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RFC 1123

Requirements for Internet Hosts - Application and Support

Pages: 98
Internet Standard: 3
STD 3 is also:  1122
Updates:  08220952
Updated by:  134921815321596677669210
Part 2 of 4 – Pages 16 to 43
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Top   ToC   RFC1123 - Page 16   prevText


      Telnet is the standard Internet application protocol for remote
      login.  It provides the encoding rules to link a user's
      keyboard/display on a client ("user") system with a command
      interpreter on a remote server system.  A subset of the Telnet
      protocol is also incorporated within other application protocols,
      e.g., FTP and SMTP.

      Telnet uses a single TCP connection, and its normal data stream
      ("Network Virtual Terminal" or "NVT" mode) is 7-bit ASCII with
      escape sequences to embed control functions.  Telnet also allows
      the negotiation of many optional modes and functions.

      The primary Telnet specification is to be found in RFC-854
      [TELNET:1], while the options are defined in many other RFCs; see
      Section 7 for references.


      3.2.1  Option Negotiation: RFC-854, pp. 2-3

         Every Telnet implementation MUST include option negotiation and
         subnegotiation machinery [TELNET:2].

         A host MUST carefully follow the rules of RFC-854 to avoid
         option-negotiation loops.  A host MUST refuse (i.e, reply
         WONT/DONT to a DO/WILL) an unsupported option.  Option
         negotiation SHOULD continue to function (even if all requests
         are refused) throughout the lifetime of a Telnet connection.

         If all option negotiations fail, a Telnet implementation MUST
         default to, and support, an NVT.

              Even though more sophisticated "terminals" and supporting
              option negotiations are becoming the norm, all
              implementations must be prepared to support an NVT for any
              user-server communication.

      3.2.2  Telnet Go-Ahead Function: RFC-854, p. 5, and RFC-858

         On a host that never sends the Telnet command Go Ahead (GA),
         the Telnet Server MUST attempt to negotiate the Suppress Go
         Ahead option (i.e., send "WILL Suppress Go Ahead").  A User or
         Server Telnet MUST always accept negotiation of the Suppress Go
Top   ToC   RFC1123 - Page 17
         Ahead option.

         When it is driving a full-duplex terminal for which GA has no
         meaning, a User Telnet implementation MAY ignore GA commands.

              Half-duplex ("locked-keyboard") line-at-a-time terminals
              for which the Go-Ahead mechanism was designed have largely
              disappeared from the scene.  It turned out to be difficult
              to implement sending the Go-Ahead signal in many operating
              systems, even some systems that support native half-duplex
              terminals.  The difficulty is typically that the Telnet
              server code does not have access to information about
              whether the user process is blocked awaiting input from
              the Telnet connection, i.e., it cannot reliably determine
              when to send a GA command.  Therefore, most Telnet Server
              hosts do not send GA commands.

              The effect of the rules in this section is to allow either
              end of a Telnet connection to veto the use of GA commands.

              There is a class of half-duplex terminals that is still
              commercially important: "data entry terminals," which
              interact in a full-screen manner.  However, supporting
              data entry terminals using the Telnet protocol does not
              require the Go Ahead signal; see Section 3.3.2.

      3.2.3  Control Functions: RFC-854, pp. 7-8

         The list of Telnet commands has been extended to include EOR
         (End-of-Record), with code 239 [TELNET:9].

         Both User and Server Telnets MAY support the control functions
         EOR, EC, EL, and Break, and MUST support AO, AYT, DM, IP, NOP,
         SB, and SE.

         A host MUST be able to receive and ignore any Telnet control
         functions that it does not support.

              Note that a Server Telnet is required to support the
              Telnet IP (Interrupt Process) function, even if the server
              host has an equivalent in-stream function (e.g., Control-C
              in many systems).  The Telnet IP function may be stronger
              than an in-stream interrupt command, because of the out-
              of-band effect of TCP urgent data.

              The EOR control function may be used to delimit the
Top   ToC   RFC1123 - Page 18
              stream.  An important application is data entry terminal
              support (see Section 3.3.2).  There was concern that since
              EOR had not been defined in RFC-854, a host that was not
              prepared to correctly ignore unknown Telnet commands might
              crash if it received an EOR.  To protect such hosts, the
              End-of-Record option [TELNET:9] was introduced; however, a
              properly implemented Telnet program will not require this

      3.2.4  Telnet "Synch" Signal: RFC-854, pp. 8-10

         When it receives "urgent" TCP data, a User or Server Telnet
         MUST discard all data except Telnet commands until the DM (and
         end of urgent) is reached.

         When it sends Telnet IP (Interrupt Process), a User Telnet
         SHOULD follow it by the Telnet "Synch" sequence, i.e., send as
         TCP urgent data the sequence "IAC IP IAC DM".  The TCP urgent
         pointer points to the DM octet.

         When it receives a Telnet IP command, a Server Telnet MAY send
         a Telnet "Synch" sequence back to the user, to flush the output
         stream.  The choice ought to be consistent with the way the
         server operating system behaves when a local user interrupts a

         When it receives a Telnet AO command, a Server Telnet MUST send
         a Telnet "Synch" sequence back to the user, to flush the output

         A User Telnet SHOULD have the capability of flushing output
         when it sends a Telnet IP; see also Section 3.4.5.

              There are three possible ways for a User Telnet to flush
              the stream of server output data:

              (1)  Send AO after IP.

                   This will cause the server host to send a "flush-
                   buffered-output" signal to its operating system.
                   However, the AO may not take effect locally, i.e.,
                   stop terminal output at the User Telnet end, until
                   the Server Telnet has received and processed the AO
                   and has sent back a "Synch".

              (2)  Send DO TIMING-MARK [TELNET:7] after IP, and discard
                   all output locally until a WILL/WONT TIMING-MARK is
Top   ToC   RFC1123 - Page 19
                   received from the Server Telnet.

                   Since the DO TIMING-MARK will be processed after the
                   IP at the server, the reply to it should be in the
                   right place in the output data stream.  However, the
                   TIMING-MARK will not send a "flush buffered output"
                   signal to the server operating system.  Whether or
                   not this is needed is dependent upon the server

              (3)  Do both.

