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    What is an RFC?

    RFC documents have been used on the Internet for more than 30 years. Researchers from universities and corporations publish these documents to solicit feedback on new technologies for the Internet. Most popular networking technologies like IP and Ethernet have been historically documented in RFC-959.

    The very first RFC, RFC 1 was published in April 1969. Although the "host software" technology discussed has long since become obsolete, documents like this one offer a very interesting glimpse into the early days of computer networking. Even today, the plain text format of the RFC remains essentially the same as it has since the beginning.

    As the basic technologies of the Internet have matured, and the Internet has grown to include many non-technical people, the need for RFCs has diminished. Yet a few RFCs are still being produced for leading-edge research in Internet-based networking.


    RFC-959

    
                                                                            
    Network Working Group                                          J. Postel
    Request for Comments: 959                                    J. Reynolds
                                                                         ISI
    Obsoletes RFC: 765 (IEN 149)                                October 1985
    
                          FILE TRANSFER PROTOCOL (FTP)
    
    
    Status of this Memo
    
       This memo is the official specification of the File Transfer
       Protocol (FTP).  Distribution of this memo is unlimited.
    
       The following new optional commands are included in this edition of
       the specification:
    
          CDUP (Change to Parent Directory), SMNT (Structure Mount), STOU
          (Store Unique), RMD (Remove Directory), MKD (Make Directory), PWD
          (Print Directory), and SYST (System).
    
       Note that this specification is compatible with the previous edition.
    
    1.  INTRODUCTION
    
       The objectives of FTP are 1) to promote sharing of files (computer
       programs and/or data), 2) to encourage indirect or implicit (via
       programs) use of remote computers, 3) to shield a user from
       variations in file storage systems among hosts, and 4) to transfer
       data reliably and efficiently.  FTP, though usable directly by a user
       at a terminal, is designed mainly for use by programs.
    
       The attempt in this specification is to satisfy the diverse needs of
       users of maxi-hosts, mini-hosts, personal workstations, and TACs,
       with a simple, and easily implemented protocol design.
    
       This paper assumes knowledge of the Transmission Control Protocol
       (TCP) [2] and the Telnet Protocol [3].  These documents are contained
       in the ARPA-Internet protocol handbook [1].
    
    2.  OVERVIEW
    
       In this section, the history, the terminology, and the FTP model are
       discussed.  The terms defined in this section are only those that
       have special significance in FTP.  Some of the terminology is very
       specific to the FTP model; some readers may wish to turn to the
       section on the FTP model while reviewing the terminology.
    
    
    
    
    
    
    
    Postel & Reynolds                                               [Page 1]
    ------------------------------------------------------------------------
                                                                            
    RFC 959                                                     October 1985
    File Transfer Protocol
    
    
       2.1.  HISTORY
    
          FTP has had a long evolution over the years.  Appendix III is a
          chronological compilation of Request for Comments documents
          relating to FTP.  These include the first proposed file transfer
          mechanisms in 1971 that were developed for implementation on hosts
          at M.I.T. (RFC 114), plus comments and discussion in RFC 141.
    
          RFC 172 provided a user-level oriented protocol for file transfer
          between host computers (including terminal IMPs).  A revision of
          this as RFC 265, restated FTP for additional review, while RFC 281
          suggested further changes.  The use of a "Set Data Type"
          transaction was proposed in RFC 294 in January 1982.
    
          RFC 354 obsoleted RFCs 264 and 265.  The File Transfer Protocol
          was now defined as a protocol for file transfer between HOSTs on
          the ARPANET, with the primary function of FTP defined as
          transfering files efficiently and reliably among hosts and
          allowing the convenient use of remote file storage capabilities.
          RFC 385 further commented on errors, emphasis points, and
          additions to the protocol, while RFC 414 provided a status report
          on the working server and user FTPs.  RFC 430, issued in 1973,
          (among other RFCs too numerous to mention) presented further
          comments on FTP.  Finally, an "official" FTP document was
          published as RFC 454.
    
          By July 1973, considerable changes from the last versions of FTP
          were made, but the general structure remained the same.  RFC 542
          was published as a new "official" specification to reflect these
          changes.  However, many implementations based on the older
          specification were not updated.
    
          In 1974, RFCs 607 and 614 continued comments on FTP.  RFC 624
          proposed further design changes and minor modifications.  In 1975,
          RFC 686 entitled, "Leaving Well Enough Alone", discussed the
          differences between all of the early and later versions of FTP.
          RFC 691 presented a minor revision of RFC 686, regarding the
          subject of print files.
    
          Motivated by the transition from the NCP to the TCP as the
          underlying protocol, a phoenix was born out of all of the above
          efforts in RFC 765 as the specification of FTP for use on TCP.
    
          This current edition of the FTP specification is intended to
          correct some minor documentation errors, to improve the
          explanation of some protocol features, and to add some new
          optional commands.
    
    
    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.
    
       2.2.  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]
    ------------------------------------------------------------------------
                                                                            
    RFC 959                                                     October 1985
    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
    
    
       2.3.  THE FTP MODEL
    
          With the above definitions in mind, the following model (shown in
          Figure 1) may be diagrammed for an FTP service.
    
                                                -------------
                                                |/---------\|
                                                ||   User  ||    --------
                                                ||Interface|<--->| User |
                                                |\----^----/|    --------
                      ----------                |     |     |
                      |/------\|  FTP Commands  |/----V----\|
                      ||Server|<---------------->|   User  ||
                      ||  PI  ||   FTP Replies  ||    PI   ||
                      |\--^---/|                |\----^----/|
                      |   |    |                |     |     |
          --------    |/--V---\|      Data      |/----V----\|    --------
          | File |<--->|Server|<---------------->|  User   |<--->| File |
          |System|    || DTP  ||   Connection   ||   DTP   ||    |System|
          --------    |\------/|                |\---------/|    --------
                      ----------                -------------
    
                      Server-FTP                   USER-FTP
    
          NOTES: 1. The data connection may be used in either direction.
                 2. The data connection need not exist all of the time.
    
                          Figure 1  Model for FTP Use
    
          In the model described in Figure 1, the user-protocol interpreter
          initiates the control connection.  The control connection follows
          the Telnet protocol.  At the initiation of the user, standard FTP
          commands are generated by the user-PI and transmitted to the
          server process via the control connection.  (The user may
          establish a direct control connection to the server-FTP, from a
          TAC terminal for example, and generate standard FTP commands
          independently, bypassing the user-FTP process.) Standard replies
          are sent from the server-PI to the user-PI over the control
          connection in response to the commands.
    
          The FTP commands specify the parameters for the data connection
          (data port, transfer mode, representation type, and structure) and
          the nature of file system operation (store, retrieve, append,
          delete, etc.).  The user-DTP or its designate should "listen" on
          the specified data port, and the server initiate the data
          connection and data transfer in accordance with the specified
          parameters.  It should be noted that the data port need not be in
    
    
    Postel & Reynolds                                               [Page 8]
    ------------------------------------------------------------------------
                                                                            
    RFC 959                                                     October 1985
    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]
    ------------------------------------------------------------------------
                                                                            
    RFC 959                                                     October 1985
    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.
    
    3.  DATA TRANSFER FUNCTIONS
    
       Files are transferred only via the data connection.  The control
       connection is used for the transfer of commands, which describe the
       functions to be performed, and the replies to these commands (see the
       Section on FTP Replies).  Several commands are concerned with the
       transfer of data between hosts.  These data transfer commands include
       the MODE command which specify how the bits of the data are to be
       transmitted, and the STRUcture and TYPE commands, which are used to
       define the way in which the data are to be represented.  The
       transmission and representation are basically independent but the
       "Stream" transmission mode is dependent on the file structure
       attribute and if "Compressed" transmission mode is used, the nature
       of the filler byte depends on the representation type.
    
       3.1.  DATA REPRESENTATION AND STORAGE
    
          Data is transferred from a storage device in the sending host to a
          storage device in the receiving host.  Often it is necessary to
          perform certain transformations on the data because data storage
          representations in the two systems are different.  For example,
          NVT-ASCII has different data storage representations in different
          systems.  DEC TOPS-20s's generally store NVT-ASCII as five 7-bit
          ASCII characters, left-justified in a 36-bit word. IBM Mainframe's
          store NVT-ASCII as 8-bit EBCDIC codes.  Multics stores NVT-ASCII
          as four 9-bit characters in a 36-bit word.  It is desirable to
          convert characters into the standard NVT-ASCII representation when
          transmitting text between dissimilar systems.  The sending and
          receiving sites would have to perform the necessary
          transformations between the standard representation and their
          internal representations.
    
          A different problem in representation arises when transmitting
          binary data (not character codes) between host systems with
          different word lengths.  It is not always clear how the sender
          should send data, and the receiver store it.  For example, when
          transmitting 32-bit bytes from a 32-bit word-length system to a
          36-bit word-length system, it may be desirable (for reasons of
          efficiency and usefulness) to store the 32-bit bytes
          right-justified in a 36-bit word in the latter system.  In any
          case, the user should have the option of specifying data
          representation and transformation functions.  It should be noted
    
    
    
    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.
    
          3.1.1.  DATA TYPES
    
             Data representations are handled in FTP by a user specifying a
             representation type.  This type may implicitly (as in ASCII or
             EBCDIC) or explicitly (as in Local byte) define a byte size for
             interpretation which is referred to as the "logical byte size."
             Note that this has nothing to do with the byte size used for
             transmission over the data connection, called the "transfer
             byte size", and the two should not be confused.  For example,
             NVT-ASCII has a logical byte size of 8 bits.  If the type is
             Local byte, then the TYPE command has an obligatory second
             parameter specifying the logical byte size.  The transfer byte
             size is always 8 bits.
    
             3.1.1.1.  ASCII TYPE
    
                This is the default type and must be accepted by all FTP
                implementations.  It is intended primarily for the transfer
                of text files, except when both hosts would find the EBCDIC
                type more convenient.
    
                The sender converts the data from an internal character
                representation to the standard 8-bit NVT-ASCII
                representation (see the Telnet specification).  The receiver
                will convert the data from the standard form to his own
                internal form.
    
                In accordance with the NVT standard, the  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
    
    
             3.1.1.2.  EBCDIC TYPE
    
                This type is intended for efficient transfer between hosts
                which use EBCDIC for their internal character
                representation.
    
                For transmission, the data are represented as 8-bit EBCDIC
                characters.  The character code is the only difference
                between the functional specifications of EBCDIC and ASCII
                types.
    
                End-of-line (as opposed to end-of-record--see the discussion
                of structure) will probably be rarely used with EBCDIC type
                for purposes of denoting structure, but where it is
                necessary the  character should be used.
    
