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rfc:ien:ien149

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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June 1980 IEN 149 File Transfer Protocol RFC 765

    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.
                                 2

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.
                                 3

June 1980 IEN 149 File Transfer Protocol RFC 765

    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.
                                 4

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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June 1980 IEN 149 File Transfer Protocol RFC 765

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

|/———\|

                                          ||   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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June 1980 IEN 149 File Transfer Protocol RFC 765

 "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.
                                 8

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.
                                 9

June 1980 IEN 149 File Transfer Protocol RFC 765

    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.
                                 10

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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June 1980 IEN 149 File Transfer Protocol RFC 765

          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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June 1980 IEN 149 File Transfer Protocol RFC 765

          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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June 1980 IEN 149 File Transfer Protocol RFC 765

    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

June 1980 IEN 149 File Transfer Protocol RFC 765

          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

June 1980 IEN 149 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

June 1980 IEN 149 File Transfer Protocol RFC 765

       USER NAME (USER)
          The argument field is a TELNET string identifying the user.
          The user identification is that which is required by the
          server for access to its file system.  This command will
          normally be the first command transmitted by the user after
          the TELNET connections are made (some servers may require
          this).  Additional identification information in the form of
          a password and/or an account command may also be required by
          some servers.  Servers may allow a new USER command to be
          entered at any point in order to change the access control
          and/or accounting information.  This has the effect of
          flushing any user, password, and account information already
          supplied and beginning the login sequence again.  All
          transfer parameters are unchanged and any file transfer in
          progress is completed under the old account.
       PASSWORD (PASS)
          The argument field is a TELNET string identifying the user's
          password.  This command must be immediately preceded by the
          user name command, and, for some sites, completes the user's
          identification for access control.  Since password
          information is quite sensitive, it is desirable in general
          to "mask" it or suppress typeout.  It appears that the
          server has no foolproof way to achieve this.  It is
          therefore the responsibility of the user-FTP process to hide
          the sensitive password information.
       ACCOUNT (ACCT)
          The argument field is a TELNET string identifying the user's
          account.  The command is not necessarily related to the USER
          command, as some sites may require an account for login and
          others only for specific access, such as storing files.  In
          the latter case the command may arrive at any time.
          There are reply codes to differentiate these cases for the
          automaton: when account information is required for login,
          the response to a successful PASSword command is reply code
          332.  On the other hand, if account information is NOT
          required for login, the reply to a successful PASSword
          command is 230; and if the account information is needed for
          a command issued later in the dialogue, the server should
                                 22

IEN 149 June 1980 RFC 765 File Transfer Protocol

          return a 332 or 532 reply depending on whether he stores
          (pending receipt of the ACCounT command) or discards the
          command, respectively.
       REINITIALIZE (REIN)
          This command terminates a USER, flushing all I/O and account
          information, except to allow any transfer in progress to be
          completed.  All parameters are reset to the default settings
          and the TELNET connection is left open.  This is identical
          to the state in which a user finds himself immediately after
          the TELNET connection is opened.  A USER command may be
          expected to follow.
       LOGOUT (QUIT)
          This command terminates a USER and if file transfer is not
          in progress, the server closes the TELNET connection.  If
          file transfer is in progress, the connection will remain
          open for result response and the server will then close it.
          If the user-process is transferring files for several USERs
          but does not wish to close and then reopen connections for
          each, then the REIN command should be used instead of QUIT.
          An unexpected close on the TELNET connection will cause the
          server to take the effective action of an abort (ABOR) and a
          logout (QUIT).
    TRANSFER PARAMETER COMMANDS
       All data transfer parameters have default values, and the
       commands specifying data transfer parameters are required only
       if the default parameter values are to be changed.  The default
       value is the last specified value, or if no value has been
       specified, the standard default value as stated here.  This
       implies that the server must "remember" the applicable default
       values.  The commands may be in any order except that they must
       precede the FTP service request.  The following commands
       specify data transfer parameters.
       DATA PORT (PORT)
          The argument is a HOST-PORT specification for the data port
          to be used in data connection.  There defaults for both the
          user and server data ports, and under normal circumstances
          this command and its reply are not needed.  If this command
                                 23

June 1980 IEN 149 File Transfer Protocol RFC 765

          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:
                                 25

June 1980 IEN 149 File Transfer Protocol RFC 765

       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.
                                 27

June 1980 IEN 149 File Transfer Protocol RFC 765

       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

June 1980 IEN 149 File Transfer Protocol RFC 765

          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 File Transfer Protocol RFC 765

       1. User system inserts the TELNET "Interrupt Process" (IP)
          signal in the TELNET stream.
       2. User system sends the TELNET "Synch" signal
       3. User system inserts the command (e.g., ABOR) in the TELNET
          stream.
       4. Server PI,, after receiving "IP", scans the TELNET stream
          for EXACTLY ONE FTP command.
    (For other servers this may not be necessary but the actions
    listed above should have no unusual effect.)
 FTP REPLIES
    Replies to File Transfer Protocol commands are devised to ensure
    the synchronization of requests and actions in the process of file
    transfer, and to guarantee that the user process always knows the
    state of the Server. Every command must generate at least one
    reply, although there may be more than one; in the latter case,
    the multiple replies must be easily distinguished.  In addition,
    some commands occur in sequential groups, such as USER, PASS and
    ACCT, or RNFR and RNTO.  The replies show the existence of an
    intermediate state if all preceding commands have been successful.
    A failure at any point in the sequence necessitates the repetition
    of the entire sequence from the beginning.
       The details of the command-reply sequence are made explicit in
       a set of state diagrams below.
    An FTP reply consists of a three digit number (transmitted as
    three alphanumeric characters) followed by some text.  The number
    is intended for use by automata to determine what state to enter
    next; the text is intended for the human user.  It is intended
    that the three digits contain enough encoded information that the
    user-process (the User-PI) will not need to examine the text and
    may either discard it or pass it on to the user, as appropriate.
    In particular, the text may be server-dependent, so there are
    likely to be varying texts for each reply code.
    Formally, a reply is defined to contain the 3-digit code, followed
    by Space <SP>, followed by one line of text (where some maximum
    line length has been specified), and terminated by the TELNET
    end-of-line code.  There will be cases, however, where the text is
    longer than a single line.  In these cases the complete text must
                                 32

