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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.
+
+
+
+
+
+
+
+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 <CRLF> sequence
+ should be used where necessary to denote the end of a line
+ of text. (See the discussion of file structure at the end
+ of the Section on Data Representation and Storage.)
+
+ Using the standard NVT-ASCII representation means that data
+ must be interpreted as 8-bit bytes.
+
+ The Format parameter for ASCII and EBCDIC types is discussed
+ below.
+
+
+
+
+
+
+
+
+Postel & Reynolds [Page 11]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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.
+
+
+
+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., <CR>, <LF>, <NL>, <VT>, <FF>) which the printer
+ process will interpret appropriately. <CRLF>, in exactly
+ this sequence, also denotes end-of-line.
+
+ 3.1.1.5.2. CARRIAGE CONTROL (ASA)
+
+ The file contains ASA (FORTRAN) vertical format control
+ characters. (See RFC 740 Appendix C; and Communications
+ of the ACM, Vol. 7, No. 10, p. 606, October 1964.) In a
+ line or a record formatted according to the ASA Standard,
+ the first character is not to be printed. Instead, it
+ should be used to determine the vertical movement of the
+ paper which should take place before the rest of the
+ record is printed.
+
+ The ASA Standard specifies the following control
+ characters:
+
+ Character Vertical Spacing
+
+ blank Move paper up one line
+ 0 Move paper up two lines
+ 1 Move paper to top of next page
+ + No movement, i.e., overprint
+
+ Clearly there must be some way for a printer process to
+ distinguish the end of the structural entity. If a file
+ has record structure (see below) this is no problem;
+ records will be explicitly marked during transfer and
+ storage. If the file has no record structure, the <CRLF>
+ end-of-line sequence is used to separate printing lines,
+ but these format effectors are overridden by the ASA
+ controls.
+
+
+
+
+
+
+Postel & Reynolds [Page 14]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 3.1.2. DATA STRUCTURES
+
+ In addition to different representation types, FTP allows the
+ structure of a file to be specified. Three file structures are
+ defined in FTP:
+
+ file-structure, where there is no internal structure and
+ the file is considered to be a
+ continuous sequence of data bytes,
+
+ record-structure, where the file is made up of sequential
+ records,
+
+ and page-structure, where the file is made up of independent
+ indexed pages.
+
+ File-structure is the default to be assumed if the STRUcture
+ command has not been used but both file and record structures
+ must be accepted for "text" files (i.e., files with TYPE ASCII
+ or EBCDIC) by all FTP implementations. The structure of a file
+ will affect both the transfer mode of a file (see the Section
+ on Transmission Modes) and the interpretation and storage of
+ the file.
+
+ The "natural" structure of a file will depend on which host
+ stores the file. A source-code file will usually be stored on
+ an IBM Mainframe in fixed length records but on a DEC TOPS-20
+ as a stream of characters partitioned into lines, for example
+ by <CRLF>. If the transfer of files between such disparate
+ sites is to be useful, there must be some way for one site to
+ recognize the other's assumptions about the file.
+
+ With some sites being naturally file-oriented and others
+ naturally record-oriented there may be problems if a file with
+ one structure is sent to a host oriented to the other. If a
+ text file is sent with record-structure to a host which is file
+ oriented, then that host should apply an internal
+ transformation to the file based on the record structure.
+ Obviously, this transformation should be useful, but it must
+ also be invertible so that an identical file may be retrieved
+ using record structure.
+
+ In the case of a file being sent with file-structure to a
+ record-oriented host, there exists the question of what
+ criteria the host should use to divide the file into records
+ which can be processed locally. If this division is necessary,
+ the FTP implementation should use the end-of-line sequence,
+
+
+Postel & Reynolds [Page 15]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ <CRLF> for ASCII, or <NL> for EBCDIC text files, as the
+ delimiter. If an FTP implementation adopts this technique, it
+ must be prepared to reverse the transformation if the file is
+ retrieved with file-structure.
+
+ 3.1.2.1. FILE STRUCTURE
+
+ File structure is the default to be assumed if the STRUcture
+ command has not been used.
+
+ In file-structure there is no internal structure and the
+ file is considered to be a continuous sequence of data
+ bytes.
+
+ 3.1.2.2. RECORD STRUCTURE
+
+ Record structures must be accepted for "text" files (i.e.,
+ files with TYPE ASCII or EBCDIC) by all FTP implementations.
+
+ In record-structure the file is made up of sequential
+ records.
+
+ 3.1.2.3. PAGE STRUCTURE
+
+ To transmit files that are discontinuous, FTP defines a page
+ structure. Files of this type are sometimes known as
+ "random access files" or even as "holey files". In these
+ files there is sometimes other information associated with
+ the file as a whole (e.g., a file descriptor), or with a
+ section of the file (e.g., page access controls), or both.
+ In FTP, the sections of the file are called pages.
+
+ To provide for various page sizes and associated
+ information, each page is sent with a page header. The page
+ header has the following defined fields:
+
+ Header Length
+
+ The number of logical bytes in the page header
+ including this byte. The minimum header length is 4.
+
+ Page Index
+
+ The logical page number of this section of the file.
+ This is not the transmission sequence number of this
+ page, but the index used to identify this page of the
+ file.
