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IEN 148                                                        J. PostelRFC 764                                                              ISI                                                               June 1980TELNET PROTOCOL SPECIFICATIONINTRODUCTION   The purpose of the TELNET Protocol is to provide a fairly general,   bi-directional, eight-bit byte oriented communications facility.  Its   primary goal is to allow a standard method of interfacing terminal   devices and terminal-oriented processes to each other.  It is   envisioned that the protocol may also be used for terminal-terminal   communication ("linking") and process-process communication   (distributed computation).GENERAL CONSIDERATIONS   A TELNET connection is a Transmission Control Protocol (TCP)   connection used to transmit data with interspersed TELNET control   information.  TCP and the connection establishment procedure are   documentented in the ARPA Internet Protocol Handbook.   The TELNET Protocol is built upon three main ideas:  first, the   concept of a "Network Virtual Terminal"; second, the principle of   negotiated options; and third, a symmetric view of terminals and   processes.   1.  When a TELNET connection is first established, each end is   assumed to originate and terminate at a "Network Virtual Terminal",   or NVT.  An NVT is an imaginary device which provides a standard,   network-wide, intermediate representation of a canonical terminal.   This eliminates the need for "server" and "user" Hosts* to keep   information about the characteristics of each other's terminals and   terminal handling conventions.  All Hosts, both user and server, map   their local device characteristics and conventions so as to appear to   be dealing with an NVT over the network, and each can assume a   similar mapping by the other party.  The NVT is intended to strike a   balance between being overly restricted (not providing Hosts a rich   enough vocabulary for mapping into their local character sets), and   being overly inclusive (penalizing users with modest terminals).      *NOTE:  The "user" Host is the Host to which the physical terminal      is normally attached, and the "server" host is the Host which is      normally providing some service.  As an alternate point of view,      applicable even in terminal-to-terminal or process-to-process      communications, the "user" Host is the Host which initiated the      communication.Postel                                                          [Page 1]

June 1980RFC 764, IEN 148Telnet Protocol Specification   2.  The principle of negotiated options takes cognizance of the fact   that many sites will wish to provide additional services over and   above those available within an NVT, and many users will have   sophisticated terminals and would like to have elegant, rather than   minimal, services.  Independent of, but structured within, the TELNET   Protocol various "options" will be sanctioned which can be used with   the "DO, DON'T, WILL, WON'T" structure (discussed below) to allow a   user and server to agree to use a more elaborate (or perhaps just   different) set of conventions for their TELNET connection.  Such   options could include changing the character set, the echo mode, the   line width, the page length, etc.   The basic strategy for setting up the use of options is to have   either party (or both) initiate a request that some option take   effect.  The other party may then either accept or reject the   request.  If the request is accepted the option immediately takes   effect; if it is rejected the associated aspect of the connection   remains as specified for an NVT.  Clearly, a party may always refuse   a request to enable, and must never refuse a request to disable, some   option since all parties must be prepared to support the NVT.   The syntax of option negotiation has been set up so that if both   parties request an option simultaneously, each will see the other's   request as the positive acknowledgment of its own.   3.  The symmetry of the negotiation syntax can potentially lead to   nonterminating acknowledgment loops -- each party seeing the incoming   commands not as acknowledgments but as new requests which must be   acknowledged.  To prevent such loops, the following rules prevail:      a.  Parties may only request a change in option status; i.e., a          party may not send out a "request" merely to announce what          mode it is in.      b.  If a party receives what appears to be a request to enter some          mode it is already in, the request should not be acknowledged.      c.  Whenever one party sends an option command to a second party,          whether as a request or an acknowledgment, and use of the          option will have any effect on the processing of the data          being sent from the first party to the second, then the          command must be inserted in the data stream at the point where          it is desired that it take effect.  (It should be noted that          some time will elapse between the transmission of a request          and the receipt of an acknowledgment, which may be negative.          Thus, a site may wish to buffer data, after requesting an[Page 2]                                                          Postel