IEN 149 June 1980 RFC 765 File Transfer Protocol

    be bracketed so the User-process knows when it may stop reading
    the reply (i.e. stop processing input on the TELNET connection)
    and go do other things.  This requires a special format on the
    first line to indicate that more than one line is coming, and
    another on the last line to designate it as the last.  At least
    one of these must contain the appropriate reply code to indicate
    the state of the transaction.  To satisfy all factions it was
    decided that both the first and last line codes should be the
    same.
       Thus the format for multi-line replies is that the first line
       will begin with the exact required reply code, followed
       immediately by a Hyphen, "-" (also known as Minus), followed by
       text.  The last line will begin with the same code, followed
       immediately by Space <SP>, optionally some text, and the TELNET
       end-of-line code.
          For example:
                              123-First line
                              Second line
                                234 A line beginning with numbers
                              123 The last line
       The user-process then simply needs to search for the second
       occurrence of the same reply code, followed by <SP> (Space), at
       the beginning of a line, and ignore all intermediary lines.  If
       an intermediary line begins with a 3-digit number, the Server
       must pad the front to avoid confusion.
          This scheme allows standard system routines to be used for
          reply information (such as for the STAT reply), with
          "artificial" first and last lines tacked on.  In the rare
          cases where these routines are able to generate three digits
          and a Space at the beginning of any line, the beginning of
          each text line should be offset by some neutral text, like
          Space.
       This scheme assumes that multi-line replies may not be nested.
       We  have found that, in general, nesting of replies will not
       occur, except for random system messages (also called
       spontaneous replies) which may interrupt another reply.  System
       messages (i.e. those not processed by the FTP server) will NOT
       carry reply codes and may occur anywhere in the command-reply
       sequence.  They may be ignored by the User-process as they are
       only information for the human user.
                                 33

June 1980 IEN 149 File Transfer Protocol RFC 765

    The three digits of the reply each have a special significance.
    This is intended to allow a range of very simple to very
    sophisticated response by the user-process.  The first digit
    denotes whether the response is good, bad or incomplete.
    (Referring to the state diagram) an unsophisticated user-process
    will be able to determine its next action (proceed as planned,
    redo, retrench, etc.) by simply examining this first digit.  A
    user-process that wants to know approximately what kind of error
    occurred (e.g. file system error, command syntax error) may
    examine the second digit, reserving the third digit for the finest
    gradation of information (e.g. RNTO command without a preceding
    RNFR.)
       There are five values for the first digit of the reply code:
          1yz   Positive Preliminary reply
             The requested action is being initiated; expect another
             reply before proceeding with a new command.  (The
             user-process sending another command before the
             completion reply would be in violation of protocol; but
             server-FTP processes should queue any commands that
             arrive while a preceding command is in progress.)  This
             type of reply can be used to indicate that the command
             was accepted and the user-process may now pay attention
             to the data connections, for implementations where
             simultaneous monitoring is difficult.
          2yz   Positive Completion reply
             The requested action has been successfully completed.  A
             new request may be initiated.
          3yz   Positive Intermediate reply
             The command has been accepted, but the requested action
             is being held in abeyance, pending receipt of further
             information.  The user should send another command
             specifying this information.  This reply is used in
             command sequence groups.
          4yz   Transient Negative Completion reply
             The command was not accepted and the requested action did
             not take place, but the error condition is temporary and
             the action may be requested again.  The user should
                                 34

IEN 149 June 1980 RFC 765 File Transfer Protocol

             return to the beginning of the command sequence, if any.
             It is difficult to assign a meaning to "transient",
             particularly when two distinct sites (Server and
             User-processes) have to agree on the interpretation.
             Each reply in the 4yz category might have a slightly
             different time value, but the intent is that the
             user-process is encouraged to try again.  A rule of thumb
             in determining if a reply fits into the 4yz or the 5yz
             (Permanent Negative) category is that replies are 4yz if
             the commands can be repeated without any change in
             command form or in properties of the User or Server (e.g.
             the command is spelled the same with the same arguments
             used; the user does not change his file access or user
             name; the server does not put up a new implementation.)
          5yz   Permanent Negative Completion reply
             The command was not accepted and the requested action did
             not take place.  The User-process is discouraged from
             repeating the exact request (in the same sequence).  Even
             some "permanent" error conditions can be corrected, so
             the human user may want to direct his User-process to
             reinitiate the command sequence by direct action at some
             point in the future (e.g. after the spelling has been
             changed, or the user has altered his directory status.)
       The following function groupings are encoded in the second
       digit:
          x0z   Syntax - These replies refer to syntax errors,
                syntactically correct  commands that don't fit any
                functional category, unimplemented or superfluous
                commands.
          x1z   Information -  These are replies to requests for
                information, such as status or help.
          x2z   Connections - Replies referring to the TELNET and data
                connections.
          x3z   Authentication and accounting - Replies for the login
                process and accounting procedures.
          x4z   Unspecified as yet
                                 35

June 1980 IEN 149 File Transfer Protocol RFC 765

          x5z   File system - These replies indicate the status of the
                Server file system vis-a-vis the requested transfer or
                other file system action.
       The third digit gives a finer gradation of meaning in each of
       the function categories, specified by the second digit.  The
       list of replies below will illustrate this.  Note that the text
       associated with each reply is recommended, rather than
       mandatory, and may even change according to the command with
       which it is associated.  The reply codes, on the other hand,
       must strictly follow the specifications in the last section;
       that is, Server implementations should not invent new codes for
       situations that are only slightly different from the ones
       described here, but rather should adapt codes already defined.
          A command such as TYPE or ALLO whose successful execution
          does not offer the user-process any new information will
          cause a 200 reply to be returned.  If the command is not
          implemented by a particular Server-FTP process because it
          has no relevance to that computer system, for example ALLO
          at a TOPS20 site, a Positive Completion reply is still
          desired so that the simple User-process knows it can proceed
          with its course of action.  A 202 reply is used in this case
          with, for example, the reply text:  "No storage allocation
          necessary."  If, on the other hand, the command requests a
          non-site-specific action and is unimplemented, the response
          is 502.  A refinement of that is the 504 reply for a command
          that IS implemented, but that requests an unimplemented
          parameter.
    Reply Codes by Function Groups
       200 Command okay
       500 Syntax error, command unrecognized
          [This may include errors such as command line too long.]
       501 Syntax error in parameters or arguments
       202 Command not implemented, superfluous at this site.
       502 Command not implemented
       503 Bad sequence of commands
       504 Command not implemented for that parameter
        
       110 Restart marker reply.
                                 36

IEN 149 June 1980 RFC 765 File Transfer Protocol

          In this case the text is exact and not left to the
          particular implementation; it must read:
               MARK yyyy = mmmm
          where yyyy is User-process data stream marker, and mmmm
          server's equivalent marker.  (note the spaces between
          markers and "=".)
       119 Terminal not available, will try mailbox.
       211 System status, or system help reply
       212 Directory status
       213 File status
       214 Help message
          (on how to use the server or the meaning of a particular
          non-standard command.  This reply is useful only to the
          human user.)
       215 <scheme> is the preferred scheme.
        
