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rfc:rfc959

Network Working Group J. Postel Request for Comments: 959 J. Reynolds

                                                                   ISI

Obsoletes RFC: 765 (IEN 149) October 1985

                    FILE TRANSFER PROTOCOL (FTP)

Status of this Memo

 This memo is the official specification of the File Transfer
 Protocol (FTP).  Distribution of this memo is unlimited.
 The following new optional commands are included in this edition of
 the specification:
    CDUP (Change to Parent Directory), SMNT (Structure Mount), STOU
    (Store Unique), RMD (Remove Directory), MKD (Make Directory), PWD
    (Print Directory), and SYST (System).
 Note that this specification is compatible with the previous edition.

1. 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, personal workstations, and TACs,
 with a simple, and easily implemented protocol design.
 This paper assumes knowledge of the Transmission Control Protocol
 (TCP) [2] and the Telnet Protocol [3].  These documents are contained
 in the ARPA-Internet protocol handbook [1].

2. OVERVIEW

 In this section, the history, 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.

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 2.1.  HISTORY
    FTP has had a long evolution over the years.  Appendix III is a
    chronological compilation of Request for Comments documents
    relating to FTP.  These include the first proposed file transfer
    mechanisms in 1971 that were developed for implementation on hosts
    at M.I.T. (RFC 114), plus comments and discussion in RFC 141.
    RFC 172 provided a user-level oriented protocol for file transfer
    between host computers (including terminal IMPs).  A revision of
    this as RFC 265, restated FTP for additional review, while RFC 281
    suggested further changes.  The use of a "Set Data Type"
    transaction was proposed in RFC 294 in January 1982.
    RFC 354 obsoleted RFCs 264 and 265.  The File Transfer Protocol
    was now defined as a protocol for file transfer between HOSTs on
    the ARPANET, with the primary function of FTP defined as
    transfering files efficiently and reliably among hosts and
    allowing the convenient use of remote file storage capabilities.
    RFC 385 further commented on errors, emphasis points, and
    additions to the protocol, while RFC 414 provided a status report
    on the working server and user FTPs.  RFC 430, issued in 1973,
    (among other RFCs too numerous to mention) presented further
    comments on FTP.  Finally, an "official" FTP document was
    published as RFC 454.
    By July 1973, considerable changes from the last versions of FTP
    were made, but the general structure remained the same.  RFC 542
    was published as a new "official" specification to reflect these
    changes.  However, many implementations based on the older
    specification were not updated.
    In 1974, RFCs 607 and 614 continued comments on FTP.  RFC 624
    proposed further design changes and minor modifications.  In 1975,
    RFC 686 entitled, "Leaving Well Enough Alone", discussed the
    differences between all of the early and later versions of FTP.
    RFC 691 presented a minor revision of RFC 686, regarding the
    subject of print files.
    Motivated by the transition from the NCP to the TCP as the
    underlying protocol, a phoenix was born out of all of the above
    efforts in RFC 765 as the specification of FTP for use on TCP.
    This current edition of the FTP specification is intended to
    correct some minor documentation errors, to improve the
    explanation of some protocol features, and to add some new
    optional commands.

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    In particular, the following new optional commands are included in
    this edition of the specification:
       CDUP - Change to Parent Directory
       SMNT - Structure Mount
       STOU - Store Unique
       RMD - Remove Directory
       MKD - Make Directory
       PWD - Print Directory
       SYST - System
    This specification is compatible with the previous edition.  A
    program implemented in conformance to the previous specification
    should automatically be in conformance to this specification.
 2.2.  TERMINOLOGY
    ASCII
       The ASCII character set is 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.
    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.

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    control connection
       The communication path between the USER-PI and SERVER-PI for
       the exchange of commands and replies.  This connection follows
       the Telnet Protocol.
    data connection
       A full duplex 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.
    DTP
       The data transfer process establishes and manages the data
       connection.  The DTP can be passive or active.
    End-of-Line
       The end-of-line sequence defines the separation of printing
       lines.  The sequence is Carriage Return, followed by Line Feed.
    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.

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    FTP commands
       A set of commands that comprise the control information flowing
       from the user-FTP to the server-FTP process.
    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.
    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.
    PI
       The protocol interpreter.  The user and server sides of the
       protocol have distinct roles implemented in a user-PI and a
       server-PI.

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    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.
    reply
       A reply is an acknowledgment (positive or negative) sent from
       server to user via the control connection 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.
       It 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 server protocol interpreter "listens" on Port L for a
       connection from a user-PI and establishes a control
       communication connection.  It receives standard FTP commands
       from the user-PI, sends replies, and governs the server-DTP.
    type
       The data representation type used for data transfer and
       storage.  Type implies certain transformations between the time
       of data storage and data transfer.  The representation types
       defined in FTP are described in the Section on Establishing
       Data Connections.

