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netwerk/protocol/ftp/doc/rfc959.txt

Fri, 16 Jan 2015 18:13:44 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Fri, 16 Jan 2015 18:13:44 +0100
branch: TOR_BUG_9701
changeset 14: 925c144e1f1f
permissions: -rw-r--r--

Integrate suggestion from review to improve consistency with existing code.

 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.
 .  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].
 .  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.
 Postel & Reynolds                                               [Page 1]
 RFC 959                                                     October 1985
 File Transfer Protocol
 .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.
 Postel & Reynolds                                               [Page 2]
 RFC 959                                                     October 1985
 File Transfer Protocol
       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.  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.
 Postel & Reynolds                                               [Page 3]
 RFC 959                                                     October 1985
 File Transfer Protocol
       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.
 Postel & Reynolds                                               [Page 4]
 RFC 959                                                     October 1985
 File Transfer Protocol
       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.
 Postel & Reynolds                                               [Page 5]
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 File Transfer Protocol
       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.
 Postel & Reynolds                                               [Page 6]
 RFC 959                                                     October 1985
 File Transfer Protocol
       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.
 Postel & Reynolds                                               [Page 7]
 RFC 959                                                     October 1985
 File Transfer Protocol
 .3.  THE FTP MODEL
       With the above definitions in mind, the following model (shown in
       Figure 1) may be diagrammed for an FTP service.
                                             -------------
                                             |/---------\|
                                             ||   User  ||    --------
                                             ||Interface|<--->| User |
                                             |\----^----/|    --------
                   ----------                |     |     |
                   |/------\|  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.
 . 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
 Postel & Reynolds                                               [Page 8]
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 File Transfer Protocol
       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
 Postel & Reynolds                                               [Page 9]
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 File Transfer Protocol
          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.
 .  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.
 .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
 -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
 Postel & Reynolds                                              [Page 10]
 RFC 959                                                     October 1985
 File Transfer Protocol
       that FTP provides for very limited data type representations.
       Transformations desired beyond this limited capability should be
       performed by the user directly.
 .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.
 .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.
 Postel & Reynolds                                              [Page 11]
 RFC 959                                                     October 1985
 File Transfer Protocol
 .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.
 .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.
 .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.
 Postel & Reynolds                                              [Page 12]
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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
 -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.
 .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:
 .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.
 Postel & Reynolds                                              [Page 13]
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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.
 .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.
 .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
 Move paper up two lines
 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.
 Postel & Reynolds                                              [Page 14]
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 .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,
 Postel & Reynolds                                              [Page 15]
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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.
 .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.
 .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.
 .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:
 = 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.
 = Simple Page
                         This is the normal type for simple paged files
                         with no page level associated control
                         information.  The header length must be 4.
 = Descriptor Page
                         This type is used to transmit the descriptive
                         information for the file as a whole.
 = 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.
 .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:
 . The server has completed sending data in a transfer mode
             that requires a close to indicate EOF.
 . The server receives an ABORT command from the user.
 . The port specification is changed by a command from the
             user.
 . The control connection is closed legally or otherwise.
 . 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.  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.
 .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:
 .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.
 .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
 End of data block is EOR
 End of data block is EOF
 Suspected errors in data block
 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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 .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:
 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:
 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:
 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.
 .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.
 .  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.
 .1.  FTP COMMANDS
 .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
 .  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).
 .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
 -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
 ).
             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.
 .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:
 . User system inserts the Telnet "Interrupt Process" (IP) signal
       in the Telnet stream.
 . User system sends the Telnet "Synch" signal.
 . User system inserts the command (e.g., ABOR) in the Telnet
       stream.
 . 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.)
 .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:
 -First line
                                 Second line
 A line beginning with numbers
 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:
 yz   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.
 yz   Positive Completion reply
                The requested action has been successfully completed.  A
                new request may be initiated.
 yz   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.
 yz   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.)
 yz   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.
 .2.1  Reply Codes by Function Groups
 Command okay.
 Syntax error, command unrecognized.
              This may include errors such as command line too long.
 Syntax error in parameters or arguments.
 Command not implemented, superfluous at this site.
 Command not implemented.
 Bad sequence of commands.
 Command not implemented for that parameter.
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 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 "=").
 System status, or system help reply.
 Directory status.
 File status.
 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.
 NAME system type.
              Where NAME is an official system name from the list in the
              Assigned Numbers document.
 Service ready in nnn minutes.
 Service ready for new user.
 Service closing control connection.
              Logged out if appropriate.
 Service not available, closing control connection.
              This may be a reply to any command if the service knows it
              must shut down.
 Data connection already open; transfer starting.
 Data connection open; no transfer in progress.
 Can't open data connection.
 Closing data connection.
              Requested file action successful (for example, file
              transfer or file abort).
 Connection closed; transfer aborted.
 Entering Passive Mode (h1,h2,h3,h4,p1,p2).
 User logged in, proceed.
 Not logged in.
 User name okay, need password.
 Need account for login.
 Need account for storing files.
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 File status okay; about to open data connection.
 Requested file action okay, completed.
 "PATHNAME" created.
 Requested file action pending further information.
 Requested file action not taken.
              File unavailable (e.g., file busy).
 Requested action not taken.
              File unavailable (e.g., file not found, no access).
 Requested action aborted. Local error in processing.
 Requested action aborted. Page type unknown.
 Requested action not taken.
              Insufficient storage space in system.
 Requested file action aborted.
              Exceeded storage allocation (for current directory or
              dataset).
 Requested action not taken.
              File name not allowed.
 .2.2 Numeric  Order List of Reply Codes
 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 "=").
 Service ready in nnn minutes.
 Data connection already open; transfer starting.
 File status okay; about to open data connection.
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 Command okay.
 Command not implemented, superfluous at this site.
 System status, or system help reply.
 Directory status.
 File status.
 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.
 NAME system type.
              Where NAME is an official system name from the list in the
              Assigned Numbers document.
 Service ready for new user.
 Service closing control connection.
              Logged out if appropriate.
 Data connection open; no transfer in progress.
 Closing data connection.
              Requested file action successful (for example, file
              transfer or file abort).
 Entering Passive Mode (h1,h2,h3,h4,p1,p2).
 User logged in, proceed.
 Requested file action okay, completed.
 "PATHNAME" created.
 User name okay, need password.
 Need account for login.
 Requested file action pending further information.
 Service not available, closing control connection.
              This may be a reply to any command if the service knows it
              must shut down.
 Can't open data connection.
 Connection closed; transfer aborted.
 Requested file action not taken.
              File unavailable (e.g., file busy).
 Requested action aborted: local error in processing.
 Requested action not taken.
              Insufficient storage space in system.
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 Syntax error, command unrecognized.
              This may include errors such as command line too long.
 Syntax error in parameters or arguments.
 Command not implemented.
 Bad sequence of commands.
 Command not implemented for that parameter.
 Not logged in.
 Need account for storing files.
 Requested action not taken.
              File unavailable (e.g., file not found, no access).
 Requested action aborted: page type unknown.
 Requested file action aborted.
              Exceeded storage allocation (for current directory or
              dataset).
 Requested action not taken.
              File name not allowed.
 .  DECLARATIVE SPECIFICATIONS
 .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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 .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.
 .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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 .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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 .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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 File Transfer Protocol
 .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
 "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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 RFC 959                                                     October 1985
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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
 
