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Obsoleted by: 959

IEN 149                                                        J. Postel
RFC 765                                                              ISI
                                                               June 1980

                         FILE TRANSFER PROTOCOL


INTRODUCTION

   The objectives of FTP are 1) to promote sharing of files (computer
   programs and/or data), 2) to encourage indirect or implicit (via
   programs) use of remote computers, 3) to shield a user from
   variations in file storage systems among Hosts, and 4) to transfer
   data reliably and efficiently.  FTP, though usable directly by a user
   at a terminal, is designed mainly for use by programs.

   The attempt in this specification is to satisfy the diverse needs of
   users of maxi-Hosts, mini-Hosts, and TIPs, with a simple, and easily
   implemented protocol design.

   This paper assumes knowledge of the following protocols described in
   the ARPA Internet Protocol Handbook.

      The Transmission Control Protocol

      The TELNET Protocol

DISCUSSION

   In this section, the terminology and the FTP model are discussed.
   The terms defined in this section are only those that have special
   significance in FTP.  Some of the terminology is very specific to the
   FTP model; some readers may wish to turn to the section on the FTP
   model while reviewing the terminology.

   TERMINOLOGY

      ASCII

         The ASCII character set as defined in the ARPA Internet
         Protocol Handbook.  In FTP, ASCII characters are defined to be
         the lower half of an eight-bit code set (i.e., the most
         significant bit is zero).

      access controls

         Access controls define users' access privileges to the use of a
         system, and to the files in that system.  Access controls are
         necessary to prevent unauthorized or accidental use of files.
         It is the prerogative of a server-FTP process to invoke access
         controls.




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      byte size

         There are two byte sizes of interest in FTP:  the logical byte
         size of the file, and the transfer byte size used for the
         transmission of the data.  The transfer byte size is always 8
         bits.  The transfer byte size is not necessarily the byte size
         in which data is to be stored in a system, nor the logical byte
         size for interpretation of the structure of the data.

      data connection

         A simplex connection over which data is transferred, in a
         specified mode and type. The data transferred may be a part of
         a file, an entire file or a number of files.  The path may be
         between a server-DTP and a user-DTP, or between two
         server-DTPs.

      data port

         The passive data transfer process "listens" on the data port
         for a connection from the active transfer process in order to
         open the data connection.

      EOF

         The end-of-file condition that defines the end of a file being
         transferred.

      EOR

         The end-of-record condition that defines the end of a record
         being transferred.

      error recovery

         A procedure that allows a user to recover from certain errors
         such as failure of either Host system or transfer process.  In
         FTP, error recovery may involve restarting a file transfer at a
         given checkpoint.

      FTP commands

         A set of commands that comprise the control information flowing
         from the user-FTP to the server-FTP process.






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      file

         An ordered set of computer data (including programs), of
         arbitrary length, uniquely identified by a pathname.

      mode

         The mode in which data is to be transferred via the data
         connection. The mode defines the data format during transfer
         including EOR and EOF.  The transfer modes defined in FTP are
         described in the Section on Transmission Modes.

      NVT

         The Network Virtual Terminal as defined in the TELNET Protocol.

      NVFS

         The Network Virtual File System.  A concept which defines a
         standard network file system with standard commands and
         pathname conventions.  FTP only partially implements the NVFS
         concept at this time.

      page

         A file may be structured as a set of independent parts called
         pages.  FTP supports the transmission of discontinuous files as
         independent indexed pages.

      pathname

         Pathname is defined to be the character string which must be
         input to a file system by a user in order to identify a file.
         Pathname normally contains device and/or directory names, and
         file name specification.  FTP does not yet specify a standard
         pathname convention.  Each user must follow the file naming
         conventions of the file systems involved in the transfer.

      record

         A sequential file may be structured as a number of contiguous
         parts called records.  Record structures are supported by FTP
         but a file need not have record structure.







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      reply

         A reply is an acknowledgment (positive or negative) sent from
         server to user via the TELNET connections in response to FTP
         commands.  The general form of a reply is a completion code
         (including error codes) followed by a text string.  The codes
         are for use by programs and the text is usually intended for
         human users.

      server-DTP

         The data transfer process, in its normal "active" state,
         establishes the data connection with the "listening" data port,
         sets up parameters for transfer and storage, and transfers data
         on command from its PI.  The DTP can be placed in a "passive"
         state to listen for, rather than initiate a, connection on the
         data port.

      server-FTP process

         A process or set of processes which perform the function of
         file transfer in cooperation with a user-FTP process and,
         possibly, another server.  The functions consist of a protocol
         interpreter (PI) and a data transfer process (DTP).

      server-PI

         The protocol interpreter "listens" on Port L for a connection
         from a user-PI and establishes a TELNET communication
         connection.  It receives standard FTP commands from the
         user-PI, sends replies, and governs the server-DTP.

      TELNET connections

         The full-duplex communication path between a user-PI and a
         server-PI, operating according to the TELNET Protocol.

      type

         The data representation type used for data transfer and
         storage.  Type implies certain transformations between the time
         of data storage and data transfer.  The representation types
         defined in FTP are described in the Section on Establishing
         Data Connections.






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      user

         A human being or a process on behalf of a human being wishing
         to obtain file transfer service.  The human user may interact
         directly with a server-FTP process, but use of a user-FTP
         process is preferred since the protocol design is weighted
         towards automata.

      user-DTP

         The data transfer process "listens" on the data port for a
         connection from a server-FTP process.  If two servers are
         transferring data between them, the user-DTP is inactive.

      user-FTP process

         A set of functions including a protocol interpreter, a data
         transfer process and a user interface which together perform
         the function of file transfer in cooperation with one or more
         server-FTP processes.  The user interface allows a local
         language to be used in the command-reply dialogue with the
         user.

      user-PI

         The protocol interpreter initiates the TELNET connection from
         its port U to the server-FTP process, initiates FTP commands,
         and governs the user-DTP if that process is part of the file
         transfer.





















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   THE FTP MODEL

      With the above definitions in mind, the following model (shown in
      Figure 1) may be diagrammed for an FTP service.

                                            -------------
                                            |/---------\|
                                            ||   User  ||    --------
                                            ||Interface|<--->| User |
                                            |\----:----/|    --------
                  ----------                |     V     |
                  |/------\|  FTP Commands  |/---------\|
                  ||Server|<---------------->|   User  ||
                  ||  PI  ||   FTP Replies  ||    PI   ||
                  |\--:---/|                |\----:----/|
                  |   V    |                |     V     |
      --------    |/------\|      Data      |/---------\|    --------
      | File |<--->|Server|<---------------->|  User   |<--->| File |
      |System|    || DTP  ||   Connection   ||   DTP   ||    |System|
      --------    |\------/|                |\---------/|    --------
                  ----------                -------------

                  Server-FTP                   User-FTP

      NOTES: 1. The data connection may be used in either direction.
             2. The data connection need not exist all of the time.

                      Figure 1  Model for FTP Use

      In the model described in Figure 1, the user-protocol interpreter
      initiates the TELNET connection. At the initiation of the user,
      standard FTP commands are generated by the user-PI and transmitted
      to the server process via the TELNET connection.  (The user may
      establish a direct TELNET connection to the server-FTP, from a TIP
      terminal for example, and generate standard FTP commands himself,
      bypassing the user-FTP process.) Standard replies are sent from
      the server-PI to the user-PI over the TELNET connection in
      response to the commands.

      The FTP commands specify the parameters for the data connection
      (data port, transfer mode, representation type, and structure) and
      the nature of file system operation (store, retrieve, append,
      delete, etc.).  The user-DTP or its designate should "listen" on
      the specified data port, and the server initiate the data
      connection and data transfer in accordance with the specified
      parameters.  It should be noted that the data port need not be in




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      the same Host that initiates the FTP commands via the TELNET
      connection, but the user or his user-FTP process must ensure a
      "listen" on the specified data port.  It should also be noted that
      the data connection may be used for simultaneous sending and
      receiving.

      In another situation a user might wish to transfer files between
      two Hosts, neither of which is his local Host. He sets up TELNET
      connections to the two servers and then arranges for a data
      connection between them.  In this manner control information is
      passed to the user-PI but data is transferred between the server
      data transfer processes.  Following is a model of this
      server-server interaction.


                    TELNET     ------------    TELNET
                    ---------->| User-FTP |<-----------
                    |          | User-PI  |           |
                    |          |   "C"    |           |
                    V          ------------           V
            --------------                        --------------
            | Server-FTP |   Data Connection      | Server-FTP |
            |    "A"     |<---------------------->|    "B"     |
            --------------  Port (A)     Port (B) --------------


                                 Figure 2

      The protocol requires that the TELNET connections be open while
      data transfer is in progress.  It is the responsibility of the
      user to request the closing of the TELNET connections when
      finished using the FTP service, while it is the server who takes
      the action.  The server may abort data transfer if the TELNET
      connections are closed without command.

DATA TRANSFER FUNCTIONS

   Files are transferred only via the data connection.  The TELNET
   connection is used for the transfer of commands, which describe the
   functions to be performed, and the replies to these commands (see the
   Section on FTP Replies).  Several commands are concerned with the
   transfer of data between Hosts.  These data transfer commands include
   the MODE command which specify how the bits of the data are to be
   transmitted, and the STRUcture and TYPE commands, which are used to
   define the way in which the data are to be represented. The
   transmission and representation are basically independent but




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   "Stream" transmission mode is dependent on the file structure
   attribute and if "Compressed" transmission mode is used the nature of
   the filler byte depends on the representation type.

   DATA REPRESENTATION AND STORAGE

      Data is transferred from a storage device in the sending Host to a
      storage device in the receiving Host.  Often it is necessary to
      perform certain transformations on the data because data storage
      representations in the two systems are different.  For example,
      NVT-ASCII has different data storage representations in different
      systems.  PDP-10's generally store NVT-ASCII as five 7-bit ASCII
      characters, left-justified in a 36-bit word. 360's store NVT-ASCII
      as 8-bit EBCDIC codes. Multics stores NVT-ASCII as four 9-bit
      characters in a 36-bit word.  It may be desirable to convert
      characters into the standard NVT-ASCII representation when
      transmitting text between dissimilar systems.  The sending and
      receiving sites would have to perform the necessary
      transformations between the standard representation and their
      internal representations.

      A different problem in representation arises when transmitting
      binary data (not character codes) between Host systems with
      different word lengths.  It is not always clear how the sender
      should send data, and the receiver store it.  For example, when
      transmitting 32-bit bytes from a 32-bit word-length system to a
      36-bit word-length system, it may be desirable (for reasons of
      efficiency and usefulness) to store the 32-bit bytes
      right-justified in a 36-bit word in the latter system.  In any
      case, the user should have the option of specifying data
      representation and transformation functions.  It should be noted
      that FTP provides for very limited data type representations.
      Transformations desired beyond this limited capability should be
      performed by the user directly.

      Data representations are handled in FTP by a user specifying a
      representation type.  This type may implicitly (as in ASCII or
      EBCDIC) or explicitly (as in Local byte) define a byte size for
      interpretation which is referred to as the "logical byte size."
      This has nothing to do with the byte size used for transmission
      over the data connection, called the "transfer byte size", and the
      two should not be confused.  For example, NVT-ASCII has a logical
      byte size of 8 bits.  If the type is Local byte, then the TYPE
      command has an obligatory second parameter specifying the logical
      byte size.  The transfer byte size is always 8 bits.





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      The types ASCII and EBCDIC also take a second (optional)
      parameter; this is to indicate what kind of vertical format
      control, if any, is associated with a file.  The following data
      representation types are defined in FTP:

         ASCII Format

            This is the default type and must be accepted by all FTP
            implementations.  It is intended primarily for the transfer
            of text files, except when both Hosts would find the EBCDIC
            type more convenient.