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    user
       A person or a process on behalf of a person 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 user protocol interpreter initiates the control 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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 2.3.  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 |
                                          |\----^----/|    --------
                ----------                |     |     |
                |/------\|  FTP Commands  |/----V----\|
                ||Server|<---------------->|   User  ||
                ||  PI  ||   FTP Replies  ||    PI   ||
                |\--^---/|                |\----^----/|
                |   |    |                |     |     |
    --------    |/--V---\|      Data      |/----V----\|    --------
    | 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 control connection.  The control connection follows
    the Telnet protocol.  At the initiation of the user, standard FTP
    commands are generated by the user-PI and transmitted to the
    server process via the control connection.  (The user may
    establish a direct control connection to the server-FTP, from a
    TAC terminal for example, and generate standard FTP commands
    independently, bypassing the user-FTP process.) Standard replies
    are sent from the server-PI to the user-PI over the control
    connection in response to the commands.
    The FTP commands specify the parameters for the data connection
    (data port, transfer mode, representation type, and structure) and
    the nature of file system operation (store, retrieve, append,
    delete, etc.).  The user-DTP or its designate should "listen" on
    the specified data port, and the server initiate the data
    connection and data transfer in accordance with the specified
    parameters.  It should be noted that the data port need not be in

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    the same host that initiates the FTP commands via the control
    connection, but the user or the user-FTP process must ensure a
    "listen" on the specified data port.  It ought to 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 a local host. The user sets up
    control 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.
    
                  Control     ------------   Control
                  ---------->| 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 control connections be open while
    data transfer is in progress.  It is the responsibility of the
    user to request the closing of the control connections when
    finished using the FTP service, while it is the server who takes
    the action.  The server may abort data transfer if the control
    connections are closed without command.
    The Relationship between FTP and Telnet:
       The FTP uses the Telnet protocol on the control connection.
       This can be achieved in two ways: first, the user-PI or the
       server-PI may implement the rules of the Telnet Protocol
       directly in their own procedures; or, second, the user-PI or
       the server-PI may make use of the existing Telnet module in the
       system.
       Ease of implementaion, sharing code, and modular programming
       argue for the second approach.  Efficiency and independence

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       argue for the first approach.  In practice, FTP relies on very
       little of the Telnet Protocol, so the first approach does not
       necessarily involve a large amount of code.

3. DATA TRANSFER FUNCTIONS

 Files are transferred only via the data connection.  The control
 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 the
 "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.
 3.1.  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.  DEC TOPS-20s's generally store NVT-ASCII as five 7-bit
    ASCII characters, left-justified in a 36-bit word. IBM Mainframe's
    store NVT-ASCII as 8-bit EBCDIC codes.  Multics stores NVT-ASCII
    as four 9-bit characters in a 36-bit word.  It is 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

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    that FTP provides for very limited data type representations.
    Transformations desired beyond this limited capability should be
    performed by the user directly.
    3.1.1.  DATA TYPES
       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."
       Note that 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.
       3.1.1.1.  ASCII TYPE
          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 an 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.

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       3.1.1.2.  EBCDIC TYPE
          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.
       3.1.1.3.  IMAGE TYPE
          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.
       3.1.1.4.  LOCAL TYPE
          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
          difference in byte sizes, then the logical bytes should be
          packed contiguously, disregarding transfer byte boundaries
          and with any necessary padding at the end.

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          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 (i.e., 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 that 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.
          In 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.
       3.1.1.5.  FORMAT CONTROL
          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:
          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 should 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.  Therefore, these types have a second parameter
          specifying one of the following three formats:
          3.1.1.5.1.  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.

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             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.
          3.1.1.5.2.  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.
          3.1.1.5.2.  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, p. 606, October 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.
             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.

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    3.1.2.  DATA STRUCTURES
       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 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 Mainframe in fixed length records but on a DEC TOPS-20
       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,

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       <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.
       3.1.2.1.  FILE STRUCTURE
          File structure is the default to be assumed if the STRUcture
          command has not been used.
          In file-structure there is no internal structure and the
          file is considered to be a continuous sequence of data
          bytes.
       3.1.2.2.  RECORD STRUCTURE
          Record structures must be accepted for "text" files (i.e.,
          files with TYPE ASCII or EBCDIC) by all FTP implementations.
          In record-structure the file is made up of sequential
          records.
       3.1.2.3.  PAGE STRUCTURE
          To transmit files that are discontinuous, FTP defines a page
          structure.  Files of this type are sometimes known 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:
             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.