       120 Service ready in nnn minutes
       220 Service ready for new user
       221 Service closing TELNET connection
          (logged out if appropriate)
       421 Service not available, closing TELNET connection.
          This may be a reply to any command if the service knows it
          must shut down.]
       125 Data connection already open; transfer starting
       225 Data connection open; no transfer in progress
       425 Can't open data connection
       226 Closing data connection;
          requested file action successful (for example, file transfer
          or file abort.)
       426 Connection closed; transfer aborted.
       227 Entering Passive Mode.  h1,h2,h3,h4,p1,p2
        
       230 User logged in, proceed
       530 Not logged in
       331 User name okay, need password
       332 Need account for login
       532 Need account for storing files
        
       150 File status okay; about to open data connection.
       151 User not local; Will forward to <user>@<host>.
       152 User Unknown; Mail will be forwarded by the operator.
       250 Requested file action okay, completed.
       350 Requested file action pending further information
       450 Requested file action not taken:
          file unavailable (e.g. file busy)
       550 Requested action not taken:
                                 37

June 1980 IEN 149 File Transfer Protocol RFC 765

          file unavailable (e.g. file not found, no access)
       451 Requested action aborted: local error in processing
       551 Requested action aborted: page type unknown
       452 Requested action not taken:
          insufficient storage space in system
       552 Requested file action aborted:
          exceeded storage allocation (for current directory or
          dataset)
       553 Requested action not taken:
          file name not allowed
       354 Start mail input; end with <CR><LF>.<CR><LF>
       
    Numeric Order List of Reply Codes
       110 Restart marker reply.
          In this case the text is exact and not left to the
          particular implementation; it must read:
               MARK yyyy = mmmm
          where yyyy is User-process data stream marker, and mmmm
          server's equivalent marker.  (note the spaces between
          markers and "=".)
       119 Terminal not available, will try mailbox.
       120 Service ready in nnn minutes
       125 Data connection already open; transfer starting
       150 File status okay; about to open data connection.
       151 User not local; Will forward to <user>@<host>.
       152 User Unknown; Mail will be forwarded by the operator.
       200 Command okay
       202 Command not implemented, superfluous at this site.
       211 System status, or system help reply
       212 Directory status
       213 File status
       214 Help message
          (on how to use the server or the meaning of a particular
          non-standard command.  This reply is useful only to the
          human user.)
       215 <scheme> is the preferred scheme.
       220 Service ready for new user
       221 Service closing TELNET connection
          (logged out if appropriate)
       225 Data connection open; no transfer in progress
       226 Closing data connection;
          requested file action successful (for example, file transfer
          or file abort.)
       227 Entering Passive Mode.  h1,h2,h3,h4,p1,p2
                                 38

IEN 149 June 1980 RFC 765 File Transfer Protocol

       230 User logged in, proceed
       250 Requested file action okay, completed.
       331 User name okay, need password
       332 Need account for login
       350 Requested file action pending further information
       354 Start mail input; end with <CR><LF>.<CR><LF>
       421 Service not available, closing TELNET connection.
          This may be a reply to any command if the service knows it
          must shut down.]
       425 Can't open data connection
       426 Connection closed; transfer aborted.
       450 Requested file action not taken:
          file unavailable (e.g. file busy)
       451 Requested action aborted: local error in processing
       452 Requested action not taken:
          insufficient storage space in system
       500 Syntax error, command unrecognized
          [This may include errors such as command line too long.]
       501 Syntax error in parameters or arguments
       502 Command not implemented
       503 Bad sequence of commands
       504 Command not implemented for that parameter
       530 Not logged in
       532 Need account for storing files
       550 Requested action not taken:
          file unavailable (e.g. file not found, no access)
       551 Requested action aborted: page type unknown
       552 Requested file action aborted:
          exceeded storage allocation (for current directory or
          dataset)
       553 Requested action not taken:
          file name not allowed
       
                                 39

June 1980 IEN 149 File Transfer Protocol RFC 765

DECLARATIVE SPECIFICATIONS

 MINIMUM IMPLEMENTATION
    In order to make FTP workable without needless error messages, the
    following minimum implementation is required for all servers:
       TYPE - ASCII Non-print
       MODE - Stream
       STRUCTURE - File, Record
       COMMANDS - USER, QUIT, PORT,
                  TYPE, MODE, STRU,
                    for the default values
                  RETR, STOR,
                  NOOP.
    The default values for transfer parameters are:
       
       TYPE - ASCII Non-print
       MODE - Stream
       STRU - File
    All Hosts must accept the above as the standard defaults.
 CONNECTIONS
    The server protocol interpreter shall "listen" on Port L.  The
    user or user protocol interpreter shall initiate the full-duplex
    TELNET connection.  Server- and user- processes should follow the
    conventions of the TELNET protocol as specified in the ARPA
    Internet Protocol Handbook.  Servers are under no obligation to
    provide for editing of command lines and may specify that it be
    done in the user Host.  The TELNET connection shall be closed by
    the server at the user's request after all transfers and replies
    are completed.
    The user-DTP must "listen" on the specified data port; this may be
    the default user port (U) or a port specified in the PORT command.
    The server shall initiate the data connection from his own default
    data port (L-1) using the specified user data port.  The direction
    of the transfer and the port used will be determined by the FTP
    service command.
                                 40

IEN 149 June 1980 RFC 765 File Transfer Protocol

    When data is to be transferred between two servers, A and B (refer
    to Figure 2), the user-PI, C, sets up TELNET connections with both
    server-PI's.  One of the servers, say A, is then sent a PASV
    command telling him to "listen" on his data port rather than
    initiate a connection when he receives a transfer service command.
    When the user-PI receives an acknowledgment to the PASV command,
    which includes the identity of the host and port being listened
    on, the user-PI then sends A's port, a, to B in a PORT command; a
    reply is returned.  The user-PI may then send the corresponding
    service commands to A and B.  Server B initiates the connection
    and the transfer proceeds.  The command-reply sequence is listed
    below where the messages are vertically synchronous but
    horizontally asynchronous:
       User-PI - Server A                User-PI - Server B
       ------------------                ------------------
       
       C->A : Connect                    C->B : Connect
       C->A : PASV
       A->C : 227 Entering Passive Mode. A1,A2,A3,A4,a1,a2
                                         C->B : PORT A1,A2,A3,A4,a1,a2
                                         B->C : 200 Okay
       C->A : STOR                       C->B : RETR
                  B->A : Connect to HOST-A, PORT-a
    The data connection shall be closed by the server under the
    conditions described in the Section on Establishing Data
    Connections.  If the server wishes to close the connection after a
    transfer where it is not required, he should do so immediately
    after the file transfer is completed.  He should not wait until
    after a new transfer command is received because the user-process
    will have already tested the data connection to see if it needs to
    do a "listen"; (recall that the user must "listen" on a closed
    data port BEFORE sending the transfer request).  To prevent a race
    condition here, the server sends a reply (226) after closing the
    data connection (or if the connection is left open, a "file
    transfer completed" reply (250) and the user-PI should wait for
    one of these replies before issuing a new transfer command.
                                 41