             3.1.1.3.  IMAGE TYPE
    
                The data are sent as contiguous bits which, for transfer,
                are packed into the 8-bit transfer bytes.  The receiving
                site must store the data as contiguous bits.  The structure
                of the storage system might necessitate the padding of the
                file (or of each record, for a record-structured file) to
                some convenient boundary (byte, word or block).  This
                padding, which must be all zeros, may occur only at the end
                of the file (or at the end of each record) and there must be
                a way of identifying the padding bits so that they may be
                stripped off if the file is retrieved.  The padding
                transformation should be well publicized to enable a user to
                process a file at the storage site.
    
                Image type is intended for the efficient storage and
                retrieval of files and for the transfer of binary data.  It
                is recommended that this type be accepted by all FTP
                implementations.
    
             3.1.1.4.  LOCAL TYPE
    
                The data is transferred in logical bytes of the size
                specified by the obligatory second parameter, Byte size.
                The value of Byte size must be a decimal integer; there is
                no default value.  The logical byte size is not necessarily
                the same as the transfer byte size.  If there is a
                difference in byte sizes, then the logical bytes should be
                packed contiguously, disregarding transfer byte boundaries
                and with any necessary padding at the end.
    
    
    
    Postel & Reynolds                                              [Page 12]
    ------------------------------------------------------------------------
                                                                            
    RFC 959                                                     October 1985
    File Transfer Protocol
    
    
                When the data reaches the receiving host, it will be
                transformed in a manner dependent on the logical byte size
                and the particular host.  This transformation must be
                invertible (i.e., an identical file can be retrieved if the
                same parameters are used) and should be well publicized by
                the FTP implementors.
    
                For example, a user sending 36-bit floating-point numbers to
                a host with a 32-bit word could send that data as Local byte
                with a logical byte size of 36.  The receiving host would
                then be expected to store the logical bytes so that they
                could be easily manipulated; in this example putting the
                36-bit logical bytes into 64-bit double words should
                suffice.
    
                In another example, a pair of hosts with a 36-bit word size
                may send data to one another in words by using TYPE L 36.
                The data would be sent in the 8-bit transmission bytes
                packed so that 9 transmission bytes carried two host words.
    
             3.1.1.5.  FORMAT CONTROL
    
                The types ASCII and EBCDIC also take a second (optional)
                parameter; this is to indicate what kind of vertical format
                control, if any, is associated with a file.  The following
                data representation types are defined in FTP:
    
                A character file may be transferred to a host for one of
                three purposes: for printing, for storage and later
                retrieval, or for processing.  If a file is sent for
                printing, the receiving host must know how the vertical
                format control is represented.  In the second case, it must
                be possible to store a file at a host and then retrieve it
                later in exactly the same form.  Finally, it should be
                possible to move a file from one host to another and process
                the file at the second host without undue trouble.  A single
                ASCII or EBCDIC format does not satisfy all these
                conditions.  Therefore, these types have a second parameter
                specifying one of the following three formats:
    
                3.1.1.5.1.  NON PRINT
    
                   This is the default format to be used if the second
                   (format) parameter is omitted.  Non-print format must be
                   accepted by all FTP implementations.
    
    
    
    
    Postel & Reynolds                                              [Page 13]
    ------------------------------------------------------------------------
                                                                            
    RFC 959                                                     October 1985
    File Transfer Protocol
    
    
                   The file need contain no vertical format information.  If
                   it is passed to a printer process, this process may
                   assume standard values for spacing and margins.
    
                   Normally, this format will be used with files destined
                   for processing or just storage.
    
                3.1.1.5.2.  TELNET FORMAT CONTROLS
    
                   The file contains ASCII/EBCDIC vertical format controls
                   (i.e., , , , , ) which the printer
                   process will interpret appropriately.  , in exactly
                   this sequence, also denotes end-of-line.
    
                3.1.1.5.2.  CARRIAGE CONTROL (ASA)
    
                   The file contains ASA (FORTRAN) vertical format control
                   characters.  (See RFC 740 Appendix C; and Communications
                   of the ACM, Vol. 7, No. 10, p. 606, October 1964.)  In a
                   line or a record formatted according to the ASA Standard,
                   the first character is not to be printed.  Instead, it
                   should be used to determine the vertical movement of the
                   paper which should take place before the rest of the
                   record is printed.
    
                   The ASA Standard specifies the following control
                   characters:
    
                      Character     Vertical Spacing
    
                      blank         Move paper up one line
                      0             Move paper up two lines
                      1             Move paper to top of next page
                      +             No movement, i.e., overprint
    
                   Clearly there must be some way for a printer process to
                   distinguish the end of the structural entity.  If a file
                   has record structure (see below) this is no problem;
                   records will be explicitly marked during transfer and
                   storage.  If the file has no record structure, the 
                   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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          3.1.2.  DATA STRUCTURES
    
             In addition to different representation types, FTP allows the
             structure of a file to be specified.  Three file structures are
             defined in FTP:
    
                file-structure,     where there is no internal structure and
                                    the file is considered to be a
                                    continuous sequence of data bytes,
    
                record-structure,   where the file is made up of sequential
                                    records,
    
                and page-structure, where the file is made up of independent
                                    indexed pages.
    
             File-structure is the default to be assumed if the STRUcture
             command has not been used but both file and record structures
             must be accepted for "text" files (i.e., files with TYPE ASCII
             or EBCDIC) by all FTP implementations.  The structure of a file
             will affect both the transfer mode of a file (see the Section
             on Transmission Modes) and the interpretation and storage of
             the file.
    
             The "natural" structure of a file will depend on which host
             stores the file.  A source-code file will usually be stored on
             an IBM Mainframe in fixed length records but on a DEC TOPS-20
             as a stream of characters partitioned into lines, for example
             by .  If the transfer of files between such disparate
             sites is to be useful, there must be some way for one site to
             recognize the other's assumptions about the file.
    
             With some sites being naturally file-oriented and others
             naturally record-oriented there may be problems if a file with
             one structure is sent to a host oriented to the other.  If a
             text file is sent with record-structure to a host which is file
             oriented, then that host should apply an internal
             transformation to the file based on the record structure.
             Obviously, this transformation should be useful, but it must
             also be invertible so that an identical file may be retrieved
             using record structure.
    
             In the case of a file being sent with file-structure to a
             record-oriented host, there exists the question of what
             criteria the host should use to divide the file into records
             which can be processed locally.  If this division is necessary,
             the FTP implementation should use the end-of-line sequence,
    
    
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              for ASCII, or  for EBCDIC text files, as the
             delimiter.  If an FTP implementation adopts this technique, it
             must be prepared to reverse the transformation if the file is
             retrieved with file-structure.
    
             3.1.2.1.  FILE STRUCTURE
    
                File structure is the default to be assumed if the STRUcture
                command has not been used.
    
                In file-structure there is no internal structure and the
                file is considered to be a continuous sequence of data
                bytes.
    
             3.1.2.2.  RECORD STRUCTURE
    
                Record structures must be accepted for "text" files (i.e.,
                files with TYPE ASCII or EBCDIC) by all FTP implementations.
    
                In record-structure the file is made up of sequential
                records.
    
             3.1.2.3.  PAGE STRUCTURE
    
                To transmit files that are discontinuous, FTP defines a page
                structure.  Files of this type are sometimes known as
                "random access files" or even as "holey files".  In these
                files there is sometimes other information associated with
                the file as a whole (e.g., a file descriptor), or with a
                section of the file (e.g., page access controls), or both.
                In FTP, the sections of the file are called pages.
    
                To provide for various page sizes and associated
                information, each page is sent with a page header.  The page
                header has the following defined fields:
    
                   Header Length
    
                      The number of logical bytes in the page header
                      including this byte.  The minimum header length is 4.
    
                   Page Index
    
                      The logical page number of this section of the file.
                      This is not the transmission sequence number of this
                      page, but the index used to identify this page of the
                      file.
    
    
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                   Data Length
    
                      The number of logical bytes in the page data.  The
                      minimum data length is 0.
    
                   Page Type
    
                      The type of page this is.  The following page types
                      are defined:
    
                         0 = Last Page
    
                            This is used to indicate the end of a paged
                            structured transmission.  The header length must
                            be 4, and the data length must be 0.
    
                         1 = Simple Page
    
                            This is the normal type for simple paged files
                            with no page level associated control
                            information.  The header length must be 4.
    
                         2 = Descriptor Page
    
                            This type is used to transmit the descriptive
                            information for the file as a whole.
    
                         3 = Access Controlled Page
    
                            This type includes an additional header field
                            for paged files with page level access control
                            information.  The header length must be 5.
    
                   Optional Fields
    
                      Further header fields may be used to supply per page
                      control information, for example, per page access
                      control.
    
                All fields are one logical byte in length.  The logical byte
                size is specified by the TYPE command.  See Appendix I for
                further details and a specific case at the page structure.
    
          A note of caution about parameters:  a file must be stored and
          retrieved with the same parameters if the retrieved version is to
    
    
    
    
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          be identical to the version originally transmitted.  Conversely,
          FTP implementations must return a file identical to the original
          if the parameters used to store and retrieve a file are the same.
    
       3.2.  ESTABLISHING DATA CONNECTIONS
    
          The mechanics of transferring data consists of setting up the data
          connection to the appropriate ports and choosing the parameters
          for transfer.  Both the user and the server-DTPs have a default
          data port.  The user-process default data port is the same as the
          control connection port (i.e., U).  The server-process default
          data port is the port adjacent to the control connection port
          (i.e., L-1).
    
          The transfer byte size is 8-bit bytes.  This byte size is relevant
          only for the actual transfer of the data; it has no bearing on
          representation of the data within a host's file system.
    
          The passive data transfer process (this may be a user-DTP or a
          second server-DTP) shall "listen" on the data port prior to
          sending a transfer request command.  The FTP request command
          determines the direction of the data transfer.  The server, upon
          receiving the transfer request, will initiate the data connection
          to the port.  When the connection is established, the data
          transfer begins between DTP's, and the server-PI sends a
          confirming reply to the user-PI.
    
          Every FTP implementation must support the use of the default data
          ports, and only the USER-PI can initiate a change to non-default
          ports.
    
          It is possible for the user to specify an alternate data port by
          use of the PORT command.  The user may want a file dumped on a TAC
          line printer or retrieved from a third party host.  In the latter
          case, the user-PI sets up control connections with both
          server-PI's.  One server is then told (by an FTP command) to
          "listen" for a connection which the other will initiate.  The
          user-PI sends one server-PI a PORT command indicating the data
          port of the other.  Finally, both are sent the appropriate
          transfer commands.  The exact sequence of commands and replies
          sent between the user-controller and the servers is defined in the
          Section on FTP Replies.
    