+
+
+Postel & Reynolds [Page 16]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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
+
+
+
+
+Postel & Reynolds [Page 17]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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
+
+
+
+
+Postel & Reynolds [Page 18]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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
+
+
+Postel & Reynolds [Page 19]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ inherently unreliable, since one can not determine if the
+ connection closed prematurely or not. The other transfer modes
+ (Block, Compressed) do not close the connection to indicate the
+ end of file. They have enough FTP encoding that the data
+ connection can be parsed to determine the end of the file.
+ Thus using these modes one can leave the data connection open
+ for multiple file transfers.
+
+ 3.4. TRANSMISSION MODES
+
+ The next consideration in transferring data is choosing the
+ appropriate transmission mode. There are three modes: one which
+ formats the data and allows for restart procedures; one which also
+ compresses the data for efficient transfer; and one which passes
+ the data with little or no processing. In this last case the mode
+ interacts with the structure attribute to determine the type of
+ processing. In the compressed mode, the representation type
+ determines the filler byte.
+
+ All data transfers must be completed with an end-of-file (EOF)
+ which may be explicitly stated or implied by the closing of the
+ data connection. For files with record structure, all the
+ end-of-record markers (EOR) are explicit, including the final one.
+ For files transmitted in page structure a "last-page" page type is
+ used.
+
+ NOTE: In the rest of this section, byte means "transfer byte"
+ except where explicitly stated otherwise.
+
+ For the purpose of standardized transfer, the sending host will
+ translate its internal end of line or end of record denotation
+ into the representation prescribed by the transfer mode and file
+ structure, and the receiving host will perform the inverse
+ translation to its internal denotation. An IBM Mainframe record
+ count field may not be recognized at another host, so the
+ end-of-record information may be transferred as a two byte control
+ code in Stream mode or as a flagged bit in a Block or Compressed
+ mode descriptor. End-of-line in an ASCII or EBCDIC file with no
+ record structure should be indicated by <CRLF> or <NL>,
+ respectively. Since these transformations imply extra work for
+ some systems, identical systems transferring non-record structured
+ text files might wish to use a binary representation and stream
+ mode for the transfer.
+
+
+
+
+
+
+Postel & Reynolds [Page 20]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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.
+
+
+
+Postel & Reynolds [Page 21]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ Block Header
+
+ +----------------+----------------+----------------+
+ | Descriptor | Byte Count |
+ | 8 bits | 16 bits |
+ +----------------+----------------+----------------+
+
+
+ The descriptor codes are indicated by bit flags in the
+ descriptor byte. Four codes have been assigned, where each
+ code number is the decimal value of the corresponding bit in
+ the byte.
+
+ Code Meaning
+
+ 128 End of data block is EOR
+ 64 End of data block is EOF
+ 32 Suspected errors in data block
+ 16 Data block is a restart marker
+
+ With this encoding, more than one descriptor coded condition
+ may exist for a particular block. As many bits as necessary
+ may be flagged.
+
+ The restart marker is embedded in the data stream as an
+ integral number of 8-bit bytes representing printable
+ characters in the language being used over the control
+ connection (e.g., default--NVT-ASCII). <SP> (Space, in the
+ appropriate language) must not be used WITHIN a restart marker.
+
+ For example, to transmit a six-character marker, the following
+ would be sent:
+
+ +--------+--------+--------+
+ |Descrptr| Byte count |
+ |code= 16| = 6 |
+ +--------+--------+--------+
+
+ +--------+--------+--------+
+ | Marker | Marker | Marker |
+ | 8 bits | 8 bits | 8 bits |
+ +--------+--------+--------+
+
+ +--------+--------+--------+
+ | Marker | Marker | Marker |
+ | 8 bits | 8 bits | 8 bits |
+ +--------+--------+--------+
+
+
+Postel & Reynolds [Page 22]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 3.4.3. COMPRESSED MODE
+
+ There are three kinds of information to be sent: regular data,
+ sent in a byte string; compressed data, consisting of
+ replications or filler; and control information, sent in a
+ two-byte escape sequence. If n>0 bytes (up to 127) of regular
+ data are sent, these n bytes are preceded by a byte with the
+ left-most bit set to 0 and the right-most 7 bits containing the
+ number n.
+
+ Byte string:
+
+ 1 7 8 8
+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
+ |0| n | | d(1) | ... | d(n) |
+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
+ ^ ^
+ |---n bytes---|
+ of data
+
+ String of n data bytes d(1),..., d(n)
+ Count n must be positive.
+
+ To compress a string of n replications of the data byte d, the
+ following 2 bytes are sent:
+
+ Replicated Byte:
+
+ 2 6 8
+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
+ |1 0| n | | d |
+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
+
+ A string of n filler bytes can be compressed into a single
+ byte, where the filler byte varies with the representation
+ type. If the type is ASCII or EBCDIC the filler byte is <SP>
+ (Space, ASCII code 32, EBCDIC code 64). If the type is Image
+ or Local byte the filler is a zero byte.
+
+ Filler String:
+
+ 2 6
+ +-+-+-+-+-+-+-+-+
+ |1 1| n |
+ +-+-+-+-+-+-+-+-+
+
+ The escape sequence is a double byte, the first of which is the
+
+
+Postel & Reynolds [Page 23]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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
+
+
+
+
+Postel & Reynolds [Page 24]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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.