June 1980 IEN 149 File Transfer Protocol RFC 765

 COMMANDS
    The commands are TELNET character string transmitted over the
    TELNET connections as described in the Section on FTP Commands.
    The command functions and semantics are described in the Section
    on Access Control Commands, Transfer Parameter Commands, FTP
    Service Commands, and Miscellaneous Commands.  The command syntax
    is specified here.
    The commands begin with a command code followed by an argument
    field.  The command codes are four or fewer alphabetic characters.
    Upper and lower case alphabetic characters are to be treated
    identically.  Thus any of the following may represent the retrieve
    command:
       RETR    Retr    retr    ReTr    rETr
    This also applies to any symbols representing parameter values,
    such as A or a for ASCII TYPE.  The command codes and the argument
    fields are separated by one or more spaces.
    The argument field consists of a variable length character string
    ending with the character sequence <CRLF> (Carriage Return,
    Linefeed) for NVT-ASCII representation; for other negotiated
    languages a different end of line character might be used.  It
    should be noted that the server is to take NO action until the end
    of line code is received.
    The syntax is specified below in NVT-ASCII.  All characters in the
    argument field are ASCII characters including any ASCII
    represented decimal integers.  Square brackets denote an optional
    argument field.  If the option is not taken, the appropriate
    default is implied.
                                 42

IEN 149 June 1980 RFC 765 File Transfer Protocol

    The following are the FTP commands:
       USER <SP> <username> <CRLF>
       PASS <SP> <password> <CRLF>
       ACCT <SP> <account information> <CRLF>
       REIN <CRLF>
       QUIT <CRLF>
       PORT <SP> <Host-port> <CRLF>
       PASV <CRLF>
       TYPE <SP> <type code> <CRLF>
       STRU <SP> <structure code> <CRLF>
       MODE <SP> <mode code> <CRLF>
       RETR <SP> <pathname> <CRLF>
       STOR <SP> <pathname> <CRLF>
       APPE <SP> <pathname> <CRLF>
       MLFL [<SP> <ident>] <CRLF>
       MAIL [<SP> <ident>] <CRLF>
       MSND [<SP> <ident>] <CRLF>
       MSOM [<SP> <ident>] <CRLF>
       MSAM [<SP> <ident>] <CRLF>
       MRSQ [<SP> <scheme>] <CRLF>
       MRCP <SP> <ident> <CRLF>
       ALLO <SP> <decimal integer>
           [<SP> R <SP> <decimal integer>] <CRLF>
       REST <SP> <marker> <CRLF>
       RNFR <SP> <pathname> <CRLF>
       RNTO <SP> <pathname> <CRLF>
       ABOR <CRLF>
       DELE <SP> <pathname> <CRLF>
       CWD <SP> <pathname> <CRLF>
       LIST [<SP> <pathname>] <CRLF>
       NLST [<SP> <pathname>] <CRLF>
       SITE <SP> <string> <CRLF>
       STAT [<SP> <pathname>] <CRLF>
       HELP [<SP> <string>] <CRLF>
       NOOP <CRLF>
                                 43

June 1980 IEN 149 File Transfer Protocol RFC 765

    The syntax of the above argument fields (using BNF notation where
    applicable ) is:
       <username> ::= <string>
       <password> ::= <string>
       <account information> ::= <string>
       <string> ::= <char> | <char><string>
       <char> ::= any of the 128 ASCII characters except <CR> and <LF>
       <marker> ::= <pr string>
       <pr string> ::= <pr char> | <pr char><pr string>
       <pr char> ::= printable characters, any
                     ASCII code 33 through 126
       <byte size> ::= any decimal integer 1 through 255
       <Host-port> ::= <Host-number>,<Port-number>
       <Host-number> ::= <number>,<number>,<number>,<number>
       <Port-number> ::= <number>,<number>
       <number> ::= any decimal integer 0 through 255
       <ident> ::= <string>
       <scheme> ::= R | T | ?
       <form code> ::= N | T | C
       <type code> ::= A [<SP> <form code>]
                     | E [<SP> <form code>]
                     | I
                     | L <SP> <byte size>
       <structure code> ::= F | R | P
       <mode code> ::= S | B | C
       <pathname> ::= <string>
                                 44

IEN 149 June 1980 RFC 765 File Transfer Protocol

 SEQUENCING OF COMMANDS AND REPLIES
    The communication between the user and server is intended to be an
    alternating dialogue.  As such, the user issues an FTP command and
    the server responds with a prompt primary reply.  The user should
    wait for this initial primary success or failure response before
    sending further commands.
    Certain commands require a second reply for which the user should
    also wait.  These replies may, for example, report on the progress
    or completion of file transfer or the closing of the data
    connection.  They are secondary replies to file transfer commands.
    One important group of informational replies is the connection
    greetings.  Under normal circumstances, a server will send a 220
    reply, "awaiting input", when the connection is completed.  The
    user should wait for this greeting message before sending any
    commands.  If the server is unable to accept input right away, he
    should send a 120 "expected delay" reply immediately and a 220
    reply when ready.  The user will then know not to hang up if there
    is a delay.
    The table below lists alternative success and failure replies for
    each command.  These must be strictly adhered to; a server may
    substitute text in the replies, but the meaning and action implied
    by the code numbers and by the specific command reply sequence
    cannot be altered.
    Command-Reply Sequences
       In this section, the command-reply sequence is presented.  Each
       command is listed with its possible replies; command groups are
       listed together.  Preliminary replies are listed first (with
       their succeeding replies indented and under them), then
       positive and negative completion, and finally intermediary
       replies with the remaining commands from the sequence
       following.  This listing forms the basis for the state
       diagrams, which will be presented separately.
          Connection Establishment
             120
                220
             220
             421
                                 45

June 1980 IEN 149 File Transfer Protocol RFC 765

          Login
             USER
                230
                530
                500, 501, 421
                331, 332
             PASS
                230
                202
                530
                500, 501, 503, 421
                332
             ACCT
                230
                202
                530
                500, 501, 503, 421
          Logout
             QUIT
                221
                500
             REIN
                120
                   220
                220
                421
                500, 502
          Transfer parameters
             PORT
                200
                500, 501, 421, 530
             PASV
                227
                500, 501, 502, 421, 530
             MODE, TYPE, STRU
                200
                500, 501, 504, 421, 530
          File action commands
             ALLO
                200
                202
                500, 501, 504, 421, 530
             REST
                500, 501, 502, 421, 530
                350
                                 46

IEN 149 June 1980 RFC 765 File Transfer Protocol

             STOR
                125, 150
                   (110)
                   226, 250
                   425, 426, 451, 551, 552
                532, 450, 452, 553
                500, 501, 421, 530
             RETR
                125, 150
                   (110)
                   226, 250
                   425, 426, 451
                450, 550
                500, 501, 421, 530
             LIST, NLST
                125, 150
                   226, 250
                   425, 426, 451
                450
                500, 501, 502, 421, 530
             APPE
                125, 150
                   (110)
                   226, 250
                   425, 426, 451, 551, 552
                532, 450, 550, 452, 553
                500, 501, 502, 421, 530
             MLFL
                125, 150, 151, 152
                   226, 250
                   425, 426, 451, 552
                532, 450, 550, 452, 553
                500, 501, 502, 421, 530
             RNFR
                450, 550
                500, 501, 502, 421, 530
                350
             RNTO
                250
                532, 553
                500, 501, 502, 503, 421, 530
             DELE, CWD
                250
                450, 550
                500, 501, 502, 421, 530
                                 47