            The sender converts the data from his internal character
            representation to the standard 8-bit NVT-ASCII
            representation (see the TELNET specification).  The receiver
            will convert the data from the standard form to his own
            internal form.

            In accordance with the NVT standard, the <CRLF> sequence
            should be used, where necessary, to denote the end of a line
            of text.  (See the discussion of file structure at the end
            of the Section on Data Representation and Storage).

            Using the standard NVT-ASCII representation means that data
            must be interpreted as 8-bit bytes.

            The Format parameter for ASCII and EBCDIC types is discussed
            below.

         EBCDIC Format

            This type is intended for efficient transfer between Hosts
            which use EBCDIC for their internal character
            representation.

            For transmission the data are represented as 8-bit EBCDIC
            characters.  The character code is the only difference
            between the functional specifications of EBCDIC and ASCII
            types.

            End-of-line (as opposed to end-of-record--see the discussion
            of structure) will probably be rarely used with EBCDIC type
            for purposes of denoting structure, but where it is
            necessary the <NL> character should be used.






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      A character file may be transferred to a Host for one of three
      purposes: for printing, for storage and later retrieval, or for
      processing.  If a file is sent for printing, the receiving Host
      must know how the vertical format control is represented.  In the
      second case, it must be possible to store a file at a Host and
      then retrieve it later in exactly the same form.  Finally, it
      ought to be possible to move a file from one Host to another and
      process the file at the second Host without undue trouble.  A
      single ASCII or EBCDIC format does not satisfy all these
      conditions and so these types have a second parameter specifying
      one of the following three formats:

         Non-print

            This is the default format to be used if the second (format)
            parameter is omitted.  Non-print format must be accepted by
            all FTP implementations.

            The file need contain no vertical format information.  If it
            is passed to a printer process, this process may assume
            standard values for spacing and margins.

            Normally, this format will be used with files destined for
            processing or just storage.

         TELNET Format Controls

            The file contains ASCII/EBCDIC vertical format controls
            (i.e., <CR>, <LF>, <NL>, <VT>, <FF>) which the printer
            process will interpret appropriately.  <CRLF>, in exactly
            this sequence, also denotes end-of-line.

         Carriage Control (ASA)

            The file contains ASA (FORTRAN) vertical format control
            characters.  (See RFC 740 Appendix C and Communications of
            the ACM, Vol. 7, No. 10, 606 (Oct. 1964)).  In a line or a
            record, formatted according to the ASA Standard, the first
            character is not to be printed.  Instead it should be used
            to determine the vertical movement of the paper which should
            take place before the rest of the record is printed.









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            The ASA Standard specifies the following control characters:

               Character     Vertical Spacing

               blank         Move paper up one line
               0             Move paper up two lines
               1             Move paper to top of next page
               +             No movement, i.e., overprint

            Clearly there must be some way for a printer process to
            distinguish the end of the structural entity.  If a file has
            record structure (see below) this is no problem; records
            will be explicitly marked during transfer and storage.  If
            the file has no record structure, the <CRLF> end-of-line
            sequence is used to separate printing lines, but these
            format effectors are overridden by the ASA controls.

         Image

            The data are sent as contiguous bits which, for transfer,
            are packed into the 8-bit transfer bytes.  The receiving
            site must store the data as contiguous bits.  The structure
            of the storage system might necessitate the padding of the
            file (or of each record, for a record-structured file) to
            some convenient boundary (byte, word or block).  This
            padding, which must be all zeros, may occur only at the end
            of the file (or at the end of each record) and there must be
            a way of identifying the padding bits so that they may be
            stripped off if the file is retrieved.  The padding
            transformation should be well publicized to enable a user to
            process a file at the storage site.

            Image type is intended for the efficient storage and
            retrieval of files and for the transfer of binary data.  It
            is recommended that this type be accepted by all FTP
            implementations.

         Local byte Byte size

            The data is transferred in logical bytes of the size
            specified by the obligatory second parameter, Byte size.
            The value of Byte size must be a decimal integer; there is
            no default value.  The logical byte size is not necessarily
            the same as the transfer byte size.  If there is a






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            difference in byte sizes, then the logical bytes should be
            packed contiguously, disregarding transfer byte boundaries
            and with any necessary padding at the end.

            When the data reaches the receiving Host it will be
            transformed in a manner dependent on the logical byte size
            and the particular Host.  This transformation must be
            invertible (that is an identical file can be retrieved if
            the same parameters are used) and should be well publicized
            by the FTP implementors.

            For example, a user sending 36-bit floating-point numbers to
            a Host with a 32-bit word could send his data as Local byte
            with a logical byte size of 36.  The receiving Host would
            then be expected to store the logical bytes so that they
            could be easily manipulated; in this example putting the
            36-bit logical bytes into 64-bit double words should
            suffice.

            Another example, a pair of hosts with a 36-bit word size may
            send data to one another in words by using TYPE L 36.  The
            data would be sent in the 8-bit transmission bytes packed so
            that 9 transmission bytes carried two host words.

      A note of caution about parameters:  a file must be stored and
      retrieved with the same parameters if the retrieved version is to
      be identical to the version originally transmitted.  Conversely,
      FTP implementations must return a file identical to the original
      if the parameters used to store and retrieve a file are the same.

      In addition to different representation types, FTP allows the
      structure of a file to be specified.  Three file structures are
      defined in FTP:

         file-structure, where there is no internal structure and the
                           file is considered to be a continuous
                           sequence of data bytes,

         record-structure, where the file is made up of sequential
                           records,

         and page-structure, where the file is made up of independent
                           indexed pages.

      File-structure is the default, to be assumed if the STRUcture
      command has not been used but both file and record structures must




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      be accepted for "text" files (i.e., files with TYPE ASCII or
      EBCDIC) by all FTP implementations.  The structure of a file will
      affect both the transfer mode of a file (see the Section on
      Transmission Modes) and the interpretation and storage of the
      file.

      The "natural" structure of a file will depend on which Host stores
      the file.  A source-code file will usually be stored on an IBM 360
      in fixed length records but on a PDP-10 as a stream of characters
      partitioned into lines, for example by <CRLF>.  If the transfer of
      files between such disparate sites is to be useful, there must be
      some way for one site to recognize the other's assumptions about
      the file.

      With some sites being naturally file-oriented and others naturally
      record-oriented there may be problems if a file with one structure
      is sent to a Host oriented to the other.  If a text file is sent
      with record-structure to a Host which is file oriented, then that
      Host should apply an internal transformation to the file based on
      the record structure.  Obviously this transformation should be
      useful but it must also be invertible so that an identical file
      may be retrieved using record structure.

      In the case of a file being sent with file-structure to a
      record-oriented Host, there exists the question of what criteria
      the Host should use to divide the file into records which can be
      processed locally.  If this division is necessary the FTP
      implementation should use the end-of-line sequence, <CRLF> for
      ASCII, or <NL> for EBCDIC text files, as the delimiter.  If an FTP
      implementation adopts this technique, it must be prepared to
      reverse the transformation if the file is retrieved with
      file-structure.

      Page Structure

         To transmit files that are discontinuous FTP defines a page
         structure.  Files of this type are sometimes know as "random
         access files" or even as "holey files".  In these files there
         is sometimes other information associated with the file as a
         whole (e.g., a file descriptor), or with a section of the file
         (e.g., page access controls), or both.  In FTP, the sections of
         the file are called pages.

         To provide for various page sizes and associated information
         each page is sent with a page header.  The page header has the
         following defined fields:




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            Header Length

               The number of logical bytes in the page header including
               this byte.  The minimum header length is 4.

            Page Index

               The logical page number of this section of the file.
               This is not the transmission sequence number of this
               page, but the index used to identify this page of the
               file.

            Data Length

               The number of logical bytes in the page data.  The
               minimum data length is 0.

            Page Type

               The type of page this is.  The following page types are
               defined:

                  0 = Last Page

                     This is used to indicate the end of a paged
                     structured transmission.  The header length must be
                     4, and the data length must be 0.

                  1 = Simple Page

                     This is the normal type for simple paged files with
                     no page level associated control information.  The
                     header length must be 4.

                  2 = Descriptor Page

                     This type is used to transmit the descriptive
                     information for the file as a whole.

                  3 = Access Controled Page

                     This is type includes an additional header field
                     for paged files with page level access control
                     information.  The header length must be 5.






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            Optional Fields

               Further header fields may be used to supply per page
               control information, for example, per page access
               control.

         All fields are one logical byte in length.  The logical byte
         size is specified by the TYPE command.

   ESTABLISHING DATA CONNECTIONS

      The mechanics of transferring data consists of setting up the data
      connection to the appropriate ports and choosing the parameters
      for transfer.  Both the user and the server-DTPs have a default
      data port.  The user-process default data port is the same as the
      control connection port, i.e., U.  The server-process default data
      port is the port adjacent to the control connection port, i.e.,
      L-1.

      The transfer byte size is 8-bit bytes.  This byte size is relevant
      only for the actual transfer of the data; it has no bearing on
      representation of the data within a Host's file system.

      The passive data transfer process (this may be a user-DTP or a
      second server-DTP) shall "listen" on the data port prior to
      sending a transfer request command.  The FTP request command
      determines the direction of the data transfer.  The server, upon
      receiving the transfer request, will initiate the data connection
      to the port.  When the connection is established, the data
      transfer begins between DTP's, and the server-PI sends a
      confirming reply to the user-PI.

      It is possible for the user to specify an alternate data port by
      use of the PORT command.  He might want a file dumped on a TIP
      line printer or retrieved from a third party Host.  In the latter
      case the user-PI sets up TELNET connections with both server-PI's.
      One server is then told (by an FTP command) to "listen" for a
      connection which the other will initiate.  The user-PI sends one
      server-PI a PORT command indicating the data port of the other.
      Finally both are sent the appropriate transfer commands.  The
      exact sequence of commands and replies sent between the
      user-controller and the servers is defined in the Section on FTP
      Replies.

      In general it is the server's responsibility to maintain the data
      connection--to initiate it and to close it.  The exception to this




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      is when the user-DTP is sending the data in a transfer mode that
      requires the connection to be closed to indicate EOF.  The server
      MUST close the data connection under the following conditions:

         1. The server has completed sending data in a transfer mode
            that requires a close to indicate EOF.

         2. The server receives an ABORT command from the user.

         3. The port specification is changed by a command from the
            user.

         4. The TELNET connection is closed legally or otherwise.

         5. An irrecoverable error condition occurs.

      Otherwise the close is a server option, the exercise of which he
      must indicate to the user-process by an appropriate reply.

   TRANSMISSION MODES

      The next consideration in transferring data is choosing the
      appropriate transmission mode.  There are three modes: one which
      formats the data and allows for restart procedures; one which also
      compresses the data for efficient transfer; and one which passes
      the data with little or no processing.  In this last case the mode
      interacts with the structure attribute to determine the type of
      processing.  In the compressed mode the representation type
      determines the filler byte.

      All data transfers must be completed with an end-of-file (EOF)
      which may be explicitly stated or implied by the closing of the
      data connection.  For files with record structure, all the
      end-of-record markers (EOR) are explicit, including the final one.
      For files transmitted in page structure a "last-page" page type is
      used.

      NOTE:  In the rest of this section, byte means "transfer byte"
      except where explicitly stated otherwise.

      For the purpose of standardized transfer, the sending Host will
      translate his internal end of line or end of record denotation
      into the representation prescribed by the transfer mode and file
      structure, and the receiving Host will perform the inverse
      translation to his internal denotation.  An IBM 360 record count
      field may not be recognized at another Host, so the end of record




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      information may be transferred as a two byte control code in
      Stream mode or as a flagged bit in a Block or Compressed mode
      descriptor. End of line in an ASCII or EBCDIC file with no record
      structure should be indicated by <CRLF> or <NL>, respectively.
      Since these transformations imply extra work for some systems,
      identical systems transferring non-record structured text files
      might wish to use a binary representation and stream mode for the
      transfer.