+
+
+
+
+
+
+Postel & Reynolds [Page 25]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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
+
+
+
+
+Postel & Reynolds [Page 26]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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:
+
+
+Postel & Reynolds [Page 27]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ DATA PORT (PORT)
+
+ The argument is a HOST-PORT specification for the data port
+ to be used in data connection. There are defaults for both
+ the user and server data ports, and under normal
+ circumstances this command and its reply are not needed. If
+ this command is used, the argument is the concatenation of a
+ 32-bit internet host address and a 16-bit TCP port address.
+ This address information is broken into 8-bit fields and the
+ value of each field is transmitted as a decimal number (in
+ character string representation). The fields are separated
+ by commas. A port command would be:
+
+ PORT h1,h2,h3,h4,p1,p2
+
+ where h1 is the high order 8 bits of the internet host
+ address.
+
+ PASSIVE (PASV)
+
+ This command requests the server-DTP to "listen" on a data
+ port (which is not its default data port) and to wait for a
+ connection rather than initiate one upon receipt of a
+ transfer command. The response to this command includes the
+ host and port address this server is listening on.
+
+ REPRESENTATION TYPE (TYPE)
+
+ The argument specifies the representation type as described
+ in the Section on Data Representation and Storage. Several
+ types take a second parameter. The first parameter is
+ denoted by a single Telnet character, as is the second
+ Format parameter for ASCII and EBCDIC; the second parameter
+ for local byte is a decimal integer to indicate Bytesize.
+ The parameters are separated by a <SP> (Space, ASCII code
+ 32).
+
+ The following codes are assigned for type:
+
+ \ /
+ A - ASCII | | N - Non-print
+ |-><-| T - Telnet format effectors
+ E - EBCDIC| | C - Carriage Control (ASA)
+ / \
+ I - Image
+
+ L <byte size> - Local byte Byte size
+
+
+Postel & Reynolds [Page 28]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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
+
+
+Postel & Reynolds [Page 29]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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
+
+
+Postel & Reynolds [Page 30]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ the command. This second argument is optional, but when
+ present should be separated from the first by the three
+ Telnet characters <SP> R <SP>. This command shall be
+ followed by a STORe or APPEnd command. The ALLO command
+ should be treated as a NOOP (no operation) by those servers
+ which do not require that the maximum size of the file be
+ declared beforehand, and those servers interested in only
+ the maximum record or page size should accept a dummy value
+ in the first argument and ignore it.
+
+ RESTART (REST)
+
+ The argument field represents the server marker at which
+ file transfer is to be restarted. This command does not
+ cause file transfer but skips over the file to the specified
+ data checkpoint. This command shall be immediately followed
+ by the appropriate FTP service command which shall cause
+ file transfer to resume.
+
+ RENAME FROM (RNFR)
+
+ This command specifies the old pathname of the file which is
+ to be renamed. This command must be immediately followed by
+ a "rename to" command specifying the new file pathname.
+
+ RENAME TO (RNTO)
+
+ This command specifies the new pathname of the file
+ specified in the immediately preceding "rename from"
+ command. Together the two commands cause a file to be
+ renamed.
+
+ ABORT (ABOR)
+
+ This command tells the server to abort the previous FTP
+ service command and any associated transfer of data. The
+ abort command may require "special action", as discussed in
+ the Section on FTP Commands, to force recognition by the
+ server. No action is to be taken if the previous command
+ has been completed (including data transfer). The control
+ connection is not to be closed by the server, but the data
+ connection must be closed.
+
+ There are two cases for the server upon receipt of this
+ command: (1) the FTP service command was already completed,
+ or (2) the FTP service command is still in progress.
+
+
+
+Postel & Reynolds [Page 31]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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
+
+
+Postel & Reynolds [Page 32]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ ensure that the TYPE is appropriately ASCII or EBCDIC).
+ Since the information on a file may vary widely from system
+ to system, this information may be hard to use automatically
+ in a program, but may be quite useful to a human user.
+
+ NAME LIST (NLST)
+
+ This command causes a directory listing to be sent from
+ server to user site. The pathname should specify a
+ directory or other system-specific file group descriptor; a
+ null argument implies the current directory. The server
+ will return a stream of names of files and no other
+ information. The data will be transferred in ASCII or
+ EBCDIC type over the data connection as valid pathname
+ strings separated by <CRLF> or <NL>. (Again the user must
+ ensure that the TYPE is correct.) This command is intended
+ to return information that can be used by a program to
+ further process the files automatically. For example, in
+ the implementation of a "multiple get" function.
+
+ SITE PARAMETERS (SITE)
+
+ This command is used by the server to provide services
+ specific to his system that are essential to file transfer
+ but not sufficiently universal to be included as commands in
+ the protocol. The nature of these services and the
+ specification of their syntax can be stated in a reply to
+ the HELP SITE command.
+
+ SYSTEM (SYST)
+
+ This command is used to find out the type of operating
+ system at the server. The reply shall have as its first
+ word one of the system names listed in the current version
+ of the Assigned Numbers document [4].
+
+ STATUS (STAT)
+
+ This command shall cause a status response to be sent over
+ the control connection in the form of a reply. The command
+ may be sent during a file transfer (along with the Telnet IP
+ and Synch signals--see the Section on FTP Commands) in which
+ case the server will respond with the status of the
+ operation in progress, or it may be sent between file
+ transfers. In the latter case, the command may have an
+ argument field. If the argument is a pathname, the command
+ is analogous to the "list" command except that data shall be
+
+
+Postel & Reynolds [Page 33]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ transferred over the control connection. If a partial
+ pathname is given, the server may respond with a list of
+ file names or attributes associated with that specification.