          In general, it is the server's responsibility to maintain the data
          connection--to initiate it and to close it.  The exception to this
    
    
    
    
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          is when the user-DTP is sending the data in a transfer mode that
          requires the connection to be closed to indicate EOF.  The server
          MUST close the data connection under the following conditions:
    
             1. The server has completed sending data in a transfer mode
                that requires a close to indicate EOF.
    
             2. The server receives an ABORT command from the user.
    
             3. The port specification is changed by a command from the
                user.
    
             4. The control connection is closed legally or otherwise.
    
             5. An irrecoverable error condition occurs.
    
          Otherwise the close is a server option, the exercise of which the
          server must indicate to the user-process by either a 250 or 226
          reply only.
    
       3.3.  DATA CONNECTION MANAGEMENT
    
          Default Data Connection Ports:  All FTP implementations must
          support use of the default data connection ports, and only the
          User-PI may initiate the use of non-default ports.
    
          Negotiating Non-Default Data Ports:   The User-PI may specify a
          non-default user side data port with the PORT command.  The
          User-PI may request the server side to identify a non-default
          server side data port with the PASV command.  Since a connection
          is defined by the pair of addresses, either of these actions is
          enough to get a different data connection, still it is permitted
          to do both commands to use new ports on both ends of the data
          connection.
    
          Reuse of the Data Connection:  When using the stream mode of data
          transfer the end of the file must be indicated by closing the
          connection.  This causes a problem if multiple files are to be
          transfered in the session, due to need for TCP to hold the
          connection record for a time out period to guarantee the reliable
          communication.  Thus the connection can not be reopened at once.
    
             There are two solutions to this problem.  The first is to
             negotiate a non-default port.  The second is to use another
             transfer mode.
    
             A comment on transfer modes.  The stream transfer mode is
    
    
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             inherently unreliable, since one can not determine if the
             connection closed prematurely or not.  The other transfer modes
             (Block, Compressed) do not close the connection to indicate the
             end of file.  They have enough FTP encoding that the data
             connection can be parsed to determine the end of the file.
             Thus using these modes one can leave the data connection open
             for multiple file transfers.
    
       3.4.  TRANSMISSION MODES
    
          The next consideration in transferring data is choosing the
          appropriate transmission mode.  There are three modes: one which
          formats the data and allows for restart procedures; one which also
          compresses the data for efficient transfer; and one which passes
          the data with little or no processing.  In this last case the mode
          interacts with the structure attribute to determine the type of
          processing.  In the compressed mode, the representation type
          determines the filler byte.
    
          All data transfers must be completed with an end-of-file (EOF)
          which may be explicitly stated or implied by the closing of the
          data connection.  For files with record structure, all the
          end-of-record markers (EOR) are explicit, including the final one.
          For files transmitted in page structure a "last-page" page type is
          used.
    
          NOTE:  In the rest of this section, byte means "transfer byte"
          except where explicitly stated otherwise.
    
          For the purpose of standardized transfer, the sending host will
          translate its internal end of line or end of record denotation
          into the representation prescribed by the transfer mode and file
          structure, and the receiving host will perform the inverse
          translation to its internal denotation.  An IBM Mainframe record
          count field may not be recognized at another host, so the
          end-of-record information may be transferred as a two byte control
          code in Stream mode or as a flagged bit in a Block or Compressed
          mode descriptor.  End-of-line in an ASCII or EBCDIC file with no
          record structure should be indicated by  or ,
          respectively.  Since these transformations imply extra work for
          some systems, identical systems transferring non-record structured
          text files might wish to use a binary representation and stream
          mode for the transfer.
    
    
    
    
    
    
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          The following transmission modes are defined in FTP:
    
          3.4.1.  STREAM MODE
    
             The data is transmitted as a stream of bytes.  There is no
             restriction on the representation type used; record structures
             are allowed.
    
             In a record structured file EOR and EOF will each be indicated
             by a two-byte control code.  The first byte of the control code
             will be all ones, the escape character.  The second byte will
             have the low order bit on and zeros elsewhere for EOR and the
             second low order bit on for EOF; that is, the byte will have
             value 1 for EOR and value 2 for EOF.  EOR and EOF may be
             indicated together on the last byte transmitted by turning both
             low order bits on (i.e., the value 3).  If a byte of all ones
             was intended to be sent as data, it should be repeated in the
             second byte of the control code.
    
             If the structure is a file structure, the EOF is indicated by
             the sending host closing the data connection and all bytes are
             data bytes.
    
          3.4.2.  BLOCK MODE
    
             The file is transmitted as a series of data blocks preceded by
             one or more header bytes.  The header bytes contain a count
             field, and descriptor code.  The count field indicates the
             total length of the data block in bytes, thus marking the
             beginning of the next data block (there are no filler bits).
             The descriptor code defines:  last block in the file (EOF) last
             block in the record (EOR), restart marker (see the Section on
             Error Recovery and Restart) or suspect data (i.e., the data
             being transferred is suspected of errors and is not reliable).
             This last code is NOT intended for error control within FTP.
             It is motivated by the desire of sites exchanging certain types
             of data (e.g., seismic or weather data) to send and receive all
             the data despite local errors (such as "magnetic tape read
             errors"), but to indicate in the transmission that certain
             portions are suspect).  Record structures are allowed in this
             mode, and any representation type may be used.
    
             The header consists of the three bytes.  Of the 24 bits of
             header information, the 16 low order bits shall represent byte
             count, and the 8 high order bits shall represent descriptor
             codes as shown below.
    
    
    
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             Block Header
    
                +----------------+----------------+----------------+
                | Descriptor     |    Byte Count                   |
                |         8 bits |                      16 bits    |
                +----------------+----------------+----------------+
                
    
             The descriptor codes are indicated by bit flags in the
             descriptor byte.  Four codes have been assigned, where each
             code number is the decimal value of the corresponding bit in
             the byte.
    
                Code     Meaning
                
                 128     End of data block is EOR
                  64     End of data block is EOF
                  32     Suspected errors in data block
                  16     Data block is a restart marker
    
             With this encoding, more than one descriptor coded condition
             may exist for a particular block.  As many bits as necessary
             may be flagged.
    
             The restart marker is embedded in the data stream as an
             integral number of 8-bit bytes representing printable
             characters in the language being used over the control
             connection (e.g., default--NVT-ASCII).   (Space, in the
             appropriate language) must not be used WITHIN a restart marker.
    
             For example, to transmit a six-character marker, the following
             would be sent:
    
                +--------+--------+--------+
                |Descrptr|  Byte count     |
                |code= 16|             = 6 |
                +--------+--------+--------+
    
                +--------+--------+--------+
                | Marker | Marker | Marker |
                | 8 bits | 8 bits | 8 bits |
                +--------+--------+--------+
    
                +--------+--------+--------+
                | Marker | Marker | Marker |
                | 8 bits | 8 bits | 8 bits |
                +--------+--------+--------+
    
    
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          3.4.3.  COMPRESSED MODE
    
             There are three kinds of information to be sent:  regular data,
             sent in a byte string; compressed data, consisting of
             replications or filler; and control information, sent in a
             two-byte escape sequence.  If n>0 bytes (up to 127) of regular
             data are sent, these n bytes are preceded by a byte with the
             left-most bit set to 0 and the right-most 7 bits containing the
             number n.
    
             Byte string:
    
                 1       7                8                     8
                +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
                |0|       n     | |    d(1)       | ... |      d(n)     |
                +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
                                              ^             ^
                                              |---n bytes---|
                                                  of data
    
                String of n data bytes d(1),..., d(n)
                Count n must be positive.
    
             To compress a string of n replications of the data byte d, the
             following 2 bytes are sent:
    
             Replicated Byte:
    
                  2       6               8
                +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
                |1 0|     n     | |       d       |
                +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
    
             A string of n filler bytes can be compressed into a single
             byte, where the filler byte varies with the representation
             type.  If the type is ASCII or EBCDIC the filler byte is 
             (Space, ASCII code 32, EBCDIC code 64).  If the type is Image
             or Local byte the filler is a zero byte.
    
             Filler String:
    
                  2       6
                +-+-+-+-+-+-+-+-+
                |1 1|     n     |
                +-+-+-+-+-+-+-+-+
    
             The escape sequence is a double byte, the first of which is the
    
    
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             escape byte (all zeros) and the second of which contains
             descriptor codes as defined in Block mode.  The descriptor
             codes have the same meaning as in Block mode and apply to the
             succeeding string of bytes.
    
             Compressed mode is useful for obtaining increased bandwidth on
             very large network transmissions at a little extra CPU cost.
             It can be most effectively used to reduce the size of printer
             files such as those generated by RJE hosts.
    
       3.5.  ERROR RECOVERY AND RESTART
    
          There is no provision for detecting bits lost or scrambled in data
          transfer; this level of error control is handled by the TCP.
          However, a restart procedure is provided to protect users from
          gross system failures (including failures of a host, an
          FTP-process, or the underlying network).
    
          The restart procedure is defined only for the block and compressed
          modes of data transfer.  It requires the sender of data to insert
          a special marker code in the data stream with some marker
          information.  The marker information has meaning only to the
          sender, but must consist of printable characters in the default or
          negotiated language of the control connection (ASCII or EBCDIC).
          The marker could represent a bit-count, a record-count, or any
          other information by which a system may identify a data
          checkpoint.  The receiver of data, if it implements the restart
          procedure, would then mark the corresponding position of this
          marker in the receiving system, and return this information to the
          user.
    
          In the event of a system failure, the user can restart the data
          transfer by identifying the marker point with the FTP restart
          procedure.  The following example illustrates the use of the
          restart procedure.
    
          The sender of the data inserts an appropriate marker block in the
          data stream at a convenient point.  The receiving host marks the
          corresponding data point in its file system and conveys the last
          known sender and receiver marker information to the user, either
          directly or over the control connection in a 110 reply (depending
          on who is the sender).  In the event of a system failure, the user
          or controller process restarts the server at the last server
          marker by sending a restart command with server's marker code as
          its argument.  The restart command is transmitted over the control
    
    
    
    
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          connection and is immediately followed by the command (such as
          RETR, STOR or LIST) which was being executed when the system
          failure occurred.
    
