Network Working Group J. Postel
Request for Comments: 959 J. Reynolds
 ISI
Obsoletes RFC: 765 (IEN 149) October 1985
 FILE TRANSFER PROTOCOL (FTP)
Status of this Memo
 This memo is the official specification of the File Transfer
 Protocol (FTP). Distribution of this memo is unlimited.
 The following new optional commands are included in this edition of
 the specification:
 CDUP (Change to Parent Directory), SMNT (Structure Mount), STOU
 (Store Unique), RMD (Remove Directory), MKD (Make Directory), PWD
 (Print Directory), and SYST (System).
 Note that this specification is compatible with the previous edition.
1. INTRODUCTION
 The objectives of FTP are 1) to promote sharing of files (computer
 programs and/or data), 2) to encourage indirect or implicit (via
 programs) use of remote computers, 3) to shield a user from
 variations in file storage systems among hosts, and 4) to transfer
 data reliably and efficiently. FTP, though usable directly by a user
 at a terminal, is designed mainly for use by programs.
 The attempt in this specification is to satisfy the diverse needs of
 users of maxi-hosts, mini-hosts, personal workstations, and TACs,
 with a simple, and easily implemented protocol design.
 This paper assumes knowledge of the Transmission Control Protocol
 (TCP) [2] and the Telnet Protocol [3]. These documents are contained
 in the ARPA-Internet protocol handbook [1].
2. OVERVIEW
 In this section, the history, the terminology, and the FTP model are
 discussed. The terms defined in this section are only those that
 have special significance in FTP. Some of the terminology is very
 specific to the FTP model; some readers may wish to turn to the
 section on the FTP model while reviewing the terminology.
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File Transfer Protocol
 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.
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.
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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.
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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.
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File Transfer Protocol
 record
 A sequential file may be structured as a number of contiguous
 parts called records. Record structures are supported by FTP
 but a file need not have record structure.
 reply
 A reply is an acknowledgment (positive or negative) sent from
 server to user via the control connection in response to FTP
 commands. The general form of a reply is a completion code
 (including error codes) followed by a text string. The codes
 are for use by programs and the text is usually intended for
 human users.
 server-DTP
 The data transfer process, in its normal "active" state,
 establishes the data connection with the "listening" data port.
 It sets up parameters for transfer and storage, and transfers
 data on command from its PI. The DTP can be placed in a
 "passive" state to listen for, rather than initiate a
 connection on the data port.
 server-FTP process
 A process or set of processes which perform the function of
 file transfer in cooperation with a user-FTP process and,
 possibly, another server. The functions consist of a protocol
 interpreter (PI) and a data transfer process (DTP).
 server-PI
 The server protocol interpreter "listens" on Port L for a
 connection from a user-PI and establishes a control
 communication connection. It receives standard FTP commands
 from the user-PI, sends replies, and governs the server-DTP.
 type
 The data representation type used for data transfer and
 storage. Type implies certain transformations between the time
 of data storage and data transfer. The representation types
 defined in FTP are described in the Section on Establishing
 Data Connections.
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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.
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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
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File Transfer Protocol
 the same host that initiates the FTP commands via the control
 connection, but the user or the user-FTP process must ensure a
 "listen" on the specified data port. It ought to also be noted
 that the data connection may be used for simultaneous sending and
 receiving.
 In another situation a user might wish to transfer files between
 two hosts, neither of which is a local host. The user sets up
 control connections to the two servers and then arranges for a
 data connection between them. In this manner, control information
 is passed to the user-PI but data is transferred between the
 server data transfer processes. Following is a model of this
 server-server interaction.
 Control ------------ Control
 ---------->| User-FTP |<-----------
 | | User-PI | |
 | | "C" | |
 V ------------ V
 -------------- --------------
 | Server-FTP | Data Connection | Server-FTP |
 | "A" |<---------------------->| "B" |
 -------------- Port (A) Port (B) --------------
 Figure 2
 The protocol requires that the control connections be open while
 data transfer is in progress. It is the responsibility of the
 user to request the closing of the control connections when
 finished using the FTP service, while it is the server who takes
 the action. The server may abort data transfer if the control
 connections are closed without command.
 The Relationship between FTP and Telnet:
 The FTP uses the Telnet protocol on the control connection.
 This can be achieved in two ways: first, the user-PI or the
 server-PI may implement the rules of the Telnet Protocol
 directly in their own procedures; or, second, the user-PI or
 the server-PI may make use of the existing Telnet module in the
 system.