      The following transmission modes are defined in FTP:

         STREAM

            The data is transmitted as a stream of bytes.  There is no
            restriction on the representation type used; record
            structures are allowed.

            In a record structured file EOR and EOF will each be
            indicated by a two-byte control code.  The first byte of the
            control code will be all ones, the escape character.  The
            second byte will have the low order bit on and zeros
            elsewhere for EOR and the second low order bit on for EOF;
            that is, the byte will have value 1 for EOR and value 2 for
            EOF.  EOR and EOF may be indicated together on the last byte
            transmitted by turning both low order bits on, i.e., the
            value 3.  If a byte of all ones was intended to be sent as
            data, it should be repeated in the second byte of the
            control code.

            If the structure is file structure, the EOF is indicated by
            the sending Host closing the data connection and all bytes
            are data bytes.

         BLOCK

            The file is transmitted as a series of data blocks preceded
            by one or more header bytes.  The header bytes contain a
            count field, and descriptor code.  The count field indicates
            the total length of the data block in bytes, thus marking
            the beginning of the next data block (there are no filler
            bits). The descriptor code defines:  last block in the file
            (EOF) last block in the record (EOR), restart marker (see
            the Section on Error Recovery and Restart) or suspect data
            (i.e., the data being transferred is suspected of errors and
            is not reliable).  This last code is NOT intended for error
            control within FTP.  It is motivated by the desire of sites




                                   17


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            exchanging certain types of data (e.g., seismic or weather
            data) to send and receive all the data despite local errors
            (such as "magnetic tape read errors"), but to indicate in
            the transmission that certain portions are suspect).  Record
            structures are allowed in this mode, and any representation
            type may be used.

            The header consists of the three bytes.  Of the 24 bits of
            header information, the 16 low order bits shall represent
            byte count, and the 8 high order bits shall represent
            descriptor codes as shown below.

            Block Header

               +----------------+----------------+----------------+
               | Descriptor     |    Byte Count                   |
               |         8 bits |                      16 bits    |
               +----------------+----------------+----------------+


            The descriptor codes are indicated by bit flags in the
            descriptor byte.  Four codes have been assigned, where each
            code number is the decimal value of the corresponding bit in
            the byte.

               Code     Meaning

                128     End of data block is EOR
                 64     End of data block is EOF
                 32     Suspected errors in data block
                 16     Data block is a restart marker



            With this encoding more than one descriptor coded condition
            may exist for a particular block.  As many bits as necessary
            may be flagged.

            The restart marker is embedded in the data stream as an
            integral number of 8-bit bytes representing printable
            characters in the language being used over the TELNET
            connection (e.g., default--NVT-ASCII).  <SP> (Space, in the
            appropriate language) must not be used WITHIN a restart
            marker.






                                   18


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            For example, to transmit a six-character marker, the
            following would be sent:

               +--------+--------+--------+
               |Descrptr|  Byte count     |
               |code= 16|             = 6 |
               +--------+--------+--------+

               +--------+--------+--------+
               | Marker | Marker | Marker |
               | 8 bits | 8 bits | 8 bits |
               +--------+--------+--------+

               +--------+--------+--------+
               | Marker | Marker | Marker |
               | 8 bits | 8 bits | 8 bits |
               +--------+--------+--------+


         COMPRESSED

            There are three kinds of information to be sent:  regular
            data, sent in a byte string; compressed data, consisting of
            replications or filler; and control information, sent in a
            two-byte escape sequence.  If n>0 bytes (up to 127) of
            regular data are sent, these n bytes are preceded by a byte
            with the left-most bit set to 0 and the right-most 7 bits
            containing the number n.

            Byte string:

                1       7                8                     8
               +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
               |0|       n     | |    d(1)       | ... |      d(n)     |
               +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
                                             ^             ^
                                             |---n bytes---|
                                                 of data

               String of n data bytes d(1),..., d(n)
               Count n must be positive.

            To compress a string of n replications of the data byte d,
            the following 2 bytes are sent:






                                   19


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            Replicated Byte:

                 2       6               8
               +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
               |1 0|     n     | |       d       |
               +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+

            A string of n filler bytes can be compressed into a single
            byte, where the filler byte varies with the representation
            type.  If the type is ASCII or EBCDIC the filler byte is
            <SP> (Space, ASCII code 32., EBCDIC code 64).  If the type
            is Image or Local byte the filler is a zero byte.

            Filler String:

                 2       6
               +-+-+-+-+-+-+-+-+
               |1 1|     n     |
               +-+-+-+-+-+-+-+-+

            The escape sequence is a double byte, the first of which is
            the escape byte (all zeros) and the second of which contains
            descriptor codes as defined in Block mode.  The descriptor
            codes have the same meaning as in Block mode and apply to
            the succeeding string of bytes.

            Compressed mode is useful for obtaining increased bandwidth
            on very large network transmissions at a little extra CPU
            cost.  It can be most effectively used to reduce the size of
            printer files such as those generated by RJE Hosts.

   ERROR RECOVERY AND RESTART

      There is no provision for detecting bits lost or scrambled in data
      transfer; this level of error control is handled by the TCP.
      However, a restart procedure is provided to protect users from
      gross system failures (including failures of a Host, an
      FTP-process, or the underlying network).

      The restart procedure is defined only for the block and compressed
      modes of data transfer.  It requires the sender of data to insert
      a special marker code in the data stream with some marker
      information.  The marker information has meaning only to the
      sender, but must consist of printable characters in the default or
      negotiated language of the TELNET connection (ASCII or EBCDIC).
      The marker could represent a bit-count, a record-count, or any




                                   20


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



      other information by which a system may identify a data
      checkpoint.  The receiver of data, if it implements the restart
      procedure, would then mark the corresponding position of this
      marker in the receiving system, and return this information to the
      user.

      In the event of a system failure, the user can restart the data
      transfer by identifying the marker point with the FTP restart
      procedure.  The following example illustrates the use of the
      restart procedure.

      The sender of the data inserts an appropriate marker block in the
      data stream at a convenient point.  The receiving Host marks the
      corresponding data point in its file system and conveys the last
      known sender and receiver marker information to the user, either
      directly or over the TELNET connection in a 110 reply (depending
      on who is the sender).  In the event of a system failure, the user
      or controller process restarts the server at the last server
      marker by sending a restart command with server's marker code as
      its argument.  The restart command is transmitted over the TELNET
      connection and is immediately followed by the command (such as
      RETR, STOR or LIST) which was being executed when the system
      failure occurred.

FILE TRANSFER FUNCTIONS

   The communication channel from the user-PI to the server-PI is
   established by a TCP connection from the user to a standard server
   port.  The user protocol interpreter is responsible for sending FTP
   commands and interpreting the replies received; the server-PI
   interprets commands, sends replies and directs its DTP to set up the
   data connection and transfer the data.  If the second party to the
   data transfer (the passive transfer process) is the user-DTP then it
   is governed through the internal protocol of the user-FTP Host; if it
   is a second server-DTP then it is governed by its PI on command from
   the user-PI.  The FTP replies are discussed in the next section.  In
   the description of a few of the commands in this section it is
   helpful to be explicit about the possible replies.

   FTP COMMANDS

      ACCESS CONTROL COMMANDS

         The following commands specify access control identifiers
         (command codes are shown in parentheses).





                                   21


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         USER NAME (USER)

            The argument field is a TELNET string identifying the user.
            The user identification is that which is required by the
            server for access to its file system.  This command will
            normally be the first command transmitted by the user after
            the TELNET connections are made (some servers may require
            this).  Additional identification information in the form of
            a password and/or an account command may also be required by
            some servers.  Servers may allow a new USER command to be
            entered at any point in order to change the access control
            and/or accounting information.  This has the effect of
            flushing any user, password, and account information already
            supplied and beginning the login sequence again.  All
            transfer parameters are unchanged and any file transfer in
            progress is completed under the old account.

         PASSWORD (PASS)

            The argument field is a TELNET string identifying the user's
            password.  This command must be immediately preceded by the
            user name command, and, for some sites, completes the user's
            identification for access control.  Since password
            information is quite sensitive, it is desirable in general
            to "mask" it or suppress typeout.  It appears that the
            server has no foolproof way to achieve this.  It is
            therefore the responsibility of the user-FTP process to hide
            the sensitive password information.

         ACCOUNT (ACCT)

            The argument field is a TELNET string identifying the user's
            account.  The command is not necessarily related to the USER
            command, as some sites may require an account for login and
            others only for specific access, such as storing files.  In
            the latter case the command may arrive at any time.

            There are reply codes to differentiate these cases for the
            automaton: when account information is required for login,
            the response to a successful PASSword command is reply code
            332.  On the other hand, if account information is NOT
            required for login, the reply to a successful PASSword
            command is 230; and if the account information is needed for
            a command issued later in the dialogue, the server should






                                   22


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



            return a 332 or 532 reply depending on whether he stores
            (pending receipt of the ACCounT command) or discards the
            command, respectively.

         REINITIALIZE (REIN)

            This command terminates a USER, flushing all I/O and account
            information, except to allow any transfer in progress to be
            completed.  All parameters are reset to the default settings
            and the TELNET connection is left open.  This is identical
            to the state in which a user finds himself immediately after
            the TELNET connection is opened.  A USER command may be
            expected to follow.

         LOGOUT (QUIT)

            This command terminates a USER and if file transfer is not
            in progress, the server closes the TELNET connection.  If
            file transfer is in progress, the connection will remain
            open for result response and the server will then close it.
            If the user-process is transferring files for several USERs
            but does not wish to close and then reopen connections for
            each, then the REIN command should be used instead of QUIT.

            An unexpected close on the TELNET connection will cause the
            server to take the effective action of an abort (ABOR) and a
            logout (QUIT).

      TRANSFER PARAMETER COMMANDS

         All data transfer parameters have default values, and the
         commands specifying data transfer parameters are required only
         if the default parameter values are to be changed.  The default
         value is the last specified value, or if no value has been
         specified, the standard default value as stated here.  This
         implies that the server must "remember" the applicable default
         values.  The commands may be in any order except that they must
         precede the FTP service request.  The following commands
         specify data transfer parameters.

         DATA PORT (PORT)

            The argument is a HOST-PORT specification for the data port
            to be used in data connection.  There defaults for both the
            user and server data ports, and under normal circumstances
            this command and its reply are not needed.  If this command




                                   23


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            is used  the argument is the concatenation of a 32-bit
            internet host address and a 16-bit TCP port address.  This
            address information is broken into 8-bit fields and the
            value of each field is transmitted as a decimal number (in
            character string representation).  The fields are separated
            by commas.  A port command would be:

               PORT h1,h2,h3,h4,p1,p2

            where, h1 is the high order 8 bits of the internet host
            address.

         PASSIVE (PASV)

            This command requests the server-DTP to "listen" on a data
            port (which is not its default data port) and to wait for a
            connection rather than initiate one upon receipt of a
            transfer command.  The response to this command includes the
            host and port address this server is listening on.

         REPRESENTATION TYPE (TYPE)

            The argument specifies the representation type as described
            in the Section on Data Representation and Storage.  Several
            types take a second parameter.  The first parameter is
            denoted by a single TELNET character, as is the second
            Format parameter for ASCII and EBCDIC; the second parameter
            for local byte is a decimal integer to indicate Bytesize.
            The parameters are separated by a <SP> (Space, ASCII code
            32.).

            The following codes are assigned for type:

                         \    /
               A - ASCII |    | N - Non-print
                         |-><-| T - TELNET format effectors
               E - EBCDIC|    | C - Carriage Control (ASA)
                         /    \
               I - Image

               L <byte size> - Local byte Byte size

            The default representation type is ASCII Non-print.  If the
            Format parameter is changed, and later just the first
            argument is changed, Format then returns to the Non-print
            default.