+ If no argument is given, the server should return general
+ status information about the server FTP process. This
+ should include current values of all transfer parameters and
+ the status of connections.
+
+ HELP (HELP)
+
+ This command shall cause the server to send helpful
+ information regarding its implementation status over the
+ control connection to the user. The command may take an
+ argument (e.g., any command name) and return more specific
+ information as a response. The reply is type 211 or 214.
+ It is suggested that HELP be allowed before entering a USER
+ command. The server may use this reply to specify
+ site-dependent parameters, e.g., in response to HELP SITE.
+
+ NOOP (NOOP)
+
+ This command does not affect any parameters or previously
+ entered commands. It specifies no action other than that the
+ server send an OK reply.
+
+ The File Transfer Protocol follows the specifications of the Telnet
+ protocol for all communications over the control connection. Since
+ the language used for Telnet communication may be a negotiated
+ option, all references in the next two sections will be to the
+ "Telnet language" and the corresponding "Telnet end-of-line code".
+ Currently, one may take these to mean NVT-ASCII and <CRLF>. No other
+ specifications of the Telnet protocol will be cited.
+
+ FTP commands are "Telnet strings" terminated by the "Telnet end of
+ line code". The command codes themselves are alphabetic characters
+ terminated by the character <SP> (Space) if parameters follow and
+ Telnet-EOL otherwise. The command codes and the semantics of
+ commands are described in this section; the detailed syntax of
+ commands is specified in the Section on Commands, the reply sequences
+ are discussed in the Section on Sequencing of Commands and Replies,
+ and scenarios illustrating the use of commands are provided in the
+ Section on Typical FTP Scenarios.
+
+ FTP commands may be partitioned as those specifying access-control
+ identifiers, data transfer parameters, or FTP service requests.
+ Certain commands (such as ABOR, STAT, QUIT) may be sent over the
+ control connection while a data transfer is in progress. Some
+
+
+Postel & Reynolds [Page 34]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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
+
+
+Postel & Reynolds [Page 35]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ <SP>, followed by one line of text (where some maximum line length
+ has been specified), and terminated by the Telnet end-of-line
+ code. There will be cases however, where the text is longer than
+ a single line. In these cases the complete text must be bracketed
+ so the User-process knows when it may stop reading the reply (i.e.
+ stop processing input on the control connection) and go do other
+ things. This requires a special format on the first line to
+ indicate that more than one line is coming, and another on the
+ last line to designate it as the last. At least one of these must
+ contain the appropriate reply code to indicate the state of the
+ transaction. To satisfy all factions, it was decided that both
+ the first and last line codes should be the same.
+
+ Thus the format for multi-line replies is that the first line
+ will begin with the exact required reply code, followed
+ immediately by a Hyphen, "-" (also known as Minus), followed by
+ text. The last line will begin with the same code, followed
+ immediately by Space <SP>, optionally some text, and the Telnet
+ end-of-line code.
+
+ For example:
+ 123-First line
+ Second line
+ 234 A line beginning with numbers
+ 123 The last line
+
+ The user-process then simply needs to search for the second
+ occurrence of the same reply code, followed by <SP> (Space), at
+ the beginning of a line, and ignore all intermediary lines. If
+ an intermediary line begins with a 3-digit number, the Server
+ must pad the front to avoid confusion.
+
+ This scheme allows standard system routines to be used for
+ reply information (such as for the STAT reply), with
+ "artificial" first and last lines tacked on. In rare cases
+ where these routines are able to generate three digits and a
+ Space at the beginning of any line, the beginning of each
+ text line should be offset by some neutral text, like Space.
+
+ This scheme assumes that multi-line replies may not be nested.
+
+ The three digits of the reply each have a special significance.
+ This is intended to allow a range of very simple to very
+ sophisticated responses by the user-process. The first digit
+ denotes whether the response is good, bad or incomplete.
+ (Referring to the state diagram), an unsophisticated user-process
+ will be able to determine its next action (proceed as planned,
+
+
+Postel & Reynolds [Page 36]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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
+
+
+Postel & Reynolds [Page 37]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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
+
+
+Postel & Reynolds [Page 38]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel & Reynolds [Page 39]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+Postel & Reynolds [Page 40]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel & Reynolds [Page 41]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+Postel & Reynolds [Page 42]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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]
+
+
+
+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:
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel & Reynolds [Page 44]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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]
+
+
+
+RFC 959 October 1985
+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 <CRLF> (Carriage Return, Line
+ Feed) for NVT-ASCII representation; for other negotiated languages
+ a different end of line character might be used. It should be
+ noted that the server is to take no action until the end of line
+ code is received.