RFC 764, IEN 148                                               June 1980                                           Telnet Protocol Specification          option, until it learns whether the request is accepted or          rejected, in order to hide the "uncertainty period" from the          user.)   Option requests are likely to flurry back and forth when a TELNET   connection is first established, as each party attempts to get the   best possible service from the other party.  Beyond that, however,   options can be used to dynamically modify the characteristics of the   connection to suit changing local conditions.  For example, the NVT,   as will be explained later, uses a transmission discipline well   suited to the many "line at a time" applications such as BASIC, but   poorly suited to the many "character at a time" applications such as   NLS.  A server might elect to devote the extra processor overhead   required for a "character at a time" discipline when it was suitable   for the local process and would negotiate an appropriate option.   However, rather than then being permanently burdened with the extra   processing overhead, it could switch (i.e., negotiate) back to NVT   when the more "taut" control was no longer necessary.   It is possible for requests initiated by processes to stimulate a   nonterminating request loop if the process responds to a rejection by   merely re-requesting the option.  To prevent such loops from   occurring, rejected requests should not be repeated until something   changes.  Operationally, this can mean the process is running a   different program, or the user has given another command, or whatever   makes sense in the context of the given process and the given option.   A good rule of thumb is that a re-request should only occur as a   result of subsequent information from the other end of the connection   or when demanded by local human intervention.   Option designers should not feel constrained by the somewhat limited   syntax available for option negotiation.  The intent of the simple   syntax is to make it easy to have options--since it is   correspondingly easy to profess ignorance about them.  If some   particular option requires a richer negotiation structure than   possible within "DO, DON'T, WILL, WON'T", the proper tack is to use   "DO, DON'T, WILL, WON'T" to establish that both parties understand   the option, and once this is accomplished a more exotic syntax can be   used freely.  For example, a party might send a request to alter   (establish) line length.  If it is accepted, then a different syntax   can be used for actually negotiating the line length--such a   "sub-negotiation" perhaps including fields for minimum allowable,   maximum allowable and desired line lengths.  The important concept is   that such expanded negotiations should never begin until some prior   (standard) negotiation has established that both parties are capable   of parsing the expanded syntax.Postel                                                          [Page 3]

June 1980RFC 764, IEN 148Telnet Protocol Specification   In summary, WILL XXX is sent, by either party, to indicate that   party's desire (offer) to begin performing option XXX, DO XXX and   DON'T XXX being its positive and negative acknowledgments; similarly,   DO XXX is sent to indicate a desire (request) that the other party   (i.e., the recipient of the DO) begin performing option XXX, WILL XXX   and WON'T XXX being the positive and negative acknowledgments.  Since   the NVT is what is left when no options are enabled, the DON'T and   WON'T responses are guaranteed to leave the connection in a state   which both ends can handle.  Thus, all Hosts may implement their   TELNET processes to be totally unaware of options that are not   supported, simply returning a rejection to (i.e., refusing) any   option request that cannot be understood.   As much as possible, the TELNET protocol has been made server-user   symmetrical so that it easily and naturally covers the user-user   (linking) and server-server (cooperating processes) cases.  It is   hoped, but not absolutely required, that options will further this   intent.  In any case, it is explicitly acknowledged that symmetry is   an operating principle rather than an ironclad rule.   A companion document, "TELNET Option Specifications," should be   consulted for information about the procedure for establishing new   options.  That document, as well as descriptions of all currently   defined options, is contained in the TELNET section of the ARPA   Internet Protocol Handbook.THE NETWORK VIRTUAL TERMINAL   The Network Virtual Terminal (NVT) is a bi-directional character   device.  The NVT has a printer and a keyboard.  The printer responds   to incoming data and the keyboard produces outgoing data which is   sent over the TELNET connection and, if "echoes" are desired, to the   NVT's printer as well.  "Echoes" will not be expected to traverse the   network (although options exist to enable a "remote" echoing mode of   operation, no Host is required to implement this option).  The code   set is seven-bit USASCII in an eight-bit field, except as modified   herein.  Any code conversion and timing considerations are local   problems and do not affect the NVT.   TRANSMISSION OF DATA      Although a TELNET connection through the network is intrinsically      full duplex, the NVT is to be viewed as a half-duplex device      operating in a line-buffered mode.  That is, unless and until[Page 4]                                                          Postel

