RFC 959 - File Transfer Protocol
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`Network Working Group J. Postel
`Request for Comments: 959 J. Reynolds
` ISI
`Obsoletes RFC: 765 (IEN 149) October 1985
`FILE TRANSFER PROTOCOL (FTP)
`
`Status of this Memo
` This memo is the official specification of the File Transfer
` Protocol (FTP). Distribution of this memo is unlimited.
` The following new optional commands are included in this edition of
` the specification:
` CDUP (Change to Parent Directory), SMNT (Structure Mount), STOU
` (Store Unique), RMD (Remove Directory), MKD (Make Directory), PWD
` (Print Directory), and SYST (System).
` Note that this specification is compatible with the previous edition.
`1. INTRODUCTION
` The objectives of FTP are 1) to promote sharing of files (computer
` programs and/or data), 2) to encourage indirect or implicit (via
` programs) use of remote computers, 3) to shield a user from
` variations in file storage systems among hosts, and 4) to transfer
` data reliably and efficiently. FTP, though usable directly by a user
` at a terminal, is designed mainly for use by programs.
` The attempt in this specification is to satisfy the diverse needs of
` users of maxi-hosts, mini-hosts, personal workstations, and TACs,
` with a simple, and easily implemented protocol design.
` This paper assumes knowledge of the Transmission Control Protocol
` (TCP) [2] and the Telnet Protocol [3]. These documents are contained
` in the ARPA-Internet protocol handbook [1].
`2. OVERVIEW
` In this section, the history, the terminology, and the FTP model are
` discussed. The terms defined in this section are only those that
` have special significance in FTP. Some of the terminology is very
` specific to the FTP model; some readers may wish to turn to the
` section on the FTP model while reviewing the terminology.
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`File Transfer Protocol
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` 2.1. HISTORY
` FTP has had a long evolution over the years. Appendix III is a
` chronological compilation of Request for Comments documents
` relating to FTP. These include the first proposed file transfer
` mechanisms in 1971 that were developed for implementation on hosts
` at M.I.T. (RFC 114), plus comments and discussion in RFC 141.
` RFC 172 provided a user-level oriented protocol for file transfer
` between host computers (including terminal IMPs). A revision of
` this as RFC 265, restated FTP for additional review, while RFC 281
` suggested further changes. The use of a "Set Data Type"
` transaction was proposed in RFC 294 in January 1982.
` RFC 354 obsoleted RFCs 264 and 265. The File Transfer Protocol
` was now defined as a protocol for file transfer between HOSTs on
` the ARPANET, with the primary function of FTP defined as
` transfering files efficiently and reliably among hosts and
` allowing the convenient use of remote file storage capabilities.
` RFC 385 further commented on errors, emphasis points, and
` additions to the protocol, while RFC 414 provided a status report
` on the working server and user FTPs. RFC 430, issued in 1973,
` (among other RFCs too numerous to mention) presented further
` comments on FTP. Finally, an "official" FTP document was
` published as RFC 454.
` By July 1973, considerable changes from the last versions of FTP
` were made, but the general structure remained the same. RFC 542
` was published as a new "official" specification to reflect these
` changes. However, many implementations based on the older
` specification were not updated.
` In 1974, RFCs 607 and 614 continued comments on FTP. RFC 624
` proposed further design changes and minor modifications. In 1975,
` RFC 686 entitled, "Leaving Well Enough Alone", discussed the
` differences between all of the early and later versions of FTP.
` RFC 691 presented a minor revision of RFC 686, regarding the
` subject of print files.
` Motivated by the transition from the NCP to the TCP as the
` underlying protocol, a phoenix was born out of all of the above
` efforts in RFC 765 as the specification of FTP for use on TCP.
` This current edition of the FTP specification is intended to
` correct some minor documentation errors, to improve the
` explanation of some protocol features, and to add some new
` optional commands.
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` In particular, the following new optional commands are included in
` this edition of the specification:
` CDUP - Change to Parent Directory
` SMNT - Structure Mount
` STOU - Store Unique
` RMD - Remove Directory
` MKD - Make Directory
` PWD - Print Directory
` SYST - System
` This specification is compatible with the previous edition. A
` program implemented in conformance to the previous specification
` should automatically be in conformance to this specification.