June 1980 IEN 149 File Transfer Protocol RFC 765

             ABOR
                225, 226
                500, 501, 502, 421
             MAIL, MSND
                151, 152
                   354
                      250
                      451, 552
                354
                   250
                   451, 552
                450, 550, 452, 553
                500, 501, 502, 421, 530
             MSOM, MSAM
                119, 151, 152
                   354
                      250
                      451, 552
                354
                   250
                   451, 552
                450, 550, 452, 553
                500, 501, 502, 421, 530
             MRSQ
                200, 215
                500, 501, 502, 421, 530
             MRCP
                151, 152
                   200
                200
                450, 550, 452, 553
                500, 501, 502, 503, 421
          Informational commands
             STAT
                211, 212, 213
                450
                500, 501, 502, 421, 530
             HELP
                211, 214
                500, 501, 502, 421
          Miscellaneous commands
             SITE
                200
                202
                500, 501, 530
                                 48

IEN 149 June 1980 RFC 765 File Transfer Protocol

             NOOP
                200
                500 421
                                 49

June 1980 IEN 149 File Transfer Protocol RFC 765

STATE DIAGRAMS

 Here we present state diagrams for a very simple minded FTP
 implementation. Only the first digit of the reply codes is used.
 There is one state diagram for each group of FTP commands or command
 sequences.
 The command groupings were determined by constructing a model for
 each command then collecting together the commands with structurally
 identical models.
 For each command or command sequence there are three possible
 outcomes: success (S), failure (F), and error (E). In the state
 diagrams below we use the symbol B for "begin", and the symbol W for
 "wait for reply".
 We first present the diagram that represents the largest group of FTP
 commands:
    
                             1,3    +---+
                        ----------->| E |
                       |            +---+
                       |
    +---+    cmd    +---+    2      +---+
    | B |---------->| W |---------->| S |
    +---+           +---+           +---+
                       |
                       |     4,5    +---+
                        ----------->| F |
                                    +---+
    
    This diagram models the commands:
       ABOR, ALLO, DELE, CWD, HELP, MODE, MRCP, MRSQ, NOOP, PASV,
       QUIT, SITE, PORT, STAT, STRU, TYPE.
                                 50

IEN 149 June 1980 RFC 765 File Transfer Protocol

 The other large group of commands is represented by a very similar
 diagram:
    
                             3      +---+
                        ----------->| E |
                       |            +---+
                       |
    +---+    cmd    +---+    2      +---+
    | B |---------->| W |---------->| S |
    +---+       --->+---+           +---+
               |     | |
               |     | |     4,5    +---+
               |  1  |  ----------->| F |
                -----               +---+
    
    This diagram models the commands:
       APPE, LIST, MLFL, NLST, REIN, RETR, STOR.
 Note that this second model could also be used to represent the first
 group of commands, the only difference being that in the first group
 the 100 series replies are unexpected and therefore treated as error,
 while the second group expects (some may require) 100 series replies.
 The remaining diagrams model command sequences, perhaps the simplest
 of these is the rename sequence:
    
    +---+   RNFR    +---+    1,2    +---+
    | B |---------->| W |---------->| E |
    +---+           +---+        -->+---+
                     | |        |
              3      | | 4,5    |
       --------------  ------   |
      |                      |  |   +---+
      |               ------------->| S |
      |              |   1,3 |  |   +---+
      |             2|  --------
      |              | |     |
      V              | |     |
    +---+   RNTO    +---+ 4,5 ----->+---+
    |   |---------->| W |---------->| F |
    +---+           +---+           +---+
    
                                 51

June 1980 IEN 149 File Transfer Protocol RFC 765

 A very similar diagram models the Mail and Send commands:
    
                 ----  1
                |    |
    +---+  cmd   -->+---+     2     +---+
    | B |---------->| W |---------->| E |
    +---+           +---+        -->+---+
                     | |        |
              3      | | 4,5    |
       --------------  ------   |
      |                      |  |   +---+
      |               ------------->| S |
      |              |   1,3 |  |   +---+
      |             2|  --------
      |              | |     |
      V              | |     |
    +---+   text    +---+ 4,5 ----->+---+
    |   |---------->| W |---------->| F |
    +---+           +---+           +---+
    
       This diagram models the commands:
          MAIL, MSND, MSOM, MSAM.
    Note that the "text" here is a series of lines sent from the user
    to the server with no response expected until the last line is
    sent, recall that the last line must consist only of a single
    period.
                                 52

IEN 149 June 1980 RFC 765 File Transfer Protocol

 The next diagram is a simple model of the Restart command:
    
    +---+   REST    +---+    1,2    +---+
    | B |---------->| W |---------->| E |
    +---+           +---+        -->+---+
                     | |        |
              3      | | 4,5    |
       --------------  ------   |
      |                      |  |   +---+
      |               ------------->| S |
      |              |   3   |  |   +---+
      |             2|  --------
      |              | |     |
      V              | |     |
    +---+   cmd     +---+ 4,5 ----->+---+
    |   |---------->| W |---------->| F |
    +---+        -->+---+           +---+
                |      |
                |  1   |
                 ------
       Where "cmd" is APPE, STOR, RETR, or MLFL.
 We note that the above three models are similar, in fact the Mail
 diagram and the Rename diagram are structurally identical. The
 Restart differs from the other two only in the treatment of 100
 series replies at the second stage.
                                 53

June 1980 IEN 149 File Transfer Protocol RFC 765

 The most complicated diagram is for the Login sequence:
    
                          1
    +---+   USER    +---+------------->+---+
    | B |---------->| W | 2       ---->| E |
    +---+           +---+------  |  -->+---+
                     | |       | | |
                   3 | | 4,5   | | |
       --------------   -----  | | |
      |                      | | | |
      |                      | | | |
      |                 ---------  |
      |               1|     | |   |
      V                |     | |   |
    +---+   PASS    +---+ 2  |  ------>+---+
    |   |---------->| W |------------->| S |
    +---+           +---+   ---------->+---+
                     | |   | |     |
                   3 | |4,5| |     |
       --------------   --------   |
      |                    | |  |  |
      |                    | |  |  |
      |                 -----------
      |             1,3|   | |  |
      V                |  2| |  |
    +---+   ACCT    +---+--  |   ----->+---+
    |   |---------->| W | 4,5 -------->| F |
    +---+           +---+------------->+---+
                                 54

IEN 149 June 1980 RFC 765 File Transfer Protocol

 Finally we present a generalized diagram that could be used to model
 the command and reply interchange:
    