                                   24


IEN 149                                                        June 1980
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         FILE STRUCTURE (STRU)

            The argument is a single TELNET character code specifying
            file structure described in the Section on Data
            Representation and Storage.

            The following codes are assigned for structure:

               F - File (no record structure)
               R - Record structure
               P - Page structure

            The default structure is File.

         TRANSFER MODE (MODE)

            The argument is a single TELNET character code specifying
            the data transfer modes described in the Section on
            Transmission Modes.

            The following codes are assigned for transfer modes:

               S - Stream
               B - Block
               C - Compressed

            The default transfer mode is Stream.

      FTP SERVICE COMMANDS

         The FTP service commands define the file transfer or the file
         system function requested by the user.  The argument of an FTP
         service command will normally be a pathname.  The syntax of
         pathnames must conform to server site conventions (with
         standard defaults applicable), and the language conventions of
         the TELNET connection.  The suggested default handling is to
         use the last specified device, directory or file name, or the
         standard default defined for local users.  The commands may be
         in any order except that a "rename from" command must be
         followed by a "rename to" command and the restart command must
         be followed by the interrupted service command.  The data, when
         transferred in response to FTP service commands, shall always
         be sent over the data connection, except for certain
         informative replies.  The following commands specify FTP
         service requests:





                                   25


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         RETRIEVE (RETR)

            This command causes the server-DTP to transfer a copy of the
            file, specified in the pathname, to the server- or user-DTP
            at the other end of the data connection.  The status and
            contents of the file at the server site shall be unaffected.

         STORE (STOR)

            This command causes the server-DTP to accept the data
            transferred via the data connection and to store the data as
            a file at the server site.  If the file specified in the
            pathname exists at the server site then its contents shall
            be replaced by the data being transferred.  A new file is
            created at the server site if the file specified in the
            pathname does not already exist.

         APPEND (with create) (APPE)

            This command causes the server-DTP to accept the data
            transferred via the data connection and to store the data in
            a file at the server site.  If the file specified in the
            pathname exists at the server site, then the data shall be
            appended to that file; otherwise the file specified in the
            pathname shall be created at the server site.

         MAIL FILE (MLFL)

            The intent of this command is to enable a user at the user
            site to mail data (in form of a file) to another user at the
            server site.  It should be noted that the files to be mailed
            are transmitted via the data connection in ASCII or EBCDIC
            type.  (It is the user's responsibility to ensure that the
            type is correct.)  These files should be inserted into the
            destination user's mailbox by the server in accordance with
            serving Host mail conventions.  The mail may be marked as
            sent from the particular user HOST and the user specified by
            the 'USER' command.  The argument field may contain a Host
            system ident, or it may be empty.  If the argument field is
            empty or blank (one or more spaces), then the mail is
            destined for a printer or other designated place for general
            delivery site mail.








                                   26


IEN 149                                                        June 1980
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         MAIL (MAIL)

            This command allows a user to send mail that is NOT in a
            file over the TELNET connection.  The argument field may
            contain system ident, or it may be empty.  The ident is
            defined as above for the MLFL command.  After the 'MAIL'
            command is received, the server is to treat the following
            lines as text of the mail sent by the user.  The mail text
            is to be terminated by a line containing only a single
            period, that is, the character sequence "CRLF.CRLF".  It is
            suggested that a modest volume of mail service should be
            free; i.e., it may be entered before a USER command.

         MAIL SEND TO TERMINAL (MSND)

            This command is like the MAIL command, except that the data
            is displayed on the addressed user's terminal, if such
            access is currently allowed, otherwise an error is returned.

         MAIL SEND TO TERMINAL OR MAILBOX (MSOM)

            This command is like the MAIL command, except that the data
            is displayed on the addressed user's terminal, if such
            access is currently allowed, otherwise the data is placed in
            the user's mailbox.

         MAIL SEND TO TERMINAL AND MAILBOX (MSAM)

            This command is like the MAIL command, except that the data
            is displayed on the addressed user's terminal, if such
            access is currently allowed, and, in any case, the data is
            placed in the user's mailbox.

         MAIL RECIPIENT SCHEME QUESTION (MRSQ)

            This FTP command is used to select a scheme for the
            transmission of mail to several users at the same host.  The
            schemes are to list the recipients first, or to send the
            mail first.

         MAIL RECIPIENT (MRCP)

            This command is used to identify the individual recipients
            of the mail in the transmission of mail for multiple users
            at one host.





                                   27


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         ALLOCATE (ALLO)

            This command may be required by some servers to reserve
            sufficient storage to accommodate the new file to be
            transferred.  The argument shall be a decimal integer
            representing the number of bytes (using the logical byte
            size) of storage to be reserved for the file.  For files
            sent with record or page structure a maximum record or page
            size (in logical bytes) might also be necessary; this is
            indicated by a decimal integer in a second argument field of
            the command.  This second argument is optional, but when
            present should be separated from the first by the three
            TELNET characters <SP> R <SP>.  This command shall be
            followed by a STORe or APPEnd command.  The ALLO command
            should be treated as a NOOP (no operation) by those servers
            which do not require that the maximum size of the file be
            declared beforehand, and those servers interested in only
            the maximum record or page size should accept a dummy value
            in the first argument and ignore it.

         RESTART (REST)

            The argument field represents the server marker at which
            file transfer is to be restarted.  This command does not
            cause file transfer but "spaces" over the file to the
            specified data checkpoint.  This command shall be
            immediately followed by the appropriate FTP service command
            which shall cause file transfer to resume.

         RENAME FROM (RNFR)

            This command specifies the file which is to be renamed.
            This command must be immediately followed by a "rename to"
            command specifying the new file pathname.

         RENAME TO (RNTO)

            This command specifies the new pathname of the file
            specified in the immediately preceding "rename from"
            command.  Together the two commands cause a file to be
            renamed.

         ABORT (ABOR)

            This command tells the server to abort the previous FTP
            service command and any associated transfer of data.  The




                                   28


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



            abort command may require "special action", as discussed in
            the Section on FTP Commands, to force recognition by the
            server.  No action is to be taken if the previous command
            has been completed (including data transfer).  The TELNET
            connection is not to be closed by the server, but the data
            connection must be closed.

            There are two cases for the server upon receipt of this
            command: (1) the FTP service command was already completed,
            or (2) the FTP service command is still in progress.

               In the first case, the server closes the data connection
               (if it is open) and responds with a 226 reply, indicating
               that the abort command was successfully processed.

               In the second case, the server aborts the FTP service in
               progress and closes the data connection, returning a 426
               reply to indicate that the service request terminated in
               abnormally.  The server then sends a 226 reply,
               indicating that the abort command was successfully
               processed.

         DELETE (DELE)

            This command causes the file specified in the pathname to be
            deleted at the server site.  If an extra level of protection
            is desired (such as the query, "DO you really wish to
            delete?"), it should be provided by the user-FTP process.

         CHANGE WORKING DIRECTORY (CWD)

            This command allows the user to work with a different
            directory or dataset for file storage or retrieval without
            altering his login or accounting information.  Transfer
            parameters are similarly unchanged.  The argument is a
            pathname specifying a directory or other system dependent
            file group designator.

         LIST (LIST)

            This command causes a list to be sent from the server to the
            passive DTP.  If the pathname specifies a directory, the
            server should transfer a list of files in the specified
            directory.  If the pathname specifies a file then the server
            should send current information on the file.  A null
            argument implies the user's current working or default




                                   29


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            directory.  The data transfer is over the data connection in
            type ASCII or type EBCDIC.  (The user must ensure that the
            TYPE is appropriately ASCII or EBCDIC).

         NAME-LIST (NLST)

            This command causes a directory listing to be sent from
            server to user site.  The pathname should specify a
            directory or other system-specific file group descriptor; a
            null argument implies the current directory.  The server
            will return a stream of names of files and no other
            information.  The data will be transferred in ASCII or
            EBCDIC type over the data connection as valid pathname
            strings separated by <CRLF> or <NL>.  (Again the user must
            ensure that the TYPE is correct.)

         SITE PARAMETERS (SITE)

            This command is used by the server to provide services
            specific to his system that are essential to file transfer
            but not sufficiently universal to be included as commands in
            the protocol.  The nature of these services and the
            specification of their syntax can be stated in a reply to
            the HELP SITE command.

         STATUS (STAT)

            This command shall cause a status response to be sent over
            the TELNET connection in the form of a reply.  The command
            may be sent during a file transfer (along with the TELNET IP
            and Synch signals--see the Section on FTP Commands) in which
            case the server will respond with the status of the
            operation in progress, or it may be sent between file
            transfers.  In the latter case the command may have an
            argument field.  If the argument is a pathname, the command
            is analogous to the "list" command except that data shall be
            transferred over the TELNET connection.  If a partial
            pathname is given, the server may respond with a list of
            file names or attributes associated with that specification.
            If no argument is given, the server should return general
            status information about the server FTP process.  This
            should include current values of all transfer parameters and
            the status of connections.







                                   30


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



         HELP (HELP)

            This command shall cause the server to send helpful
            information regarding its implementation status over the
            TELNET connection to the user.  The command may take an
            argument (e.g., any command name) and return more specific
            information as a response.  The reply is type 211 or 214.
            It is suggested that HELP be allowed before entering a USER
            command. The server may use this reply to specify
            site-dependent parameters, e.g., in response to HELP SITE.

         NOOP (NOOP)

            This command does not affect any parameters or previously
            entered commands. It specifies no action other than that the
            server send an OK reply.

      The File Transfer Protocol follows the specifications of the
      TELNET protocol for all communications over the TELNET connection.
      Since, the language used for TELNET communication may be a
      negotiated option, all references in the next two sections will be
      to the "TELNET language" and the corresponding "TELNET end of line
      code".  Currently one may take these to mean NVT-ASCII and <CRLF>.
      No other specifications of the TELNET protocol will be cited.

      FTP commands are "TELNET strings" terminated by the "TELNET end of
      line code".  The command codes themselves are alphabetic
      characters terminated by the character <SP> (Space) if parameters
      follow and TELNET-EOL otherwise.  The command codes and the
      semantics of commands are described in this section; the detailed
      syntax of commands is specified in the Section on Commands, the
      reply sequences are discussed in the Section on Sequencing of
      Commands and Replies, and scenarios illustrating the use of
      commands are provided in the Section on Typical FTP Scenarios.

      FTP commands may be partitioned as those specifying access-control
      identifiers, data transfer parameters, or FTP service requests.
      Certain commands (such as ABOR, STAT, QUIT) may be sent over the
      TELNET connection while a data transfer is in progress.  Some
      servers may not be able to monitor the TELNET and data connections
      simultaneously, in which case some special action will be
      necessary to get the server's attention.  The exact form of the
      "special action" is undefined; but the following ordered format is
      tentatively recommended:






                                   31


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         1. User system inserts the TELNET "Interrupt Process" (IP)
            signal in the TELNET stream.

         2. User system sends the TELNET "Synch" signal

         3. User system inserts the command (e.g., ABOR) in the TELNET
            stream.

         4. Server PI,, after receiving "IP", scans the TELNET stream
            for EXACTLY ONE FTP command.

      (For other servers this may not be necessary but the actions
      listed above should have no unusual effect.)

   FTP REPLIES

      Replies to File Transfer Protocol commands are devised to ensure
      the synchronization of requests and actions in the process of file
      transfer, and to guarantee that the user process always knows the
      state of the Server. Every command must generate at least one
      reply, although there may be more than one; in the latter case,
      the multiple replies must be easily distinguished.  In addition,
      some commands occur in sequential groups, such as USER, PASS and
      ACCT, or RNFR and RNTO.  The replies show the existence of an
      intermediate state if all preceding commands have been successful.
      A failure at any point in the sequence necessitates the repetition
      of the entire sequence from the beginning.

         The details of the command-reply sequence are made explicit in
         a set of state diagrams below.