+
+ The syntax is specified below in NVT-ASCII. All characters in the
+ argument field are ASCII characters including any ASCII
+ represented decimal integers. Square brackets denote an optional
+ argument field. If the option is not taken, the appropriate
+ default is implied.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel & Reynolds [Page 46]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 5.3.1. FTP COMMANDS
+
+ The following are the FTP commands:
+
+ USER <SP> <username> <CRLF>
+ PASS <SP> <password> <CRLF>
+ ACCT <SP> <account-information> <CRLF>
+ CWD <SP> <pathname> <CRLF>
+ CDUP <CRLF>
+ SMNT <SP> <pathname> <CRLF>
+ QUIT <CRLF>
+ REIN <CRLF>
+ PORT <SP> <host-port> <CRLF>
+ PASV <CRLF>
+ TYPE <SP> <type-code> <CRLF>
+ STRU <SP> <structure-code> <CRLF>
+ MODE <SP> <mode-code> <CRLF>
+ RETR <SP> <pathname> <CRLF>
+ STOR <SP> <pathname> <CRLF>
+ STOU <CRLF>
+ APPE <SP> <pathname> <CRLF>
+ ALLO <SP> <decimal-integer>
+ [<SP> R <SP> <decimal-integer>] <CRLF>
+ REST <SP> <marker> <CRLF>
+ RNFR <SP> <pathname> <CRLF>
+ RNTO <SP> <pathname> <CRLF>
+ ABOR <CRLF>
+ DELE <SP> <pathname> <CRLF>
+ RMD <SP> <pathname> <CRLF>
+ MKD <SP> <pathname> <CRLF>
+ PWD <CRLF>
+ LIST [<SP> <pathname>] <CRLF>
+ NLST [<SP> <pathname>] <CRLF>
+ SITE <SP> <string> <CRLF>
+ SYST <CRLF>
+ STAT [<SP> <pathname>] <CRLF>
+ HELP [<SP> <string>] <CRLF>
+ NOOP <CRLF>
+
+
+
+
+
+
+
+
+
+
+
+Postel & Reynolds [Page 47]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 5.3.2. FTP COMMAND ARGUMENTS
+
+ The syntax of the above argument fields (using BNF notation
+ where applicable) is:
+
+ <username> ::= <string>
+ <password> ::= <string>
+ <account-information> ::= <string>
+ <string> ::= <char> | <char><string>
+ <char> ::= any of the 128 ASCII characters except <CR> and
+ <LF>
+ <marker> ::= <pr-string>
+ <pr-string> ::= <pr-char> | <pr-char><pr-string>
+ <pr-char> ::= printable characters, any
+ ASCII code 33 through 126
+ <byte-size> ::= <number>
+ <host-port> ::= <host-number>,<port-number>
+ <host-number> ::= <number>,<number>,<number>,<number>
+ <port-number> ::= <number>,<number>
+ <number> ::= any decimal integer 1 through 255
+ <form-code> ::= N | T | C
+ <type-code> ::= A [<sp> <form-code>]
+ | E [<sp> <form-code>]
+ | I
+ | L <sp> <byte-size>
+ <structure-code> ::= F | R | P
+ <mode-code> ::= S | B | C
+ <pathname> ::= <string>
+ <decimal-integer> ::= any decimal integer
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel & Reynolds [Page 48]
+
+
+
+RFC 959 October 1985
+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]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ Transfer parameters
+ PORT
+ 200
+ 500, 501, 421, 530
+ PASV
+ 227
+ 500, 501, 502, 421, 530
+ MODE
+ 200
+ 500, 501, 504, 421, 530
+ TYPE
+ 200
+ 500, 501, 504, 421, 530
+ STRU
+ 200
+ 500, 501, 504, 421, 530
+ File action commands
+ ALLO
+ 200
+ 202
+ 500, 501, 504, 421, 530
+ REST
+ 500, 501, 502, 421, 530
+ 350
+ STOR
+ 125, 150
+ (110)
+ 226, 250
+ 425, 426, 451, 551, 552
+ 532, 450, 452, 553
+ 500, 501, 421, 530
+ STOU
+ 125, 150
+ (110)
+ 226, 250
+ 425, 426, 451, 551, 552
+ 532, 450, 452, 553
+ 500, 501, 421, 530
+ RETR
+ 125, 150
+ (110)
+ 226, 250
+ 425, 426, 451
+ 450, 550
+ 500, 501, 421, 530
+
+
+
+
+Postel & Reynolds [Page 51]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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]
+
+
+
+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]
+
+
+
+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]
+
+
+
+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]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ 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]
+
+
+
+RFC 959 October 1985
+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]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ Finally, we present a generalized diagram that could be used to model
+ the command and reply interchange:
+
+
+ ------------------------------------
+ | |
+ Begin | |
+ | V |
+ | +---+ cmd +---+ 2 +---+ |
+ -->| |------->| |---------->| | |
+ | | | W | | S |-----|
+ -->| | -->| |----- | | |
+ | +---+ | +---+ 4,5 | +---+ |
+ | | | | | | |
+ | | | 1| |3 | +---+ |
+ | | | | | | | | |
+ | | ---- | ---->| F |-----
+ | | | | |
+ | | | +---+
+ -------------------
+ |
+ |
+ V
+ End
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel & Reynolds [Page 58]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+7. TYPICAL FTP SCENARIO
+
+ User at host U wanting to transfer files to/from host S:
+
+ In general, the user will communicate to the server via a mediating
+ user-FTP process. The following may be a typical scenario. The
+ user-FTP prompts are shown in parentheses, '---->' represents
+ commands from host U to host S, and '<----' represents replies from
+ host S to host U.
