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]