      An FTP reply consists of a three digit number (transmitted as
      three alphanumeric characters) followed by some text.  The number
      is intended for use by automata to determine what state to enter
      next; the text is intended for the human user.  It is intended
      that the three digits contain enough encoded information that the
      user-process (the User-PI) will not need to examine the text and
      may either discard it or pass it on to the user, as appropriate.
      In particular, the text may be server-dependent, so there are
      likely to be varying texts for each reply code.

      Formally, a reply is defined to contain the 3-digit code, followed
      by Space <SP>, followed by one line of text (where some maximum
      line length has been specified), and terminated by the TELNET
      end-of-line code.  There will be cases, however, where the text is
      longer than a single line.  In these cases the complete text must




                                   32


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



      be bracketed so the User-process knows when it may stop reading
      the reply (i.e. stop processing input on the TELNET connection)
      and go do other things.  This requires a special format on the
      first line to indicate that more than one line is coming, and
      another on the last line to designate it as the last.  At least
      one of these must contain the appropriate reply code to indicate
      the state of the transaction.  To satisfy all factions it was
      decided that both the first and last line codes should be the
      same.

         Thus the format for multi-line replies is that the first line
         will begin with the exact required reply code, followed
         immediately by a Hyphen, "-" (also known as Minus), followed by
         text.  The last line will begin with the same code, followed
         immediately by Space <SP>, optionally some text, and the TELNET
         end-of-line code.

            For example:
                                123-First line
                                Second line
                                  234 A line beginning with numbers
                                123 The last line

         The user-process then simply needs to search for the second
         occurrence of the same reply code, followed by <SP> (Space), at
         the beginning of a line, and ignore all intermediary lines.  If
         an intermediary line begins with a 3-digit number, the Server
         must pad the front to avoid confusion.

            This scheme allows standard system routines to be used for
            reply information (such as for the STAT reply), with
            "artificial" first and last lines tacked on.  In the rare
            cases where these routines are able to generate three digits
            and a Space at the beginning of any line, the beginning of
            each text line should be offset by some neutral text, like
            Space.

         This scheme assumes that multi-line replies may not be nested.
         We  have found that, in general, nesting of replies will not
         occur, except for random system messages (also called
         spontaneous replies) which may interrupt another reply.  System
         messages (i.e. those not processed by the FTP server) will NOT
         carry reply codes and may occur anywhere in the command-reply
         sequence.  They may be ignored by the User-process as they are
         only information for the human user.





                                   33


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



      The three digits of the reply each have a special significance.
      This is intended to allow a range of very simple to very
      sophisticated response by the user-process.  The first digit
      denotes whether the response is good, bad or incomplete.
      (Referring to the state diagram) an unsophisticated user-process
      will be able to determine its next action (proceed as planned,
      redo, retrench, etc.) by simply examining this first digit.  A
      user-process that wants to know approximately what kind of error
      occurred (e.g. file system error, command syntax error) may
      examine the second digit, reserving the third digit for the finest
      gradation of information (e.g. RNTO command without a preceding
      RNFR.)

         There are five values for the first digit of the reply code:

            1yz   Positive Preliminary reply

               The requested action is being initiated; expect another
               reply before proceeding with a new command.  (The
               user-process sending another command before the
               completion reply would be in violation of protocol; but
               server-FTP processes should queue any commands that
               arrive while a preceding command is in progress.)  This
               type of reply can be used to indicate that the command
               was accepted and the user-process may now pay attention
               to the data connections, for implementations where
               simultaneous monitoring is difficult.

            2yz   Positive Completion reply

               The requested action has been successfully completed.  A
               new request may be initiated.

            3yz   Positive Intermediate reply

               The command has been accepted, but the requested action
               is being held in abeyance, pending receipt of further
               information.  The user should send another command
               specifying this information.  This reply is used in
               command sequence groups.

            4yz   Transient Negative Completion reply

               The command was not accepted and the requested action did
               not take place, but the error condition is temporary and
               the action may be requested again.  The user should




                                   34


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



               return to the beginning of the command sequence, if any.
               It is difficult to assign a meaning to "transient",
               particularly when two distinct sites (Server and
               User-processes) have to agree on the interpretation.
               Each reply in the 4yz category might have a slightly
               different time value, but the intent is that the
               user-process is encouraged to try again.  A rule of thumb
               in determining if a reply fits into the 4yz or the 5yz
               (Permanent Negative) category is that replies are 4yz if
               the commands can be repeated without any change in
               command form or in properties of the User or Server (e.g.
               the command is spelled the same with the same arguments
               used; the user does not change his file access or user
               name; the server does not put up a new implementation.)

            5yz   Permanent Negative Completion reply

               The command was not accepted and the requested action did
               not take place.  The User-process is discouraged from
               repeating the exact request (in the same sequence).  Even
               some "permanent" error conditions can be corrected, so
               the human user may want to direct his User-process to
               reinitiate the command sequence by direct action at some
               point in the future (e.g. after the spelling has been
               changed, or the user has altered his directory status.)

         The following function groupings are encoded in the second
         digit:

            x0z   Syntax - These replies refer to syntax errors,
                  syntactically correct  commands that don't fit any
                  functional category, unimplemented or superfluous
                  commands.

            x1z   Information -  These are replies to requests for
                  information, such as status or help.

            x2z   Connections - Replies referring to the TELNET and data
                  connections.

            x3z   Authentication and accounting - Replies for the login
                  process and accounting procedures.

            x4z   Unspecified as yet






                                   35


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



            x5z   File system - These replies indicate the status of the
                  Server file system vis-a-vis the requested transfer or
                  other file system action.

         The third digit gives a finer gradation of meaning in each of
         the function categories, specified by the second digit.  The
         list of replies below will illustrate this.  Note that the text
         associated with each reply is recommended, rather than
         mandatory, and may even change according to the command with
         which it is associated.  The reply codes, on the other hand,
         must strictly follow the specifications in the last section;
         that is, Server implementations should not invent new codes for
         situations that are only slightly different from the ones
         described here, but rather should adapt codes already defined.

            A command such as TYPE or ALLO whose successful execution
            does not offer the user-process any new information will
            cause a 200 reply to be returned.  If the command is not
            implemented by a particular Server-FTP process because it
            has no relevance to that computer system, for example ALLO
            at a TOPS20 site, a Positive Completion reply is still
            desired so that the simple User-process knows it can proceed
            with its course of action.  A 202 reply is used in this case
            with, for example, the reply text:  "No storage allocation
            necessary."  If, on the other hand, the command requests a
            non-site-specific action and is unimplemented, the response
            is 502.  A refinement of that is the 504 reply for a command
            that IS implemented, but that requests an unimplemented
            parameter.

      Reply Codes by Function Groups

         200 Command okay
         500 Syntax error, command unrecognized
            [This may include errors such as command line too long.]
         501 Syntax error in parameters or arguments
         202 Command not implemented, superfluous at this site.
         502 Command not implemented
         503 Bad sequence of commands
         504 Command not implemented for that parameter

         110 Restart marker reply.








                                   36


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



            In this case the text is exact and not left to the
            particular implementation; it must read:
                 MARK yyyy = mmmm
            where yyyy is User-process data stream marker, and mmmm
            server's equivalent marker.  (note the spaces between
            markers and "=".)
         119 Terminal not available, will try mailbox.
         211 System status, or system help reply
         212 Directory status
         213 File status
         214 Help message
            (on how to use the server or the meaning of a particular
            non-standard command.  This reply is useful only to the
            human user.)
         215 <scheme> is the preferred scheme.

         120 Service ready in nnn minutes
         220 Service ready for new user
         221 Service closing TELNET connection
            (logged out if appropriate)
         421 Service not available, closing TELNET connection.
            This may be a reply to any command if the service knows it
            must shut down.]
         125 Data connection already open; transfer starting
         225 Data connection open; no transfer in progress
         425 Can't open data connection
         226 Closing data connection;
            requested file action successful (for example, file transfer
            or file abort.)
         426 Connection closed; transfer aborted.
         227 Entering Passive Mode.  h1,h2,h3,h4,p1,p2

         230 User logged in, proceed
         530 Not logged in
         331 User name okay, need password
         332 Need account for login
         532 Need account for storing files

         150 File status okay; about to open data connection.
         151 User not local; Will forward to <user>@<host>.
         152 User Unknown; Mail will be forwarded by the operator.
         250 Requested file action okay, completed.
         350 Requested file action pending further information
         450 Requested file action not taken:
            file unavailable (e.g. file busy)
         550 Requested action not taken:




                                   37


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



            file unavailable (e.g. file not found, no access)
         451 Requested action aborted: local error in processing
         551 Requested action aborted: page type unknown
         452 Requested action not taken:
            insufficient storage space in system
         552 Requested file action aborted:
            exceeded storage allocation (for current directory or
            dataset)
         553 Requested action not taken:
            file name not allowed
         354 Start mail input; end with <CR><LF>.<CR><LF>


      Numeric Order List of Reply Codes

         110 Restart marker reply.
            In this case the text is exact and not left to the
            particular implementation; it must read:
                 MARK yyyy = mmmm
            where yyyy is User-process data stream marker, and mmmm
            server's equivalent marker.  (note the spaces between
            markers and "=".)
         119 Terminal not available, will try mailbox.
         120 Service ready in nnn minutes
         125 Data connection already open; transfer starting
         150 File status okay; about to open data connection.
         151 User not local; Will forward to <user>@<host>.
         152 User Unknown; Mail will be forwarded by the operator.
         200 Command okay
         202 Command not implemented, superfluous at this site.
         211 System status, or system help reply
         212 Directory status
         213 File status
         214 Help message
            (on how to use the server or the meaning of a particular
            non-standard command.  This reply is useful only to the
            human user.)
         215 <scheme> is the preferred scheme.
         220 Service ready for new user
         221 Service closing TELNET connection
            (logged out if appropriate)
         225 Data connection open; no transfer in progress
         226 Closing data connection;
            requested file action successful (for example, file transfer
            or file abort.)
         227 Entering Passive Mode.  h1,h2,h3,h4,p1,p2




                                   38


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



         230 User logged in, proceed
         250 Requested file action okay, completed.
         331 User name okay, need password
         332 Need account for login
         350 Requested file action pending further information
         354 Start mail input; end with <CR><LF>.<CR><LF>
         421 Service not available, closing TELNET connection.
            This may be a reply to any command if the service knows it
            must shut down.]
         425 Can't open data connection
         426 Connection closed; transfer aborted.
         450 Requested file action not taken:
            file unavailable (e.g. file busy)
         451 Requested action aborted: local error in processing
         452 Requested action not taken:
            insufficient storage space in system
         500 Syntax error, command unrecognized
            [This may include errors such as command line too long.]
         501 Syntax error in parameters or arguments
         502 Command not implemented
         503 Bad sequence of commands
         504 Command not implemented for that parameter
         530 Not logged in
         532 Need account for storing files
         550 Requested action not taken:
            file unavailable (e.g. file not found, no access)
         551 Requested action aborted: page type unknown
         552 Requested file action aborted:
            exceeded storage allocation (for current directory or
            dataset)
         553 Requested action not taken:
            file name not allowed


















                                   39


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



DECLARATIVE SPECIFICATIONS

   MINIMUM IMPLEMENTATION

      In order to make FTP workable without needless error messages, the
      following minimum implementation is required for all servers:

         TYPE - ASCII Non-print
         MODE - Stream
         STRUCTURE - File, Record
         COMMANDS - USER, QUIT, PORT,
                    TYPE, MODE, STRU,
                      for the default values
                    RETR, STOR,
                    NOOP.

      The default values for transfer parameters are:


         TYPE - ASCII Non-print
         MODE - Stream
         STRU - File

      All Hosts must accept the above as the standard defaults.