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             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 Controlled Page
                      This type includes an additional header field
                      for paged files with page level access control
                      information.  The header length must be 5.
             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.  See Appendix I for
          further details and a specific case at the page structure.
    A note of caution about parameters:  a file must be stored and
    retrieved with the same parameters if the retrieved version is to

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    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.
 3.2.  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.
    Every FTP implementation must support the use of the default data
    ports, and only the USER-PI can initiate a change to non-default
    ports.
    It is possible for the user to specify an alternate data port by
    use of the PORT command.  The user may want a file dumped on a TAC
    line printer or retrieved from a third party host.  In the latter
    case, the user-PI sets up control connections with both
    server-PI's.  One server is then told (by an FTP command) to
    "listen" for a connection which the other will initiate.  The
    user-PI sends one server-PI a PORT command indicating the data
    port of the other.  Finally, both are sent the appropriate
    transfer commands.  The exact sequence of commands and replies
    sent between the user-controller and the servers is defined in the
    Section on FTP Replies.
    In general, it is the server's responsibility to maintain the data
    connection--to initiate it and to close it.  The exception to this

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    is when the user-DTP is sending the data in a transfer mode that
    requires the connection to be closed to indicate EOF.  The server
    MUST close the data connection under the following conditions:
       1. The server has completed sending data in a transfer mode
          that requires a close to indicate EOF.
       2. The server receives an ABORT command from the user.
       3. The port specification is changed by a command from the
          user.
       4. The control connection is closed legally or otherwise.
       5. An irrecoverable error condition occurs.
    Otherwise the close is a server option, the exercise of which the
    server must indicate to the user-process by either a 250 or 226
    reply only.
 3.3.  DATA CONNECTION MANAGEMENT
    Default Data Connection Ports:  All FTP implementations must
    support use of the default data connection ports, and only the
    User-PI may initiate the use of non-default ports.
    Negotiating Non-Default Data Ports:   The User-PI may specify a
    non-default user side data port with the PORT command.  The
    User-PI may request the server side to identify a non-default
    server side data port with the PASV command.  Since a connection
    is defined by the pair of addresses, either of these actions is
    enough to get a different data connection, still it is permitted
    to do both commands to use new ports on both ends of the data
    connection.
    Reuse of the Data Connection:  When using the stream mode of data
    transfer the end of the file must be indicated by closing the
    connection.  This causes a problem if multiple files are to be
    transfered in the session, due to need for TCP to hold the
    connection record for a time out period to guarantee the reliable
    communication.  Thus the connection can not be reopened at once.
       There are two solutions to this problem.  The first is to
       negotiate a non-default port.  The second is to use another
       transfer mode.
       A comment on transfer modes.  The stream transfer mode is

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       inherently unreliable, since one can not determine if the
       connection closed prematurely or not.  The other transfer modes
       (Block, Compressed) do not close the connection to indicate the
       end of file.  They have enough FTP encoding that the data
       connection can be parsed to determine the end of the file.
       Thus using these modes one can leave the data connection open
       for multiple file transfers.
 3.4.  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 its 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 its internal denotation.  An IBM Mainframe record
    count field may not be recognized at another host, so the
    end-of-record 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.

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    The following transmission modes are defined in FTP:
    3.4.1.  STREAM MODE
       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 a file structure, the EOF is indicated by
       the sending host closing the data connection and all bytes are
       data bytes.
    3.4.2.  BLOCK MODE
       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 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.

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       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 control
       connection (e.g., default--NVT-ASCII).  <SP> (Space, in the
       appropriate language) must not be used WITHIN a restart marker.
       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 |
          +--------+--------+--------+

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    3.4.3.  COMPRESSED MODE
       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:
       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

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       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.
 3.5.  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 control connection (ASCII or EBCDIC).
    The marker could represent a bit-count, a record-count, or any
    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 control 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 control

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    connection and is immediately followed by the command (such as
    RETR, STOR or LIST) which was being executed when the system
    failure occurred.

4. FILE TRANSFER FUNCTIONS

 The communication channel from the user-PI to the server-PI is
 established as a TCP connection from the user to the 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.
 4.1.  FTP COMMANDS
    4.1.1.  ACCESS CONTROL COMMANDS
       The following commands specify access control identifiers
       (command codes are shown in parentheses).
       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 control 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 access control
          parameters.

