    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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    IEN 149                                                        June 1980
    RFC 765                                           File Transfer Protocol

          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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    IEN 149                                                        June 1980
    RFC 765                                           File Transfer Protocol

          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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    IEN 149                                                        June 1980
    RFC 765                                           File Transfer Protocol

          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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    IEN 149                                                        June 1980
    RFC 765                                           File Transfer Protocol

          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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    IEN 149                                                        June 1980
    RFC 765                                           File Transfer Protocol

                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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    IEN 149                                                        June 1980
    RFC 765                                           File Transfer Protocol

          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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    IEN 149                                                        June 1980
    RFC 765                                           File Transfer Protocol

                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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    IEN 149                                                        June 1980
    RFC 765                                           File Transfer Protocol

          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

    IEN 149                                                        June 1980
    RFC 765                                           File Transfer Protocol

                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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    File Transfer Protocol                                           RFC 765

                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

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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
    RFC 765                                           File Transfer Protocol

             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:

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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
    RFC 765                                           File Transfer Protocol

             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.

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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

    June 1980                                                        IEN 149
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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.

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    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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