+
+ LOCAL COMMANDS BY USER ACTION INVOLVED
+
+ ftp (host) multics<CR> Connect to host S, port L,
+ establishing control connections.
+ <---- 220 Service ready <CRLF>.
+ username Doe <CR> USER Doe<CRLF>---->
+ <---- 331 User name ok,
+ need password<CRLF>.
+ password mumble <CR> PASS mumble<CRLF>---->
+ <---- 230 User logged in<CRLF>.
+ retrieve (local type) ASCII<CR>
+ (local pathname) test 1 <CR> User-FTP opens local file in ASCII.
+ (for. pathname) test.pl1<CR> RETR test.pl1<CRLF> ---->
+ <---- 150 File status okay;
+ about to open data
+ connection<CRLF>.
+ Server makes data connection
+ to port U.
+
+ <---- 226 Closing data connection,
+ file transfer successful<CRLF>.
+ type Image<CR> TYPE I<CRLF> ---->
+ <---- 200 Command OK<CRLF>
+ store (local type) image<CR>
+ (local pathname) file dump<CR> User-FTP opens local file in Image.
+ (for.pathname) >udd>cn>fd<CR> STOR >udd>cn>fd<CRLF> ---->
+ <---- 550 Access denied<CRLF>
+ terminate QUIT <CRLF> ---->
+ Server closes all
+ connections.
+
+8. CONNECTION ESTABLISHMENT
+
+ The FTP control connection is established via TCP between the user
+ process port U and the server process port L. This protocol is
+ assigned the service port 21 (25 octal), that is L=21.
+
+
+
+Postel & Reynolds [Page 59]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+APPENDIX I - PAGE STRUCTURE
+
+ The need for FTP to support page structure derives principally from
+ the need to support efficient transmission of files between TOPS-20
+ systems, particularly the files used by NLS.
+
+ The file system of TOPS-20 is based on the concept of pages. The
+ operating system is most efficient at manipulating files as pages.
+ The operating system provides an interface to the file system so that
+ many applications view files as sequential streams of characters.
+ However, a few applications use the underlying page structures
+ directly, and some of these create holey files.
+
+ A TOPS-20 disk file consists of four things: a pathname, a page
+ table, a (possibly empty) set of pages, and a set of attributes.
+
+ The pathname is specified in the RETR or STOR command. It includes
+ the directory name, file name, file name extension, and generation
+ number.
+
+ The page table contains up to 2**18 entries. Each entry may be
+ EMPTY, or may point to a page. If it is not empty, there are also
+ some page-specific access bits; not all pages of a file need have the
+ same access protection.
+
+ A page is a contiguous set of 512 words of 36 bits each.
+
+ The attributes of the file, in the File Descriptor Block (FDB),
+ contain such things as creation time, write time, read time, writer's
+ byte-size, end-of-file pointer, count of reads and writes, backup
+ system tape numbers, etc.
+
+ Note that there is NO requirement that entries in the page table be
+ contiguous. There may be empty page table slots between occupied
+ ones. Also, the end of file pointer is simply a number. There is no
+ requirement that it in fact point at the "last" datum in the file.
+ Ordinary sequential I/O calls in TOPS-20 will cause the end of file
+ pointer to be left after the last datum written, but other operations
+ may cause it not to be so, if a particular programming system so
+ requires.
+
+ In fact, in both of these special cases, "holey" files and
+ end-of-file pointers NOT at the end of the file, occur with NLS data
+ files.
+
+
+
+
+
+Postel & Reynolds [Page 60]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ The TOPS-20 paged files can be sent with the FTP transfer parameters:
+ TYPE L 36, STRU P, and MODE S (in fact, any mode could be used).
+
+ Each page of information has a header. Each header field, which is a
+ logical byte, is a TOPS-20 word, since the TYPE is L 36.
+
+ The header fields are:
+
+ Word 0: Header Length.
+
+ The header length is 5.
+
+ Word 1: Page Index.
+
+ If the data is a disk file page, this is the number of that
+ page in the file's page map. Empty pages (holes) in the file
+ are simply not sent. Note that a hole is NOT the same as a
+ page of zeros.
+
+ Word 2: Data Length.
+
+ The number of data words in this page, following the header.
+ Thus, the total length of the transmission unit is the Header
+ Length plus the Data Length.
+
+ Word 3: Page Type.
+
+ A code for what type of chunk this is. A data page is type 3,
+ the FDB page is type 2.
+
+ Word 4: Page Access Control.
+
+ The access bits associated with the page in the file's page
+ map. (This full word quantity is put into AC2 of an SPACS by
+ the program reading from net to disk.)
+
+ After the header are Data Length data words. Data Length is
+ currently either 512 for a data page or 31 for an FDB. Trailing
+ zeros in a disk file page may be discarded, making Data Length less
+ than 512 in that case.
+
+
+
+
+
+
+
+
+
+Postel & Reynolds [Page 61]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+APPENDIX II - DIRECTORY COMMANDS
+
+ Since UNIX has a tree-like directory structure in which directories
+ are as easy to manipulate as ordinary files, it is useful to expand
+ the FTP servers on these machines to include commands which deal with
+ the creation of directories. Since there are other hosts on the
+ ARPA-Internet which have tree-like directories (including TOPS-20 and
+ Multics), these commands are as general as possible.