    4.  FILE TRANSFER FUNCTIONS
    
       The communication channel from the user-PI to the server-PI is
       established as a TCP connection from the user to the standard server
       port.  The user protocol interpreter is responsible for sending FTP
       commands and interpreting the replies received; the server-PI
       interprets commands, sends replies and directs its DTP to set up the
       data connection and transfer the data.  If the second party to the
       data transfer (the passive transfer process) is the user-DTP, then it
       is governed through the internal protocol of the user-FTP host; if it
       is a second server-DTP, then it is governed by its PI on command from
       the user-PI.  The FTP replies are discussed in the next section.  In
       the description of a few of the commands in this section, it is
       helpful to be explicit about the possible replies.
    
       4.1.  FTP COMMANDS
    
          4.1.1.  ACCESS CONTROL COMMANDS
    
             The following commands specify access control identifiers
             (command codes are shown in parentheses).
    
             USER NAME (USER)
    
                The argument field is a Telnet string identifying the user.
                The user identification is that which is required by the
                server for access to its file system.  This command will
                normally be the first command transmitted by the user after
                the control connections are made (some servers may require
                this).  Additional identification information in the form of
                a password and/or an account command may also be required by
                some servers.  Servers may allow a new USER command to be
                entered at any point in order to change the access control
                and/or accounting information.  This has the effect of
                flushing any user, password, and account information already
                supplied and beginning the login sequence again.  All
                transfer parameters are unchanged and any file transfer in
                progress is completed under the old access control
                parameters.
    
    
    
    
    
    
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             PASSWORD (PASS)
    
                The argument field is a Telnet string specifying the user's
                password.  This command must be immediately preceded by the
                user name command, and, for some sites, completes the user's
                identification for access control.  Since password
                information is quite sensitive, it is desirable in general
                to "mask" it or suppress typeout.  It appears that the
                server has no foolproof way to achieve this.  It is
                therefore the responsibility of the user-FTP process to hide
                the sensitive password information.
    
             ACCOUNT (ACCT)
    
                The argument field is a Telnet string identifying the user's
                account.  The command is not necessarily related to the USER
                command, as some sites may require an account for login and
                others only for specific access, such as storing files.  In
                the latter case the command may arrive at any time.
    
                There are reply codes to differentiate these cases for the
                automation: when account information is required for login,
                the response to a successful PASSword command is reply code
                332.  On the other hand, if account information is NOT
                required for login, the reply to a successful PASSword
                command is 230; and if the account information is needed for
                a command issued later in the dialogue, the server should
                return a 332 or 532 reply depending on whether it stores
                (pending receipt of the ACCounT command) or discards the
                command, respectively.
    
             CHANGE WORKING DIRECTORY (CWD)
    
                This command allows the user to work with a different
                directory or dataset for file storage or retrieval without
                altering his login or accounting information.  Transfer
                parameters are similarly unchanged.  The argument is a
                pathname specifying a directory or other system dependent
                file group designator.
    
             CHANGE TO PARENT DIRECTORY (CDUP)
    
                This command is a special case of CWD, and is included to
                simplify the implementation of programs for transferring
                directory trees between operating systems having different
    
    
    
    
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                syntaxes for naming the parent directory.  The reply codes
                shall be identical to the reply codes of CWD.  See
                Appendix II for further details.
    
             STRUCTURE MOUNT (SMNT)
    
                This command allows the user to mount a different file
                system data structure without altering his login or
                accounting information.  Transfer parameters are similarly
                unchanged.  The argument is a pathname specifying a
                directory or other system dependent file group designator.
    
             REINITIALIZE (REIN)
    
                This command terminates a USER, flushing all I/O and account
                information, except to allow any transfer in progress to be
                completed.  All parameters are reset to the default settings
                and the control connection is left open.  This is identical
                to the state in which a user finds himself immediately after
                the control connection is opened.  A USER command may be
                expected to follow.
    
             LOGOUT (QUIT)
    
                This command terminates a USER and if file transfer is not
                in progress, the server closes the control connection.  If
                file transfer is in progress, the connection will remain
                open for result response and the server will then close it.
                If the user-process is transferring files for several USERs
                but does not wish to close and then reopen connections for
                each, then the REIN command should be used instead of QUIT.
    
                An unexpected close on the control connection will cause the
                server to take the effective action of an abort (ABOR) and a
                logout (QUIT).
    
          4.1.2.  TRANSFER PARAMETER COMMANDS
    
             All data transfer parameters have default values, and the
             commands specifying data transfer parameters are required only
             if the default parameter values are to be changed.  The default
             value is the last specified value, or if no value has been
             specified, the standard default value is as stated here.  This
             implies that the server must "remember" the applicable default
             values.  The commands may be in any order except that they must
             precede the FTP service request.  The following commands
             specify data transfer parameters:
    
    
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             DATA PORT (PORT)
    
                The argument is a HOST-PORT specification for the data port
                to be used in data connection.  There are defaults for both
                the user and server data ports, and under normal
                circumstances this command and its reply are not needed.  If
                this command is used, the argument is the concatenation of a
                32-bit internet host address and a 16-bit TCP port address.
                This address information is broken into 8-bit fields and the
                value of each field is transmitted as a decimal number (in
                character string representation).  The fields are separated
                by commas.  A port command would be:
    
                   PORT h1,h2,h3,h4,p1,p2
    
                where h1 is the high order 8 bits of the internet host
                address.
    
             PASSIVE (PASV)
    
                This command requests the server-DTP to "listen" on a data
                port (which is not its default data port) and to wait for a
                connection rather than initiate one upon receipt of a
                transfer command.  The response to this command includes the
                host and port address this server is listening on.
    
             REPRESENTATION TYPE (TYPE)
    
                The argument specifies the representation type as described
                in the Section on Data Representation and Storage.  Several
                types take a second parameter.  The first parameter is
                denoted by a single Telnet character, as is the second
                Format parameter for ASCII and EBCDIC; the second parameter
                for local byte is a decimal integer to indicate Bytesize.
                The parameters are separated by a  (Space, ASCII code
                32).
    
                The following codes are assigned for type:
    
                             \    /
                   A - ASCII |    | N - Non-print
                             |-><-| T - Telnet format effectors
                   E - EBCDIC|    | C - Carriage Control (ASA)
                             /    \
                   I - Image
                   
                   L  - Local byte Byte size
    
    
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                The default representation type is ASCII Non-print.  If the
                Format parameter is changed, and later just the first
                argument is changed, Format then returns to the Non-print
                default.
    
             FILE STRUCTURE (STRU)
    
                The argument is a single Telnet character code specifying
                file structure described in the Section on Data
                Representation and Storage.
    
                The following codes are assigned for structure:
    
                   F - File (no record structure)
                   R - Record structure
                   P - Page structure
    
                The default structure is File.
    
             TRANSFER MODE (MODE)
    
                The argument is a single Telnet character code specifying
                the data transfer modes described in the Section on
                Transmission Modes.
    
                The following codes are assigned for transfer modes:
    
                   S - Stream
                   B - Block
                   C - Compressed
    
                The default transfer mode is Stream.
    
          4.1.3.  FTP SERVICE COMMANDS
    
             The FTP service commands define the file transfer or the file
             system function requested by the user.  The argument of an FTP
             service command will normally be a pathname.  The syntax of
             pathnames must conform to server site conventions (with
             standard defaults applicable), and the language conventions of
             the control connection.  The suggested default handling is to
             use the last specified device, directory or file name, or the
             standard default defined for local users.  The commands may be
             in any order except that a "rename from" command must be
             followed by a "rename to" command and the restart command must
             be followed by the interrupted service command (e.g., STOR or
             RETR).  The data, when transferred in response to FTP service
    
    
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             commands, shall always be sent over the data connection, except
             for certain informative replies.  The following commands
             specify FTP service requests:
    
             RETRIEVE (RETR)
    
                This command causes the server-DTP to transfer a copy of the
                file, specified in the pathname, to the server- or user-DTP
                at the other end of the data connection.  The status and
                contents of the file at the server site shall be unaffected.
    
             STORE (STOR)
    
                This command causes the server-DTP to accept the data
                transferred via the data connection and to store the data as
                a file at the server site.  If the file specified in the
                pathname exists at the server site, then its contents shall
                be replaced by the data being transferred.  A new file is
                created at the server site if the file specified in the
                pathname does not already exist.
    
             STORE UNIQUE (STOU)
    
                This command behaves like STOR except that the resultant
                file is to be created in the current directory under a name
                unique to that directory.  The 250 Transfer Started response
                must include the name generated.
    
             APPEND (with create) (APPE)
    
                This command causes the server-DTP to accept the data
                transferred via the data connection and to store the data in
                a file at the server site.  If the file specified in the
                pathname exists at the server site, then the data shall be
                appended to that file; otherwise the file specified in the
                pathname shall be created at the server site.
    
             ALLOCATE (ALLO)
    
                This command may be required by some servers to reserve
                sufficient storage to accommodate the new file to be
                transferred.  The argument shall be a decimal integer
                representing the number of bytes (using the logical byte
                size) of storage to be reserved for the file.  For files
                sent with record or page structure a maximum record or page
                size (in logical bytes) might also be necessary; this is
                indicated by a decimal integer in a second argument field of
    
    
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                the command.  This second argument is optional, but when
                present should be separated from the first by the three
                Telnet characters  R .  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  or .  (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 .  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  (Space) if parameters follow and
       Telnet-EOL otherwise.  The command codes and the semantics of
       commands are described in this section; the detailed syntax of
       commands is specified in the Section on Commands, the reply sequences
       are discussed in the Section on Sequencing of Commands and Replies,
       and scenarios illustrating the use of commands are provided in the
       Section on Typical FTP Scenarios.
    
       FTP commands may be partitioned as those specifying access-control
       identifiers, data transfer parameters, or FTP service requests.
       Certain commands (such as ABOR, STAT, QUIT) may be sent over the
       control connection while a data transfer is in progress.  Some
    
    
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       servers may not be able to monitor the control and data connections
       simultaneously, in which case some special action will be necessary
       to get the server's attention.  The following ordered format is
       tentatively recommended:
    
          1. User system inserts the Telnet "Interrupt Process" (IP) signal
          in the Telnet stream.
    
          2. User system sends the Telnet "Synch" signal.
    
          3. User system inserts the command (e.g., ABOR) in the Telnet
          stream.
    
          4. Server PI, after receiving "IP", scans the Telnet stream for
          EXACTLY ONE FTP command.
    
       (For other servers this may not be necessary but the actions listed
       above should have no unusual effect.)
    
       4.2.  FTP REPLIES
    
          Replies to File Transfer Protocol commands are devised to ensure
          the synchronization of requests and actions in the process of file
          transfer, and to guarantee that the user process always knows the
          state of the Server.  Every command must generate at least one
          reply, although there may be more than one; in the latter case,
          the multiple replies must be easily distinguished.  In addition,
          some commands occur in sequential groups, such as USER, PASS and
          ACCT, or RNFR and RNTO.  The replies show the existence of an
          intermediate state if all preceding commands have been successful.
          A failure at any point in the sequence necessitates the repetition
          of the entire sequence from the beginning.
    