 
 
 
             Login
                USER
 
 
 , 501, 421
 , 332
                PASS
 
 
 
 , 501, 503, 421
 
                ACCT
 
 
 
 , 501, 503, 421
                CWD
 
 , 501, 502, 421, 530, 550
                CDUP
 
 , 501, 502, 421, 530, 550
                SMNT
 , 250
 , 501, 502, 421, 530, 550
             Logout
                REIN
 
 
 
 
 , 502
                QUIT
 
 
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 RFC 959                                                     October 1985
 File Transfer Protocol
             Transfer parameters
                PORT
 
 , 501, 421, 530
                PASV
 
 , 501, 502, 421, 530
                MODE
 
 , 501, 504, 421, 530
                TYPE
 
 , 501, 504, 421, 530
                STRU
 
 , 501, 504, 421, 530
             File action commands
                ALLO
 
 
 , 501, 504, 421, 530
                REST
 , 501, 502, 421, 530
 
                STOR
 , 150
                      (110)
 , 250
 , 426, 451, 551, 552
 , 450, 452, 553
 , 501, 421, 530
                STOU
 , 150
                      (110)
 , 250
 , 426, 451, 551, 552
 , 450, 452, 553
 , 501, 421, 530
                RETR
 , 150
                      (110)
 , 250
 , 426, 451
 , 550
 , 501, 421, 530
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 File Transfer Protocol
                LIST
 , 150
 , 250
 , 426, 451
 
 , 501, 502, 421, 530
                NLST
 , 150
 , 250
 , 426, 451
 
 , 501, 502, 421, 530
                APPE
 , 150
                      (110)
 , 250
 , 426, 451, 551, 552
 , 450, 550, 452, 553
 , 501, 502, 421, 530
                RNFR
 , 550
 , 501, 502, 421, 530
 