RFC 764, IEN 148                                               June 1980                                           Telnet Protocol Specification      options are negotiated to the contrary, the following default      conditions pertain to the transmission of data over the TELNET      connection:         1)  Insofar as the availability of local buffer space permits,         data should be accumulated in the Host where it is generated         until a complete line of data is ready for transmission, or         until some locally-defined explicit signal to transmit occurs.         This signal could be generated either by a process or by a         human user.         The motivation for this rule is the high cost, to some Hosts,         of processing network input interrupts, coupled with the         default NVT specification that "echoes" do not traverse the         network.  Thus, it is reasonable to buffer some amount of data         at its source.  Many systems take some processing action at the         end of each input line (even line printers or card punches         frequently tend to work this way), so the transmission should         be triggered at the end of a line.  On the other hand, a user         or process may sometimes find it necessary or desirable to         provide data which does not terminate at the end of a line;         therefore implementers are cautioned to provide methods of         locally signaling that all buffered data should be transmitted         immediately.         2)  When a process has completed sending data to an NVT printer         and has no queued input from the NVT keyboard for further         processing (i.e., when a process at one end of a TELNET         connection cannot proceed without input from the other end),         the process must transmit the TELNET Go Ahead (GA) command.         This rule is not intended to require that the TELNET GA command         be sent from a terminal at the end of each line, since server         Hosts do not normally require a special signal (in addition to         end-of-line or other locally-defined characters) in order to         commence processing.  Rather, the TELNET GA is designed to help         a user's local Host operate a physically half duplex terminal         which has a "lockable" keyboard such as the IBM 2741.  A         description of this type of terminal may help to explain the         proper use of the GA command.         The terminal-computer connection is always under control of         either the user or the computer.  Neither can unilaterally         seize control from the other; rather the controlling end must         relinguish its control explicitly.  At the terminal end, the         hardware is constructed so as to relinquish control each timePostel                                                          [Page 5]

June 1980RFC 764, IEN 148Telnet Protocol Specification         that a "line" is terminated (i.e., when the "New Line" key is         typed by the user).  When this occurs, the attached (local)         computer processes the input data, decides if output should be         generated, and if not returns control to the terminal.  If         output should be generated, control is retained by the computer         until all output has been transmitted.         The difficulties of using this type of terminal through the         network should be obvious.  The "local" computer is no longer         able to decide whether to retain control after seeing an         end-of-line signal or not; this decision can only be made by         the "remote" computer which is processing the data.  Therefore,         the TELNET GA command provides a mechanism whereby the "remote"         (server) computer can signal the "local" (user) computer that         it is time to pass control to the user of the terminal.  It         should be transmitted at those times, and only at those times,         when the user should be given control of the terminal.  Note         that premature transmission of the GA command may result in the         blocking of output, since the user is likely to assume that the         transmitting system has paused, and therefore he will fail to         turn the line around manually.      The foregoing, of course, does not apply to the user-to-server      direction of communication.  In this direction, GAs may be sent at      any time, but need not ever be sent.  Also, if the TELNET      connection is being used for process-to-process communication, GAs      need not be sent in either direction.  Finally, for      terminal-to-terminal communication, GAs may be required in      neither, one, or both directions.  If a Host plans to support      terminal-to-terminal communication it is suggested that the Host      provide the user with a means of manually signaling that it is      time for a GA to be sent over the TELNET connection; this,      however, is not a requirement on the implementer of a TELNET      process.   STANDARD REPRESENTATION OF CONTROL FUNCTIONS      As stated in the Introduction to this document, the primary goal      of the TELNET protocol is the provision of a standard interfacing      of terminal devices and terminal-oriented processes through the      network.  Early experiences with this type of interconnection have      shown that certain functions are implemented by most servers, but      that the methods of invoking these functions differ widely.  For a      human user who interacts with several server systems, these      differences are highly frustrating.  TELNET, therefore, defines a      standard representation for five of these functions, as described[Page 6]                                                          Postel

RFC 764, IEN 148                                               June 1980                                           Telnet Protocol Specification      below.  These standard representations have standard, but not      required, meanings (with the exception that the IP function may be      required by other protocols which use TELNET); that is, a system      which does not provide the function to local users need not      provide it to network users and may treat the standard      representation for the function as a No-operation.  On the other      hand, a system which does provide the function to local is obliged      to provide the same function as a network user who transmits the      standard representation for the function.      Interrupt Process (IP)         Many systems provide a function which suspends, interrupts,         aborts, or terminates the operation of a user process.  This         function is frequently used when a user believes his process is         in an unending loop, or when an unwanted process has been         inadvertently activated.  IP is the standard representation for         invoking this function.  It should be noted by implementers         that IP may be required by other protocols which use TELNET,         and therefore should be implemented if these other protocols         are to be supported.      Abort Output (AO)         Many systems provide a function which allows a process, which         is generating output, to run to completion (or to reach the         same stopping point it would reach if running to completion)         but without sending the output to the user's terminal.         Further, this function typically clears any output already         produced but not yet actually printed (or displayed) on the         user's terminal.  AO is the standard representation for         invoking this function.  For example, some subsystem might         normally accept a user's command, send a long text string to         the user's terminal in response, and finally signal readiness         to accept the next command by sending a "prompt" character         (preceded by <CR><LF>) to the user's terminal.  If the AO were         received during the transmission of the text string, a         reasonable implementation would be to suppress the remainder of         the text string, but transmit the prompt character and the         preceding <CR><LF>.  (This is possibly in distinction to the         action which might be taken if an IP were received; the IP         might cause suppression of the text string and an exit from the         subsystem.)         It should be noted, by systems which provide this function,         that there may be buffers external to the system (in thePostel                                                          [Page 7]