   CONNECTIONS

      The server protocol interpreter shall "listen" on Port L.  The
      user or user protocol interpreter shall initiate the full-duplex
      TELNET connection.  Server- and user- processes should follow the
      conventions of the TELNET protocol as specified in the ARPA
      Internet Protocol Handbook.  Servers are under no obligation to
      provide for editing of command lines and may specify that it be
      done in the user Host.  The TELNET connection shall be closed by
      the server at the user's request after all transfers and replies
      are completed.

      The user-DTP must "listen" on the specified data port; this may be
      the default user port (U) or a port specified in the PORT command.
      The server shall initiate the data connection from his own default
      data port (L-1) using the specified user data port.  The direction
      of the transfer and the port used will be determined by the FTP
      service command.







                                   40


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



      When data is to be transferred between two servers, A and B (refer
      to Figure 2), the user-PI, C, sets up TELNET connections with both
      server-PI's.  One of the servers, say A, is then sent a PASV
      command telling him to "listen" on his data port rather than
      initiate a connection when he receives a transfer service command.
      When the user-PI receives an acknowledgment to the PASV command,
      which includes the identity of the host and port being listened
      on, the user-PI then sends A's port, a, to B in a PORT command; a
      reply is returned.  The user-PI may then send the corresponding
      service commands to A and B.  Server B initiates the connection
      and the transfer proceeds.  The command-reply sequence is listed
      below where the messages are vertically synchronous but
      horizontally asynchronous:

         User-PI - Server A                User-PI - Server B
         ------------------                ------------------

         C->A : Connect                    C->B : Connect
         C->A : PASV
         A->C : 227 Entering Passive Mode. A1,A2,A3,A4,a1,a2
                                           C->B : PORT A1,A2,A3,A4,a1,a2
                                           B->C : 200 Okay
         C->A : STOR                       C->B : RETR
                    B->A : Connect to HOST-A, PORT-a

      The data connection shall be closed by the server under the
      conditions described in the Section on Establishing Data
      Connections.  If the server wishes to close the connection after a
      transfer where it is not required, he should do so immediately
      after the file transfer is completed.  He should not wait until
      after a new transfer command is received because the user-process
      will have already tested the data connection to see if it needs to
      do a "listen"; (recall that the user must "listen" on a closed
      data port BEFORE sending the transfer request).  To prevent a race
      condition here, the server sends a reply (226) after closing the
      data connection (or if the connection is left open, a "file
      transfer completed" reply (250) and the user-PI should wait for
      one of these replies before issuing a new transfer command.












                                   41


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



   COMMANDS

      The commands are TELNET character string transmitted over the
      TELNET connections as described in the Section on FTP Commands.
      The command functions and semantics are described in the Section
      on Access Control Commands, Transfer Parameter Commands, FTP
      Service Commands, and Miscellaneous Commands.  The command syntax
      is specified here.

      The commands begin with a command code followed by an argument
      field.  The command codes are four or fewer alphabetic characters.
      Upper and lower case alphabetic characters are to be treated
      identically.  Thus any of the following may represent the retrieve
      command:

         RETR    Retr    retr    ReTr    rETr

      This also applies to any symbols representing parameter values,
      such as A or a for ASCII TYPE.  The command codes and the argument
      fields are separated by one or more spaces.

      The argument field consists of a variable length character string
      ending with the character sequence <CRLF> (Carriage Return,
      Linefeed) for NVT-ASCII representation; for other negotiated
      languages a different end of line character might be used.  It
      should be noted that the server is to take NO action until the end
      of line code is received.

      The syntax is specified below in NVT-ASCII.  All characters in the
      argument field are ASCII characters including any ASCII
      represented decimal integers.  Square brackets denote an optional
      argument field.  If the option is not taken, the appropriate
      default is implied.

















                                   42


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



      The following are the FTP commands:

         USER <SP> <username> <CRLF>
         PASS <SP> <password> <CRLF>
         ACCT <SP> <account information> <CRLF>
         REIN <CRLF>
         QUIT <CRLF>
         PORT <SP> <Host-port> <CRLF>
         PASV <CRLF>
         TYPE <SP> <type code> <CRLF>
         STRU <SP> <structure code> <CRLF>
         MODE <SP> <mode code> <CRLF>
         RETR <SP> <pathname> <CRLF>
         STOR <SP> <pathname> <CRLF>
         APPE <SP> <pathname> <CRLF>
         MLFL [<SP> <ident>] <CRLF>
         MAIL [<SP> <ident>] <CRLF>
         MSND [<SP> <ident>] <CRLF>
         MSOM [<SP> <ident>] <CRLF>
         MSAM [<SP> <ident>] <CRLF>
         MRSQ [<SP> <scheme>] <CRLF>
         MRCP <SP> <ident> <CRLF>
         ALLO <SP> <decimal integer>
             [<SP> R <SP> <decimal integer>] <CRLF>
         REST <SP> <marker> <CRLF>
         RNFR <SP> <pathname> <CRLF>
         RNTO <SP> <pathname> <CRLF>
         ABOR <CRLF>
         DELE <SP> <pathname> <CRLF>
         CWD <SP> <pathname> <CRLF>
         LIST [<SP> <pathname>] <CRLF>
         NLST [<SP> <pathname>] <CRLF>
         SITE <SP> <string> <CRLF>
         STAT [<SP> <pathname>] <CRLF>
         HELP [<SP> <string>] <CRLF>
         NOOP <CRLF>














                                   43


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



      The syntax of the above argument fields (using BNF notation where
      applicable ) is:

         <username> ::= <string>
         <password> ::= <string>
         <account information> ::= <string>
         <string> ::= <char> | <char><string>
         <char> ::= any of the 128 ASCII characters except <CR> and <LF>
         <marker> ::= <pr string>
         <pr string> ::= <pr char> | <pr char><pr string>
         <pr char> ::= printable characters, any
                       ASCII code 33 through 126
         <byte size> ::= any decimal integer 1 through 255
         <Host-port> ::= <Host-number>,<Port-number>
         <Host-number> ::= <number>,<number>,<number>,<number>
         <Port-number> ::= <number>,<number>
         <number> ::= any decimal integer 0 through 255
         <ident> ::= <string>
         <scheme> ::= R | T | ?
         <form code> ::= N | T | C
         <type code> ::= A [<SP> <form code>]
                       | E [<SP> <form code>]
                       | I
                       | L <SP> <byte size>
         <structure code> ::= F | R | P
         <mode code> ::= S | B | C
         <pathname> ::= <string>























                                   44


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



   SEQUENCING OF COMMANDS AND REPLIES

      The communication between the user and server is intended to be an
      alternating dialogue.  As such, the user issues an FTP command and
      the server responds with a prompt primary reply.  The user should
      wait for this initial primary success or failure response before
      sending further commands.

      Certain commands require a second reply for which the user should
      also wait.  These replies may, for example, report on the progress
      or completion of file transfer or the closing of the data
      connection.  They are secondary replies to file transfer commands.

      One important group of informational replies is the connection
      greetings.  Under normal circumstances, a server will send a 220
      reply, "awaiting input", when the connection is completed.  The
      user should wait for this greeting message before sending any
      commands.  If the server is unable to accept input right away, he
      should send a 120 "expected delay" reply immediately and a 220
      reply when ready.  The user will then know not to hang up if there
      is a delay.

      The table below lists alternative success and failure replies for
      each command.  These must be strictly adhered to; a server may
      substitute text in the replies, but the meaning and action implied
      by the code numbers and by the specific command reply sequence
      cannot be altered.

      Command-Reply Sequences

         In this section, the command-reply sequence is presented.  Each
         command is listed with its possible replies; command groups are
         listed together.  Preliminary replies are listed first (with
         their succeeding replies indented and under them), then
         positive and negative completion, and finally intermediary
         replies with the remaining commands from the sequence
         following.  This listing forms the basis for the state
         diagrams, which will be presented separately.

            Connection Establishment
               120
                  220
               220
               421






                                   45


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



            Login
               USER
                  230
                  530
                  500, 501, 421
                  331, 332
               PASS
                  230
                  202
                  530
                  500, 501, 503, 421
                  332
               ACCT
                  230
                  202
                  530
                  500, 501, 503, 421
            Logout
               QUIT
                  221
                  500
               REIN
                  120
                     220
                  220
                  421
                  500, 502
            Transfer parameters
               PORT
                  200
                  500, 501, 421, 530
               PASV
                  227
                  500, 501, 502, 421, 530
               MODE, TYPE, STRU
                  200
                  500, 501, 504, 421, 530
            File action commands
               ALLO
                  200
                  202
                  500, 501, 504, 421, 530
               REST
                  500, 501, 502, 421, 530
                  350





                                   46


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



               STOR
                  125, 150
                     (110)
                     226, 250
                     425, 426, 451, 551, 552
                  532, 450, 452, 553
                  500, 501, 421, 530
               RETR
                  125, 150
                     (110)
                     226, 250
                     425, 426, 451
                  450, 550
                  500, 501, 421, 530
               LIST, NLST
                  125, 150
                     226, 250
                     425, 426, 451
                  450
                  500, 501, 502, 421, 530
               APPE
                  125, 150
                     (110)
                     226, 250
                     425, 426, 451, 551, 552
                  532, 450, 550, 452, 553
                  500, 501, 502, 421, 530
               MLFL
                  125, 150, 151, 152
                     226, 250
                     425, 426, 451, 552
                  532, 450, 550, 452, 553
                  500, 501, 502, 421, 530
               RNFR
                  450, 550
                  500, 501, 502, 421, 530
                  350
               RNTO
                  250
                  532, 553
                  500, 501, 502, 503, 421, 530
               DELE, CWD
                  250
                  450, 550
                  500, 501, 502, 421, 530





                                   47


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



               ABOR
                  225, 226
                  500, 501, 502, 421
               MAIL, MSND
                  151, 152
                     354
                        250
                        451, 552
                  354
                     250
                     451, 552
                  450, 550, 452, 553
                  500, 501, 502, 421, 530
               MSOM, MSAM
                  119, 151, 152
                     354
                        250
                        451, 552
                  354
                     250
                     451, 552
                  450, 550, 452, 553
                  500, 501, 502, 421, 530
               MRSQ
                  200, 215
                  500, 501, 502, 421, 530
               MRCP
                  151, 152
                     200
                  200
                  450, 550, 452, 553
                  500, 501, 502, 503, 421
            Informational commands
               STAT
                  211, 212, 213
                  450
                  500, 501, 502, 421, 530
               HELP
                  211, 214
                  500, 501, 502, 421
            Miscellaneous commands
               SITE
                  200
                  202
                  500, 501, 530





                                   48


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



               NOOP
                  200
                  500 421















































                                   49


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



STATE DIAGRAMS

   Here we present state diagrams for a very simple minded FTP
   implementation. Only the first digit of the reply codes is used.
   There is one state diagram for each group of FTP commands or command
   sequences.

   The command groupings were determined by constructing a model for
   each command then collecting together the commands with structurally
   identical models.

   For each command or command sequence there are three possible
   outcomes: success (S), failure (F), and error (E). In the state
   diagrams below we use the symbol B for "begin", and the symbol W for
   "wait for reply".

   We first present the diagram that represents the largest group of FTP
   commands:


                               1,3    +---+
                          ----------->| E |
                         |            +---+
                         |
      +---+    cmd    +---+    2      +---+
      | B |---------->| W |---------->| S |
      +---+           +---+           +---+
                         |
                         |     4,5    +---+
                          ----------->| F |
                                      +---+


      This diagram models the commands:

         ABOR, ALLO, DELE, CWD, HELP, MODE, MRCP, MRSQ, NOOP, PASV,
         QUIT, SITE, PORT, STAT, STRU, TYPE.