              The best method is not entirely clear, since it must
              accommodate a number of existing server hosts that do not
              follow the Telnet standards in various ways.  The safest
              approach is probably to provide a user-controllable option
              to select (1), (2), or (3).

      3.2.5  NVT Printer and Keyboard: RFC-854, p. 11

         In NVT mode, a Telnet SHOULD NOT send characters with the
         high-order bit 1, and MUST NOT send it as a parity bit.
         Implementations that pass the high-order bit to applications
         SHOULD negotiate binary mode (see Section 3.2.6).

              Implementors should be aware that a strict reading of
              RFC-854 allows a client or server expecting NVT ASCII to
              ignore characters with the high-order bit set.  In
              general, binary mode is expected to be used for
              transmission of an extended (beyond 7-bit) character set
              with Telnet.

              However, there exist applications that really need an 8-
              bit NVT mode, which is currently not defined, and these
              existing applications do set the high-order bit during
              part or all of the life of a Telnet connection.  Note that
              binary mode is not the same as 8-bit NVT mode, since
              binary mode turns off end-of-line processing.  For this
              reason, the requirements on the high-order bit are stated
              as SHOULD, not MUST.

              RFC-854 defines a minimal set of properties of a "network
              virtual terminal" or NVT; this is not meant to preclude
              additional features in a real terminal.  A Telnet
              connection is fully transparent to all 7-bit ASCII
              characters, including arbitrary ASCII control characters.
Top   ToC   RFC1123 - Page 20
              For example, a terminal might support full-screen commands
              coded as ASCII escape sequences; a Telnet implementation
              would pass these sequences as uninterpreted data.  Thus,
              an NVT should not be conceived as a terminal type of a
              highly-restricted device.

      3.2.6  Telnet Command Structure: RFC-854, p. 13

         Since options may appear at any point in the data stream, a
         Telnet escape character (known as IAC, with the value 255) to
         be sent as data MUST be doubled.

      3.2.7  Telnet Binary Option: RFC-856

         When the Binary option has been successfully negotiated,
         arbitrary 8-bit characters are allowed.  However, the data
         stream MUST still be scanned for IAC characters, any embedded
         Telnet commands MUST be obeyed, and data bytes equal to IAC
         MUST be doubled.  Other character processing (e.g., replacing
         CR by CR NUL or by CR LF) MUST NOT be done.  In particular,
         there is no end-of-line convention (see Section 3.3.1) in
         binary mode.

              The Binary option is normally negotiated in both
              directions, to change the Telnet connection from NVT mode
              to "binary mode".

              The sequence IAC EOR can be used to delimit blocks of data
              within a binary-mode Telnet stream.

      3.2.8  Telnet Terminal-Type Option: RFC-1091

         The Terminal-Type option MUST use the terminal type names
         officially defined in the Assigned Numbers RFC [INTRO:5], when
         they are available for the particular terminal.  However, the
         receiver of a Terminal-Type option MUST accept any name.

              RFC-1091 [TELNET:10] updates an earlier version of the
              Terminal-Type option defined in RFC-930.  The earlier
              version allowed a server host capable of supporting
              multiple terminal types to learn the type of a particular
              client's terminal, assuming that each physical terminal
              had an intrinsic type.  However, today a "terminal" is
              often really a terminal emulator program running in a PC,
              perhaps capable of emulating a range of terminal types.
              Therefore, RFC-1091 extends the specification to allow a
Top   ToC   RFC1123 - Page 21
              more general terminal-type negotiation between User and
              Server Telnets.


      3.3.1  Telnet End-of-Line Convention

         The Telnet protocol defines the sequence CR LF to mean "end-
         of-line".  For terminal input, this corresponds to a command-
         completion or "end-of-line" key being pressed on a user
         terminal; on an ASCII terminal, this is the CR key, but it may
         also be labelled "Return" or "Enter".

         When a Server Telnet receives the Telnet end-of-line sequence
         CR LF as input from a remote terminal, the effect MUST be the
         same as if the user had pressed the "end-of-line" key on a
         local terminal.  On server hosts that use ASCII, in particular,
         receipt of the Telnet sequence CR LF must cause the same effect
         as a local user pressing the CR key on a local terminal.  Thus,
         CR LF and CR NUL MUST have the same effect on an ASCII server
         host when received as input over a Telnet connection.

         A User Telnet MUST be able to send any of the forms: CR LF, CR
         NUL, and LF.  A User Telnet on an ASCII host SHOULD have a
         user-controllable mode to send either CR LF or CR NUL when the
         user presses the "end-of-line" key, and CR LF SHOULD be the

         The Telnet end-of-line sequence CR LF MUST be used to send
         Telnet data that is not terminal-to-computer (e.g., for Server
         Telnet sending output, or the Telnet protocol incorporated
         another application protocol).

              To allow interoperability between arbitrary Telnet clients
              and servers, the Telnet protocol defined a standard
              representation for a line terminator.  Since the ASCII
              character set includes no explicit end-of-line character,
              systems have chosen various representations, e.g., CR, LF,
              and the sequence CR LF.  The Telnet protocol chose the CR
              LF sequence as the standard for network transmission.

              Unfortunately, the Telnet protocol specification in RFC-
              854 [TELNET:1] has turned out to be somewhat ambiguous on
              what character(s) should be sent from client to server for
              the "end-of-line" key.  The result has been a massive and
              continuing interoperability headache, made worse by
              various faulty implementations of both User and Server
Top   ToC   RFC1123 - Page 22

              Although the Telnet protocol is based on a perfectly
              symmetric model, in a remote login session the role of the
              user at a terminal differs from the role of the server
              host.  For example, RFC-854 defines the meaning of CR, LF,
              and CR LF as output from the server, but does not specify
              what the User Telnet should send when the user presses the
              "end-of-line" key on the terminal; this turns out to be
              the point at issue.

              When a user presses the "end-of-line" key, some User
              Telnet implementations send CR LF, while others send CR
              NUL (based on a different interpretation of the same
              sentence in RFC-854).  These will be equivalent for a
              correctly-implemented ASCII server host, as discussed
              above.  For other servers, a mode in the User Telnet is

              The existence of User Telnets that send only CR NUL when
              CR is pressed creates a dilemma for non-ASCII hosts: they
              can either treat CR NUL as equivalent to CR LF in input,
              thus precluding the possibility of entering a "bare" CR,
              or else lose complete interworking.