` 2.2. TERMINOLOGY
` ASCII
` The ASCII character set is as defined in the ARPA-Internet
` Protocol Handbook. In FTP, ASCII characters are defined to be
` the lower half of an eight-bit code set (i.e., the most
` significant bit is zero).
` access controls
` Access controls define users' access privileges to the use of a
` system, and to the files in that system. Access controls are
` necessary to prevent unauthorized or accidental use of files.
` It is the prerogative of a server-FTP process to invoke access
` controls.
` byte size
` There are two byte sizes of interest in FTP: the logical byte
` size of the file, and the transfer byte size used for the
` transmission of the data. The transfer byte size is always 8
` bits. The transfer byte size is not necessarily the byte size
` in which data is to be stored in a system, nor the logical byte
` size for interpretation of the structure of the data.
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` control connection
` The communication path between the USER-PI and SERVER-PI for
` the exchange of commands and replies. This connection follows
` the Telnet Protocol.
` data connection
` A full duplex connection over which data is transferred, in a
` specified mode and type. The data transferred may be a part of
` a file, an entire file or a number of files. The path may be
` between a server-DTP and a user-DTP, or between two
` server-DTPs.
` data port
` The passive data transfer process "listens" on the data port
` for a connection from the active transfer process in order to
` open the data connection.
` DTP
` The data transfer process establishes and manages the data
` connection. The DTP can be passive or active.
` End-of-Line
` The end-of-line sequence defines the separation of printing
` lines. The sequence is Carriage Return, followed by Line Feed.
` EOF
` The end-of-file condition that defines the end of a file being
` transferred.
` EOR
` The end-of-record condition that defines the end of a record
` being transferred.
` error recovery
` A procedure that allows a user to recover from certain errors
` such as failure of either host system or transfer process. In
` FTP, error recovery may involve restarting a file transfer at a
` given checkpoint.
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` FTP commands
` A set of commands that comprise the control information flowing
` from the user-FTP to the server-FTP process.
` file
` An ordered set of computer data (including programs), of
` arbitrary length, uniquely identified by a pathname.
` mode
` The mode in which data is to be transferred via the data
` connection. The mode defines the data format during transfer
` including EOR and EOF. The transfer modes defined in FTP are
` described in the Section on Transmission Modes.
` NVT
` The Network Virtual Terminal as defined in the Telnet Protocol.
` NVFS
` The Network Virtual File System. A concept which defines a
` standard network file system with standard commands and
` pathname conventions.
` page
` A file may be structured as a set of independent parts called
` pages. FTP supports the transmission of discontinuous files as
` independent indexed pages.
` pathname
` Pathname is defined to be the character string which must be
` input to a file system by a user in order to identify a file.
` Pathname normally contains device and/or directory names, and
` file name specification. FTP does not yet specify a standard
` pathname convention. Each user must follow the file naming
` conventions of the file systems involved in the transfer.
` PI
` The protocol interpreter. The user and server sides of the
` protocol have distinct roles implemented in a user-PI and a
` server-PI.
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` record
` A sequential file may be structured as a number of contiguous
` parts called records. Record structures are supported by FTP
` but a file need not have record structure.
` reply
` A reply is an acknowledgment (positive or negative) sent from
` server to user via the control connection in response to FTP
` commands. The general form of a reply is a completion code
` (including error codes) followed by a text string. The codes
` are for use by programs and the text is usually intended for
` human users.
` server-DTP
` The data transfer process, in its normal "active" state,
` establishes the data connection with the "listening" data port.
` It sets up parameters for transfer and storage, and transfers
` data on command from its PI. The DTP can be placed in a
` "passive" state to listen for, rather than initiate a
` connection on the data port.
` server-FTP process
` A process or set of processes which perform the function of
` file transfer in cooperation with a user-FTP process and,
` possibly, another server. The functions consist of a protocol
` interpreter (PI) and a data transfer process (DTP).