+
+ Four directory commands have been added to FTP:
+
+ MKD pathname
+
+ Make a directory with the name "pathname".
+
+ RMD pathname
+
+ Remove the directory with the name "pathname".
+
+ PWD
+
+ Print the current working directory name.
+
+ CDUP
+
+ Change to the parent of the current working directory.
+
+ The "pathname" argument should be created (removed) as a
+ subdirectory of the current working directory, unless the "pathname"
+ string contains sufficient information to specify otherwise to the
+ server, e.g., "pathname" is an absolute pathname (in UNIX and
+ Multics), or pathname is something like "<abso.lute.path>" to
+ TOPS-20.
+
+ REPLY CODES
+
+ The CDUP command is a special case of CWD, and is included to
+ simplify the implementation of programs for transferring directory
+ trees between operating systems having different syntaxes for
+ naming the parent directory. The reply codes for CDUP be
+ identical to the reply codes of CWD.
+
+ The reply codes for RMD be identical to the reply codes for its
+ file analogue, DELE.
+
+ The reply codes for MKD, however, are a bit more complicated. A
+ freshly created directory will probably be the object of a future
+
+
+Postel & Reynolds [Page 62]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ CWD command. Unfortunately, the argument to MKD may not always be
+ a suitable argument for CWD. This is the case, for example, when
+ a TOPS-20 subdirectory is created by giving just the subdirectory
+ name. That is, with a TOPS-20 server FTP, the command sequence
+
+ MKD MYDIR
+ CWD MYDIR
+
+ will fail. The new directory may only be referred to by its
+ "absolute" name; e.g., if the MKD command above were issued while
+ connected to the directory <DFRANKLIN>, the new subdirectory
+ could only be referred to by the name <DFRANKLIN.MYDIR>.
+
+ Even on UNIX and Multics, however, the argument given to MKD may
+ not be suitable. If it is a "relative" pathname (i.e., a pathname
+ which is interpreted relative to the current directory), the user
+ would need to be in the same current directory in order to reach
+ the subdirectory. Depending on the application, this may be
+ inconvenient. It is not very robust in any case.
+
+ To solve these problems, upon successful completion of an MKD
+ command, the server should return a line of the form:
+
+ 257<space>"<directory-name>"<space><commentary>
+
+ That is, the server will tell the user what string to use when
+ referring to the created directory. The directory name can
+ contain any character; embedded double-quotes should be escaped by
+ double-quotes (the "quote-doubling" convention).
+
+ For example, a user connects to the directory /usr/dm, and creates
+ a subdirectory, named pathname:
+
+ CWD /usr/dm
+ 200 directory changed to /usr/dm
+ MKD pathname
+ 257 "/usr/dm/pathname" directory created
+
+ An example with an embedded double quote:
+
+ MKD foo"bar
+ 257 "/usr/dm/foo""bar" directory created
+ CWD /usr/dm/foo"bar
+ 200 directory changed to /usr/dm/foo"bar
+
+
+
+
+
+Postel & Reynolds [Page 63]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ The prior existence of a subdirectory with the same name is an
+ error, and the server must return an "access denied" error reply
+ in that case.
+
+ CWD /usr/dm
+ 200 directory changed to /usr/dm
+ MKD pathname
+ 521-"/usr/dm/pathname" directory already exists;
+ 521 taking no action.
+
+ The failure replies for MKD are analogous to its file creating
+ cousin, STOR. Also, an "access denied" return is given if a file
+ name with the same name as the subdirectory will conflict with the
+ creation of the subdirectory (this is a problem on UNIX, but
+ shouldn't be one on TOPS-20).
+
+ Essentially because the PWD command returns the same type of
+ information as the successful MKD command, the successful PWD
+ command uses the 257 reply code as well.
+
+ SUBTLETIES
+
+ Because these commands will be most useful in transferring
+ subtrees from one machine to another, carefully observe that the
+ argument to MKD is to be interpreted as a sub-directory of the
+ current working directory, unless it contains enough information
+ for the destination host to tell otherwise. A hypothetical
+ example of its use in the TOPS-20 world:
+
+ CWD <some.where>
+ 200 Working directory changed
+ MKD overrainbow
+ 257 "<some.where.overrainbow>" directory created
+ CWD overrainbow
+ 431 No such directory
+ CWD <some.where.overrainbow>
+ 200 Working directory changed
+
+ CWD <some.where>
+ 200 Working directory changed to <some.where>
+ MKD <unambiguous>
+ 257 "<unambiguous>" directory created
+ CWD <unambiguous>
+
+ Note that the first example results in a subdirectory of the
+ connected directory. In contrast, the argument in the second
+ example contains enough information for TOPS-20 to tell that the
+
+
+Postel & Reynolds [Page 64]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ <unambiguous> directory is a top-level directory. Note also that
+ in the first example the user "violated" the protocol by
+ attempting to access the freshly created directory with a name
+ other than the one returned by TOPS-20. Problems could have
+ resulted in this case had there been an <overrainbow> directory;
+ this is an ambiguity inherent in some TOPS-20 implementations.
+ Similar considerations apply to the RMD command. The point is
+ this: except where to do so would violate a host's conventions for
+ denoting relative versus absolute pathnames, the host should treat
+ the operands of the MKD and RMD commands as subdirectories. The
+ 257 reply to the MKD command must always contain the absolute
+ pathname of the created directory.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel & Reynolds [Page 65]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+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.