June 1980RFC 764, IEN 148Telnet Protocol Specification         network and the user's "local" Host) which should be cleared;         the appropriate way to do this is to transmit the "Synch"         signal described below.      Are You There (AYT)         Many systems provide a function which provides the user with         some visible (e.g., printable) evidence that the system is         still up and running.  This function may be invoked by the user         when the system is unexpectedly "silent" for a long time,         because of the unanticipated (by the user) length of a         computation, an unusually heavy system load, etc.  AYT is the         standard representation for invoking this function.      Erase Character (EC)         Many systems provide a function which deletes the last         preceding undeleted character or "print position"* from the         stream of data being supplied by the user.  This function is         typically used to edit keyboard input when typing mistakes are         made.  EC is the standard representation for invoking this         function.            *NOTE:  A "print position" may contain several characters            which are the result of overstrikes, or of sequences such as            <char1> BS <char2>...      Erase Line (EL)         Many systems provide a function which deletes all the data in         the current "line" of input.  This function is typically used         to edit keyboard input.  EL is the standard representation for         invoking this function.   THE TELNET "SYNCH" SIGNAL      Most time-sharing systems provide mechanisms which allow a      terminal user to regain control of a "runaway" process; the IP and      AO functions described above are examples of these mechanisms.      Such systems, when used locally, have access to all of the signals      supplied by the user, whether these are normal characters or      special "out of band" signals such as those supplied by the      teletype "BREAK" key or the IBM 2741 "ATTN" key.  This is not      necessarily true when terminals are connected to the system[Page 8]                                                          Postel

RFC 764, IEN 148                                               June 1980                                           Telnet Protocol Specification      through the network; the network's flow control mechanisms may      cause such a signal to be buffered elsewhere, for example in the      user's Host.      To counter this problem, the TELNET "Synch" mechanism is      introduced.  A Synch signal consists of a TCP Urgent notification,      coupled with the TELNET command DATA MARK.  The Urgent      notification, which is not subject to the flow control pertaining      to the TELNET connection, is used to invoke special handling of      the data stream by the process which receives it.  In this mode,      the data stream is immediately scanned for "interesting" signals      as defined below, discarding intervening data.  The TELNET command      DATA MARK (DM) is the synchronizing mark in the data stream which      indicates that any special signal has already occurred and the      recipient can return to normal processing of the data stream.         The Synch is sent via the TCP send operation with the Urgent         flag set and the DM as the last (or only) data octet.      When several Synchs are sent in rapid succession, the Urgent      notifications may be merged.  It is not possible to count Urgents      since the number received will be less than or equal the number      sent.  When in normal mode a DM is a no operation, when in urgent      mode it signals the end of the urgent processing (this should      correspond with the end of Urgent pointer indicated by TCP).         If TCP indicates the end of Urgent data before the DM is found,         TELNET should continue the special handling of the data stream         until the DM is found.      "Interesting" signals are defined to be:  the TELNET standard      representations of IP, AO, and AYT (but not EC or EL); the local      analogs of these standard representations (if any); all other      TELNET commands; other site-defined signals which can be acted on      without delaying the scan of the data stream.      Since one effect of the SYNCH mechanism is the discarding of      essentially all characters (except TELNET commands) between the      sender of the Synch and its recipient, this mechanism is specified      as the standard way to clear the data path when that is desired.      For example, if a user at a terminal causes an AO to be      transmitted, the server which receives the AO (if it provides that      function at all) should return a Synch to the user.      Finally, just as the TCP Urgent notification is needed at thePostel                                                          [Page 9]