             The details of the command-reply sequence are made explicit in
             a set of state diagrams below.
    
          An FTP reply consists of a three digit number (transmitted as
          three alphanumeric characters) followed by some text.  The number
          is intended for use by automata to determine what state to enter
          next; the text is intended for the human user.  It is intended
          that the three digits contain enough encoded information that the
          user-process (the User-PI) will not need to examine the text and
          may either discard it or pass it on to the user, as appropriate.
          In particular, the text may be server-dependent, so there are
          likely to be varying texts for each reply code.
    
          A reply is defined to contain the 3-digit code, followed by Space
    
    
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          , 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 , optionally some text, and the Telnet
             end-of-line code.
    
                For example:
                                    123-First line
                                    Second line
                                      234 A line beginning with numbers
                                    123 The last line
    
             The user-process then simply needs to search for the second
             occurrence of the same reply code, followed by  (Space), at
             the beginning of a line, and ignore all intermediary lines.  If
             an intermediary line begins with a 3-digit number, the Server
             must pad the front  to avoid confusion.
    
                This scheme allows standard system routines to be used for
                reply information (such as for the STAT reply), with
                "artificial" first and last lines tacked on.  In rare cases
                where these routines are able to generate three digits and a
                Space at the beginning of any line, the beginning of each
                text line should be offset by some neutral text, like Space.
    
             This scheme assumes that multi-line replies may not be nested.
    
          The three digits of the reply each have a special significance.
          This is intended to allow a range of very simple to very
          sophisticated responses by the user-process.  The first digit
          denotes whether the response is good, bad or incomplete.
          (Referring to the state diagram), an unsophisticated user-process
          will be able to determine its next action (proceed as planned,
    
    
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          redo, retrench, etc.) by simply examining this first digit.  A
          user-process that wants to know approximately what kind of error
          occurred (e.g. file system error, command syntax error) may
          examine the second digit, reserving the third digit for the finest
          gradation of information (e.g., RNTO command without a preceding
          RNFR).
    
             There are five values for the first digit of the reply code:
    
                1yz   Positive Preliminary reply
    
                   The requested action is being initiated; expect another
                   reply before proceeding with a new command.  (The
                   user-process sending another command before the
                   completion reply would be in violation of protocol; but
                   server-FTP processes should queue any commands that
                   arrive while a preceding command is in progress.)  This
                   type of reply can be used to indicate that the command
                   was accepted and the user-process may now pay attention
                   to the data connections, for implementations where
                   simultaneous monitoring is difficult.  The server-FTP
                   process may send at most, one 1yz reply per command.
    
                2yz   Positive Completion reply
    
                   The requested action has been successfully completed.  A
                   new request may be initiated.
    
                3yz   Positive Intermediate reply
    
                   The command has been accepted, but the requested action
                   is being held in abeyance, pending receipt of further
                   information.  The user should send another command
                   specifying this information.  This reply is used in
                   command sequence groups.
    
                4yz   Transient Negative Completion reply
    
                   The command was not accepted and the requested action did
                   not take place, but the error condition is temporary and
                   the action may be requested again.  The user should
                   return to the beginning of the command sequence, if any.
                   It is difficult to assign a meaning to "transient",
                   particularly when two distinct sites (Server- and
                   User-processes) have to agree on the interpretation.
                   Each reply in the 4yz category might have a slightly
                   different time value, but the intent is that the
    
    
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                   user-process is encouraged to try again.  A rule of thumb
                   in determining if a reply fits into the 4yz or the 5yz
                   (Permanent Negative) category is that replies are 4yz if
                   the commands can be repeated without any change in
                   command form or in properties of the User or Server
                   (e.g., the command is spelled the same with the same
                   arguments used; the user does not change his file access
                   or user name; the server does not put up a new
                   implementation.)
    
                5yz   Permanent Negative Completion reply
    
                   The command was not accepted and the requested action did
                   not take place.  The User-process is discouraged from
                   repeating the exact request (in the same sequence).  Even
                   some "permanent" error conditions can be corrected, so
                   the human user may want to direct his User-process to
                   reinitiate the command sequence by direct action at some
                   point in the future (e.g., after the spelling has been
                   changed, or the user has altered his directory status.)
    
             The following function groupings are encoded in the second
             digit:
    
                x0z   Syntax - These replies refer to syntax errors,
                      syntactically correct commands that don't fit any
                      functional category, unimplemented or superfluous
                      commands.
    
                x1z   Information -  These are replies to requests for
                      information, such as status or help.
    
                x2z   Connections - Replies referring to the control and
                      data connections.
    
                x3z   Authentication and accounting - Replies for the login
                      process and accounting procedures.
    
                x4z   Unspecified as yet.
    
                x5z   File system - These replies indicate the status of the
                      Server file system vis-a-vis the requested transfer or
                      other file system action.
    
             The third digit gives a finer gradation of meaning in each of
             the function categories, specified by the second digit.  The
             list of replies below will illustrate this.  Note that the text
    
    
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             associated with each reply is recommended, rather than
             mandatory, and may even change according to the command with
             which it is associated.  The reply codes, on the other hand,
             must strictly follow the specifications in the last section;
             that is, Server implementations should not invent new codes for
             situations that are only slightly different from the ones
             described here, but rather should adapt codes already defined.
    
                A command such as TYPE or ALLO whose successful execution
                does not offer the user-process any new information will
                cause a 200 reply to be returned.  If the command is not
                implemented by a particular Server-FTP process because it
                has no relevance to that computer system, for example ALLO
                at a TOPS20 site, a Positive Completion reply is still
                desired so that the simple User-process knows it can proceed
                with its course of action.  A 202 reply is used in this case
                with, for example, the reply text:  "No storage allocation
                necessary."  If, on the other hand, the command requests a
                non-site-specific action and is unimplemented, the response
                is 502.  A refinement of that is the 504 reply for a command
                that is implemented, but that requests an unimplemented
                parameter.
    
          4.2.1  Reply Codes by Function Groups
    
             200 Command okay.
             500 Syntax error, command unrecognized.
                 This may include errors such as command line too long.
             501 Syntax error in parameters or arguments.
             202 Command not implemented, superfluous at this site.
             502 Command not implemented.
             503 Bad sequence of commands.
             504 Command not implemented for that parameter.
              
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
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             110 Restart marker reply.
                 In this case, the text is exact and not left to the
                 particular implementation; it must read:
                      MARK yyyy = mmmm
                 Where yyyy is User-process data stream marker, and mmmm
                 server's equivalent marker (note the spaces between markers
                 and "=").
             211 System status, or system help reply.
             212 Directory status.
             213 File status.
             214 Help message.
                 On how to use the server or the meaning of a particular
                 non-standard command.  This reply is useful only to the
                 human user.
             215 NAME system type.
                 Where NAME is an official system name from the list in the
                 Assigned Numbers document.
              
             120 Service ready in nnn minutes.
             220 Service ready for new user.
             221 Service closing control connection.
                 Logged out if appropriate.
             421 Service not available, closing control connection.
                 This may be a reply to any command if the service knows it
                 must shut down.
             125 Data connection already open; transfer starting.
             225 Data connection open; no transfer in progress.
             425 Can't open data connection.
             226 Closing data connection.
                 Requested file action successful (for example, file
                 transfer or file abort).
             426 Connection closed; transfer aborted.
             227 Entering Passive Mode (h1,h2,h3,h4,p1,p2).
              
             230 User logged in, proceed.
             530 Not logged in.
             331 User name okay, need password.
             332 Need account for login.
             532 Need account for storing files.
              
    
    
    
    
    
    
    
    
    
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             150 File status okay; about to open data connection.
             250 Requested file action okay, completed.
             257 "PATHNAME" created.
             350 Requested file action pending further information.
             450 Requested file action not taken.
                 File unavailable (e.g., file busy).
             550 Requested action not taken.
                 File unavailable (e.g., file not found, no access).
             451 Requested action aborted. Local error in processing.
             551 Requested action aborted. Page type unknown.
             452 Requested action not taken.
                 Insufficient storage space in system.
             552 Requested file action aborted.
                 Exceeded storage allocation (for current directory or
                 dataset).
             553 Requested action not taken.
                 File name not allowed.
             
    
          4.2.2 Numeric  Order List of Reply Codes
    
             110 Restart marker reply.
                 In this case, the text is exact and not left to the
                 particular implementation; it must read:
                      MARK yyyy = mmmm
                 Where yyyy is User-process data stream marker, and mmmm
                 server's equivalent marker (note the spaces between markers
                 and "=").
             120 Service ready in nnn minutes.
             125 Data connection already open; transfer starting.
             150 File status okay; about to open data connection.
              
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
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             200 Command okay.
             202 Command not implemented, superfluous at this site.
             211 System status, or system help reply.
             212 Directory status.
             213 File status.
             214 Help message.
                 On how to use the server or the meaning of a particular
                 non-standard command.  This reply is useful only to the
                 human user.
             215 NAME system type.
                 Where NAME is an official system name from the list in the
                 Assigned Numbers document.
             220 Service ready for new user.
             221 Service closing control connection.
                 Logged out if appropriate.
             225 Data connection open; no transfer in progress.
             226 Closing data connection.
                 Requested file action successful (for example, file
                 transfer or file abort).
             227 Entering Passive Mode (h1,h2,h3,h4,p1,p2).
             230 User logged in, proceed.
             250 Requested file action okay, completed.
             257 "PATHNAME" created.
              
             331 User name okay, need password.
             332 Need account for login.
             350 Requested file action pending further information.
              
             421 Service not available, closing control connection.
                 This may be a reply to any command if the service knows it
                 must shut down.
             425 Can't open data connection.
             426 Connection closed; transfer aborted.
             450 Requested file action not taken.
                 File unavailable (e.g., file busy).
             451 Requested action aborted: local error in processing.
             452 Requested action not taken.
                 Insufficient storage space in system.
              
    
    
    
    
    
    
    
    
    
    
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             500 Syntax error, command unrecognized.
                 This may include errors such as command line too long.
             501 Syntax error in parameters or arguments.
             502 Command not implemented.
             503 Bad sequence of commands.
             504 Command not implemented for that parameter.
             530 Not logged in.
             532 Need account for storing files.
             550 Requested action not taken.
                 File unavailable (e.g., file not found, no access).
             551 Requested action aborted: page type unknown.
             552 Requested file action aborted.
                 Exceeded storage allocation (for current directory or
                 dataset).
             553 Requested action not taken.
                 File name not allowed.
             