+
+
+Postel & Reynolds [Page 66]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ Bressler, Bob, and Bob Thomas, "Mail Retrieval via FTP", RFC 458
+ (NIC 14378), BBN-NET and BBN-TENEX, 20 February 1973.
+
+ Neigus, Nancy, "File Transfer Protocol", RFC 542 (NIC 17759), BBN,
+ 12 July 1973.
+
+ Krilanovich, Mark, and George Gregg, "Comments on the File Transfer
+ Protocol", RFC 607 (NIC 21255), UCSB, 7 January 1974.
+
+ Pogran, Ken, and Nancy Neigus, "Response to RFC 607 - Comments on the
+ File Transfer Protocol", RFC 614 (NIC 21530), BBN, 28 January 1974.
+
+ Krilanovich, Mark, George Gregg, Wayne Hathaway, and Jim White,
+ "Comments on the File Transfer Protocol", RFC 624 (NIC 22054), UCSB,
+ Ames Research Center, SRI-ARC, 28 February 1974.
+
+ Bhushan, Abhay, "FTP Comments and Response to RFC 430", RFC 463
+ (NIC 14573), MIT-DMCG, 21 February 1973.
+
+ Braden, Bob, "FTP Data Compression", RFC 468 (NIC 14742), UCLA/CCN,
+ 8 March 1973.
+
+ Bhushan, Abhay, "FTP and Network Mail System", RFC 475 (NIC 14919),
+ MIT-DMCG, 6 March 1973.
+
+ Bressler, Bob, and Bob Thomas "FTP Server-Server Interaction - II",
+ RFC 478 (NIC 14947), BBN-NET and BBN-TENEX, 26 March 1973.
+
+ White, Jim, "Use of FTP by the NIC Journal", RFC 479 (NIC 14948),
+ SRI-ARC, 8 March 1973.
+
+ White, Jim, "Host-Dependent FTP Parameters", RFC 480 (NIC 14949),
+ SRI-ARC, 8 March 1973.
+
+ Padlipsky, Mike, "An FTP Command-Naming Problem", RFC 506
+ (NIC 16157), MIT-Multics, 26 June 1973.
+
+ Day, John, "Memo to FTP Group (Proposal for File Access Protocol)",
+ RFC 520 (NIC 16819), Illinois, 25 June 1973.
+
+ Merryman, Robert, "The UCSD-CC Server-FTP Facility", RFC 532
+ (NIC 17451), UCSD-CC, 22 June 1973.
+
+ Braden, Bob, "TENEX FTP Problem", RFC 571 (NIC 18974), UCLA/CCN,
+ 15 November 1973.
+
+
+
+
+Postel & Reynolds [Page 67]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+ McKenzie, Alex, and Jon Postel, "Telnet and FTP Implementation -
+ Schedule Change", RFC 593 (NIC 20615), BBN and MITRE,
+ 29 November 1973.
+
+ Sussman, Julie, "FTP Error Code Usage for More Reliable Mail
+ Service", RFC 630 (NIC 30237), BBN, 10 April 1974.
+
+ Postel, Jon, "Revised FTP Reply Codes", RFC 640 (NIC 30843),
+ UCLA/NMC, 5 June 1974.
+
+ Harvey, Brian, "Leaving Well Enough Alone", RFC 686 (NIC 32481),
+ SU-AI, 10 May 1975.
+
+ Harvey, Brian, "One More Try on the FTP", RFC 691 (NIC 32700), SU-AI,
+ 28 May 1975.
+
+ Lieb, J., "CWD Command of FTP", RFC 697 (NIC 32963), 14 July 1975.
+
+ Harrenstien, Ken, "FTP Extension: XSEN", RFC 737 (NIC 42217), SRI-KL,
+ 31 October 1977.
+
+ Harrenstien, Ken, "FTP Extension: XRSQ/XRCP", RFC 743 (NIC 42758),
+ SRI-KL, 30 December 1977.
+
+ Lebling, P. David, "Survey of FTP Mail and MLFL", RFC 751, MIT,
+ 10 December 1978.
+
+ Postel, Jon, "File Transfer Protocol Specification", RFC 765, ISI,
+ June 1980.
+
+ Mankins, David, Dan Franklin, and Buzz Owen, "Directory Oriented FTP
+ Commands", RFC 776, BBN, December 1980.
+
+ Padlipsky, Michael, "FTP Unique-Named Store Command", RFC 949, MITRE,
+ July 1985.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Postel & Reynolds [Page 68]
+
+
+
+RFC 959 October 1985
+File Transfer Protocol
+
+
+REFERENCES
+
+ [1] Feinler, Elizabeth, "Internet Protocol Transition Workbook",
+ Network Information Center, SRI International, March 1982.
+
+ [2] Postel, Jon, "Transmission Control Protocol - DARPA Internet
+ Program Protocol Specification", RFC 793, DARPA, September 1981.
+
+ [3] Postel, Jon, and Joyce Reynolds, "Telnet Protocol
+ Specification", RFC 854, ISI, May 1983.
+
+ [4] Reynolds, Joyce, and Jon Postel, "Assigned Numbers", RFC 943,
+ ISI, April 1985.
+
+
+
+
+
+
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+
+
+
+
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+
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+Postel & Reynolds [Page 69]
+
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