June 1980RFC 764, IEN 148Telnet Protocol Specification      TELNET level as an out-of-band signal, so other protocols which      make use of TELNET may require a TELNET command which can be      viewed as an out-of-band signal at a different level.      By convention the sequence [IP, Synch] is to be used as such a      signal.  For example, suppose that some other protocol, which uses      TELNET, defines the character string STOP analogously to the      TELNET command AO.  Imagine that a user of this protocol wishes a      server to process the STOP string, but the connection is blocked      because the server is processing other commands.  The user should      instruct his system to:         1. Send the TELNET IP character;         2. Send the TELNET SYNC sequence, that is:            Send the Data Mark (DM) as the only character            in a TCP urgent mode send operation.         3. Send the character string STOP; and         4. Send the other protocol's analog of the TELNET DM, if any.      The user (or process acting on his behalf) must transmit the      TELNET SYNCH sequence of step 2 above to ensure that the TELNET IP      gets through to the server's TELNET interpreter.         The Urgent should wake up the TELNET process, the IP should         wake up the next higher level process.   THE NVT PRINTER AND KEYBOARD      The NVT printer has an unspecified carriage width and page length      and can produce representations of all 95 USASCII graphics (codes      32 through 126).  Of the 33 USASCII control codes (0 through 31      and 127), and the 128 uncovered codes (128 through 255), the      following have specified meaning to the NVT printer:         NAME                  CODE         MEANING         NULL (NUL)              0   A no operation         Line Feed (LF)         10   Moves the printer to the                                     next print line, keeping the                                     same horizontal position         Carriage Return (CR)   13   Moves the printer to the left                                     margin of the current line.[Page 10]                                                         Postel

RFC 764, IEN 148                                               June 1980                                           Telnet Protocol Specification         In addition, the following codes shall have defined, but not         required, effects on the NVT printer.  Neither end of a TELNET         connection may assume that the other party will take, or will         have taken, any particular action upon receipt or transmission         of these:         BELL (BEL)              7   Produces an audible or                                     visible signal (which does                                     NOT move the print head)         Back Space (BS)         8   Moves the print head one                                     character position towards                                     the left margin.         Horizontal Tab (HT)     9   Moves the printer to the                                     next horizontal tab stop.                                     It remains unspecified how                                     either party determines or                                     establishes where such tab                                     stops are located.         Vertical Tab (VT)       11  Moves the printer to the                                     next horizontal tab stop.It                                     remains unspecified how                                     either party determines or                                     establishes where such tab                                     stops are located.         Form Feed (FF)          12  Moves the printer to the top                                     of the next page, keeping                                     the same horizontal position      All remaining codes do not cause the NVT printer to take any      action.      The sequence "CR LF", as defined, will cause the NVT to be      positioned at the left margin of the next print line (as would,      for example, the sequence "LF CR").  However, many systems and      terminals do not treat CR and LF independently, and will have to      go to some effort to simulate their effect.  (For example, some      terminals do not have a CR independent of the LF, but on such      terminals it may be possible to simulate a CR by backspacing.)      Therefore, the sequence "CR LF" must be treated as a single "new      line" character and used whenever their combined action is      intended; the sequence "CR NUL" must be used where a carriage      return alone is actually desired; and the CR character must be      avoided in other contexts.  This rule gives assurance to systems      which must decide whether to perform a "new line" function or a      multiple-backspace that the TELNET stream contains a character      following a CR that will allow a rational decision.Postel                                                         [Page 11]

June 1980RFC 764, IEN 148Telnet Protocol Specification         Note that "CR LF" or "CR NUL" is required in both directions         (in the default ASCII mode), to preserve the symmetry of the         NVT model.  Even though it may be known in some situations         (e.g., with remote echo and suppress go ahead options in         effect) that characters are not being sent to an actual         printer, none the less, for the sake of consistency, the         protocol requires that a NUL be inserted following a CR not         followed by a LF in the data stream.  The converse of this is         that a NUL received in the data stream after a CR (in the         absence of options negotiations which explicitly specify         otherwise) should be stripped out prior to applying the NVT to         local character set mapping.      The NVT keyboard has keys, or key combinations, or key sequences,      for generating all 128 USASCII codes.  Note that although many      have no effect on the NVT printer, the NVT keyboard is capable of      generating them.      In addition to these codes, the NVT keyboard shall be capable of      generating the following additional codes which, except as noted,      have defined, but not reguired, meanings.  The actual code      assignments for these "characters" are in the TELNET Command      section, because they are viewed as being, in some sense, generic      and should be available even when the data stream is interpreted      as being some other character set.      Synch         This key allows the user to clear his data path to the other         party.  The activation of this key causes a DM (see command         section) to be sent in the data stream and a TCP Urgent         notification is associated with it.  The pair DM-Urgent is to         have required meaning as defined previously.      Break (BRK)         This code is provided because it is a signal outside the         USASCII set which is currently given local meaning within many         systems.  It is intended to indicate that the Break Key or the         Attention Key was hit.  Note, however, that this is intended to         provide a 129th code for systems which require it, not as a         synonym for the IP standard representation.[Page 12]                                                         Postel