    
    5.  DECLARATIVE SPECIFICATIONS
    
       5.1.  MINIMUM IMPLEMENTATION
    
          In order to make FTP workable without needless error messages, the
          following minimum implementation is required for all servers:
    
             TYPE - ASCII Non-print
             MODE - Stream
             STRUCTURE - File, Record
             COMMANDS - USER, QUIT, PORT,
                        TYPE, MODE, STRU,
                          for the default values
                        RETR, STOR,
                        NOOP.
    
          The default values for transfer parameters are:
    
             TYPE - ASCII Non-print
             MODE - Stream
             STRU - File
    
          All hosts must accept the above as the standard defaults.
    
    
    
    
    
    
    
    
    Postel & Reynolds                                              [Page 43]
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    RFC 959                                                     October 1985
    File Transfer Protocol
    
    
       5.2.  CONNECTIONS
    
          The server protocol interpreter shall "listen" on Port L.  The
          user or user protocol interpreter shall initiate the full-duplex
          control connection.  Server- and user- processes should follow the
          conventions of the Telnet protocol as specified in the
          ARPA-Internet Protocol Handbook [1].  Servers are under no
          obligation to provide for editing of command lines and may require
          that it be done in the user host.  The control connection shall be
          closed by the server at the user's request after all transfers and
          replies are completed.
    
          The user-DTP must "listen" on the specified data port; this may be
          the default user port (U) or a port specified in the PORT command.
          The server shall initiate the data connection from his own default
          data port (L-1) using the specified user data port.  The direction
          of the transfer and the port used will be determined by the FTP
          service command.
    
          Note that all FTP implementation must support data transfer using
          the default port, and that only the USER-PI may initiate the use
          of non-default ports.
    
          When data is to be transferred between two servers, A and B (refer
          to Figure 2), the user-PI, C, sets up control connections with
          both server-PI's.  One of the servers, say A, is then sent a PASV
          command telling him to "listen" on his data port rather than
          initiate a connection when he receives a transfer service command.
          When the user-PI receives an acknowledgment to the PASV command,
          which includes the identity of the host and port being listened
          on, the user-PI then sends A's port, a, to B in a PORT command; a
          reply is returned.  The user-PI may then send the corresponding
          service commands to A and B.  Server B initiates the connection
          and the transfer proceeds.  The command-reply sequence is listed
          below where the messages are vertically synchronous but
          horizontally asynchronous:
    
    
    
    
    
    
    
    
    
    
    
    
    
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             User-PI - Server A                User-PI - Server B
             ------------------                ------------------
             
             C->A : Connect                    C->B : Connect
             C->A : PASV
             A->C : 227 Entering Passive Mode. A1,A2,A3,A4,a1,a2
                                               C->B : PORT A1,A2,A3,A4,a1,a2
                                               B->C : 200 Okay
             C->A : STOR                       C->B : RETR
                        B->A : Connect to HOST-A, PORT-a
    
                                    Figure 3
    
          The data connection shall be closed by the server under the
          conditions described in the Section on Establishing Data
          Connections.  If the data connection is to be closed following a
          data transfer where closing the connection is not required to
          indicate the end-of-file, the server must do so immediately.
          Waiting until after a new transfer command is not permitted
          because the user-process will have already tested the data
          connection to see if it needs to do a "listen"; (remember that the
          user must "listen" on a closed data port BEFORE sending the
          transfer request).  To prevent a race condition here, the server
          sends a reply (226) after closing the data connection (or if the
          connection is left open, a "file transfer completed" reply (250)
          and the user-PI should wait for one of these replies before
          issuing a new transfer command).
    
          Any time either the user or server see that the connection is
          being closed by the other side, it should promptly read any
          remaining data queued on the connection and issue the close on its
          own side.
    
       5.3.  COMMANDS
    
          The commands are Telnet character strings transmitted over the
          control connections as described in the Section on FTP Commands.
          The command functions and semantics are described in the Section
          on Access Control Commands, Transfer Parameter Commands, FTP
          Service Commands, and Miscellaneous Commands.  The command syntax
          is specified here.
    
          The commands begin with a command code followed by an argument
          field.  The command codes are four or fewer alphabetic characters.
          Upper and lower case alphabetic characters are to be treated
          identically.  Thus, any of the following may represent the
          retrieve command:
    
    
    Postel & Reynolds                                              [Page 45]
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    File Transfer Protocol
    
    
                      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  (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.
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    Postel & Reynolds                                              [Page 46]
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    RFC 959                                                     October 1985
    File Transfer Protocol
    
    
          5.3.1.  FTP COMMANDS
    
             The following are the FTP commands:
    
                USER   
                PASS   
                ACCT   
                CWD    
                CDUP 
                SMNT   
                QUIT 
                REIN 
                PORT   
                PASV 
                TYPE   
                STRU   
                MODE   
                RETR   
                STOR   
                STOU 
                APPE   
                ALLO  
                    [ R  ] 
                REST   
                RNFR   
                RNTO   
                ABOR 
                DELE   
                RMD    
                MKD    
                PWD  
                LIST [ ] 
                NLST [ ] 
                SITE   
                SYST 
                STAT [ ] 
                HELP [ ] 
                NOOP 
    
    
    
    
    
    
    
    
    
    
    
    Postel & Reynolds                                              [Page 47]
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    File Transfer Protocol
    
    
          5.3.2.  FTP COMMAND ARGUMENTS
    
             The syntax of the above argument fields (using BNF notation
             where applicable) is:
    
                 ::= 
                 ::= 
                 ::= 
                 ::=  | 
                 ::= any of the 128 ASCII characters except  and
                
                 ::= 
                 ::=  | 
                 ::= printable characters, any
                              ASCII code 33 through 126
                 ::= 
                 ::= ,
                 ::= ,,,
                 ::= ,
                 ::= any decimal integer 1 through 255
                 ::= N | T | C
                 ::= A [ ]
                              | E [ ]
                              | I
                              | L  
                 ::= F | R | P
                 ::= S | B | C
                 ::= 
                 ::= any decimal integer
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    Postel & Reynolds                                              [Page 48]
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    File Transfer Protocol
    
    
       5.4.  SEQUENCING OF COMMANDS AND REPLIES
    
          The communication between the user and server is intended to be an
          alternating dialogue.  As such, the user issues an FTP command and
          the server responds with a prompt primary reply.  The user should
          wait for this initial primary success or failure response before
          sending further commands.
    
          Certain commands require a second reply for which the user should
          also wait.  These replies may, for example, report on the progress
          or completion of file transfer or the closing of the data
          connection.  They are secondary replies to file transfer commands.
    
          One important group of informational replies is the connection
          greetings.  Under normal circumstances, a server will send a 220
          reply, "awaiting input", when the connection is completed.  The
          user should wait for this greeting message before sending any
          commands.  If the server is unable to accept input right away, a
          120 "expected delay" reply should be sent immediately and a 220
          reply when ready.  The user will then know not to hang up if there
          is a delay.
    
          Spontaneous Replies
    
             Sometimes "the system" spontaneously has a message to be sent
             to a user (usually all users).  For example, "System going down
             in 15 minutes".  There is no provision in FTP for such
             spontaneous information to be sent from the server to the user.
             It is recommended that such information be queued in the
             server-PI and delivered to the user-PI in the next reply
             (possibly making it a multi-line reply).
    
          The table below lists alternative success and failure replies for
          each command.  These must be strictly adhered to; a server may
          substitute text in the replies, but the meaning and action implied
          by the code numbers and by the specific command reply sequence
          cannot be altered.
    
          Command-Reply Sequences
    
             In this section, the command-reply sequence is presented.  Each
             command is listed with its possible replies; command groups are
             listed together.  Preliminary replies are listed first (with
             their succeeding replies indented and under them), then
             positive and negative completion, and finally intermediary
    
    
    
    
    Postel & Reynolds                                              [Page 49]
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             replies with the remaining commands from the sequence
             following.  This listing forms the basis for the state
             diagrams, which will be presented separately.
    
                Connection Establishment
                   120
                      220
                   220
                   421
                Login
                   USER
                      230
                      530
                      500, 501, 421
                      331, 332
                   PASS
                      230
                      202
                      530
                      500, 501, 503, 421
                      332
                   ACCT
                      230
                      202
                      530
                      500, 501, 503, 421
                   CWD
                      250
                      500, 501, 502, 421, 530, 550
                   CDUP
                      200
                      500, 501, 502, 421, 530, 550
                   SMNT
                      202, 250
                      500, 501, 502, 421, 530, 550
                Logout
                   REIN
                      120
                         220
                      220
                      421
                      500, 502
                   QUIT
                      221
                      500
    
    
    
    
    Postel & Reynolds                                              [Page 50]
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    RFC 959                                                     October 1985
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                Transfer parameters
                   PORT
                      200
                      500, 501, 421, 530
                   PASV
                      227
                      500, 501, 502, 421, 530
                   MODE
                      200
                      500, 501, 504, 421, 530
                   TYPE
                      200
                      500, 501, 504, 421, 530
                   STRU
                      200
                      500, 501, 504, 421, 530
                File action commands
                   ALLO
                      200
                      202
                      500, 501, 504, 421, 530
                   REST
                      500, 501, 502, 421, 530
                      350
                   STOR
                      125, 150
                         (110)
                         226, 250
                         425, 426, 451, 551, 552
                      532, 450, 452, 553
                      500, 501, 421, 530
                   STOU
                      125, 150
                         (110)
                         226, 250
                         425, 426, 451, 551, 552
                      532, 450, 452, 553
                      500, 501, 421, 530
                   RETR
                      125, 150
                         (110)
                         226, 250
                         425, 426, 451
                      450, 550
                      500, 501, 421, 530
    
    
    
    
    Postel & Reynolds                                              [Page 51]
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    RFC 959                                                     October 1985
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                   LIST
                      125, 150
                         226, 250
                         425, 426, 451
                      450
                      500, 501, 502, 421, 530
                   NLST
                      125, 150
                         226, 250
                         425, 426, 451
                      450
                      500, 501, 502, 421, 530
                   APPE
                      125, 150
                         (110)
                         226, 250
                         425, 426, 451, 551, 552
                      532, 450, 550, 452, 553
                      500, 501, 502, 421, 530
                   RNFR
                      450, 550
                      500, 501, 502, 421, 530
                      350
                   RNTO
                      250
                      532, 553
                      500, 501, 502, 503, 421, 530
                   DELE
                      250
                      450, 550
                      500, 501, 502, 421, 530
                   RMD
                      250
                      500, 501, 502, 421, 530, 550
                   MKD
                      257
                      500, 501, 502, 421, 530, 550
                   PWD
                      257
                      500, 501, 502, 421, 550
                   ABOR
                      225, 226
                      500, 501, 502, 421
    
    
    
    
    
    
    Postel & Reynolds                                              [Page 52]
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    RFC 959                                                     October 1985
    File Transfer Protocol
    
    
                Informational commands
                   SYST
                      215
                      500, 501, 502, 421
                   STAT
                      211, 212, 213
                      450
                      500, 501, 502, 421, 530
                   HELP
                      211, 214
                      500, 501, 502, 421
                Miscellaneous commands
                   SITE
                      200
                      202
                      500, 501, 530
                   NOOP
                      200
                      500 421
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    Postel & Reynolds                                              [Page 53]
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    RFC 959                                                     October 1985
    File Transfer Protocol
    
    
    6.  STATE DIAGRAMS
    
       Here we present state diagrams for a very simple minded FTP
       implementation.  Only the first digit of the reply codes is used.
       There is one state diagram for each group of FTP commands or command
       sequences.
    