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       PASSWORD (PASS)
          The argument field is a Telnet string specifying 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
          automation: 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
          return a 332 or 532 reply depending on whether it stores
          (pending receipt of the ACCounT command) or discards the
          command, respectively.
       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.
       CHANGE TO PARENT DIRECTORY (CDUP)
          This command is a special case of CWD, and is included to
          simplify the implementation of programs for transferring
          directory trees between operating systems having different

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          syntaxes for naming the parent directory.  The reply codes
          shall be identical to the reply codes of CWD.  See
          Appendix II for further details.
       STRUCTURE MOUNT (SMNT)
          This command allows the user to mount a different file
          system data structure 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.
       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 control connection is left open.  This is identical
          to the state in which a user finds himself immediately after
          the control 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 control 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 control connection will cause the
          server to take the effective action of an abort (ABOR) and a
          logout (QUIT).
    4.1.2.  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 is 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:

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

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          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.
       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.
    4.1.3.  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 control 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 (e.g., STOR or
       RETR).  The data, when transferred in response to FTP service

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       commands, shall always be sent over the data connection, except
       for certain informative replies.  The following commands
       specify FTP service requests:
       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.
       STORE UNIQUE (STOU)
          This command behaves like STOR except that the resultant
          file is to be created in the current directory under a name
          unique to that directory.  The 250 Transfer Started response
          must include the name generated.
       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.
       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

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          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 skips 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 old pathname of 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
          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 control
          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.

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             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
             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.
       REMOVE DIRECTORY (RMD)
          This command causes the directory specified in the pathname
          to be removed as a directory (if the pathname is absolute)
          or as a subdirectory of the current working directory (if
          the pathname is relative).  See Appendix II.
       MAKE DIRECTORY (MKD)
          This command causes the directory specified in the pathname
          to be created as a directory (if the pathname is absolute)
          or as a subdirectory of the current working directory (if
          the pathname is relative).  See Appendix II.
       PRINT WORKING DIRECTORY (PWD)
          This command causes the name of the current working
          directory to be returned in the reply.  See Appendix II.
       LIST (LIST)
          This command causes a list to be sent from the server to the
          passive DTP.  If the pathname specifies a directory or other
          group of files, 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 directory.  The data transfer is over the data
          connection in type ASCII or type EBCDIC.  (The user must

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          ensure that the TYPE is appropriately ASCII or EBCDIC).
          Since the information on a file may vary widely from system
          to system, this information may be hard to use automatically
          in a program, but may be quite useful to a human user.
       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.)  This command is intended
          to return information that can be used by a program to
          further process the files automatically.  For example, in
          the implementation of a "multiple get" function.
       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.
       SYSTEM (SYST)
          This command is used to find out the type of operating
          system at the server.  The reply shall have as its first
          word one of the system names listed in the current version
          of the Assigned Numbers document [4].
       STATUS (STAT)
          This command shall cause a status response to be sent over
          the control 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

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          transferred over the control 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.
       HELP (HELP)
          This command shall cause the server to send helpful
          information regarding its implementation status over the
          control 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 control 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
 control connection while a data transfer is in progress.  Some

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 servers may not be able to monitor the control and data connections
 simultaneously, in which case some special action will be necessary
 to get the server's attention.  The following ordered format is
 tentatively recommended:
    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.)
 4.2.  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.
    A reply is defined to contain the 3-digit code, followed by Space

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    <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 be bracketed
    so the User-process knows when it may stop reading the reply (i.e.
    stop processing input on the control 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 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.
    The three digits of the reply each have a special significance.
    This is intended to allow a range of very simple to very
    sophisticated responses 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,

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    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.  The server-FTP
             process may send at most, one 1yz reply per command.
          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
             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

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             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 control and
                data connections.
          x3z   Authentication and accounting - Replies for the login
                process and accounting procedures.
          x4z   Unspecified as yet.
          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

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       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.
    4.2.1  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.
        

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       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 "=").
       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 NAME system type.
           Where NAME is an official system name from the list in the
           Assigned Numbers document.
        
       120 Service ready in nnn minutes.
       220 Service ready for new user.
       221 Service closing control connection.
           Logged out if appropriate.
       421 Service not available, closing control 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.
        

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       150 File status okay; about to open data connection.
       250 Requested file action okay, completed.
       257 "PATHNAME" created.
       350 Requested file action pending further information.
       450 Requested file action not taken.
           File unavailable (e.g., file busy).
       550 Requested action not taken.
           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.
       
    4.2.2 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 "=").
       120 Service ready in nnn minutes.
       125 Data connection already open; transfer starting.
       150 File status okay; about to open data connection.
        

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       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 NAME system type.
           Where NAME is an official system name from the list in the
           Assigned Numbers document.
       220 Service ready for new user.
       221 Service closing control 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).
       230 User logged in, proceed.
       250 Requested file action okay, completed.
       257 "PATHNAME" created.
        
       331 User name okay, need password.
       332 Need account for login.
       350 Requested file action pending further information.
        
       421 Service not available, closing control 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.
        

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       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.
       

5. DECLARATIVE SPECIFICATIONS

 5.1.  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.