              Suppose a user on host A uses Telnet to log into a server
              host B, and then execute B's User Telnet program to log
              into server host C.  It is desirable for the Server/User
              Telnet combination on B to be as transparent as possible,
              i.e., to appear as if A were connected directly to C.  In
              particular, correct implementation will make B transparent
              to Telnet end-of-line sequences, except that CR LF may be
              translated to CR NUL or vice versa.

              To understand Telnet end-of-line issues, one must have at
              least a general model of the relationship of Telnet to the
              local operating system.  The Server Telnet process is
              typically coupled into the terminal driver software of the
              operating system as a pseudo-terminal.  A Telnet end-of-
              line sequence received by the Server Telnet must have the
              same effect as pressing the end-of-line key on a real
              locally-connected terminal.

              Operating systems that support interactive character-at-
              a-time applications (e.g., editors) typically have two
              internal modes for their terminal I/O: a formatted mode,
              in which local conventions for end-of-line and other
Top   ToC   RFC1123 - Page 23
              formatting rules have been applied to the data stream, and
              a "raw" mode, in which the application has direct access
              to every character as it was entered.  A Server Telnet
              must be implemented in such a way that these modes have
              the same effect for remote as for local terminals.  For
              example, suppose a CR LF or CR NUL is received by the
              Server Telnet on an ASCII host.  In raw mode, a CR
              character is passed to the application; in formatted mode,
              the local system's end-of-line convention is used.

      3.3.2  Data Entry Terminals

              In addition to the line-oriented and character-oriented
              ASCII terminals for which Telnet was designed, there are
              several families of video display terminals that are
              sometimes known as "data entry terminals" or DETs.  The
              IBM 3270 family is a well-known example.

              Two Internet protocols have been designed to support
              generic DETs: SUPDUP [TELNET:16, TELNET:17], and the DET
              option [TELNET:18, TELNET:19].  The DET option drives a
              data entry terminal over a Telnet connection using (sub-)
              negotiation.  SUPDUP is a completely separate terminal
              protocol, which can be entered from Telnet by negotiation.
              Although both SUPDUP and the DET option have been used
              successfully in particular environments, neither has
              gained general acceptance or wide implementation.

              A different approach to DET interaction has been developed
              for supporting the IBM 3270 family through Telnet,
              although the same approach would be applicable to any DET.
              The idea is to enter a "native DET" mode, in which the
              native DET input/output stream is sent as binary data.
              The Telnet EOR command is used to delimit logical records
              (e.g., "screens") within this binary stream.

              The rules for entering and leaving native DET mode are as

              o    The Server uses the Terminal-Type option [TELNET:10]
                   to learn that the client is a DET.

              o    It is conventional, but not required, that both ends
                   negotiate the EOR option [TELNET:9].

              o    Both ends negotiate the Binary option [TELNET:3] to
Top   ToC   RFC1123 - Page 24
                   enter native DET mode.

              o    When either end negotiates out of binary mode, the
                   other end does too, and the mode then reverts to
                   normal NVT.

      3.3.3  Option Requirements

         Every Telnet implementation MUST support the Binary option
         [TELNET:3] and the Suppress Go Ahead option [TELNET:5], and
         SHOULD support the Echo [TELNET:4], Status [TELNET:6], End-of-
         Record [TELNET:9], and Extended Options List [TELNET:8]

         A User or Server Telnet SHOULD support the Window Size Option
         [TELNET:12] if the local operating system provides the
         corresponding capability.

              Note that the End-of-Record option only signifies that a
              Telnet can receive a Telnet EOR without crashing;
              therefore, every Telnet ought to be willing to accept
              negotiation of the End-of-Record option.  See also the
              discussion in Section 3.2.3.

      3.3.4  Option Initiation

         When the Telnet protocol is used in a client/server situation,
         the server SHOULD initiate negotiation of the terminal
         interaction mode it expects.

              The Telnet protocol was defined to be perfectly
              symmetrical, but its application is generally asymmetric.
              Remote login has been known to fail because NEITHER side
              initiated negotiation of the required non-default terminal
              modes.  It is generally the server that determines the
              preferred mode, so the server needs to initiate the
              negotiation; since the negotiation is symmetric, the user
              can also initiate it.

         A client (User Telnet) SHOULD provide a means for users to
         enable and disable the initiation of option negotiation.

              A user sometimes needs to connect to an application
              service (e.g., FTP or SMTP) that uses Telnet for its
Top   ToC   RFC1123 - Page 25
              control stream but does not support Telnet options.  User
              Telnet may be used for this purpose if initiation of
              option negotiation is  disabled.

      3.3.5  Telnet Linemode Option

              An important new Telnet option, LINEMODE [TELNET:12], has
              been proposed.  The LINEMODE option provides a standard
              way for a User Telnet and a Server Telnet to agree that
              the client rather than the server will perform terminal
              character processing.  When the client has prepared a
              complete line of text, it will send it to the server in
              (usually) one TCP packet.  This option will greatly
              decrease the packet cost of Telnet sessions and will also
              give much better user response over congested or long-
              delay networks.

              The LINEMODE option allows dynamic switching between local
              and remote character processing.  For example, the Telnet
              connection will automatically negotiate into single-
              character mode while a full screen editor is running, and
              then return to linemode when the editor is finished.

              We expect that when this RFC is released, hosts should
              implement the client side of this option, and may
              implement the server side of this option.  To properly
              implement the server side, the server needs to be able to
              tell the local system not to do any input character
              processing, but to remember its current terminal state and
              notify the Server Telnet process whenever the state
              changes.  This will allow password echoing and full screen
              editors to be handled properly, for example.


      3.4.1  Character Set Transparency

         User Telnet implementations SHOULD be able to send or receive
         any 7-bit ASCII character.  Where possible, any special
         character interpretations by the user host's operating system
         SHOULD be bypassed so that these characters can conveniently be
         sent and received on the connection.