` server-PI
` The server protocol interpreter "listens" on Port L for a
` connection from a user-PI and establishes a control
` communication connection. It receives standard FTP commands
` from the user-PI, sends replies, and governs the server-DTP.
` type
` The data representation type used for data transfer and
` storage. Type implies certain transformations between the time
` of data storage and data transfer. The representation types
` defined in FTP are described in the Section on Establishing
` Data Connections.
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` user
` A person or a process on behalf of a person wishing to obtain
` file transfer service. The human user may interact directly
` with a server-FTP process, but use of a user-FTP process is
` preferred since the protocol design is weighted towards
` automata.
` user-DTP
` The data transfer process "listens" on the data port for a
` connection from a server-FTP process. If two servers are
` transferring data between them, the user-DTP is inactive.
` user-FTP process
` A set of functions including a protocol interpreter, a data
` transfer process and a user interface which together perform
` the function of file transfer in cooperation with one or more
` server-FTP processes. The user interface allows a local
` language to be used in the command-reply dialogue with the
` user.
` user-PI
` The user protocol interpreter initiates the control connection
` from its port U to the server-FTP process, initiates FTP
` commands, and governs the user-DTP if that process is part of
` the file transfer.
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` 2.3. THE FTP MODEL
` With the above definitions in mind, the following model (shown in
` Figure 1) may be diagrammed for an FTP service.
` -------------
` |/---------\|
` || User || --------
` ||Interface|<--->| User |
` |\----^----/| --------
` ---------- | | |
` |/------\| FTP Commands |/----V----\|
` ||Server|<---------------->| User ||
` || PI || FTP Replies || PI ||
` |\--^---/| |\----^----/|
` | | | | | |
` -------- |/--V---\| Data |/----V----\| --------
` | File |<--->|Server|<---------------->| User |<--->| File |
` |System| || DTP || Connection || DTP || |System|
` -------- |\------/| |\---------/| --------
` ---------- -------------
` Server-FTP USER-FTP
` NOTES: 1. The data connection may be used in either direction.
` 2. The data connection need not exist all of the time.
` Figure 1 Model for FTP Use
` In the model described in Figure 1, the user-protocol interpreter
` initiates the control connection. The control connection follows
` the Telnet protocol. At the initiation of the user, standard FTP
` commands are generated by the user-PI and transmitted to the
` server process via the control connection. (The user may
` establish a direct control connection to the server-FTP, from a
` TAC terminal for example, and generate standard FTP commands
` independently, bypassing the user-FTP process.) Standard replies
` are sent from the server-PI to the user-PI over the control
` connection in response to the commands.
` The FTP commands specify the parameters for the data connection
` (data port, transfer mode, representation type, and structure) and
` the nature of file system operation (store, retrieve, append,
` delete, etc.). The user-DTP or its designate should "listen" on
` the specified data port, and the server initiate the data
` connection and data transfer in accordance with the specified
` parameters. It should be noted that the data port need not be in
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` the same host that initiates the FTP commands via the control
` connection, but the user or the user-FTP process must ensure a
` "listen" on the specified data port. It ought to also be noted
` that the data connection may be used for simultaneous sending and
` receiving.
` In another situation a user might wish to transfer files between
` two hosts, neither of which is a local host. The user sets up
` control connections to the two servers and then arranges for a
` data connection between them. In this manner, control information
` is passed to the user-PI but data is transferred between the
` server data transfer processes. Following is a model of this
` server-server interaction.
`
` Control ------------ Control
` ---------->| User-FTP |<-----------
` | | User-PI | |
` | | "C" | |
` V ------------ V
` -------------- --------------
` | Server-FTP | Data Connection | Server-FTP |
` | "A" |<---------------------->| "B" |
` -------------- Port (A) Port (B) --------------
`
` Figure 2
` The protocol requires that the control connections be open while
` data transfer is in progress. It is the responsibility of the
` user to request the closing of the control connections when
` finished using the FTP service, while it is the server who takes
` the action. The server may abort data transfer if the control
` connections are closed without command.
` The Relationship between FTP and Telnet:
` The FTP uses the Telnet protocol on the control connection.