                RNTO
 
 , 553
 , 501, 502, 503, 421, 530
                DELE
 
 , 550
 , 501, 502, 421, 530
                RMD
 
 , 501, 502, 421, 530, 550
                MKD
 
 , 501, 502, 421, 530, 550
                PWD
 
 , 501, 502, 421, 550
                ABOR
 , 226
 , 501, 502, 421
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 RFC 959                                                     October 1985
 File Transfer Protocol
             Informational commands
                SYST
 
 , 501, 502, 421
                STAT
 , 212, 213
 
 , 501, 502, 421, 530
                HELP
 , 214
 , 501, 502, 421
             Miscellaneous commands
                SITE
 
 
 , 501, 530
                NOOP
 
 421
 Postel & Reynolds                                              [Page 53]
 RFC 959                                                     October 1985
 File Transfer Protocol
 .  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:
 ,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:
 +---+
                           ----------->| 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 |
       +---+           +---+        -->+---+
                        | |        |
 | | 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 |
       +---+           +---+        -->+---+
                        | |        |
 | | 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)
 series replies.  Remember that at most, one 100 series reply is
    allowed per command.
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 RFC 959                                                     October 1985
 File Transfer Protocol
    The most complicated diagram is for the Login sequence:
 
       +---+   USER    +---+------------->+---+
       | B |---------->| W | 2       ---->| E |
       +---+           +---+------  |  -->+---+
                        | |       | | |
 | | 4,5   | | |
          --------------   -----  | | |
         |                      | | | |
         |                      | | | |
         |                 ---------  |
         |               1|     | |   |
         V                |     | |   |
       +---+   PASS    +---+ 2  |  ------>+---+
       |   |---------->| W |------------->| S |
       +---+           +---+   ---------->+---+
                        | |   | |     |
 | |4,5| |     |
          --------------   --------   |
         |                    | |  |  |
         |                    | |  |  |
         |                 -----------
         |             1,3|   | |  |
         V                |  2| |  |
       +---+   ACCT    +---+--  |   ----->+---+
       |   |---------->| W | 4,5 -------->| F |
       +---+           +---+------------->+---+
 Postel & Reynolds                                              [Page 57]
 RFC 959                                                     October 1985
 File Transfer Protocol
    Finally, we present a generalized diagram that could be used to model
    the command and reply interchange:
                ------------------------------------
               |                                    |
       Begin   |                                    |
         |     V                                    |
         |   +---+  cmd   +---+ 2         +---+     |
          -->|   |------->|   |---------->|   |     |
             |   |        | W |           | S |-----|
          -->|   |     -->|   |-----      |   |     |
         |   +---+    |   +---+ 4,5 |     +---+     |
         |     |      |    | |      |               |
         |     |      |   1| |3     |     +---+     |
         |     |      |    | |      |     |   |     |
         |     |       ----  |       ---->| F |-----
         |     |             |            |   |
         |     |             |            +---+
          -------------------
               |
               |
               V
              End
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 File Transfer Protocol
 .  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.
 .  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.
 Postel & Reynolds                                              [Page 59]
 RFC 959                                                     October 1985
 File Transfer Protocol
 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.
 Postel & Reynolds                                              [Page 61]
 RFC 959                                                     October 1985
 File Transfer Protocol
 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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 RFC 959                                                     October 1985
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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:
 <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
 directory changed to /usr/dm
          MKD pathname
 "/usr/dm/pathname" directory created
       An example with an embedded double quote:
          MKD foo"bar
 "/usr/dm/foo""bar" directory created
          CWD /usr/dm/foo"bar
 directory changed to /usr/dm/foo"bar
 Postel & Reynolds                                              [Page 63]
 RFC 959                                                     October 1985
 File Transfer Protocol
       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
 directory changed to /usr/dm
          MKD pathname
 -"/usr/dm/pathname" directory already exists;
 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>
 Working directory changed
          MKD overrainbow
 "<some.where.overrainbow>" directory created
          CWD overrainbow
 No such directory
          CWD <some.where.overrainbow>
 Working directory changed
          CWD <some.where>
 Working directory changed to <some.where>
          MKD <unambiguous>
 "<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
 Postel & Reynolds                                              [Page 64]
 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
 reply to the MKD command must always contain the absolute
       pathname of the created directory.
 Postel & Reynolds                                              [Page 65]
 RFC 959                                                     October 1985
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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,
 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,
 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,
 February 1973.
    McKenzie, Alex, "File Transfer Protocol", RFC 454 (NIC 14333), BBN,
 February 1973.
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    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,
 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,
 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,
 November 1973.
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    McKenzie, Alex, and Jon Postel, "Telnet and FTP Implementation -
    Schedule Change", RFC 593 (NIC 20615), BBN and MITRE,
 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,
 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,
 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,
 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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