         Some character value MUST be reserved as "escape to command
         mode"; conventionally, doubling this character allows it to be
         entered as data.  The specific character used SHOULD be user
Top   ToC   RFC1123 - Page 26
         On binary-mode connections, a User Telnet program MAY provide
         an escape mechanism for entering arbitrary 8-bit values, if the
         host operating system doesn't allow them to be entered directly
         from the keyboard.

              The transparency issues are less pressing on servers, but
              implementors should take care in dealing with issues like:
              masking off parity bits (sent by an older, non-conforming
              client) before they reach programs that expect only NVT
              ASCII, and properly handling programs that request 8-bit
              data streams.

      3.4.2  Telnet Commands

         A User Telnet program MUST provide a user the capability of
         entering any of the Telnet control functions IP, AO, or AYT,
         and SHOULD provide the capability of entering EC, EL, and

      3.4.3  TCP Connection Errors

         A User Telnet program SHOULD report to the user any TCP errors
         that are reported by the transport layer (see "TCP/Application
         Layer Interface" section in [INTRO:1]).

      3.4.4  Non-Default Telnet Contact Port

         A User Telnet program SHOULD allow the user to optionally
         specify a non-standard contact port number at the Server Telnet

      3.4.5  Flushing Output

         A User Telnet program SHOULD provide the user the ability to
         specify whether or not output should be flushed when an IP is
         sent; see Section 3.2.4.

         For any output flushing scheme that causes the User Telnet to
         flush output locally until a Telnet signal is received from the
         Server, there SHOULD be a way for the user to manually restore
         normal output, in case the Server fails to send the expected
Top   ToC   RFC1123 - Page 27

                                                 |        | | | |S| |
                                                 |        | | | |H| |F
                                                 |        | | | |O|M|o
                                                 |        | |S| |U|U|o
                                                 |        | |H| |L|S|t
                                                 |        |M|O| |D|T|n
                                                 |        |U|U|M| | |o
                                                 |        |S|L|A|N|N|t
                                                 |        |T|D|Y|O|O|t
FEATURE                                          |SECTION | | | |T|T|e
                                                 |        | | | | | |
Option Negotiation                               |3.2.1   |x| | | | |
  Avoid negotiation loops                        |3.2.1   |x| | | | |
  Refuse unsupported options                     |3.2.1   |x| | | | |
  Negotiation OK anytime on connection           |3.2.1   | |x| | | |
  Default to NVT                                 |3.2.1   |x| | | | |
  Send official name in Term-Type option         |3.2.8   |x| | | | |
  Accept any name in Term-Type option            |3.2.8   |x| | | | |
  Implement Binary, Suppress-GA options          |3.3.3   |x| | | | |
  Echo, Status, EOL, Ext-Opt-List options        |3.3.3   | |x| | | |
  Implement Window-Size option if appropriate    |3.3.3   | |x| | | |
  Server initiate mode negotiations              |3.3.4   | |x| | | |
  User can enable/disable init negotiations      |3.3.4   | |x| | | |
                                                 |        | | | | | |
Go-Aheads                                        |        | | | | | |
  Non-GA server negotiate SUPPRESS-GA option     |3.2.2   |x| | | | |
  User or Server accept SUPPRESS-GA option       |3.2.2   |x| | | | |
  User Telnet ignore GA's                        |3.2.2   | | |x| | |
                                                 |        | | | | | |
Control Functions                                |        | | | | | |
  Support SE NOP DM IP AO AYT SB                 |3.2.3   |x| | | | |
  Support EOR EC EL Break                        |3.2.3   | | |x| | |
  Ignore unsupported control functions           |3.2.3   |x| | | | |
  User, Server discard urgent data up to DM      |3.2.4   |x| | | | |
  User Telnet send "Synch" after IP, AO, AYT     |3.2.4   | |x| | | |
  Server Telnet reply Synch to IP                |3.2.4   | | |x| | |
  Server Telnet reply Synch to AO                |3.2.4   |x| | | | |
  User Telnet can flush output when send IP      |3.2.4   | |x| | | |
                                                 |        | | | | | |
Encoding                                         |        | | | | | |
  Send high-order bit in NVT mode                |3.2.5   | | | |x| |
  Send high-order bit as parity bit              |3.2.5   | | | | |x|
  Negot. BINARY if pass high-ord. bit to applic  |3.2.5   | |x| | | |
  Always double IAC data byte                    |3.2.6   |x| | | | |
Top   ToC   RFC1123 - Page 28
  Double IAC data byte in binary mode            |3.2.7   |x| | | | |
  Obey Telnet cmds in binary mode                |3.2.7   |x| | | | |
  End-of-line, CR NUL in binary mode             |3.2.7   | | | | |x|
                                                 |        | | | | | |
End-of-Line                                      |        | | | | | |
  EOL at Server same as local end-of-line        |3.3.1   |x| | | | |
  ASCII Server accept CR LF or CR NUL for EOL    |3.3.1   |x| | | | |
  User Telnet able to send CR LF, CR NUL, or LF  |3.3.1   |x| | | | |
    ASCII user able to select CR LF/CR NUL       |3.3.1   | |x| | | |
    User Telnet default mode is CR LF            |3.3.1   | |x| | | |
  Non-interactive uses CR LF for EOL             |3.3.1   |x| | | | |
                                                 |        | | | | | |
User Telnet interface                            |        | | | | | |
  Input & output all 7-bit characters            |3.4.1   | |x| | | |
  Bypass local op sys interpretation             |3.4.1   | |x| | | |
  Escape character                               |3.4.1   |x| | | | |
     User-settable escape character              |3.4.1   | |x| | | |
  Escape to enter 8-bit values                   |3.4.1   | | |x| | |
  Can input IP, AO, AYT                          |3.4.2   |x| | | | |
  Can input EC, EL, Break                        |3.4.2   | |x| | | |
  Report TCP connection errors to user           |3.4.3   | |x| | | |
  Optional non-default contact port              |3.4.4   | |x| | | |
  Can spec: output flushed when IP sent          |3.4.5   | |x| | | |
  Can manually restore output mode               |3.4.5   | |x| | | |
                                                 |        | | | | | |
Top   ToC   RFC1123 - Page 29


      4.1.1  INTRODUCTION

         The File Transfer Protocol FTP is the primary Internet standard
         for file transfer.  The current specification is contained in
         RFC-959 [FTP:1].