             ------------------------------------
            |                                    |
    Begin   |                                    |
      |     V                                    |
      |   +---+  cmd   +---+ 2         +---+     |
       -->|   |------->|   |---------->|   |     |
          |   |        | W |           | S |-----|
       -->|   |     -->|   |-----      |   |     |
      |   +---+    |   +---+ 4,5 |     +---+     |
      |     |      |    | |      |               |
      |     |      |   1| |3     |     +---+     |
      |     |      |    | |      |     |   |     |
      |     |       ----  |       ---->| F |-----
      |     |             |            |   |
      |     |             |            +---+
       -------------------
            |
            |
            V
           End
    
                                 55

June 1980 IEN 149 File Transfer Protocol RFC 765

TYPICAL FTP SCENARIO

 User at Host U wanting to transfer files to/from Host S:
 In general the user will communicate to the server via a mediating
 user-FTP process.  The following may be a typical scenario.  The
 user-FTP prompts are shown in parentheses, '---->' represents
 commands from Host U to Host S, and '<----' represents replies from
 Host S to Host U.
    LOCAL COMMANDS BY USER              ACTION INVOLVED
    ftp (host) multics<CR>         Connect to Host S, port L,
                                   establishing TELNET connections
                                   <---- 220 Service ready <CRLF>
    username Doe <CR>              USER Doe<CRLF>---->
                                   <---- 331 User name ok,
                                             need password<CRLF>
    password mumble <CR>           PASS mumble<CRLF>---->
                                   <---- 230 User logged in.<CRLF>
    retrieve (local type) ASCII<CR>
    (local pathname) test 1 <CR>   User-FTP opens local file in ASCII.
    (for.pathname) test.pl1<CR>    RETR test.pl1<CRLF> ---->
                                   <---- 150 File status okay;
                                         about to open data connection
                                   Server makes data connection
                                   to port U
    <CRLF>
                                   <---- 226 Closing data connection,
                                       file transfer successful<CRLF>
    type Image<CR>                 TYPE I<CRLF> ---->
                                   <---- 200 Command OK<CRLF>
    store (local type) image<CR>
    (local pathname) file dump<CR> User-FTP opens local file in Image.
    (for.pathname) >udd>cn>fd<CR>  STOR >udd>cn>fd<CRLF> ---->
                                   <---- 450 Access denied<CRLF>
    terminate                      QUIT <CRLF> ---->
                                   Server closes all
                                   connections.
                                 56

IEN 149 June 1980 RFC 765 File Transfer Protocol

CONNECTION ESTABLISHMENT

 The FTP control connection is established via TCP between the user
 process port U and the server process port L.  This protocol is
 assigned the service port 21 (25 octal), that is L=21.
                                 57

June 1980 IEN 149 File Transfer Protocol RFC 765

APPENDIX ON MAIL

 The basic commands transmitting mail are the MAIL and the MLFL
 commands.  These commands cause the transmitted data to be entered
 into the recipients mailbox.
    MAIL <SP> <recipient name> <CRLF>
       If accepted, returns 354 reply and considers all succeeding
       lines to be the message text, terminated by a line containing
       only a period, upon which a 250 completion reply is returned.
       Various errors are possible.
    MLFL <SP> <recipient name> <CRLF>
       If accepted, acts like a STOR command, except that the data is
       considered to be the message text.  Various errors are
       possible.
 There are two possible preliminary replies that a server may use to
 indicate that it is accepting mail for a user whose mailbox is not at
 that server.
    151 User not local; Will forward to <user>@<host>.
       This reply indicates that the server knows the user's mailbox
       is on another host and will take responsibility for forwarding
       the mail to that host.  For example, at BBN (or ISI) there are
       several host which each have a list of many of the users on
       several of the host.  These hosts then can accept mail for any
       user on their list and forward it to the correct host.
    152 User Unknown; Mail will be forwarded by the operator.
       This reply indicates that the host does not recognize the user
       name, but that it will accept the mail and have the operator
       attempt to deliver it.  This is useful if the user name is
       misspelled, but may be a disservice if the mail is really
       undeliverable.
 Three FTP commands provide for "sending" a message to a logged-in
 user's terminal, as well as variants for mailing it normally whether
 the user is logged in or not.
                                 58

IEN 149 June 1980 RFC 765 File Transfer Protocol

    MSND -- SeND to terminal.
       Returns 450 failure reply if the addressee is refusing or not
       logged in.
    MSOM -- Send to terminal Or Mailbox.
       Returns 119 notification reply if terminal is not accessible.
    MSAM -- Send to terminal And Mailbox.
       Returns 119 notification reply if terminal is not accessible.
 Note that for MSOM and MSAM, it is the mailing which determines
 success, not the sending, although MSOM as implemented uses a 119
 reply (in addition to the normal success/failure code) to indicate
 that because the SEND failed, an attempt is being made to mail the
 message instead.  There are no corresponding variants for MLFL, since
 messages transmitted in this way are generally short.
 There are two FTP commands which allow one to mail the text of a
 message to several recipients simultaneously; such message
 transmission is far more efficient than the practice of sending the
 text again and again for each additional recipient at a site.
 There are two basic ways of sending a single text to several
 recipients.  In one, all recipients are specified first, and then the
 text is sent; in the other, the order is reversed and the text is
 sent first, followed by the recipients.  Both schemes are necessary
 because neither by itself is optimal for all systems, as will be
 explained later.  To select a particular scheme, the MRSQ command is
 used; to specify recipients after a scheme is chosen, MRCP commands
 are given; and to furnish text, the MAIL or MLFL commands are used.
 Scheme Selection: MRSQ
    MRSQ is the means by which a user program can test for
    implementation of MRSQ/MRCP, select a particular scheme, reset its
    state thereof, and even do some rudimentary negotiation.  Its
    format is like that of the TYPE command, as follows:
                                 59

June 1980 IEN 149 File Transfer Protocol RFC 765

       MRSQ [<SP> <scheme>] <CRLF>
       <scheme> = a single character.  The following are defined:
          R  Recipients first.  If not implemented, T must be.
          T  Text first.  If this is not implemented, R must be.
          ?  Request for preference.  Must always be implemented.
          No argument means a "selection" of none of the schemes (the
          default).
       Replies:
          200 OK, we'll use specified scheme.
          215 <scheme> This is the scheme I prefer.
          501 I understand MRSQ but can't use that scheme.
          5xx Command unrecognized or unimplemented.
    Three aspects of MRSQ need to be pointed out here.  The first is
    that an MRSQ with no argument must always return a 200 reply and
    restore the default state of having no scheme selected.  Any other
    reply implies that MRSQ and hence MRCP are not understood or
    cannot be performed correctly.
    The second is that the use of "?" as a <scheme> asks the FTP
    server to return a 215 reply in which the server specifies a
    "preferred" scheme.  The format of this reply is simple:
       215 <SP> <scheme> [<SP> <arbitrary text>] <CRLF>
       Any other reply (e.g. 4xx or 5xx) implies that MRSQ and MRCP
       are not implemented, because "?" must always be implemented if
       MRSQ is.
    The third important thing about MRSQ is that it always has the
    side effect of resetting all schemes to their initial state.  This
    reset must be done no matter what the reply will be - 200, 215, or
    501.  The actions necessary for a reset will be explained when
    discussing how each scheme actually works.
 Message Text Specification: MAIL/MLFL
    Regardless of which scheme (if any) has been selected, a MAIL or
    MLFL with a non-null argument will behave exactly as before; the
    MRSQ/MRCP commands have no effect on them.  However, such normal
    MAIL/MLFL commands do have the same side effect as MRSQ; they
    "reset" the current scheme to its initial state.
                                 60