` This can be achieved in two ways: first, the user-PI or the
` server-PI may implement the rules of the Telnet Protocol
` directly in their own procedures; or, second, the user-PI or
` the server-PI may make use of the existing Telnet module in the
` system.
` Ease of implementaion, sharing code, and modular programming
` argue for the second approach. Efficiency and independence
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` argue for the first approach. In practice, FTP relies on very
` little of the Telnet Protocol, so the first approach does not
` necessarily involve a large amount of code.
`3. DATA TRANSFER FUNCTIONS
` Files are transferred only via the data connection. The control
` connection is used for the transfer of commands, which describe the
` functions to be performed, and the replies to these commands (see the
` Section on FTP Replies). Several commands are concerned with the
` transfer of data between hosts. These data transfer commands include
` the MODE command which specify how the bits of the data are to be
` transmitted, and the STRUcture and TYPE commands, which are used to
` define the way in which the data are to be represented. The
` transmission and representation are basically independent but the
` "Stream" transmission mode is dependent on the file structure
` attribute and if "Compressed" transmission mode is used, the nature
` of the filler byte depends on the representation type.
` 3.1. DATA REPRESENTATION AND STORAGE
` Data is transferred from a storage device in the sending host to a
` storage device in the receiving host. Often it is necessary to
` perform certain transformations on the data because data storage
` representations in the two systems are different. For example,
` NVT-ASCII has different data storage representations in different
` systems. DEC TOPS-20s's generally store NVT-ASCII as five 7-bit
` ASCII characters, left-justified in a 36-bit word. IBM Mainframe's
` store NVT-ASCII as 8-bit EBCDIC codes. Multics stores NVT-ASCII
` as four 9-bit characters in a 36-bit word. It is desirable to
` convert characters into the standard NVT-ASCII representation when
` transmitting text between dissimilar systems. The sending and
` receiving sites would have to perform the necessary
` transformations between the standard representation and their
` internal representations.
` A different problem in representation arises when transmitting
` binary data (not character codes) between host systems with
` different word lengths. It is not always clear how the sender
` should send data, and the receiver store it. For example, when
` transmitting 32-bit bytes from a 32-bit word-length system to a
` 36-bit word-length system, it may be desirable (for reasons of
` efficiency and usefulness) to store the 32-bit bytes
` right-justified in a 36-bit word in the latter system. In any
` case, the user should have the option of specifying data
` representation and transformation functions. It should be noted
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` that FTP provides for very limited data type representations.
` Transformations desired beyond this limited capability should be
` performed by the user directly.
` 3.1.1. DATA TYPES
` Data representations are handled in FTP by a user specifying a
` representation type. This type may implicitly (as in ASCII or
` EBCDIC) or explicitly (as in Local byte) define a byte size for
` interpretation which is referred to as the "logical byte size."
` Note that this has nothing to do with the byte size used for
` transmission over the data connection, called the "transfer
` byte size", and the two should not be confused. For example,
` NVT-ASCII has a logical byte size of 8 bits. If the type is
` Local byte, then the TYPE command has an obligatory second
` parameter specifying the logical byte size. The transfer byte
` size is always 8 bits.
` 3.1.1.1. ASCII TYPE
` This is the default type and must be accepted by all FTP
` implementations. It is intended primarily for the transfer
` of text files, except when both hosts would find the EBCDIC
` type more convenient.
` The sender converts the data from an internal character
` representation to the standard 8-bit NVT-ASCII
` representation (see the Telnet specification). The receiver
` will convert the data from the standard form to his own
` internal form.
` In accordance with the NVT standard, the <CRLF> sequence
` should be used where necessary to denote the end of a line
` of text. (See the discussion of file structure at the end
` of the Section on Data Representation and Storage.)
` Using the standard NVT-ASCII representation means that data
` must be interpreted as 8-bit bytes.
` The Format parameter for ASCII and EBCDIC types is discussed
` below.
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` 3.1.1.2. EBCDIC TYPE
` This type is intended for efficient transfer between hosts
` which use EBCDIC for their internal character
` representation.
` For transmission, the data are represented as 8-bit EBCDIC
` characters. The character code is the only difference
` between the functional specifications of EBCDIC and ASCII
` types.