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 5.2.  CONNECTIONS
    The server protocol interpreter shall "listen" on Port L.  The
    user or user protocol interpreter shall initiate the full-duplex
    control connection.  Server- and user- processes should follow the
    conventions of the Telnet protocol as specified in the
    ARPA-Internet Protocol Handbook [1].  Servers are under no
    obligation to provide for editing of command lines and may require
    that it be done in the user host.  The control 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.
    Note that all FTP implementation must support data transfer using
    the default port, and that only the USER-PI may initiate the use
    of non-default ports.
    When data is to be transferred between two servers, A and B (refer
    to Figure 2), the user-PI, C, sets up control 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:

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       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
                              Figure 3
    The data connection shall be closed by the server under the
    conditions described in the Section on Establishing Data
    Connections.  If the data connection is to be closed following a
    data transfer where closing the connection is not required to
    indicate the end-of-file, the server must do so immediately.
    Waiting until after a new transfer command is not permitted
    because the user-process will have already tested the data
    connection to see if it needs to do a "listen"; (remember 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).
    Any time either the user or server see that the connection is
    being closed by the other side, it should promptly read any
    remaining data queued on the connection and issue the close on its
    own side.
 5.3.  COMMANDS
    The commands are Telnet character strings transmitted over the
    control 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:

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                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, Line
    Feed) 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.

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    5.3.1.  FTP COMMANDS
       The following are the FTP commands:
          USER <SP> <username> <CRLF>
          PASS <SP> <password> <CRLF>
          ACCT <SP> <account-information> <CRLF>
          CWD  <SP> <pathname> <CRLF>
          CDUP <CRLF>
          SMNT <SP> <pathname> <CRLF>
          QUIT <CRLF>
          REIN <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>
          STOU <CRLF>
          APPE <SP> <pathname> <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>
          RMD  <SP> <pathname> <CRLF>
          MKD  <SP> <pathname> <CRLF>
          PWD  <CRLF>
          LIST [<SP> <pathname>] <CRLF>
          NLST [<SP> <pathname>] <CRLF>
          SITE <SP> <string> <CRLF>
          SYST <CRLF>
          STAT [<SP> <pathname>] <CRLF>
          HELP [<SP> <string>] <CRLF>
          NOOP <CRLF>

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    5.3.2.  FTP COMMAND ARGUMENTS
       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> ::= <number>
          <host-port> ::= <host-number>,<port-number>
          <host-number> ::= <number>,<number>,<number>,<number>
          <port-number> ::= <number>,<number>
          <number> ::= any decimal integer 1 through 255
          <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>
          <decimal-integer> ::= any decimal integer

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 5.4.  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, a
    120 "expected delay" reply should be sent immediately and a 220
    reply when ready.  The user will then know not to hang up if there
    is a delay.
    Spontaneous Replies
       Sometimes "the system" spontaneously has a message to be sent
       to a user (usually all users).  For example, "System going down
       in 15 minutes".  There is no provision in FTP for such
       spontaneous information to be sent from the server to the user.
       It is recommended that such information be queued in the
       server-PI and delivered to the user-PI in the next reply
       (possibly making it a multi-line reply).
    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

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       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
          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
             CWD
                250
                500, 501, 502, 421, 530, 550
             CDUP
                200
                500, 501, 502, 421, 530, 550
             SMNT
                202, 250
                500, 501, 502, 421, 530, 550
          Logout
             REIN
                120
                   220
                220
                421
                500, 502
             QUIT
                221
                500

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RFC 959 October 1985 File Transfer Protocol

          Transfer parameters
             PORT
                200
                500, 501, 421, 530
             PASV
                227
                500, 501, 502, 421, 530
             MODE
                200
                500, 501, 504, 421, 530
             TYPE
                200
                500, 501, 504, 421, 530
             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
             STOR
                125, 150
                   (110)
                   226, 250
                   425, 426, 451, 551, 552
                532, 450, 452, 553
                500, 501, 421, 530
             STOU
                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

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             LIST
                125, 150
                   226, 250
                   425, 426, 451
                450
                500, 501, 502, 421, 530
             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
             RNFR
                450, 550
                500, 501, 502, 421, 530
                350
             RNTO
                250
                532, 553
                500, 501, 502, 503, 421, 530
             DELE
                250
                450, 550
                500, 501, 502, 421, 530
             RMD
                250
                500, 501, 502, 421, 530, 550
             MKD
                257
                500, 501, 502, 421, 530, 550
             PWD
                257
                500, 501, 502, 421, 550
             ABOR
                225, 226
                500, 501, 502, 421

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          Informational commands
             SYST
                215
                500, 501, 502, 421
             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
             NOOP
                200
                500 421

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6. 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, CDUP, SMNT, HELP, MODE, NOOP, PASV,
       QUIT, SITE, PORT, SYST, STAT, RMD, MKD, PWD, STRU, and TYPE.