                                   50


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



   The other large group of commands is represented by a very similar
   diagram:


                               3      +---+
                          ----------->| E |
                         |            +---+
                         |
      +---+    cmd    +---+    2      +---+
      | B |---------->| W |---------->| S |
      +---+       --->+---+           +---+
                 |     | |
                 |     | |     4,5    +---+
                 |  1  |  ----------->| F |
                  -----               +---+


      This diagram models the commands:

         APPE, LIST, MLFL, NLST, REIN, RETR, STOR.

   Note that this second model could also be used to represent the first
   group of commands, the only difference being that in the first group
   the 100 series replies are unexpected and therefore treated as error,
   while the second group expects (some may require) 100 series replies.

   The remaining diagrams model command sequences, perhaps the simplest
   of these is the rename sequence:


      +---+   RNFR    +---+    1,2    +---+
      | B |---------->| W |---------->| E |
      +---+           +---+        -->+---+
                       | |        |
                3      | | 4,5    |
         --------------  ------   |
        |                      |  |   +---+
        |               ------------->| S |
        |              |   1,3 |  |   +---+
        |             2|  --------
        |              | |     |
        V              | |     |
      +---+   RNTO    +---+ 4,5 ----->+---+
      |   |---------->| W |---------->| F |
      +---+           +---+           +---+





                                   51


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



   A very similar diagram models the Mail and Send commands:


                   ----  1
                  |    |
      +---+  cmd   -->+---+     2     +---+
      | B |---------->| W |---------->| E |
      +---+           +---+        -->+---+
                       | |        |
                3      | | 4,5    |
         --------------  ------   |
        |                      |  |   +---+
        |               ------------->| S |
        |              |   1,3 |  |   +---+
        |             2|  --------
        |              | |     |
        V              | |     |
      +---+   text    +---+ 4,5 ----->+---+
      |   |---------->| W |---------->| F |
      +---+           +---+           +---+


         This diagram models the commands:

            MAIL, MSND, MSOM, MSAM.

      Note that the "text" here is a series of lines sent from the user
      to the server with no response expected until the last line is
      sent, recall that the last line must consist only of a single
      period.




















                                   52


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



   The next diagram is a simple model of the Restart command:


      +---+   REST    +---+    1,2    +---+
      | B |---------->| W |---------->| E |
      +---+           +---+        -->+---+
                       | |        |
                3      | | 4,5    |
         --------------  ------   |
        |                      |  |   +---+
        |               ------------->| S |
        |              |   3   |  |   +---+
        |             2|  --------
        |              | |     |
        V              | |     |
      +---+   cmd     +---+ 4,5 ----->+---+
      |   |---------->| W |---------->| F |
      +---+        -->+---+           +---+
                  |      |
                  |  1   |
                   ------


         Where "cmd" is APPE, STOR, RETR, or MLFL.

   We note that the above three models are similar, in fact the Mail
   diagram and the Rename diagram are structurally identical. The
   Restart differs from the other two only in the treatment of 100
   series replies at the second stage.





















                                   53


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



   The most complicated diagram is for the Login sequence:


                            1
      +---+   USER    +---+------------->+---+
      | B |---------->| W | 2       ---->| E |
      +---+           +---+------  |  -->+---+
                       | |       | | |
                     3 | | 4,5   | | |
         --------------   -----  | | |
        |                      | | | |
        |                      | | | |
        |                 ---------  |
        |               1|     | |   |
        V                |     | |   |
      +---+   PASS    +---+ 2  |  ------>+---+
      |   |---------->| W |------------->| S |
      +---+           +---+   ---------->+---+
                       | |   | |     |
                     3 | |4,5| |     |
         --------------   --------   |
        |                    | |  |  |
        |                    | |  |  |
        |                 -----------
        |             1,3|   | |  |
        V                |  2| |  |
      +---+   ACCT    +---+--  |   ----->+---+
      |   |---------->| W | 4,5 -------->| F |
      +---+           +---+------------->+---+





















                                   54


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



   Finally we present a generalized diagram that could be used to model
   the command and reply interchange:


               ------------------------------------
              |                                    |
      Begin   |                                    |
        |     V                                    |
        |   +---+  cmd   +---+ 2         +---+     |
         -->|   |------->|   |---------->|   |     |
            |   |        | W |           | S |-----|
         -->|   |     -->|   |-----      |   |     |
        |   +---+    |   +---+ 4,5 |     +---+     |
        |     |      |    | |      |               |
        |     |      |   1| |3     |     +---+     |
        |     |      |    | |      |     |   |     |
        |     |       ----  |       ---->| F |-----
        |     |             |            |   |
        |     |             |            +---+
         -------------------
              |
              |
              V
             End


























                                   55


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



TYPICAL FTP SCENARIO

   User at Host U wanting to transfer files to/from Host S:

   In general the user will communicate to the server via a mediating
   user-FTP process.  The following may be a typical scenario.  The
   user-FTP prompts are shown in parentheses, '---->' represents
   commands from Host U to Host S, and '<----' represents replies from
   Host S to Host U.

      LOCAL COMMANDS BY USER              ACTION INVOLVED

      ftp (host) multics<CR>         Connect to Host S, port L,
                                     establishing TELNET connections
                                     <---- 220 Service ready <CRLF>
      username Doe <CR>              USER Doe<CRLF>---->
                                     <---- 331 User name ok,
                                               need password<CRLF>
      password mumble <CR>           PASS mumble<CRLF>---->
                                     <---- 230 User logged in.<CRLF>
      retrieve (local type) ASCII<CR>
      (local pathname) test 1 <CR>   User-FTP opens local file in ASCII.
      (for.pathname) test.pl1<CR>    RETR test.pl1<CRLF> ---->
                                     <---- 150 File status okay;
                                           about to open data connection
                                     Server makes data connection
                                     to port U
      <CRLF>
                                     <---- 226 Closing data connection,
                                         file transfer successful<CRLF>
      type Image<CR>                 TYPE I<CRLF> ---->
                                     <---- 200 Command OK<CRLF>
      store (local type) image<CR>
      (local pathname) file dump<CR> User-FTP opens local file in Image.
      (for.pathname) >udd>cn>fd<CR>  STOR >udd>cn>fd<CRLF> ---->
                                     <---- 450 Access denied<CRLF>
      terminate                      QUIT <CRLF> ---->
                                     Server closes all
                                     connections.











                                   56


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



CONNECTION ESTABLISHMENT

   The FTP control connection is established via TCP between the user
   process port U and the server process port L.  This protocol is
   assigned the service port 21 (25 octal), that is L=21.













































                                   57


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



APPENDIX ON MAIL

   The basic commands transmitting mail are the MAIL and the MLFL
   commands.  These commands cause the transmitted data to be entered
   into the recipients mailbox.

      MAIL <SP> <recipient name> <CRLF>

         If accepted, returns 354 reply and considers all succeeding
         lines to be the message text, terminated by a line containing
         only a period, upon which a 250 completion reply is returned.
         Various errors are possible.

      MLFL <SP> <recipient name> <CRLF>

         If accepted, acts like a STOR command, except that the data is
         considered to be the message text.  Various errors are
         possible.

   There are two possible preliminary replies that a server may use to
   indicate that it is accepting mail for a user whose mailbox is not at
   that server.

      151 User not local; Will forward to <user>@<host>.

         This reply indicates that the server knows the user's mailbox
         is on another host and will take responsibility for forwarding
         the mail to that host.  For example, at BBN (or ISI) there are
         several host which each have a list of many of the users on
         several of the host.  These hosts then can accept mail for any
         user on their list and forward it to the correct host.

      152 User Unknown; Mail will be forwarded by the operator.

         This reply indicates that the host does not recognize the user
         name, but that it will accept the mail and have the operator
         attempt to deliver it.  This is useful if the user name is
         misspelled, but may be a disservice if the mail is really
         undeliverable.

   Three FTP commands provide for "sending" a message to a logged-in
   user's terminal, as well as variants for mailing it normally whether
   the user is logged in or not.







                                   58


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



      MSND -- SeND to terminal.

         Returns 450 failure reply if the addressee is refusing or not
         logged in.

      MSOM -- Send to terminal Or Mailbox.

         Returns 119 notification reply if terminal is not accessible.

      MSAM -- Send to terminal And Mailbox.

         Returns 119 notification reply if terminal is not accessible.

   Note that for MSOM and MSAM, it is the mailing which determines
   success, not the sending, although MSOM as implemented uses a 119
   reply (in addition to the normal success/failure code) to indicate
   that because the SEND failed, an attempt is being made to mail the
   message instead.  There are no corresponding variants for MLFL, since
   messages transmitted in this way are generally short.

   There are two FTP commands which allow one to mail the text of a
   message to several recipients simultaneously; such message
   transmission is far more efficient than the practice of sending the
   text again and again for each additional recipient at a site.

   There are two basic ways of sending a single text to several
   recipients.  In one, all recipients are specified first, and then the
   text is sent; in the other, the order is reversed and the text is
   sent first, followed by the recipients.  Both schemes are necessary
   because neither by itself is optimal for all systems, as will be
   explained later.  To select a particular scheme, the MRSQ command is
   used; to specify recipients after a scheme is chosen, MRCP commands
   are given; and to furnish text, the MAIL or MLFL commands are used.

   Scheme Selection: MRSQ

      MRSQ is the means by which a user program can test for
      implementation of MRSQ/MRCP, select a particular scheme, reset its
      state thereof, and even do some rudimentary negotiation.  Its
      format is like that of the TYPE command, as follows:










                                   59


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



         MRSQ [<SP> <scheme>] <CRLF>

         <scheme> = a single character.  The following are defined:
            R  Recipients first.  If not implemented, T must be.
            T  Text first.  If this is not implemented, R must be.
            ?  Request for preference.  Must always be implemented.

            No argument means a "selection" of none of the schemes (the
            default).

         Replies:
            200 OK, we'll use specified scheme.
            215 <scheme> This is the scheme I prefer.
            501 I understand MRSQ but can't use that scheme.
            5xx Command unrecognized or unimplemented.

      Three aspects of MRSQ need to be pointed out here.  The first is
      that an MRSQ with no argument must always return a 200 reply and
      restore the default state of having no scheme selected.  Any other
      reply implies that MRSQ and hence MRCP are not understood or
      cannot be performed correctly.

      The second is that the use of "?" as a <scheme> asks the FTP
      server to return a 215 reply in which the server specifies a
      "preferred" scheme.  The format of this reply is simple:

         215 <SP> <scheme> [<SP> <arbitrary text>] <CRLF>

         Any other reply (e.g. 4xx or 5xx) implies that MRSQ and MRCP
         are not implemented, because "?" must always be implemented if
         MRSQ is.

      The third important thing about MRSQ is that it always has the
      side effect of resetting all schemes to their initial state.  This
      reset must be done no matter what the reply will be - 200, 215, or
      501.  The actions necessary for a reset will be explained when
      discussing how each scheme actually works.

   Message Text Specification: MAIL/MLFL

      Regardless of which scheme (if any) has been selected, a MAIL or
      MLFL with a non-null argument will behave exactly as before; the
      MRSQ/MRCP commands have no effect on them.  However, such normal
      MAIL/MLFL commands do have the same side effect as MRSQ; they
      "reset" the current scheme to its initial state.





                                   60


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



      It is only when the argument is null (e.g. MAIL<CRLF> or
      MLFL<CRLF>) that the particular scheme being used is important,
      because rather than producing an error (as most servers currently
      do), the server will accept message text for this "null"
      specification; what it does with it depends on which scheme is in
      effect, and will be described in "Scheme Mechanics".

   Recipient specification: MRCP

      In order to specify recipient names  (i.e., idents) and receive
      some acknowledgment (or refusal) for each name, the following
      command is used:

         MRCP <SP> <ident> <CRLF>

         Reply for no scheme:
            503 No scheme specified yet; use MRSQ.
         Replies for scheme T are identical to those for MAIL/MLFL.
         Replies for scheme R (recipients first):
            200 OK, name stored.
            452 Recipient table full, this name not stored.
            553 Recipient name rejected.
            4xx Temporary error, try this name again later.
            5xx Permanent error, report to sender.