IEN 149 June 1980 RFC 765 File Transfer Protocol

    It is only when the argument is null (e.g. MAIL<CRLF> or
    MLFL<CRLF>) that the particular scheme being used is important,
    because rather than producing an error (as most servers currently
    do), the server will accept message text for this "null"
    specification; what it does with it depends on which scheme is in
    effect, and will be described in "Scheme Mechanics".
 Recipient specification: MRCP
    In order to specify recipient names  (i.e., idents) and receive
    some acknowledgment (or refusal) for each name, the following
    command is used:
       MRCP <SP> <ident> <CRLF>
       Reply for no scheme:
          503 No scheme specified yet; use MRSQ.
       Replies for scheme T are identical to those for MAIL/MLFL.
       Replies for scheme R (recipients first):
          200 OK, name stored.
          452 Recipient table full, this name not stored.
          553 Recipient name rejected.
          4xx Temporary error, try this name again later.
          5xx Permanent error, report to sender.
    Note that use of this command is an error if no scheme has been
    selected yet; an MRSQ <scheme> must have been given if MRCP is to
    be used.
 Scheme mechanics: MRSQ R (Recipients first)
    In the recipients-first scheme, MRCP is used to specify names
    which the FTP server stores in a list or table.  Normally the
    reply for each MRCP will be either a 200 for acceptance, or a
    4xx/5xx code for rejection; all 5xx codes are permanent rejections
    (e.g. user not known) which should be reported to the human
    sender, whereas 4xx codes in general connote some temporary error
    that may be rectified later.  None of the 4xx/5xx replies impinge
    on previous or succeeding MRCP commands, except for 452 which
    indicates that no further MRCP's will succeed unless a message is
    sent to the already stored recipients or a reset is done.
    Sending message text to stored recipients is done by giving a MAIL
    or MLFL command with no argument; that is, just MAIL<CRLF> or
    MLFL<CRLF>.  Transmission of the message text is exactly the same
    as for normal MAIL/MLFL; however, a positive acknowledgment at the
                                 61

June 1980 IEN 149 File Transfer Protocol RFC 765

    end of transmission means that the message has been sent to ALL
    recipients that were remembered with MRCP, and a failure code
    means that it should be considered to have failed for ALL of these
    specified recipients.  This applies regardless of the actual error
    code; and whether the reply signifies success or failure, all
    stored recipient names are flushed and forgotten - in other words,
    things are reset to their initial state.  This purging of the
    recipient name list must also be done as the "reset" side effect
    of any use of MRSQ.
    A 452 reply to an MRCP can thus be handled by using a MAIL/MLFL to
    specify the message for currently stored recipients, and then
    sending more MRCP's and another MAIL/MLFL, as many times as
    necessary; for example, if a server only had room for 10 names
    this would result in a 50-recipient message being sent 5 times, to
    10 different recipients each time.
    If a user attempts to specify message text (MAIL/MLFL with no
    argument) before any successful MRCP's have been given, this
    should be treated exactly as a "normal" MAIL/MLFL with a null
    recipient would be; some servers will return an error of some
    type, such as "550 Null recipient".
    See Example 1 for an example using MRSQ R.
 Scheme mechanics: MRSQ T (Text first)
    In the text-first scheme, MAIL/MLFL with no argument is used to
    specify message text, which the server stores away.  Succeeding
    MRCP's are then treated as if they were MAIL/MLFL commands, except
    that none of the text transfer manipulations are done; the stored
    message text is sent to the specified recipient, and a reply code
    is returned identical to that which an actual MAIL/MLFL would
    invoke. (Note ANY 2xx code indicates success.)
    The stored message text is not forgotten until the next MAIL/MLFL
    or MRSQ, which will either replace it with new text or flush it
    entirely.  Any use of MRSQ will reset this scheme by flushing
    stored text, as will any use of MAIL/MLFL with a non-null
    argument.
    If an MRCP is seen before any message text has been stored, the
    user in effect is trying to send a null message; some servers
    might allow this, others would return an error code.
    See Example 2 for an example using MRSQ T.
                                 62

IEN 149 June 1980 RFC 765 File Transfer Protocol

 Why two schemes anyway?
    Because neither by itself is optimal for all systems.  MRSQ R
    allows more of a "bulk" mailing, because everything is saved up
    and then mailed simultaneously; this is very useful for systems
    such as ITS where the FTP server does not itself write mail
    directly, but hands it on to a central mailer demon of great
    power; the more information (e.g. recipients) associated with a
    single "hand-off", the more efficiently mail can be delivered.
    By contrast, MRSQ T is geared to FTP servers which want to deliver
    mail directly, in one-by-one incremental fashion.  This way they
    can return an individual success/failure reply code for each
    recipient given which may depend on variable file system factors
    such as exceeding disk allocation, mailbox access conflicts, and
    so forth; if they tried to emulate MRSQ R's bulk mailing, they
    would have to ensure that a success reply to the MAIL/MLFL indeed
    meant that it had been delivered to ALL recipients specified - not
    just some.
 Notes:
  • Because these commands are not required in the minimum

implementation of FTP, one must be prepared to deal with sites

      which don't recognize either MRSQ or MRCP.  "MRSQ" and "MRSQ ?"
      are explicitly designed as tests to see whether either scheme is
      implemented; MRCP is not, and a failure return of the
      "unimplemented" variety could be confused with "No scheme
      selected yet", or even with "Recipient unknown".  Be safe, be
      sure, use MRSQ!
  • There is no way to indicate in a positive response to "MRSQ ?"

that the preferred "scheme" for a server is that of the default

      state; i.e. none of the multi-recipient schemes.  The rationale
      is that in this case, it would be pointless to implement
      MRSQ/MRCP at all, and the response would therefore be negative.
  • One reason that the use of MAIL/MLFL is restricted to null

arguments with this multi-recipient extension is the ambiguity

      that would result if a non-null argument were allowed; for
      example, if MRSQ R was in effect and some MRCP's had been given,
      and a MAIL FOO<CRLF> was done, there would be no way to
      distinguish a failure reply for mailbox "FOO" from a global
      failure for all recipients specified.  A similar situation
      exists for MRSQ T; it would not be clear whether the text was
      stored and the mailbox failed, or vice versa, or both.
                                 63

June 1980 IEN 149 File Transfer Protocol RFC 765

  • "Resets" are done by all MRSQ's and "normal" MAIL/MLFL's to

avoid confusion and overly complicated implementation. The MRSQ

      command implies a change or uncertainty of status, and the
      latter commands would otherwise have to use some independent
      mechanisms to avoid clobbering the data bases (e.g., message
      text storage area) used by the T/R schemes.  However, once a
      scheme is selected, it remains "in effect" just as a "TYPE A"
      remains selected.  The recommended way for doing a reset,
      without changing the current selection, is with "MRSQ ?".
      Remember that "MRSQ" alone reverts to the no-scheme state.
  • It is permissible to intersperse other FTP commands among the

MRSQ/MRCP/MAIL sequences.