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 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, NLST, REIN, RETR, STOR, and STOU.
 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.
 Remember that at most, one 100 series reply is allowed per command.
 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 |
    +---+           +---+           +---+
    

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 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, or RETR.
 We note that the above three models are similar.  The Restart differs
 from the Rename two only in the treatment of 100 series replies at
 the second stage, while the second group expects (some may require)
 100 series replies.  Remember that at most, one 100 series reply is
 allowed per command.

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 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 |
    +---+           +---+------------->+---+

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 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
    

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7. 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 control 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<CRLF>.
                                   Server makes data connection
                                   to port U.
    
                                   <---- 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> ---->
                                   <---- 550 Access denied<CRLF>
    terminate                      QUIT <CRLF> ---->
                                   Server closes all
                                   connections.

8. 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.

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APPENDIX I - PAGE STRUCTURE

 The need for FTP to support page structure derives principally from
 the  need to support efficient transmission of files between TOPS-20
 systems, particularly the files used by NLS.
 The file system of TOPS-20 is based on the concept of pages.  The
 operating system is most efficient at manipulating files as pages.
 The operating system provides 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 TOPS-20 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 generation
 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 entries 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 TOPS-20 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.
 In fact, in both of these special cases, "holey" files and
 end-of-file pointers NOT at the end of the file, occur with NLS data
 files.

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 The TOPS-20 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 TOPS-20 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 31 for an FDB.  Trailing
 zeros in a disk file page may be discarded, making Data Length less
 than 512 in that case.

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APPENDIX II - DIRECTORY COMMANDS

 Since UNIX has a tree-like directory structure in which directories
 are as easy to manipulate as ordinary files, it is useful to expand
 the FTP servers on these machines to include commands which deal with
 the creation of directories.  Since there are other hosts on the
 ARPA-Internet which have tree-like directories (including TOPS-20 and
 Multics), these commands are as general as possible.
    Four directory commands have been added to FTP:
       MKD pathname
          Make a directory with the name "pathname".
       RMD pathname
          Remove the directory with the name "pathname".
       PWD
          Print the current working directory name.
       CDUP
          Change to the parent of the current working directory.
 The  "pathname"  argument should be created (removed) as a
 subdirectory of the current working directory, unless the "pathname"
 string contains sufficient information to specify otherwise to the
 server, e.g., "pathname" is an absolute pathname (in UNIX and
 Multics), or pathname is something like "<abso.lute.path>" to
 TOPS-20.
 REPLY CODES
    The CDUP command is a special case of CWD, and is included to
    simplify the implementation of programs for transferring directory
    trees between operating systems having different syntaxes for
    naming the parent directory.  The reply codes for CDUP be
    identical to the reply codes of CWD.
    The reply codes for RMD be identical to the reply codes for its
    file analogue, DELE.
    The reply codes for MKD, however, are a bit more complicated.  A
    freshly created directory will probably be the object of a future

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    CWD command.  Unfortunately, the argument to MKD may not always be
    a suitable argument for CWD.  This is the case, for example, when
    a TOPS-20 subdirectory is created by giving just the subdirectory
    name.  That is, with a TOPS-20 server FTP, the command sequence
       MKD MYDIR
       CWD MYDIR
    will fail.  The new directory may only be referred to by its
    "absolute" name; e.g., if the MKD command above were issued while
    connected to the directory <DFRANKLIN>, the new subdirectory
    could only be referred to by the name <DFRANKLIN.MYDIR>.
    Even on UNIX and Multics, however, the argument given to MKD may
    not be suitable.  If it is a "relative" pathname (i.e., a pathname
    which is interpreted relative to the current directory), the user
    would need to be in the same current directory in order to reach
    the subdirectory.  Depending on the application, this may be
    inconvenient.  It is not very robust in any case.
    To solve these problems, upon successful completion of an MKD
    command, the server should return a line of the form:
       257<space>"<directory-name>"<space><commentary>
    That is, the server will tell the user what string to use when
    referring to the created  directory.  The directory name can
    contain any character; embedded double-quotes should be escaped by
    double-quotes (the "quote-doubling" convention).
    For example, a user connects to the directory /usr/dm, and creates
    a subdirectory, named pathname:
       CWD /usr/dm
       200 directory changed to /usr/dm
       MKD pathname
       257 "/usr/dm/pathname" directory created
    An example with an embedded double quote:
       MKD foo"bar
       257 "/usr/dm/foo""bar" directory created
       CWD /usr/dm/foo"bar
       200 directory changed to /usr/dm/foo"bar