 Ease of implementaion, sharing code, and modular programming
 argue for the second approach. Efficiency and independence
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File Transfer Protocol
 argue for the first approach. In practice, FTP relies on very
 little of the Telnet Protocol, so the first approach does not
 necessarily involve a large amount of code.
3. DATA TRANSFER FUNCTIONS
 Files are transferred only via the data connection. The control
 connection is used for the transfer of commands, which describe the
 functions to be performed, and the replies to these commands (see the
 Section on FTP Replies). Several commands are concerned with the
 transfer of data between hosts. These data transfer commands include
 the MODE command which specify how the bits of the data are to be
 transmitted, and the STRUcture and TYPE commands, which are used to
 define the way in which the data are to be represented. The
 transmission and representation are basically independent but the
 "Stream" transmission mode is dependent on the file structure
 attribute and if "Compressed" transmission mode is used, the nature
 of the filler byte depends on the representation type.
 3.1. DATA REPRESENTATION AND STORAGE
 Data is transferred from a storage device in the sending host to a
 storage device in the receiving host. Often it is necessary to
 perform certain transformations on the data because data storage
 representations in the two systems are different. For example,
 NVT-ASCII has different data storage representations in different
 systems. DEC TOPS-20s's generally store NVT-ASCII as five 7-bit
 ASCII characters, left-justified in a 36-bit word. IBM Mainframe's
 store NVT-ASCII as 8-bit EBCDIC codes. Multics stores NVT-ASCII
 as four 9-bit characters in a 36-bit word. It is desirable to
 convert characters into the standard NVT-ASCII representation when
 transmitting text between dissimilar systems. The sending and
 receiving sites would have to perform the necessary
 transformations between the standard representation and their
 internal representations.
 A different problem in representation arises when transmitting
 binary data (not character codes) between host systems with
 different word lengths. It is not always clear how the sender
 should send data, and the receiver store it. For example, when
 transmitting 32-bit bytes from a 32-bit word-length system to a
 36-bit word-length system, it may be desirable (for reasons of
 efficiency and usefulness) to store the 32-bit bytes
 right-justified in a 36-bit word in the latter system. In any
 case, the user should have the option of specifying data
 representation and transformation functions. It should be noted
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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.
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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.
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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.
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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.
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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,
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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.
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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
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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
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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
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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.
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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.
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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 |
 +--------+--------+--------+
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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
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
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.
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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
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:
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
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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
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
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.
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
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
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
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
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,
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
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
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.
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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.
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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.
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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.
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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.
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File Transfer Protocol
 5.2. CONNECTIONS
 The server protocol interpreter shall "listen" on Port L. The
 user or user protocol interpreter shall initiate the full-duplex
 control connection. Server- and user- processes should follow the
 conventions of the Telnet protocol as specified in the
 ARPA-Internet Protocol Handbook [1]. Servers are under no
 obligation to provide for editing of command lines and may require
 that it be done in the user host. The control connection shall be
 closed by the server at the user's request after all transfers and
 replies are completed.
 The user-DTP must "listen" on the specified data port; this may be
 the default user port (U) or a port specified in the PORT command.
 The server shall initiate the data connection from his own default
 data port (L-1) using the specified user data port. The direction
 of the transfer and the port used will be determined by the FTP
 service command.
 Note that all FTP implementation must support data transfer using
 the default port, and that only the USER-PI may initiate the use
 of non-default ports.
 When data is to be transferred between two servers, A and B (refer
 to Figure 2), the user-PI, C, sets up control connections with
 both server-PI's. One of the servers, say A, is then sent a PASV
 command telling him to "listen" on his data port rather than
 initiate a connection when he receives a transfer service command.
 When the user-PI receives an acknowledgment to the PASV command,
 which includes the identity of the host and port being listened
 on, the user-PI then sends A's port, a, to B in a PORT command; a
 reply is returned. The user-PI may then send the corresponding
 service commands to A and B. Server B initiates the connection
 and the transfer proceeds. The command-reply sequence is listed
 below where the messages are vertically synchronous but
 horizontally asynchronous:
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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:
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.
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>
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File Transfer Protocol
 5.3.2. FTP COMMAND ARGUMENTS
 The syntax of the above argument fields (using BNF notation
 where applicable) is:
 <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
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
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
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
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
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
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.
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 |
 +---+ +---+ +---+
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.
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 |
 +---+ +---+------------->+---+
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
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.
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.
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.
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
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
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
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.
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.
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.
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.
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.