RFC 764, IEN 148                                               June 1980                                           Telnet Protocol Specification      Interrupt Process (IP)         Suspend, interrupt, abort or terminate the process to which the         NVT is connected.  Also, part of the out-of-band signal for         other protocols which use TELNET.      Abort Output (AO)         Allow the current process to (appear to) run to completion, but         do not send its output to the user.  Also, send a Synch to the         user.      Are You There (AYT)         Send back to the NVT some visible (i.e., printable) evidence         that the AYT was received.      Erase Character (EC)         The recipient should delete the last preceding undeleted         character or "print position" from the data stream.      Erase Line (EL)         The recipient should delete characters from the data stream         back to, but not including, the last "CR LF" sequence sent over         the TELNET connection.      The spirit of these "extra" keys, and also the printer format      effectors, is that they should represent a natural extension of      the mapping that already must be done from "NVT" into "local".      Just as the NVT data byte 104 should be mapped into whatever the      local code for "uppercase D" is, so the EC character should be      mapped into whatever the local "Erase Character" function is.      Further, just as the mapping for 174 is somewhat arbitrary in an      environment that has no "vertical bar" character, the EL character      may have a somewhat arbitrary mapping (or none at all) if there is      no local "Erase Line" facility.  Similarly for format effectors:      if the terminal actually does have a "Vertical tab", then the      mapping for VT is obvious, and only when the terminal does not      have a vertical tab should the effect of VT be unpredictable.Postel                                                         [Page 13]

June 1980RFC 764, IEN 148Telnet Protocol SpecificationTELNET COMMAND STRUCTURE   All TELNET commands consist of at least a two byte sequence:  the   "Interpret as Command" (IAC) escape character followed by the code   for the command.  The commands dealing with option negotiation are   three byte sequences, the third byte being the code for the option   referenced.  This format was chosen so that as more comprehensive use   of the "data space" is made -- by negotiations from the basic NVT, of   course -- collisions of data bytes with reserved command values will   be minimized, all such collisions requiring the inconvenience, and   inefficiency, of "escaping" the data bytes into the stream.  With the   current set-up, only the IAC need be doubled to be sent as data, and   the other 255 codes may be passed transparently.   The following are the defined TELNET commands.  Note that these codes   and code sequences have the indicated meaning only when immediately   preceded by an IAC.      NAME               CODE              MEANING      SE                  240 End of subnegotiation parameters      NOP                 241 No operation      Data Mark           242 The data stream portion of a Synch                              This should always be accompanied                              by a TCP Urgent notification.      Break               243 NVT character BRK      Interrupt Process   244 The function IP      Abort output        245 The function AO      Are You There       246 The function AYT      Erase character     247 The function EC      Erase Line          248 The function EL      Go ahead            249 The GA signal      SB                  250 Indicates that what follows is                              subnegotiation of the indicated                              option      WILL (option code)  251 Indicates the desire to begin                              performing, or confirmation that                              you are now performing, the                              indicated option      WON't (option code) 252 Indicates the refusal to perform,                              or continue performing, the                              indicated option.      DO (option code)    253 Indicates the request that the                              other party perform, or                              confirmation that you are expecting                              the other party to perform, the[Page 14]                                                         Postel

RFC 764, IEN 148                                               June 1980                                           Telnet Protocol Specification                              indicated option.      DON'T (option code) 254 Indicates the demand that the                              other party stop performing,                              or confirmation that you are no                              longer expecting the other party                              to perform, the indicated option.      IAC                 255 Data Byte 255.CONNECTION ESTABLISHMENT   The TELNET TCP connection is established between the user's port U   and the server's port L.  The server listens on its well known port L   for such connections.  Since a TCP connection is full duplex and   identified by the pair of ports, the server can engage in many   simultaneous connections involving it's port L and different user   ports U.   Port Assignment      When used for remote user access to service hosts (i.e., remote      terminal access) this protocol is assigned server port 23 (27      octal).  That is L=23.Postel                                                         [Page 15]