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    The prior existence of a subdirectory with the same name is an
    error, and the server must return an "access denied" error reply
    in that case.
       CWD /usr/dm
       200 directory changed to /usr/dm
       MKD pathname
       521-"/usr/dm/pathname" directory already exists;
       521 taking no action.
    The failure replies for MKD are analogous to its file  creating
    cousin, STOR.  Also, an "access denied" return is given if a file
    name with the same name as the subdirectory will conflict with the
    creation of the subdirectory (this is a problem on UNIX, but
    shouldn't be one on TOPS-20).
    Essentially because the PWD command returns the same type of
    information as the successful MKD command, the successful PWD
    command uses the 257 reply code as well.
 SUBTLETIES
    Because these commands will be most useful in transferring
    subtrees from one machine to another, carefully observe that the
    argument to MKD is to be interpreted as a sub-directory of  the
    current working directory, unless it contains enough information
    for the destination host to tell otherwise.  A hypothetical
    example of its use in the TOPS-20 world:
       CWD <some.where>
       200 Working directory changed
       MKD overrainbow
       257 "<some.where.overrainbow>" directory created
       CWD overrainbow
       431 No such directory
       CWD <some.where.overrainbow>
       200 Working directory changed
       CWD <some.where>
       200 Working directory changed to <some.where>
       MKD <unambiguous>
       257 "<unambiguous>" directory created
       CWD <unambiguous>
    Note that the first example results in a subdirectory of the
    connected directory.  In contrast, the argument in the second
    example contains enough information for TOPS-20 to tell that  the

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RFC 959 October 1985 File Transfer Protocol

    <unambiguous> directory is a top-level directory.  Note also that
    in the first example the user "violated" the protocol by
    attempting to access the freshly created directory with a name
    other than the one returned by TOPS-20.  Problems could have
    resulted in this case had there been an <overrainbow> directory;
    this is an ambiguity inherent in some TOPS-20 implementations.
    Similar considerations apply to the RMD command.  The point is
    this: except where to do so would violate a host's conventions for
    denoting relative versus absolute pathnames, the host should treat
    the operands of the MKD and RMD commands as subdirectories.  The
    257 reply to the MKD command must always contain the absolute
    pathname of the created directory.

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APPENDIX III - RFCs on FTP

 Bhushan, Abhay, "A File Transfer Protocol", RFC 114 (NIC 5823),
 MIT-Project MAC, 16 April 1971.
 Harslem, Eric, and John Heafner, "Comments on RFC 114 (A File
 Transfer Protocol)", RFC 141 (NIC 6726), RAND, 29 April 1971.
 Bhushan, Abhay, et al, "The File Transfer Protocol", RFC 172
 (NIC 6794), MIT-Project MAC, 23 June 1971.
 Braden, Bob, "Comments on DTP and FTP Proposals", RFC 238 (NIC 7663),
 UCLA/CCN, 29 September 1971.
 Bhushan, Abhay, et al, "The File Transfer Protocol", RFC 265
 (NIC 7813), MIT-Project MAC, 17 November 1971.
 McKenzie, Alex, "A Suggested Addition to File Transfer Protocol",
 RFC 281 (NIC 8163), BBN, 8 December 1971.
 Bhushan, Abhay, "The Use of "Set Data Type" Transaction in File
 Transfer Protocol", RFC 294 (NIC 8304), MIT-Project MAC,
 25 January 1972.
 Bhushan, Abhay, "The File Transfer Protocol", RFC 354 (NIC 10596),
 MIT-Project MAC, 8 July 1972.
 Bhushan, Abhay, "Comments on the File Transfer Protocol (RFC 354)",
 RFC 385 (NIC 11357), MIT-Project MAC, 18 August 1972.
 Hicks, Greg, "User FTP Documentation", RFC 412 (NIC 12404), Utah,
 27 November 1972.
 Bhushan, Abhay, "File Transfer Protocol (FTP) Status and Further
 Comments", RFC 414 (NIC 12406), MIT-Project MAC, 20 November 1972.
 Braden, Bob, "Comments on File Transfer Protocol", RFC 430
 (NIC 13299), UCLA/CCN, 7 February 1973.
 Thomas, Bob, and Bob Clements, "FTP Server-Server Interaction",
 RFC 438 (NIC 13770), BBN, 15 January 1973.
 Braden, Bob, "Print Files in FTP", RFC 448 (NIC 13299), UCLA/CCN,
 27 February 1973.
 McKenzie, Alex, "File Transfer Protocol", RFC 454 (NIC 14333), BBN,
 16 February 1973.