      Note that use of this command is an error if no scheme has been
      selected yet; an MRSQ <scheme> must have been given if MRCP is to
      be used.

   Scheme mechanics: MRSQ R (Recipients first)

      In the recipients-first scheme, MRCP is used to specify names
      which the FTP server stores in a list or table.  Normally the
      reply for each MRCP will be either a 200 for acceptance, or a
      4xx/5xx code for rejection; all 5xx codes are permanent rejections
      (e.g. user not known) which should be reported to the human
      sender, whereas 4xx codes in general connote some temporary error
      that may be rectified later.  None of the 4xx/5xx replies impinge
      on previous or succeeding MRCP commands, except for 452 which
      indicates that no further MRCP's will succeed unless a message is
      sent to the already stored recipients or a reset is done.

      Sending message text to stored recipients is done by giving a MAIL
      or MLFL command with no argument; that is, just MAIL<CRLF> or
      MLFL<CRLF>.  Transmission of the message text is exactly the same
      as for normal MAIL/MLFL; however, a positive acknowledgment at the




                                   61


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



      end of transmission means that the message has been sent to ALL
      recipients that were remembered with MRCP, and a failure code
      means that it should be considered to have failed for ALL of these
      specified recipients.  This applies regardless of the actual error
      code; and whether the reply signifies success or failure, all
      stored recipient names are flushed and forgotten - in other words,
      things are reset to their initial state.  This purging of the
      recipient name list must also be done as the "reset" side effect
      of any use of MRSQ.

      A 452 reply to an MRCP can thus be handled by using a MAIL/MLFL to
      specify the message for currently stored recipients, and then
      sending more MRCP's and another MAIL/MLFL, as many times as
      necessary; for example, if a server only had room for 10 names
      this would result in a 50-recipient message being sent 5 times, to
      10 different recipients each time.

      If a user attempts to specify message text (MAIL/MLFL with no
      argument) before any successful MRCP's have been given, this
      should be treated exactly as a "normal" MAIL/MLFL with a null
      recipient would be; some servers will return an error of some
      type, such as "550 Null recipient".

      See Example 1 for an example using MRSQ R.

   Scheme mechanics: MRSQ T (Text first)

      In the text-first scheme, MAIL/MLFL with no argument is used to
      specify message text, which the server stores away.  Succeeding
      MRCP's are then treated as if they were MAIL/MLFL commands, except
      that none of the text transfer manipulations are done; the stored
      message text is sent to the specified recipient, and a reply code
      is returned identical to that which an actual MAIL/MLFL would
      invoke. (Note ANY 2xx code indicates success.)

      The stored message text is not forgotten until the next MAIL/MLFL
      or MRSQ, which will either replace it with new text or flush it
      entirely.  Any use of MRSQ will reset this scheme by flushing
      stored text, as will any use of MAIL/MLFL with a non-null
      argument.

      If an MRCP is seen before any message text has been stored, the
      user in effect is trying to send a null message; some servers
      might allow this, others would return an error code.

      See Example 2 for an example using MRSQ T.




                                   62


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



   Why two schemes anyway?

      Because neither by itself is optimal for all systems.  MRSQ R
      allows more of a "bulk" mailing, because everything is saved up
      and then mailed simultaneously; this is very useful for systems
      such as ITS where the FTP server does not itself write mail
      directly, but hands it on to a central mailer demon of great
      power; the more information (e.g. recipients) associated with a
      single "hand-off", the more efficiently mail can be delivered.

      By contrast, MRSQ T is geared to FTP servers which want to deliver
      mail directly, in one-by-one incremental fashion.  This way they
      can return an individual success/failure reply code for each
      recipient given which may depend on variable file system factors
      such as exceeding disk allocation, mailbox access conflicts, and
      so forth; if they tried to emulate MRSQ R's bulk mailing, they
      would have to ensure that a success reply to the MAIL/MLFL indeed
      meant that it had been delivered to ALL recipients specified - not
      just some.

   Notes:

      * Because these commands are not required in the minimum
        implementation of FTP, one must be prepared to deal with sites
        which don't recognize either MRSQ or MRCP.  "MRSQ" and "MRSQ ?"
        are explicitly designed as tests to see whether either scheme is
        implemented; MRCP is not, and a failure return of the
        "unimplemented" variety could be confused with "No scheme
        selected yet", or even with "Recipient unknown".  Be safe, be
        sure, use MRSQ!

      * There is no way to indicate in a positive response to "MRSQ ?"
        that the preferred "scheme" for a server is that of the default
        state; i.e. none of the multi-recipient schemes.  The rationale
        is that in this case, it would be pointless to implement
        MRSQ/MRCP at all, and the response would therefore be negative.

      * One reason that the use of MAIL/MLFL is restricted to null
        arguments with this multi-recipient extension is the ambiguity
        that would result if a non-null argument were allowed; for
        example, if MRSQ R was in effect and some MRCP's had been given,
        and a MAIL FOO<CRLF> was done, there would be no way to
        distinguish a failure reply for mailbox "FOO" from a global
        failure for all recipients specified.  A similar situation
        exists for MRSQ T; it would not be clear whether the text was
        stored and the mailbox failed, or vice versa, or both.




                                   63


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



      * "Resets" are done by all MRSQ's and "normal" MAIL/MLFL's to
        avoid confusion and overly complicated implementation.  The MRSQ
        command implies a change or uncertainty of status, and the
        latter commands would otherwise have to use some independent
        mechanisms to avoid clobbering the data bases (e.g., message
        text storage area) used by the T/R schemes.  However, once a
        scheme is selected, it remains "in effect" just as a "TYPE A"
        remains selected.  The recommended way for doing a reset,
        without changing the current selection, is with "MRSQ ?".
        Remember that "MRSQ" alone reverts to the no-scheme state.

      * It is permissible to intersperse other FTP commands among the
        MRSQ/MRCP/MAIL sequences.





































                                   64


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



   Example 1

                  Example of MRSQ R (Recipients first)

      This is an example of how MRSQ R is used; first the user must
      establish that the server in fact implements MRSQ:

         U: MRSQ
         S: 200 OK, no scheme selected.

      An MRSQ with a null argument always returns a 200 if implemented,
      selecting the "scheme" of null, i.e. none of them.  If MRSQ were
      not implemented, a code of 4xx or 5xx would be returned.

         U: MRSQ R
         S: 200 OK, using that scheme

      All's well; now the recipients can be specified.

         U: MRCP Foo
         S: 200 OK

         U: MRCP Raboof
         S: 553 Who's that?  No such user here.

         U: MRCP bar
         S: 200 OK

      Well, two out of three ain't bad.  Note that the demise of
      "Raboof" has no effect on the storage of "Foo" or "bar".  Now to
      furnish the message text, by giving a MAIL or MLFL with no
      argument:

         U: MAIL
         S: 354 Type mail, ended by <CRLF>.<CRLF>
         U: Blah blah blah blah....etc etc etc
         U: .
         S: 250 Mail sent.

      The text has now been sent to both "Foo" and "bar".










                                   65


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



   Example 2

                     Example of MRSQ T (Text first)

      Using the same message as the previous example:

         U: MRSQ ?
         S: 215 T Text first, please.

      MRSQ is indeed implemented, and the server says that it prefers
      "T", but that needn't stop the user from trying something else:

         U: MRSQ R
         S: 501 Sorry, I really can't do that.

      It's possible that it could have understood "R" also, but in
      general it's best to use the "preferred" scheme, since the server
      knows which is most efficient for its particular site.  Anyway:

         U: MRSQ T
         S: 200 OK, using that scheme.

      Scheme "T" is now selected, and the text must be sent:

         U: MAIL
         S: 354 Type mail, ended by <CRLF>.<CRLF>
         U: Blah blah blah blah....etc etc etc
         U: .
         S: 250 Mail stored.

      Now recipients can be specified:

         U: MRCP Foo
         S: 250 Stored mail sent.

         U: MRCP Raboof
         S: 553 Who's that?  No such user here.

         U: MRCP bar
         S: 250 Stored mail sent.










                                   66


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



      Again, the text has now been sent to both "Foo" and "bar", and
      still remains stored.  A new message can be sent with another
      MAIL/MRCP... sequence, but the fastidious or paranoid could chose
      to do:

         U: MRSQ ?
         S: 215 T Text first, please.

      Which resets things without altering the scheme in effect.









































                                   67


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



APPENDIX ON PAGE STRUCTURE

   The need for FTP to support page structure derives principally from
   the  need to support efficient transmission of files between TOPS20
   systems, particularly the files used by NLS.

   The file system of TOPS20 is based on the concept of pages.  The
   system level is most efficient at manipulating files as pages.
   System level programs provide an interface to the file system so that
   many applications view files as sequential streams of characters.
   However, a few applications use the underlying page structures
   directly, and some of these create holey files.

   A TOPS20 file is just a bunch of words pointed to by a page table.
   If those words contain CRLF's, fine -- but that doesn't mean "record"
   to TOPS20.

   A TOPS20 disk file consists of four things: a pathname, a page table,
   a (possibly empty) set of pages, and a set of attributes.

   The pathname is specified in the RETR or STOR command.  It includes
   the directory name, file name, file name extension, and version
   number.

   The page table contains up to 2**18 entries.  Each entry may be
   EMPTY, or may point to a page.  If it is not empty, there are also
   some page-specific access bits; not all pages of a file need have the
   same access protection.

      A page is a contiguous set of 512 words of 36 bits each.

   The attributes of the file, in the File Descriptor Block (FDB),
   contain such things as creation time, write time, read time, writer's
   byte-size, end of file pointer, count of reads and writes, backup
   system tape numbers, etc.

   Note that there is NO requirement that pages in the page table be
   contiguous.  There may be empty page table slots between occupied
   ones.  Also, the end of file pointer is simply a number.  There is no
   requirement that it in fact point at the "last" datum in the file.
   Ordinary sequential I/O calls in TOPS20 will cause the end of file
   pointer to be left after the last datum written, but other operations
   may cause it not to be so, if a particular programming system so
   requires.






                                   68


IEN 149                                                        June 1980
RFC 765                                           File Transfer Protocol



   In fact both of these special cases, "holey" files and
   end-of-file pointers not at the end of the file, occur with NLS data
   files.

   The TOPS20 paged files can be sent with the FTP transfer parameters:
   TYPE L 36, STRU P, and MODE S (in fact any mode could be used).

   Each page of information has a header.  Each header field, which is a
   logical byte, is a TOPS20 word, since the TYPE is L 36.

   The header fields are:

      Word 0: Header Length.

         The header length is 5.

      Word 1: Page Index.

         If the data is a disk file page, this is the number of that
         page in the file's page map.  Empty pages (holes) in the file
         are simply not sent.  Note that a hole is NOT the same as a
         page of zeros.

      Word 2: Data Length.

         The number of data words in this page, following the header.
         Thus the total length of the transmission unit is the Header
         Length plus the Data Length.

      Word 3: Page Type.

         A code for what type of chunk this is. A data page is type 3,
         the FDB page is type 2.

      Word 4: Page Access Control.

         The access bits associated with the page in the file's page
         map.  (This full word quantity is put into AC2 of an SPACS by
         the program reading from net to disk.)

   After the header are Data Length data words.  Data Length is
   currently either 512 for a data page or 21 for an FDB.  Trailing
   zeros in a disk file page may be discarded, making Data Length less
   than 512 in that case.






                                   69


June 1980                                                        IEN 149
File Transfer Protocol                                           RFC 765



   Data transfers are implemented like the layers of an onion: some
   characters are packaged into a line.  Some lines are packaged into a
   file.  The file is broken into other manageable units for
   transmission.  Those units have compression applied to them.  The
   units may be flagged by restart markers.  On the other end, the
   process is reversed.












































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