       The command groupings were determined by constructing a model for
       each command then collecting together the commands with structurally
       identical models.
    
       For each command or command sequence there are three possible
       outcomes: success (S), failure (F), and error (E).  In the state
       diagrams below we use the symbol B for "begin", and the symbol W for
       "wait for reply".
    
       We first present the diagram that represents the largest group of FTP
       commands:
    
          
                                   1,3    +---+
                              ----------->| E |
                             |            +---+
                             |
          +---+    cmd    +---+    2      +---+
          | B |---------->| W |---------->| S |
          +---+           +---+           +---+
                             |
                             |     4,5    +---+
                              ----------->| F |
                                          +---+
          
    
          This diagram models the commands:
    
             ABOR, ALLO, DELE, CWD, CDUP, SMNT, HELP, MODE, NOOP, PASV,
             QUIT, SITE, PORT, SYST, STAT, RMD, MKD, PWD, STRU, and TYPE.
    
    
    
    
    
    
    
    
    
    
    
    
    Postel & Reynolds                                              [Page 54]
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    RFC 959                                                     October 1985
    File Transfer Protocol
    
    
       The other large group of commands is represented by a very similar
       diagram:
    
          
                                   3      +---+
                              ----------->| E |
                             |            +---+
                             |
          +---+    cmd    +---+    2      +---+
          | B |---------->| W |---------->| S |
          +---+       --->+---+           +---+
                     |     | |
                     |     | |     4,5    +---+
                     |  1  |  ----------->| F |
                      -----               +---+
          
    
          This diagram models the commands:
    
             APPE, LIST, NLST, REIN, RETR, STOR, and STOU.
    
       Note that this second model could also be used to represent the first
       group of commands, the only difference being that in the first group
       the 100 series replies are unexpected and therefore treated as error,
       while the second group expects (some may require) 100 series replies.
       Remember that at most, one 100 series reply is allowed per command.
    
       The remaining diagrams model command sequences, perhaps the simplest
       of these is the rename sequence:
    
          
          +---+   RNFR    +---+    1,2    +---+
          | B |---------->| W |---------->| E |
          +---+           +---+        -->+---+
                           | |        |
                    3      | | 4,5    |
             --------------  ------   |
            |                      |  |   +---+
            |               ------------->| S |
            |              |   1,3 |  |   +---+
            |             2|  --------
            |              | |     |
            V              | |     |
          +---+   RNTO    +---+ 4,5 ----->+---+
          |   |---------->| W |---------->| F |
          +---+           +---+           +---+
          
    
    
    Postel & Reynolds                                              [Page 55]
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    RFC 959                                                     October 1985
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       The next diagram is a simple model of the Restart command:
    
          
          +---+   REST    +---+    1,2    +---+
          | B |---------->| W |---------->| E |
          +---+           +---+        -->+---+
                           | |        |
                    3      | | 4,5    |
             --------------  ------   |
            |                      |  |   +---+
            |               ------------->| S |
            |              |   3   |  |   +---+
            |             2|  --------
            |              | |     |
            V              | |     |
          +---+   cmd     +---+ 4,5 ----->+---+
          |   |---------->| W |---------->| F |
          +---+        -->+---+           +---+
                      |      |
                      |  1   |
                       ------
          
    
             Where "cmd" is APPE, STOR, or RETR.
    
       We note that the above three models are similar.  The Restart differs
       from the Rename two only in the treatment of 100 series replies at
       the second stage, while the second group expects (some may require)
       100 series replies.  Remember that at most, one 100 series reply is
       allowed per command.
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    Postel & Reynolds                                              [Page 56]
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    File Transfer Protocol
    
    
       The most complicated diagram is for the Login sequence:
    
          
                                1
          +---+   USER    +---+------------->+---+
          | B |---------->| W | 2       ---->| E |
          +---+           +---+------  |  -->+---+
                           | |       | | |
                         3 | | 4,5   | | |
             --------------   -----  | | |
            |                      | | | |
            |                      | | | |
            |                 ---------  |
            |               1|     | |   |
            V                |     | |   |
          +---+   PASS    +---+ 2  |  ------>+---+
          |   |---------->| W |------------->| S |
          +---+           +---+   ---------->+---+
                           | |   | |     |
                         3 | |4,5| |     |
             --------------   --------   |
            |                    | |  |  |
            |                    | |  |  |
            |                 -----------
            |             1,3|   | |  |
            V                |  2| |  |
          +---+   ACCT    +---+--  |   ----->+---+
          |   |---------->| W | 4,5 -------->| F |
          +---+           +---+------------->+---+
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    Postel & Reynolds                                              [Page 57]
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       Finally, we present a generalized diagram that could be used to model
       the command and reply interchange:
    
          
                   ------------------------------------
                  |                                    |
          Begin   |                                    |
            |     V                                    |
            |   +---+  cmd   +---+ 2         +---+     |
             -->|   |------->|   |---------->|   |     |
                |   |        | W |           | S |-----|
             -->|   |     -->|   |-----      |   |     |
            |   +---+    |   +---+ 4,5 |     +---+     |
            |     |      |    | |      |               |
            |     |      |   1| |3     |     +---+     |
            |     |      |    | |      |     |   |     |
            |     |       ----  |       ---->| F |-----
            |     |             |            |   |
            |     |             |            +---+
             -------------------
                  |
                  |
                  V
                 End
          
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    Postel & Reynolds                                              [Page 58]
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    RFC 959                                                     October 1985
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    7.  TYPICAL FTP SCENARIO
    
       User at host U wanting to transfer files to/from host S:
    
       In general, the user will communicate to the server via a mediating
       user-FTP process.  The following may be a typical scenario.  The
       user-FTP prompts are shown in parentheses, '---->' represents
       commands from host U to host S, and '<----' represents replies from
       host S to host U.
    
          LOCAL COMMANDS BY USER              ACTION INVOLVED
    
          ftp (host) multics         Connect to host S, port L,
                                         establishing control connections.
                                         <---- 220 Service ready .
          username Doe               USER Doe---->
                                         <---- 331 User name ok,
                                                   need password.
          password mumble            PASS mumble---->
                                         <---- 230 User logged in.
          retrieve (local type) ASCII
          (local pathname) test 1    User-FTP opens local file in ASCII.
          (for. pathname) test.pl1   RETR test.pl1 ---->
                                         <---- 150 File status okay;
                                               about to open data
                                               connection.
                                         Server makes data connection
                                         to port U.
          
                                         <---- 226 Closing data connection,
                                             file transfer successful.
          type Image                 TYPE I ---->
                                         <---- 200 Command OK
          store (local type) image
          (local pathname) file dump User-FTP opens local file in Image.
          (for.pathname) >udd>cn>fd  STOR >udd>cn>fd ---->
                                         <---- 550 Access denied
          terminate                      QUIT  ---->
                                         Server closes all
                                         connections.
    
    8.  CONNECTION ESTABLISHMENT
    
       The FTP control connection is established via TCP between the user
       process port U and the server process port L.  This protocol is
       assigned the service port 21 (25 octal), that is L=21.
    
    
    
    Postel & Reynolds                                              [Page 59]
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    APPENDIX I -  PAGE STRUCTURE
    
       The need for FTP to support page structure derives principally from
       the  need to support efficient transmission of files between TOPS-20
       systems, particularly the files used by NLS.
    
       The file system of TOPS-20 is based on the concept of pages.  The
       operating system is most efficient at manipulating files as pages.
       The operating system provides an interface to the file system so that
       many applications view files as sequential streams of characters.
       However, a few applications use the underlying page structures
       directly, and some of these create holey files.
    
       A TOPS-20 disk file consists of four things: a pathname, a page
       table, a (possibly empty) set of pages, and a set of attributes.
    
       The pathname is specified in the RETR or STOR command.  It includes
       the directory name, file name, file name extension, and generation
       number.
    
       The page table contains up to 2**18 entries.  Each entry may be
       EMPTY, or may point to a page.  If it is not empty, there are also
       some page-specific access bits; not all pages of a file need have the
       same access protection.
    
          A page is a contiguous set of 512 words of 36 bits each.
    
       The attributes of the file, in the File Descriptor Block (FDB),
       contain such things as creation time, write time, read time, writer's
       byte-size, end-of-file pointer, count of reads and writes, backup
       system tape numbers, etc.
    
       Note that there is NO requirement that entries in the page table be
       contiguous.  There may be empty page table slots between occupied
       ones.  Also, the end of file pointer is simply a number.  There is no
       requirement that it in fact point at the "last" datum in the file.
       Ordinary sequential I/O calls in TOPS-20 will cause the end of file
       pointer to be left after the last datum written, but other operations
       may cause it not to be so, if a particular programming system so
       requires.
    
       In fact, in both of these special cases, "holey" files and
       end-of-file pointers NOT at the end of the file, occur with NLS data
       files.
    
    
    
    
    
    Postel & Reynolds                                              [Page 60]
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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]
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    APPENDIX II -  DIRECTORY COMMANDS
    
       Since UNIX has a tree-like directory structure in which directories
       are as easy to manipulate as ordinary files, it is useful to expand
       the FTP servers on these machines to include commands which deal with
       the creation of directories.  Since there are other hosts on the
       ARPA-Internet which have tree-like directories (including TOPS-20 and
       Multics), these commands are as general as possible.
    
          Four directory commands have been added to FTP:
    
             MKD pathname
    
                Make a directory with the name "pathname".
    
             RMD pathname
    
                Remove the directory with the name "pathname".
    
             PWD
    
                Print the current working directory name.
    
             CDUP
    
                Change to the parent of the current working directory.
    