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RFC 959 October 1985 File Transfer Protocol

 Bressler, Bob, and Bob Thomas, "Mail Retrieval via FTP", RFC 458
 (NIC 14378), BBN-NET and BBN-TENEX, 20 February 1973.
 Neigus, Nancy, "File Transfer Protocol", RFC 542 (NIC 17759), BBN,
 12 July 1973.
 Krilanovich, Mark, and George Gregg, "Comments on the File Transfer
 Protocol", RFC 607 (NIC 21255), UCSB, 7 January 1974.
 Pogran, Ken, and Nancy Neigus, "Response to RFC 607 - Comments on the
 File Transfer Protocol", RFC 614 (NIC 21530), BBN, 28 January 1974.
 Krilanovich, Mark, George Gregg, Wayne Hathaway, and Jim White,
 "Comments on the File Transfer Protocol", RFC 624 (NIC 22054), UCSB,
 Ames Research Center, SRI-ARC, 28 February 1974.
 Bhushan, Abhay, "FTP Comments and Response to RFC 430", RFC 463
 (NIC 14573), MIT-DMCG, 21 February 1973.
 Braden, Bob, "FTP Data Compression", RFC 468 (NIC 14742), UCLA/CCN,
 8 March 1973.
 Bhushan, Abhay, "FTP and Network Mail System", RFC 475 (NIC 14919),
 MIT-DMCG, 6 March 1973.
 Bressler, Bob, and Bob Thomas "FTP Server-Server Interaction - II",
 RFC 478 (NIC 14947), BBN-NET and BBN-TENEX, 26 March 1973.
 White, Jim, "Use of FTP by the NIC Journal", RFC 479 (NIC 14948),
 SRI-ARC, 8 March 1973.
 White, Jim, "Host-Dependent FTP Parameters", RFC 480 (NIC 14949),
 SRI-ARC, 8 March 1973.
 Padlipsky, Mike, "An FTP Command-Naming Problem", RFC 506
 (NIC 16157), MIT-Multics, 26 June 1973.
 Day, John, "Memo to FTP Group (Proposal for File Access Protocol)",
 RFC 520 (NIC 16819), Illinois, 25 June 1973.
 Merryman, Robert, "The UCSD-CC Server-FTP Facility", RFC 532
 (NIC 17451), UCSD-CC, 22 June 1973.
 Braden, Bob, "TENEX FTP Problem", RFC 571 (NIC 18974), UCLA/CCN,
 15 November 1973.

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RFC 959 October 1985 File Transfer Protocol

 McKenzie, Alex, and Jon Postel, "Telnet and FTP Implementation -
 Schedule Change", RFC 593 (NIC 20615), BBN and MITRE,
 29 November 1973.
 Sussman, Julie, "FTP Error Code Usage for More Reliable Mail
 Service", RFC 630 (NIC 30237), BBN, 10 April 1974.
 Postel, Jon, "Revised FTP Reply Codes", RFC 640 (NIC 30843),
 UCLA/NMC, 5 June 1974.
 Harvey, Brian, "Leaving Well Enough Alone", RFC 686 (NIC 32481),
 SU-AI, 10 May 1975.
 Harvey, Brian, "One More Try on the FTP", RFC 691 (NIC 32700), SU-AI,
 28 May 1975.
 Lieb, J., "CWD Command of FTP", RFC 697 (NIC 32963), 14 July 1975.
 Harrenstien, Ken, "FTP Extension: XSEN", RFC 737 (NIC 42217), SRI-KL,
 31 October 1977.
 Harrenstien, Ken, "FTP Extension: XRSQ/XRCP", RFC 743 (NIC 42758),
 SRI-KL, 30 December 1977.
 Lebling, P. David, "Survey of FTP Mail and MLFL", RFC 751, MIT,
 10 December 1978.
 Postel, Jon, "File Transfer Protocol Specification", RFC 765, ISI,
 June 1980.
 Mankins, David, Dan Franklin, and Buzz Owen, "Directory Oriented FTP
 Commands", RFC 776, BBN, December 1980.
 Padlipsky, Michael, "FTP Unique-Named Store Command", RFC 949, MITRE,
 July 1985.

Postel & Reynolds [Page 68]

RFC 959 October 1985 File Transfer Protocol

REFERENCES

 [1]  Feinler, Elizabeth, "Internet Protocol Transition Workbook",
      Network Information Center, SRI International, March 1982.
 [2]  Postel, Jon, "Transmission Control Protocol - DARPA Internet
      Program Protocol Specification", RFC 793, DARPA, September 1981.
 [3]  Postel, Jon, and Joyce Reynolds, "Telnet Protocol
      Specification", RFC 854, ISI, May 1983.
 [4]  Reynolds, Joyce, and Jon Postel, "Assigned Numbers", RFC 943,
      ISI, April 1985.

Postel & Reynolds [Page 69]

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