         FTP uses separate simultaneous TCP connections for control and
         for data transfer.  The FTP protocol includes many features,
         some of which are not commonly implemented.  However, for every
         feature in FTP, there exists at least one implementation.  The
         minimum implementation defined in RFC-959 was too small, so a
         somewhat larger minimum implementation is defined here.

         Internet users have been unnecessarily burdened for years by
         deficient FTP implementations.  Protocol implementors have
         suffered from the erroneous opinion that implementing FTP ought
         to be a small and trivial task.  This is wrong, because FTP has
         a user interface, because it has to deal (correctly) with the
         whole variety of communication and operating system errors that
         may occur, and because it has to handle the great diversity of
         real file systems in the world.

  LOCAL Type: RFC-959 Section

            An FTP program MUST support TYPE I ("IMAGE" or binary type)
            as well as TYPE L 8 ("LOCAL" type with logical byte size 8).
            A machine whose memory is organized into m-bit words, where
            m is not a multiple of 8, MAY also support TYPE L m.

                 The command "TYPE L 8" is often required to transfer
                 binary data between a machine whose memory is organized
                 into (e.g.) 36-bit words and a machine with an 8-bit
                 byte organization.  For an 8-bit byte machine, TYPE L 8
                 is equivalent to IMAGE.

                 "TYPE L m" is sometimes specified to the FTP programs
                 on two m-bit word machines to ensure the correct
                 transfer of a native-mode binary file from one machine
                 to the other.  However, this command should have the
                 same effect on these machines as "TYPE I".
Top   ToC   RFC1123 - Page 30  Telnet Format Control: RFC-959 Section

            A host that makes no distinction between TYPE N and TYPE T
            SHOULD implement TYPE T to be identical to TYPE N.

                 This provision should ease interoperation with hosts
                 that do make this distinction.

                 Many hosts represent text files internally as strings
                 of ASCII characters, using the embedded ASCII format
                 effector characters (LF, BS, FF, ...) to control the
                 format when a file is printed.  For such hosts, there
                 is no distinction between "print" files and other
                 files.  However, systems that use record structured
                 files typically need a special format for printable
                 files (e.g., ASA carriage control).   For the latter
                 hosts, FTP allows a choice of TYPE N or TYPE T.
  Page Structure: RFC-959 Section and Appendix I

            Implementation of page structure is NOT RECOMMENDED in
            general. However, if a host system does need to implement
            FTP for "random access" or "holey" files, it MUST use the
            defined page structure format rather than define a new
            private FTP format.
  Data Structure Transformations: RFC-959 Section 3.1.2

            An FTP transformation between record-structure and file-
            structure SHOULD be invertible, to the extent possible while
            making the result useful on the target host.

                 RFC-959 required strict invertibility between record-
                 structure and file-structure, but in practice,
                 efficiency and convenience often preclude it.
                 Therefore, the requirement is being relaxed.  There are
                 two different objectives for transferring a file:
                 processing it on the target host, or just storage.  For
                 storage, strict invertibility is important.  For
                 processing, the file created on the target host needs
                 to be in the format expected by application programs on
                 that host.

                 As an example of the conflict, imagine a record-
                 oriented operating system that requires some data files
                 to have exactly 80 bytes in each record.  While STORing
Top   ToC   RFC1123 - Page 31
                 a file on such a host, an FTP Server must be able to
                 pad each line or record to 80 bytes; a later retrieval
                 of such a file cannot be strictly invertible.
  Data Connection Management: RFC-959 Section 3.3

            A User-FTP that uses STREAM mode SHOULD send a PORT command
            to assign a non-default data port before each transfer
            command is issued.

                 This is required because of the long delay after a TCP
                 connection is closed until its socket pair can be
                 reused, to allow multiple transfers during a single FTP
                 session.  Sending a port command can avoided if a
                 transfer mode other than stream is used, by leaving the
                 data transfer connection open between transfers.
  PASV Command: RFC-959 Section 4.1.2

            A server-FTP MUST implement the PASV command.

            If multiple third-party transfers are to be executed during
            the same session, a new PASV command MUST be issued before
            each transfer command, to obtain a unique port pair.

                 The format of the 227 reply to a PASV command is not
                 well standardized.  In particular, an FTP client cannot
                 assume that the parentheses shown on page 40 of RFC-959
                 will be present (and in fact, Figure 3 on page 43 omits
                 them).  Therefore, a User-FTP program that interprets
                 the PASV reply must scan the reply for the first digit
                 of the host and port numbers.

                 Note that the host number h1,h2,h3,h4 is the IP address
                 of the server host that is sending the reply, and that
                 p1,p2 is a non-default data transfer port that PASV has
  LIST and NLST Commands: RFC-959 Section 4.1.3

            The data returned by an NLST command MUST contain only a
            simple list of legal pathnames, such that the server can use
            them directly as the arguments of subsequent data transfer
            commands for the individual files.

            The data returned by a LIST or NLST command SHOULD use an
Top   ToC   RFC1123 - Page 32
            implied TYPE AN, unless the current type is EBCDIC, in which
            case an implied TYPE EN SHOULD be used.

                 Many FTP clients support macro-commands that will get
                 or put files matching a wildcard specification, using
                 NLST to obtain a list of pathnames.  The expansion of
                 "multiple-put" is local to the client, but "multiple-
                 get" requires cooperation by the server.

                 The implied type for LIST and NLST is designed to
                 provide compatibility with existing User-FTPs, and in
                 particular with multiple-get commands.
  SITE Command: RFC-959 Section 4.1.3

            A Server-FTP SHOULD use the SITE command for non-standard
            features, rather than invent new private commands or
            unstandardized extensions to existing commands.
  STOU Command: RFC-959 Section 4.1.3

            The STOU command stores into a uniquely named file.  When it
            receives an STOU command, a Server-FTP MUST return the
            actual file name in the "125 Transfer Starting" or the "150
            Opening Data Connection" message that precedes the transfer
            (the 250 reply code mentioned in RFC-959 is incorrect).  The
            exact format of these messages is hereby defined to be as

                125 FILE: pppp
                150 FILE: pppp

            where pppp represents the unique pathname of the file that
            will be written.
  Telnet End-of-line Code: RFC-959, Page 34

            Implementors MUST NOT assume any correspondence between READ
            boundaries on the control connection and the Telnet EOL
            sequences (CR LF).