` End-of-line (as opposed to end-of-record--see the discussion
` of structure) will probably be rarely used with EBCDIC type
` for purposes of denoting structure, but where it is
` necessary the <NL> character should be used.
` 3.1.1.3. IMAGE TYPE
` The data are sent as contiguous bits which, for transfer,
` are packed into the 8-bit transfer bytes. The receiving
` site must store the data as contiguous bits. The structure
` of the storage system might necessitate the padding of the
` file (or of each record, for a record-structured file) to
` some convenient boundary (byte, word or block). This
` padding, which must be all zeros, may occur only at the end
` of the file (or at the end of each record) and there must be
` a way of identifying the padding bits so that they may be
` stripped off if the file is retrieved. The padding
` transformation should be well publicized to enable a user to
` process a file at the storage site.
` Image type is intended for the efficient storage and
` retrieval of files and for the transfer of binary data. It
` is recommended that this type be accepted by all FTP
` implementations.
` 3.1.1.4. LOCAL TYPE
` The data is transferred in logical bytes of the size
` specified by the obligatory second parameter, Byte size.
` The value of Byte size must be a decimal integer; there is
` no default value. The logical byte size is not necessarily
` the same as the transfer byte size. If there is a
` difference in byte sizes, then the logical bytes should be
` packed contiguously, disregarding transfer byte boundaries
` and with any necessary padding at the end.
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` When the data reaches the receiving host, it will be
` transformed in a manner dependent on the logical byte size
` and the particular host. This transformation must be
` invertible (i.e., an identical file can be retrieved if the
` same parameters are used) and should be well publicized by
` the FTP implementors.
` For example, a user sending 36-bit floating-point numbers to
` a host with a 32-bit word could send that data as Local byte
` with a logical byte size of 36. The receiving host would
` then be expected to store the logical bytes so that they
` could be easily manipulated; in this example putting the
` 36-bit logical bytes into 64-bit double words should
` suffice.
` In another example, a pair of hosts with a 36-bit word size
` may send data to one another in words by using TYPE L 36.
` The data would be sent in the 8-bit transmission bytes
` packed so that 9 transmission bytes carried two host words.
` 3.1.1.5. FORMAT CONTROL
` The types ASCII and EBCDIC also take a second (optional)
` parameter; this is to indicate what kind of vertical format
` control, if any, is associated with a file. The following
` data representation types are defined in FTP:
` A character file may be transferred to a host for one of
` three purposes: for printing, for storage and later
` retrieval, or for processing. If a file is sent for
` printing, the receiving host must know how the vertical
` format control is represented. In the second case, it must
` be possible to store a file at a host and then retrieve it
` later in exactly the same form. Finally, it should be
` possible to move a file from one host to another and process
` the file at the second host without undue trouble. A single
` ASCII or EBCDIC format does not satisfy all these
` conditions. Therefore, these types have a second parameter
` specifying one of the following three formats:
` 3.1.1.5.1. NON PRINT
` This is the default format to be used if the second
` (format) parameter is omitted. Non-print format must be
` accepted by all FTP implementations.
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` The file need contain no vertical format information. If
` it is passed to a printer process, this process may
` assume standard values for spacing and margins.
` Normally, this format will be used with files destined
` for processing or just storage.
` 3.1.1.5.2. TELNET FORMAT CONTROLS
` The file contains ASCII/EBCDIC vertical format controls
` (i.e., <CR>, <LF>, <NL>, <VT>, <FF>) which the printer
` process will interpret appropriately. <CRLF>, in exactly
` this sequence, also denotes end-of-line.
` 3.1.1.5.2. CARRIAGE CONTROL (ASA)
` The file contains ASA (FORTRAN) vertical format control
` characters. (See RFC 740 Appendix C; and Communications
` of the ACM, Vol. 7, No. 10, p. 606, October 1964.) In a
` line or a record formatted according to the ASA Standard,
` the first character is not to be printed. Instead, it
` should be used to determine the vertical movement of the
` paper which should take place before the rest of the
` record is printed.
` The ASA Standard specifies the following control
` characters:
` Character Vertical Spacing
` blank Move paper up one line
` 0