       The  "pathname"  argument should be created (removed) as a
       subdirectory of the current working directory, unless the "pathname"
       string contains sufficient information to specify otherwise to the
       server, e.g., "pathname" is an absolute pathname (in UNIX and
       Multics), or pathname is something like "" to
       TOPS-20.
    
       REPLY CODES
    
          The CDUP command is a special case of CWD, and is included to
          simplify the implementation of programs for transferring directory
          trees between operating systems having different syntaxes for
          naming the parent directory.  The reply codes for CDUP be
          identical to the reply codes of CWD.
    
          The reply codes for RMD be identical to the reply codes for its
          file analogue, DELE.
    
          The reply codes for MKD, however, are a bit more complicated.  A
          freshly created directory will probably be the object of a future
    
    
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          CWD command.  Unfortunately, the argument to MKD may not always be
          a suitable argument for CWD.  This is the case, for example, when
          a TOPS-20 subdirectory is created by giving just the subdirectory
          name.  That is, with a TOPS-20 server FTP, the command sequence
    
             MKD MYDIR
             CWD MYDIR
    
          will fail.  The new directory may only be referred to by its
          "absolute" name; e.g., if the MKD command above were issued while
          connected to the directory , the new subdirectory
          could only be referred to by the name .
    
          Even on UNIX and Multics, however, the argument given to MKD may
          not be suitable.  If it is a "relative" pathname (i.e., a pathname
          which is interpreted relative to the current directory), the user
          would need to be in the same current directory in order to reach
          the subdirectory.  Depending on the application, this may be
          inconvenient.  It is not very robust in any case.
    
          To solve these problems, upon successful completion of an MKD
          command, the server should return a line of the form:
    
             257""
    
          That is, the server will tell the user what string to use when
          referring to the created  directory.  The directory name can
          contain any character; embedded double-quotes should be escaped by
          double-quotes (the "quote-doubling" convention).
    
          For example, a user connects to the directory /usr/dm, and creates
          a subdirectory, named pathname:
    
             CWD /usr/dm
             200 directory changed to /usr/dm
             MKD pathname
             257 "/usr/dm/pathname" directory created
    
          An example with an embedded double quote:
    
             MKD foo"bar
             257 "/usr/dm/foo""bar" directory created
             CWD /usr/dm/foo"bar
             200 directory changed to /usr/dm/foo"bar
    
    
    
    
    
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          The prior existence of a subdirectory with the same name is an
          error, and the server must return an "access denied" error reply
          in that case.
    
             CWD /usr/dm
             200 directory changed to /usr/dm
             MKD pathname
             521-"/usr/dm/pathname" directory already exists;
             521 taking no action.
    
          The failure replies for MKD are analogous to its file  creating
          cousin, STOR.  Also, an "access denied" return is given if a file
          name with the same name as the subdirectory will conflict with the
          creation of the subdirectory (this is a problem on UNIX, but
          shouldn't be one on TOPS-20).
    
          Essentially because the PWD command returns the same type of
          information as the successful MKD command, the successful PWD
          command uses the 257 reply code as well.
    
       SUBTLETIES
    
          Because these commands will be most useful in transferring
          subtrees from one machine to another, carefully observe that the
          argument to MKD is to be interpreted as a sub-directory of  the
          current working directory, unless it contains enough information
          for the destination host to tell otherwise.  A hypothetical
          example of its use in the TOPS-20 world:
    
             CWD 
             200 Working directory changed
             MKD overrainbow
             257 "" directory created
             CWD overrainbow
             431 No such directory
             CWD 
             200 Working directory changed
    
             CWD 
             200 Working directory changed to 
             MKD 
             257 "" directory created
             CWD 
    
          Note that the first example results in a subdirectory of the
          connected directory.  In contrast, the argument in the second
          example contains enough information for TOPS-20 to tell that  the
    
    
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           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  directory;
          this is an ambiguity inherent in some TOPS-20 implementations.
          Similar considerations apply to the RMD command.  The point is
          this: except where to do so would violate a host's conventions for
          denoting relative versus absolute pathnames, the host should treat
          the operands of the MKD and RMD commands as subdirectories.  The
          257 reply to the MKD command must always contain the absolute
          pathname of the created directory.
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
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    APPENDIX III - RFCs on FTP
    
       Bhushan, Abhay, "A File Transfer Protocol", RFC 114 (NIC 5823),
       MIT-Project MAC, 16 April 1971.
    
       Harslem, Eric, and John Heafner, "Comments on RFC 114 (A File
       Transfer Protocol)", RFC 141 (NIC 6726), RAND, 29 April 1971.
    
       Bhushan, Abhay, et al, "The File Transfer Protocol", RFC 172
       (NIC 6794), MIT-Project MAC, 23 June 1971.
    
       Braden, Bob, "Comments on DTP and FTP Proposals", RFC 238 (NIC 7663),
       UCLA/CCN, 29 September 1971.
    
       Bhushan, Abhay, et al, "The File Transfer Protocol", RFC 265
       (NIC 7813), MIT-Project MAC, 17 November 1971.
    
       McKenzie, Alex, "A Suggested Addition to File Transfer Protocol",
       RFC 281 (NIC 8163), BBN, 8 December 1971.
    
       Bhushan, Abhay, "The Use of "Set Data Type" Transaction in File
       Transfer Protocol", RFC 294 (NIC 8304), MIT-Project MAC,
       25 January 1972.
    
       Bhushan, Abhay, "The File Transfer Protocol", RFC 354 (NIC 10596),
       MIT-Project MAC, 8 July 1972.
    
       Bhushan, Abhay, "Comments on the File Transfer Protocol (RFC 354)",
       RFC 385 (NIC 11357), MIT-Project MAC, 18 August 1972.
    
       Hicks, Greg, "User FTP Documentation", RFC 412 (NIC 12404), Utah,
       27 November 1972.
    
       Bhushan, Abhay, "File Transfer Protocol (FTP) Status and Further
       Comments", RFC 414 (NIC 12406), MIT-Project MAC, 20 November 1972.
    
       Braden, Bob, "Comments on File Transfer Protocol", RFC 430
       (NIC 13299), UCLA/CCN, 7 February 1973.
    
       Thomas, Bob, and Bob Clements, "FTP Server-Server Interaction",
       RFC 438 (NIC 13770), BBN, 15 January 1973.
    
       Braden, Bob, "Print Files in FTP", RFC 448 (NIC 13299), UCLA/CCN,
       27 February 1973.
    
       McKenzie, Alex, "File Transfer Protocol", RFC 454 (NIC 14333), BBN,
       16 February 1973.
    
    
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       Bressler, Bob, and Bob Thomas, "Mail Retrieval via FTP", RFC 458
       (NIC 14378), BBN-NET and BBN-TENEX, 20 February 1973.
    
       Neigus, Nancy, "File Transfer Protocol", RFC 542 (NIC 17759), BBN,
       12 July 1973.
    
       Krilanovich, Mark, and George Gregg, "Comments on the File Transfer
       Protocol", RFC 607 (NIC 21255), UCSB, 7 January 1974.
    
       Pogran, Ken, and Nancy Neigus, "Response to RFC 607 - Comments on the
       File Transfer Protocol", RFC 614 (NIC 21530), BBN, 28 January 1974.
    
       Krilanovich, Mark, George Gregg, Wayne Hathaway, and Jim White,
       "Comments on the File Transfer Protocol", RFC 624 (NIC 22054), UCSB,
       Ames Research Center, SRI-ARC, 28 February 1974.
    
       Bhushan, Abhay, "FTP Comments and Response to RFC 430", RFC 463
       (NIC 14573), MIT-DMCG, 21 February 1973.
    
       Braden, Bob, "FTP Data Compression", RFC 468 (NIC 14742), UCLA/CCN,
       8 March 1973.
    
       Bhushan, Abhay, "FTP and Network Mail System", RFC 475 (NIC 14919),
       MIT-DMCG, 6 March 1973.
    
       Bressler, Bob, and Bob Thomas "FTP Server-Server Interaction - II",
       RFC 478 (NIC 14947), BBN-NET and BBN-TENEX, 26 March 1973.
    
       White, Jim, "Use of FTP by the NIC Journal", RFC 479 (NIC 14948),
       SRI-ARC, 8 March 1973.
    
       White, Jim, "Host-Dependent FTP Parameters", RFC 480 (NIC 14949),
       SRI-ARC, 8 March 1973.
    
       Padlipsky, Mike, "An FTP Command-Naming Problem", RFC 506
       (NIC 16157), MIT-Multics, 26 June 1973.
    
       Day, John, "Memo to FTP Group (Proposal for File Access Protocol)",
       RFC 520 (NIC 16819), Illinois, 25 June 1973.
    
       Merryman, Robert, "The UCSD-CC Server-FTP Facility", RFC 532
       (NIC 17451), UCSD-CC, 22 June 1973.
    
       Braden, Bob, "TENEX FTP Problem", RFC 571 (NIC 18974), UCLA/CCN,
       15 November 1973.
    
    
    
    
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       McKenzie, Alex, and Jon Postel, "Telnet and FTP Implementation -
       Schedule Change", RFC 593 (NIC 20615), BBN and MITRE,
       29 November 1973.
    
       Sussman, Julie, "FTP Error Code Usage for More Reliable Mail
       Service", RFC 630 (NIC 30237), BBN, 10 April 1974.
    
       Postel, Jon, "Revised FTP Reply Codes", RFC 640 (NIC 30843),
       UCLA/NMC, 5 June 1974.
    
       Harvey, Brian, "Leaving Well Enough Alone", RFC 686 (NIC 32481),
       SU-AI, 10 May 1975.
    
       Harvey, Brian, "One More Try on the FTP", RFC 691 (NIC 32700), SU-AI,
       28 May 1975.
    
       Lieb, J., "CWD Command of FTP", RFC 697 (NIC 32963), 14 July 1975.
    
       Harrenstien, Ken, "FTP Extension: XSEN", RFC 737 (NIC 42217), SRI-KL,
       31 October 1977.
    
       Harrenstien, Ken, "FTP Extension: XRSQ/XRCP", RFC 743 (NIC 42758),
       SRI-KL, 30 December 1977.
    
       Lebling, P. David, "Survey of FTP Mail and MLFL", RFC 751, MIT,
       10 December 1978.
    
       Postel, Jon, "File Transfer Protocol Specification", RFC 765, ISI,
       June 1980.
    
       Mankins, David, Dan Franklin, and Buzz Owen, "Directory Oriented FTP
       Commands", RFC 776, BBN, December 1980.
    
       Padlipsky, Michael, "FTP Unique-Named Store Command", RFC 949, MITRE,
       July 1985.
    
    
    
    
    
    
    
    
    
    
    
    
    
    
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    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.
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
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