Network Working Group                                   T. C. O'SullivanRequest for Comments: 137                                       RaytheonNIC 6714                                                   30 April 1971                            TELNET Protocol   This is a request for comment and is being distributed in advance of   the Atlantic City meetings for review and comment prior to or during   discussions on TELNET in preparation for issuing an official   document.   It is also being distributed so that selected installations planning   to implement early versions of TELNET will have a common basis for   such implementation.   The proposed document is the result of the work of the committee.  It   represents a TELNET protocol felt to be adequate for initial   implementation.  A few recent suggestions by committee members and   others have been incorporated where even though not thoroughly   cleared with all members, the chairman felt that they clarified the   protocol or would tend to simplify implementation but not   substantially change the agreed-upon approach.   Readers are referenced to the following previous releases of   information:   1. Conventions for Using an IBM 2741 Terminal or a User Console for      Access to Network Server HOSTS         Joel Winett, RFC 110 (NIC #5809)   2. Level III Server Protocol for the Lincoln Laboratory 360/67 HOST         Joel Winett, RFC 109 (NIC #5808)   3. First Cut at a Proposed TELNET Protocol         J. Melvin, D. Watson, RFC 97 (NIC #5740)   4. ASCII Format for Network Interchange         V. Cerf, RFC 20 (NIC# 4722)   Another RFC will be distributed prior to the Atlantic City Meetings   containing many of the arguments supporting the proposal.O'Sullivan                                                      [Page 1]RFC 137                     TELNET Protocol                   April 1971                            TELNET PROTOCOL                          A Proposed Document                T. O'Sullivan for the TELNET Committee               Will Crowther                 BBN               Bob Long                      SDC               John Melvin                   SRI-ARC               Bob Metcalf                   Harvard               Ed Meyer                      MAC               Tom O'Sullivan (Chairman)     Raytheon               Joel Winett                   MIT-LLO'Sullivan                                                      [Page 2]RFC 137                     TELNET Protocol                   April 1971   TELNET is a third-level protocol, the function of which is to make a   terminal (or process) at a using site appear to the system or a   process at a serving site as logically equivalent to a terminal   "directly" connected to the serving site.  In performing this   function, the protocol attempts to minimize the amount of information   each HOST must keep about the characteristics of other HOSTS.   Definitions   Protocol Levels (see Figure 1)      Level 1         HOST-IMP protocol specified by BBN in NIC 5735, Specifications         for the Interconnection of a HOST, and an IMP (BBN Report 1822)      Level 2         HOST-HOST protocol performed by NCPs as described in Document         Number 1 (NIC 5413) and subsequent amendments, see RFC 107 (NIC         #5806)            One view of the NCP's function is that it takes information            from the net and routes it to receiving processes via            mechanisms internal to each HOST; conversely, processes use            the NCP, via internal system calls, to have information            routed to other processes in the net (via the other            processes' NCPs).      Level 3 (see Figure 2)         Level 3 is, by definition, the place to which and from which         the NCP communicates internally in its own host.            This level may be equivalent to the user process level in            some systems, but this may not be the case in all systems.            In using sites, the TELNET process operates at this level.            In serving sites, the TELNET server operates at this level.   Initial Connection Protocol (ICP)      An agreed-upon sequence of level 3 exchanges between two processes      which is, in general, used to synchronize the dialogue between the      processes, e.g., RFC 80 (NIC #5608) #1.O'Sullivan                                                      [Page 3]RFC 137                     TELNET Protocol                   April 1971   Serving Site      The HOST into which the TELNET process is directing the user's      keyboard input and from which the TELNET process is receiving      control information and data effecting the user's terminal.  At      the serving site, a TELNET server is executing.   Using Site      The HOST in which the TELNET process is executing.   Sending Site      The HOST transmitting data, could be either using site or serving      site.   Receiving Site      Converse of sending site.   User      The person or process "driving" the TELNET process.   In providing services the TELNET protocol will use established   network conventions, specifically the Network Control Program, and   Initial Connection Protocol referenced in the above definitions.   The TELNET protocol provides for a Network Virtual Terminal (NVT)   through which users may transmit and receive data over connections   between the using site and the serving site.   The code of the NVT will be full ASCII.  The seven-bit code will be   transmitted in eight-bit bytes, the high order bit set to zero.   It will be the responsibility of the using site to provide its users   with a means of producing all 128 ASCII codes, as well as a selected   set of special TELNET control signals (see Figure 3).O'Sullivan                                                      [Page 4]RFC 137                     TELNET Protocol                   April 1971   The ASCII character ESC will be employed by the user as an escape   signal indicating that the next character(s) has special meaning.   The meaning assigned to escape code will be serving site defined and   therefore may not be consistant across the network.   It will be the responsibility of the serving site to specify for   users how the NVT code will be used to represent the codes normally   generated by a local terminal.  The serving sites specification of   this representation is expected, where reasonable, to map on a one-   for-one basis for ASCII graphics and controls that are provided   through local terminals.  The serving site will also specify how the   escape conventions will be interpreted by the system.   The end of a line will be represented in the NVT as carriage return   followed by line feed.   The protocol assumes that at initially the serving site will not   provide any echo to the using site.   Each TELNET control signal for which code must be sent over the   connection will be represented in the NVT by an eight-bit code, with   the high order bit set to one.  Following are the special codes   established to date. (U) indicates that in most implementations the   user would be expected to have the ability to signal the TELNET   process from his terminal to initiate the code.   Code X'A0'      Source:  Both Sites (U)      Meaning: A DATA TYPE[1] signal indicating that code will be               transmitted by NVT, i.e., using the seven-bit ASCII               conventions.   Code X'80'      Source:  Using Site (U)      Meaning: Order using site NCP to send an INS and insert X'80' in               data stream.   Code X'81'      Source:  Using Site (U)      Meaning: Break or AttentionO'Sullivan                                                      [Page 5]RFC 137                     TELNET Protocol                   April 1971   Code X'82'      Source:  Serving System      Meaning: Reverse Break   Code X'83'      Source:  Both Sites      Meaning: I Echo   Code X'84'      Source:  Both Sites      Meaning: You Echo   Code X'85'      Reserved   Code X'86'      Reserved   Code X'87'