                 Thus, a server-FTP (or User-FTP) must continue reading
                 characters from the control connection until a complete
                 Telnet EOL sequence is encountered, before processing
                 the command (or response, respectively).  Conversely, a
                 single READ from the control connection may include
Top   ToC   RFC1123 - Page 33
                 more than one FTP command.
  FTP Replies: RFC-959 Section 4.2, Page 35

            A Server-FTP MUST send only correctly formatted replies on
            the control connection.  Note that RFC-959 (unlike earlier
            versions of the FTP spec) contains no provision for a
            "spontaneous" reply message.

            A Server-FTP SHOULD use the reply codes defined in RFC-959
            whenever they apply.  However, a server-FTP MAY use a
            different reply code when needed, as long as the general
            rules of Section 4.2 are followed. When the implementor has
            a choice between a 4xx and 5xx reply code, a Server-FTP
            SHOULD send a 4xx (temporary failure) code when there is any
            reasonable possibility that a failed FTP will succeed a few
            hours later.

            A User-FTP SHOULD generally use only the highest-order digit
            of a 3-digit reply code for making a procedural decision, to
            prevent difficulties when a Server-FTP uses non-standard
            reply codes.

            A User-FTP MUST be able to handle multi-line replies.  If
            the implementation imposes a limit on the number of lines
            and if this limit is exceeded, the User-FTP MUST recover,
            e.g., by ignoring the excess lines until the end of the
            multi-line reply is reached.

            A User-FTP SHOULD NOT interpret a 421 reply code ("Service
            not available, closing control connection") specially, but
            SHOULD detect closing of the control connection by the

                 Server implementations that fail to strictly follow the
                 reply rules often cause FTP user programs to hang.
                 Note that RFC-959 resolved ambiguities in the reply
                 rules found in earlier FTP specifications and must be

                 It is important to choose FTP reply codes that properly
                 distinguish between temporary and permanent failures,
                 to allow the successful use of file transfer client
                 daemons.  These programs depend on the reply codes to
                 decide whether or not to retry a failed transfer; using
                 a permanent failure code (5xx) for a temporary error
                 will cause these programs to give up unnecessarily.
Top   ToC   RFC1123 - Page 34
                 When the meaning of a reply matches exactly the text
                 shown in RFC-959, uniformity will be enhanced by using
                 the RFC-959 text verbatim.  However, a Server-FTP
                 implementor is encouraged to choose reply text that
                 conveys specific system-dependent information, when
  Connections: RFC-959 Section 5.2

            The words "and the port used" in the second paragraph of
            this section of RFC-959 are erroneous (historical), and they
            should be ignored.

            On a multihomed server host, the default data transfer port
            (L-1) MUST be associated with the same local IP address as
            the corresponding control connection to port L.

            A user-FTP MUST NOT send any Telnet controls other than
            SYNCH and IP on an FTP control connection. In particular, it
            MUST NOT attempt to negotiate Telnet options on the control
            connection.  However, a server-FTP MUST be capable of
            accepting and refusing Telnet negotiations (i.e., sending

                 Although the RFC says: "Server- and User- processes
                 should follow the conventions for the Telnet
                 protocol...[on the control connection]", it is not the
                 intent that Telnet option negotiation is to be
  Minimum Implementation; RFC-959 Section 5.1

            The following commands and options MUST be supported by
            every server-FTP and user-FTP, except in cases where the
            underlying file system or operating system does not allow or
            support a particular command.

                 Type: ASCII Non-print, IMAGE, LOCAL 8
                 Mode: Stream
                 Structure: File, Record*
                    USER, PASS, ACCT,
                    PORT, PASV,
                    TYPE, MODE, STRU,
                    RETR, STOR, APPE,
                    RNFR, RNTO, DELE,
                    CWD,  CDUP, RMD,  MKD,  PWD,
Top   ToC   RFC1123 - Page 35
                    LIST, NLST,
                    SYST, STAT,
                    HELP, NOOP, QUIT.

            *Record structure is REQUIRED only for hosts whose file
            systems support record structure.

                 Vendors are encouraged to implement a larger subset of
                 the protocol.  For example, there are important
                 robustness features in the protocol (e.g., Restart,
                 ABOR, block mode) that would be an aid to some Internet
                 users but are not widely implemented.

                 A host that does not have record structures in its file
                 system may still accept files with STRU R, recording
                 the byte stream literally.

      4.1.3  SPECIFIC ISSUES
  Non-standard Command Verbs

            FTP allows "experimental" commands, whose names begin with
            "X".  If these commands are subsequently adopted as
            standards, there may still be existing implementations using
            the "X" form.  At present, this is true for the directory

                RFC-959   "Experimental"

                  MKD        XMKD
                  RMD        XRMD
                  PWD        XPWD
                  CDUP       XCUP
                  CWD        XCWD

            All FTP implementations SHOULD recognize both forms of these
            commands, by simply equating them with extra entries in the
            command lookup table.

                 A User-FTP can access a server that supports only the
                 "X" forms by implementing a mode switch, or
                 automatically using the following procedure: if the
                 RFC-959 form of one of the above commands is rejected
                 with a 500 or 502 response code, then try the
                 experimental form; any other response would be passed
                 to the user.
Top   ToC   RFC1123 - Page 36  Idle Timeout

            A Server-FTP process SHOULD have an idle timeout, which will
            terminate the process and close the control connection if
            the server is inactive (i.e., no command or data transfer in
            progress) for a long period of time.  The idle timeout time
            SHOULD be configurable, and the default should be at least 5

            A client FTP process ("User-PI" in RFC-959) will need
            timeouts on responses only if it is invoked from a program.

