tech-invite   World Map     

IETF     RFCs     Groups     SIP     ABNFs    |    3GPP     Specs     Gloss.     Arch.     IMS     UICC    |    Misc.    |    search     info

RFC 0959

Pages: 69
Top     in Index     Prev     Next
in Group Index     Prev in Group     Next in Group     Group: ~ft

File Transfer Protocol

Part 1 of 3, p. 1 to 18
None       Next RFC Part

Obsoletes:    0765
Updated by:    2228    2640    2773    3659    5797    7151

Top       Page 1 
Network Working Group                                          J. Postel
Request for Comments: 959                                    J. Reynolds
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.


   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].


   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.

Top       Page 2 
   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.

Top       Page 3 
      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.



         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

      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.

Top       Page 4 
      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

      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.


         The data transfer process establishes and manages the data
         connection.  The DTP can be passive or active.


         The end-of-line sequence defines the separation of printing
         lines.  The sequence is Carriage Return, followed by Line Feed.


         The end-of-file condition that defines the end of a file being


         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.

Top       Page 5 
      FTP commands

         A set of commands that comprise the control information flowing
         from the user-FTP to the server-FTP process.


         An ordered set of computer data (including programs), of
         arbitrary length, uniquely identified by a pathname.


         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.


         The Network Virtual Terminal as defined in the Telnet Protocol.


         The Network Virtual File System.  A concept which defines a
         standard network file system with standard commands and
         pathname conventions.


         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 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.


         The protocol interpreter.  The user and server sides of the
         protocol have distinct roles implemented in a user-PI and a

Top       Page 6 

         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.


         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.


         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).


         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.


         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.

Top       Page 7 

         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


         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


         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.

Top       Page 8 
   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

Top       Page 9 
      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

      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

         Ease of implementaion, sharing code, and modular programming
         argue for the second approach.  Efficiency and independence

Top       Page 10 
         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.


   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.


      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

Top       Page 11 
      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.

            This is the default type and must be accepted by all FTP
            implementations.  It is intended primarily for the transfer
            of text files, except when both hosts would find the EBCDIC
            type more convenient.

            The sender converts the data from an internal character
            representation to the standard 8-bit NVT-ASCII
            representation (see the Telnet specification).  The receiver
            will convert the data from the standard form to his own
            internal form.

            In accordance with the NVT standard, the <CRLF> sequence
            should be used where necessary to denote the end of a line
            of text.  (See the discussion of file structure at the end
            of the Section on Data Representation and Storage.)

            Using the standard NVT-ASCII representation means that data
            must be interpreted as 8-bit bytes.

            The Format parameter for ASCII and EBCDIC types is discussed

Top       Page 12   EBCDIC TYPE

            This type is intended for efficient transfer between hosts
            which use EBCDIC for their internal character

            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

            End-of-line (as opposed to end-of-record--see the discussion
            of structure) will probably be rarely used with EBCDIC type
            for purposes of denoting structure, but where it is
            necessary the <NL> character should be used.

            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

            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.

Top       Page 13 
            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

            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.

            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:


               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.

Top       Page 14 
               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.


               The file contains ASCII/EBCDIC vertical format controls
               (i.e., <CR>, <LF>, <NL>, <VT>, <FF>) which the printer
               process will interpret appropriately.  <CRLF>, in exactly
               this sequence, also denotes end-of-line.


               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

                  Character     Vertical Spacing

                  blank         Move paper up one line
                  0             Move paper up two lines
                  1             Move paper to top of next page
                  +             No movement, i.e., overprint

               Clearly there must be some way for a printer process to
               distinguish the end of the structural entity.  If a file
               has record structure (see below) this is no problem;
               records will be explicitly marked during transfer and
               storage.  If the file has no record structure, the <CRLF>
               end-of-line sequence is used to separate printing lines,
               but these format effectors are overridden by the ASA

Top       Page 15 
      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

            and page-structure, where the file is made up of independent
                                indexed pages.

         File-structure is the default to be assumed if the STRUcture
         command has not been used but both file and record structures
         must be accepted for "text" files (i.e., files with TYPE ASCII
         or EBCDIC) by all FTP implementations.  The structure of a file
         will affect both the transfer mode of a file (see the Section
         on Transmission Modes) and the interpretation and storage of
         the file.

         The "natural" structure of a file will depend on which host
         stores the file.  A source-code file will usually be stored on
         an IBM Mainframe in fixed length records but on a DEC TOPS-20
         as a stream of characters partitioned into lines, for example
         by <CRLF>.  If the transfer of files between such disparate
         sites is to be useful, there must be some way for one site to
         recognize the other's assumptions about the file.

         With some sites being naturally file-oriented and others
         naturally record-oriented there may be problems if a file with
         one structure is sent to a host oriented to the other.  If a
         text file is sent with record-structure to a host which is file
         oriented, then that host should apply an internal
         transformation to the file based on the record structure.
         Obviously, this transformation should be useful, but it must
         also be invertible so that an identical file may be retrieved
         using record structure.

         In the case of a file being sent with file-structure to a
         record-oriented host, there exists the question of what
         criteria the host should use to divide the file into records
         which can be processed locally.  If this division is necessary,
         the FTP implementation should use the end-of-line sequence,

Top       Page 16 
         <CRLF> for ASCII, or <NL> for EBCDIC text files, as the
         delimiter.  If an FTP implementation adopts this technique, it
         must be prepared to reverse the transformation if the file is
         retrieved with file-structure.

            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

            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

            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

Top       Page 17 
               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

            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

Top       Page 18 
      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.

(page 18 continued on part 2)

Next RFC Part