                                 64

IEN 149 June 1980 RFC 765 File Transfer Protocol

 Example 1
                Example of MRSQ R (Recipients first)
    This is an example of how MRSQ R is used; first the user must
    establish that the server in fact implements MRSQ:
       U: MRSQ
       S: 200 OK, no scheme selected.
    An MRSQ with a null argument always returns a 200 if implemented,
    selecting the "scheme" of null, i.e. none of them.  If MRSQ were
    not implemented, a code of 4xx or 5xx would be returned.
       U: MRSQ R
       S: 200 OK, using that scheme
    All's well; now the recipients can be specified.
       U: MRCP Foo
       S: 200 OK
       U: MRCP Raboof
       S: 553 Who's that?  No such user here.
       U: MRCP bar
       S: 200 OK
    Well, two out of three ain't bad.  Note that the demise of
    "Raboof" has no effect on the storage of "Foo" or "bar".  Now to
    furnish the message text, by giving a MAIL or MLFL with no
    argument:
       U: MAIL
       S: 354 Type mail, ended by <CRLF>.<CRLF>
       U: Blah blah blah blah....etc etc etc
       U: .
       S: 250 Mail sent.
    The text has now been sent to both "Foo" and "bar".
                                 65

June 1980 IEN 149 File Transfer Protocol RFC 765

 Example 2
                   Example of MRSQ T (Text first)
    Using the same message as the previous example:
       U: MRSQ ?
       S: 215 T Text first, please.
    MRSQ is indeed implemented, and the server says that it prefers
    "T", but that needn't stop the user from trying something else:
       U: MRSQ R
       S: 501 Sorry, I really can't do that.
    It's possible that it could have understood "R" also, but in
    general it's best to use the "preferred" scheme, since the server
    knows which is most efficient for its particular site.  Anyway:
       U: MRSQ T
       S: 200 OK, using that scheme.
    Scheme "T" is now selected, and the text must be sent:
       U: MAIL
       S: 354 Type mail, ended by <CRLF>.<CRLF>
       U: Blah blah blah blah....etc etc etc
       U: .
       S: 250 Mail stored.
    Now recipients can be specified:
       U: MRCP Foo
       S: 250 Stored mail sent.
       U: MRCP Raboof
       S: 553 Who's that?  No such user here.
       U: MRCP bar
       S: 250 Stored mail sent.
                                 66

IEN 149 June 1980 RFC 765 File Transfer Protocol

    Again, the text has now been sent to both "Foo" and "bar", and
    still remains stored.  A new message can be sent with another
    MAIL/MRCP... sequence, but the fastidious or paranoid could chose
    to do:
       U: MRSQ ?
       S: 215 T Text first, please.
    Which resets things without altering the scheme in effect.
                                 67

June 1980 IEN 149 File Transfer Protocol RFC 765

APPENDIX ON PAGE STRUCTURE

 The need for FTP to support page structure derives principally from
 the  need to support efficient transmission of files between TOPS20
 systems, particularly the files used by NLS.
 The file system of TOPS20 is based on the concept of pages.  The
 system level is most efficient at manipulating files as pages.
 System level programs provide an interface to the file system so that
 many applications view files as sequential streams of characters.
 However, a few applications use the underlying page structures
 directly, and some of these create holey files.
 A TOPS20 file is just a bunch of words pointed to by a page table.
 If those words contain CRLF's, fine -- but that doesn't mean "record"
 to TOPS20.
 A TOPS20 disk file consists of four things: a pathname, a page table,
 a (possibly empty) set of pages, and a set of attributes.
 The pathname is specified in the RETR or STOR command.  It includes
 the directory name, file name, file name extension, and version
 number.
 The page table contains up to 2**18 entries.  Each entry may be
 EMPTY, or may point to a page.  If it is not empty, there are also
 some page-specific access bits; not all pages of a file need have the
 same access protection.
    A page is a contiguous set of 512 words of 36 bits each.
 The attributes of the file, in the File Descriptor Block (FDB),
 contain such things as creation time, write time, read time, writer's
 byte-size, end of file pointer, count of reads and writes, backup
 system tape numbers, etc.
 Note that there is NO requirement that pages in the page table be
 contiguous.  There may be empty page table slots between occupied
 ones.  Also, the end of file pointer is simply a number.  There is no
 requirement that it in fact point at the "last" datum in the file.
 Ordinary sequential I/O calls in TOPS20 will cause the end of file
 pointer to be left after the last datum written, but other operations
 may cause it not to be so, if a particular programming system so
 requires.
                                 68

IEN 149 June 1980 RFC 765 File Transfer Protocol

 In fact both of these special cases, "holey" files and
 end-of-file pointers not at the end of the file, occur with NLS data
 files.
 The TOPS20 paged files can be sent with the FTP transfer parameters:
 TYPE L 36, STRU P, and MODE S (in fact any mode could be used).
 Each page of information has a header.  Each header field, which is a
 logical byte, is a TOPS20 word, since the TYPE is L 36.
 The header fields are:
    Word 0: Header Length.
       The header length is 5.
    Word 1: Page Index.
       If the data is a disk file page, this is the number of that
       page in the file's page map.  Empty pages (holes) in the file
       are simply not sent.  Note that a hole is NOT the same as a
       page of zeros.
    Word 2: Data Length.
       The number of data words in this page, following the header.
       Thus the total length of the transmission unit is the Header
       Length plus the Data Length.
    Word 3: Page Type.
       A code for what type of chunk this is. A data page is type 3,
       the FDB page is type 2.
    Word 4: Page Access Control.
       The access bits associated with the page in the file's page
       map.  (This full word quantity is put into AC2 of an SPACS by
       the program reading from net to disk.)
 After the header are Data Length data words.  Data Length is
 currently either 512 for a data page or 21 for an FDB.  Trailing
 zeros in a disk file page may be discarded, making Data Length less
 than 512 in that case.
                                 69

June 1980 IEN 149 File Transfer Protocol RFC 765

 Data transfers are implemented like the layers of an onion: some
 characters are packaged into a line.  Some lines are packaged into a
 file.  The file is broken into other manageable units for
 transmission.  Those units have compression applied to them.  The
 units may be flagged by restart markers.  On the other end, the
 process is reversed.
                                 70
/data/webs/external/dokuwiki/data/pages/rfc/ien/ien149.txt · Last modified: 2001/06/25 19:12 by 127.0.0.1

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