                 Without a timeout, a Server-FTP process may be left
                 pending indefinitely if the corresponding client
                 crashes without closing the control connection.
  Concurrency of Data and Control

                 The intent of the designers of FTP was that a user
                 should be able to send a STAT command at any time while
                 data transfer was in progress and that the server-FTP
                 would reply immediately with status -- e.g., the number
                 of bytes transferred so far.  Similarly, an ABOR
                 command should be possible at any time during a data

                 Unfortunately, some small-machine operating systems
                 make such concurrent programming difficult, and some
                 other implementers seek minimal solutions, so some FTP
                 implementations do not allow concurrent use of the data
                 and control connections.  Even such a minimal server
                 must be prepared to accept and defer a STAT or ABOR
                 command that arrives during data transfer.
  FTP Restart Mechanism

            The description of the 110 reply on pp. 40-41 of RFC-959 is
            incorrect; the correct description is as follows.  A restart
            reply message, sent over the control connection from the
            receiving FTP to the User-FTP, has the format:

                110 MARK ssss = rrrr


            *    ssss is a text string that appeared in a Restart Marker
Top   ToC   RFC1123 - Page 37
                 in the data stream and encodes a position in the
                 sender's file system;

            *    rrrr encodes the corresponding position in the
                 receiver's file system.

            The encoding, which is specific to a particular file system
            and network implementation, is always generated and
            interpreted by the same system, either sender or receiver.

            When an FTP that implements restart receives a Restart
            Marker in the data stream, it SHOULD force the data to that
            point to be written to stable storage before encoding the
            corresponding position rrrr.  An FTP sending Restart Markers
            MUST NOT assume that 110 replies will be returned
            synchronously with the data, i.e., it must not await a 110
            reply before sending more data.

            Two new reply codes are hereby defined for errors
            encountered in restarting a transfer:

              554 Requested action not taken: invalid REST parameter.

                 A 554 reply may result from a FTP service command that
                 follows a REST command.  The reply indicates that the
                 existing file at the Server-FTP cannot be repositioned
                 as specified in the REST.

              555 Requested action not taken: type or stru mismatch.

                 A 555 reply may result from an APPE command or from any
                 FTP service command following a REST command.  The
                 reply indicates that there is some mismatch between the
                 current transfer parameters (type and stru) and the
                 attributes of the existing file.

                 Note that the FTP Restart mechanism requires that Block
                 or Compressed mode be used for data transfer, to allow
                 the Restart Markers to be included within the data
                 stream.  The frequency of Restart Markers can be low.

                 Restart Markers mark a place in the data stream, but
                 the receiver may be performing some transformation on
                 the data as it is stored into stable storage.  In
                 general, the receiver's encoding must include any state
                 information necessary to restart this transformation at
                 any point of the FTP data stream.  For example, in TYPE
Top   ToC   RFC1123 - Page 38
                 A transfers, some receiver hosts transform CR LF
                 sequences into a single LF character on disk.   If a
                 Restart Marker happens to fall between CR and LF, the
                 receiver must encode in rrrr that the transfer must be
                 restarted in a "CR has been seen and discarded" state.

                 Note that the Restart Marker is required to be encoded
                 as a string of printable ASCII characters, regardless
                 of the type of the data.

                 RFC-959 says that restart information is to be returned
                 "to the user".  This should not be taken literally.  In
                 general, the User-FTP should save the restart
                 information (ssss,rrrr) in stable storage, e.g., append
                 it to a restart control file.  An empty restart control
                 file should be created when the transfer first starts
                 and deleted automatically when the transfer completes
                 successfully.  It is suggested that this file have a
                 name derived in an easily-identifiable manner from the
                 name of the file being transferred and the remote host
                 name; this is analogous to the means used by many text
                 editors for naming "backup" files.

                 There are three cases for FTP restart.

                 (1)  User-to-Server Transfer

                      The User-FTP puts Restart Markers <ssss> at
                      convenient places in the data stream.  When the
                      Server-FTP receives a Marker, it writes all prior
                      data to disk, encodes its file system position and
                      transformation state as rrrr, and returns a "110
                      MARK ssss = rrrr" reply over the control
                      connection.  The User-FTP appends the pair
                      (ssss,rrrr) to its restart control file.

                      To restart the transfer, the User-FTP fetches the
                      last (ssss,rrrr) pair from the restart control
                      file, repositions its local file system and
                      transformation state using ssss, and sends the
                      command "REST rrrr" to the Server-FTP.

                 (2)  Server-to-User Transfer

                      The Server-FTP puts Restart Markers <ssss> at
                      convenient places in the data stream.  When the
                      User-FTP receives a Marker, it writes all prior
                      data to disk, encodes its file system position and
Top   ToC   RFC1123 - Page 39
                      transformation state as rrrr, and appends the pair
                      (rrrr,ssss) to its restart control file.

                      To restart the transfer, the User-FTP fetches the
                      last (rrrr,ssss) pair from the restart control
                      file, repositions its local file system and
                      transformation state using rrrr, and sends the
                      command "REST ssss" to the Server-FTP.

                 (3)  Server-to-Server ("Third-Party") Transfer

                      The sending Server-FTP puts Restart Markers <ssss>
                      at convenient places in the data stream.  When it
                      receives a Marker, the receiving Server-FTP writes
                      all prior data to disk, encodes its file system
                      position and transformation state as rrrr, and
                      sends a "110 MARK ssss = rrrr" reply over the
                      control connection to the User.  The User-FTP
                      appends the pair (ssss,rrrr) to its restart
                      control file.

                      To restart the transfer, the User-FTP fetches the
                      last (ssss,rrrr) pair from the restart control
                      file, sends "REST ssss" to the sending Server-FTP,
                      and sends "REST rrrr" to the receiving Server-FTP.


         This section discusses the user interface for a User-FTP
  Pathname Specification

            Since FTP is intended for use in a heterogeneous
            environment, User-FTP implementations MUST support remote
            pathnames as arbitrary character strings, so that their form
            and content are not limited by the conventions of the local
            operating system.

                 In particular, remote pathnames can be of arbitrary
                 length, and all the printing ASCII characters as well
                 as space (0x20) must be allowed.  RFC-959 allows a
                 pathname to contain any 7-bit ASCII character except CR
                 or LF.
Top   ToC   RFC1123 - Page 40  "QUOTE" Command

            A User-FTP program MUST implement a "QUOTE" command that
            will pass an arbitrary character string to the server and
            display all resulting response messages to the user.

            To make the "QUOTE" command useful, a User-FTP SHOULD send
            transfer control commands to the server as the user enters
            them, rather than saving all the commands and sending them
            to the server only when a data transfer is started.

                 The "QUOTE" command is essential to allow the user to
                 access servers that require system-specific commands
                 (e.g., SITE or ALLO), or to invoke new or optional
                 features that are not implemented by the User-FTP.  For
                 example, "QUOTE" may be used to specify "TYPE A T" to
                 send a print file to hosts that require the
                 distinction, even if the User-FTP does not recognize
                 that TYPE.
  Displaying Replies to User

            A User-FTP SHOULD display to the user the full text of all
            error reply messages it receives.  It SHOULD have a
            "verbose" mode in which all commands it sends and the full
            text and reply codes it receives are displayed, for
            diagnosis of problems.
  Maintaining Synchronization

            The state machine in a User-FTP SHOULD be forgiving of
            missing and unexpected reply messages, in order to maintain
            command synchronization with the server.
Top   ToC   RFC1123 - Page 41

                                           |               | | | |S| |
                                           |               | | | |H| |F
                                           |               | | | |O|M|o
                                           |               | |S| |U|U|o
                                           |               | |H| |L|S|t
                                           |               |M|O| |D|T|n
                                           |               |U|U|M| | |o
                                           |               |S|L|A|N|N|t
                                           |               |T|D|Y|O|O|t
FEATURE                                    |SECTION        | | | |T|T|e
Implement TYPE T if same as TYPE N         |        | |x| | | |
File/Record transform invertible if poss.  |        | |x| | | |
User-FTP send PORT cmd for stream mode     |        | |x| | | |
Server-FTP implement PASV                  |        |x| | | | |
  PASV is per-transfer                     |        |x| | | | |
NLST reply usable in RETR cmds             |        |x| | | | |
Implied type for LIST and NLST             |        | |x| | | |
SITE cmd for non-standard features         |        | |x| | | |
STOU cmd return pathname as specified      |        |x| | | | |
Use TCP READ boundaries on control conn.   |       | | | | |x|
                                           |               | | | | | |
Server-FTP send only correct reply format  |       |x| | | | |
Server-FTP use defined reply code if poss. |       | |x| | | |
  New reply code following Section 4.2     |       | | |x| | |
User-FTP use only high digit of reply      |       | |x| | | |
User-FTP handle multi-line reply lines     |       |x| | | | |
User-FTP handle 421 reply specially        |       | | | |x| |
                                           |               | | | | | |
Default data port same IP addr as ctl conn |       |x| | | | |
User-FTP send Telnet cmds exc. SYNCH, IP   |       | | | | |x|
User-FTP negotiate Telnet options          |       | | | | |x|
Server-FTP handle Telnet options           |       |x| | | | |
Handle "Experimental" directory cmds       |        | |x| | | |
Idle timeout in server-FTP                 |        | |x| | | |
    Configurable idle timeout              |        | |x| | | |
Receiver checkpoint data at Restart Marker |        | |x| | | |
Sender assume 110 replies are synchronous  |        | | | | |x|
                                           |               | | | | | |
Support TYPE:                              |               | | | | | |
  ASCII - Non-Print (AN)                   |       |x| | | | |
  ASCII - Telnet (AT) -- if same as AN     |        | |x| | | |
  ASCII - Carriage Control (AC)            |959  | | |x| | |
  EBCDIC - (any form)                      |959    | | |x| | |
  IMAGE                                    |        |x| | | | |
  LOCAL 8                                  |        |x| | | | |
Top   ToC   RFC1123 - Page 42
  LOCAL m                                  |        | | |x| | |2
                                           |               | | | | | |
Support MODE:                              |               | | | | | |
  Stream                                   |       |x| | | | |
  Block                                    |959 3.4.2      | | |x| | |
                                           |               | | | | | |
Support STRUCTURE:                         |               | | | | | |
  File                                     |       |x| | | | |
  Record                                   |       |x| | | | |3
  Page                                     |        | | | |x| |
                                           |               | | | | | |
Support commands:                          |               | | | | | |
  USER                                     |       |x| | | | |
  PASS                                     |       |x| | | | |
  ACCT                                     |       |x| | | | |
  CWD                                      |       |x| | | | |
  CDUP                                     |       |x| | | | |
  SMNT                                     |959 5.3.1      | | |x| | |
  REIN                                     |959 5.3.1      | | |x| | |
  QUIT                                     |       |x| | | | |
                                           |               | | | | | |
  PORT                                     |       |x| | | | |
  PASV                                     |        |x| | | | |
  TYPE                                     |       |x| | | | |1
  STRU                                     |       |x| | | | |1
  MODE                                     |       |x| | | | |1
                                           |               | | | | | |
  RETR                                     |       |x| | | | |
  STOR                                     |       |x| | | | |
  STOU                                     |959 5.3.1      | | |x| | |
  APPE                                     |       |x| | | | |
  ALLO                                     |959 5.3.1      | | |x| | |
  REST                                     |959 5.3.1      | | |x| | |
  RNFR                                     |       |x| | | | |
  RNTO                                     |       |x| | | | |
  ABOR                                     |959 5.3.1      | | |x| | |
  DELE                                     |       |x| | | | |
  RMD                                      |       |x| | | | |
  MKD                                      |       |x| | | | |
  PWD                                      |       |x| | | | |
  LIST                                     |       |x| | | | |
  NLST                                     |       |x| | | | |
  SITE                                     |        | | |x| | |
  STAT                                     |       |x| | | | |
  SYST                                     |       |x| | | | |
  HELP                                     |       |x| | | | |
  NOOP                                     |       |x| | | | |
                                           |               | | | | | |
Top   ToC   RFC1123 - Page 43
User Interface:                            |               | | | | | |
  Arbitrary pathnames                      |        |x| | | | |
  Implement "QUOTE" command                |        |x| | | | |
  Transfer control commands immediately    |        | |x| | | |
  Display error messages to user           |        | |x| | | |
    Verbose mode                           |        | |x| | | |
  Maintain synchronization with server     |        | |x| | | |


(1)  For the values shown earlier.

(2)  Here m is number of bits in a memory word.

(3)  Required for host with record-structured file system, optional

(next page on part 3)

Next Section