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rfc:rfc454

Network Working Group A. McKenzie Request for Comments: 454 BBN NIC: 14333 16 February 1973

                       FILE TRANSFER PROTOCOL
          Meeting Announcement and a New Proposed Document
 Attached is a new proposal for a File Transfer Protocol.  The
 document is an extensive update to RFC 354 and, I believe,
 incorporates solutions to most of the objections to RFC 354.
 It now seems appropriate to make another attempt to reach final
 agreement on FTP.  Accordingly, I am calling a meeting of interested
 parties, to be held at BBN on March 16, for discussion of this and
 other proposals.
 This note is directed to the network community at large, rather than
 specifically to the old FTP committee, because I don't believe that
 the FTP committee membership includes all the individuals who have
 contributed to the current state of FTP design.  Nevertheless, it is
 intended that the meeting proceed from the current state, rather than
 bringing new members up-to-speed.  Prospective attendees should
 therefore be familiar with at least the following documents:
    RFC 354
    RFC 385
    RFC 414
    RFC 418
    RFC 438
 Anyone wishing to attend this meeting should contact Alex McKenzie
 (NIC Ident aam) at BBN, 50 Moulton Street, Cambridge, Mass. 02138.
 My telephone number is:
                           (617) 491-1850 ext.441
 When there is some indication of the number of individuals planning
 to attend, a meeting room will be reserved and more specific
 information will be directed to attendees.

McKenzie [Page 1] RFC 454 File Transfer Protocol July 1972

                    PROPOSED FILE TRANSFER PROTOCOL
 This document is the outcome of a meeting held 25 January 1973 in
 Cambridge, Massachusetts, by the following people:
    Abhay Bhushan (MIT - DMCG)
    Bob Bressler (BBN - NET)
    Bob Clements (BBN - TENEX)
    Alex McKenzie (BBN - NET)
    Nancy Neigus (BBN - NET)
    Ken Pogran (MIT - MULTICS)
    Marc Seriff (MIT - DMCG)
 The basis of the document is RFC 354 with considerations drawn from
 RFC's 385, 414, 418, and 438 and personal communication with network
 participants.

McKenzie [Page 2] RFC 454 File Transfer Protocol July 1972

                    PROPOSED FILE TRANSFER PROTOCOL

INTRODUCTION

 The File Transfer Protocol (FTP) is a protocol for file transfer
 between HOSTs (including terminal IMPs), on the ARPA Computer Network
 (ARPANET).  The primary function of FTP is to transfer files
 efficiently and reliably among HOSTs and to allow the convenient use
 of remote file storage capabilities.
 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, TIPs, and the Datacomputer, with a
 simple, elegant, and easily implemented protocol design.
 This paper assumes knowledge of the following protocols:
    1) The HOST-HOST Protocol (NIC #8246)
    2) The Initial Connection Protocol (NIC #7101)
    3) The TELNET Protocol (NWG/RFC #318, NIC #9348)

II. DISCUSSION

 In this section, the terminology and the FTP model are discussed.
 The terms defined in this section are only those that have special
 significance in FTP.

II.A Terminology

 ASCII               The USASCII character set as defined in NIC
                     #7104.  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
                     provide access controls.

McKenzie [Page 3] RFC 454 File Transfer Protocol July 1972

 byte size           The byte size specified for the transfer od data.
                     The data connection is opened with this byte
                     size.  Data connection byte size is not
                     necessarily the byte size in which data is to be
                     stored in a system, and may not be related to the
                     structure of data.
 data connection     A simplex connection over which data is
                     transferred, in a specified byte size, mode and
                     type.  The data transferred may be a part of a
                     file, an entire file or a number of files.  The
                     data connection may be in either direction
                     (server-to-user or user-to-server).
 data socket         The socket on which a User-FTP process "listens"
                     for a data connection.
 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.
 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 including EOR and EOF.  The transfer modes
                     defined in FTP are described in Section III.C.
 NVT                 The Network Virtual Terminal as defined in the
                     ARPANET TELNET Protocol.

McKenzie [Page 4] RFC 454 File Transfer Protocol July 1972

 NVFS                The Network Virtual File System.  A concept which
                     defines a standard network file system with
                     standard commands and pathname conventions.  FTP
                     only partially embraces the NVFS concept at this
                     time.
 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 he wishes to use.
 record              A sequential file may be structured as a number
                     of contiguous parts called records.  Record
                     structures are supported by FTP but are not
                     mandatory.
 reply               A reply is an acknowledgement (positive or
                     negative) sent from server to user via the TELNET
                     connections in response to FTP commands.  The
                     general form of a reply is a completion code
                     (including error codes) followed by an ASCII text
                     string.  The codes are for use by programs and
                     the text is for human users.
 server-FTP process  A process or set of processes which perform the
                     function of file transfer in cooperation with a
                     user-FTP process.  The server-FTP process must
                     interpret and respond to user commands and
                     initiate the data connection.
 server site         A HOST site which has a server-FTP process.
 server-TELNET       A TELNET process which listens on a specified
                     socket for an ICP initiated by a user-TELNET, and
                     performs in accordance with the ARPANET TELNET
                     Protocol.
 TELNET connections  The full-duplex communication path between a
                     user-TELNET and a server-TELNET.  The TELNET
                     connections are established via the standard
                     ARPANET Initial Connection Protocol (ICP).

McKenzie [Page 5] RFC 454 File Transfer Protocol July 1972

 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 Section III.B.
 user                A process on behalf of a human being or a human
                     being wishing to obtain file transfer service.
 user site           A HOST site satisfying any of the following
                     conditions: 1) The site where a user is located,
                     2) a site where a user-FTP process is located, 3)
                     a site to which a data connection is made by a
                     server.  In the normal case, the sites defined by
                     1, 2, and 3 are the same site, but nothing in FTP
                     requires that this be so.
 user-FTP process    A process or set of processes which perform the
                     function of file transfer in cooperation with a
                     server-FTP process.  The user-FTP process 1)
                     initiates the ICP (via a user-TELNET), 2)
                     initiates FTP commands and 3) "listens" on the
                     data socket for the data connection.  In some
                     obvious cases (use from TIPs and other mini-
                     HOSTs) a user-FTP process will be subsumed under
                     the term "user".
 user-TELNET         A TELNET process which initiates an ICP to a
                     specified server-TELNET socket, and performs in
                     accordance with the ARPANET TELNET protocol.

II.B The FTP Model

 With the above definitions in mind, the following model (shown in
 Figure 1) may be diagramed for an FTP service.
 In the model described in Figure 1, the user-TELNET initiates the
 TELNET connections.  Standard FTP commands are then generated by the
 user and transmitted to the server site via the TELNET connections.
 FTP commands are in ASCII, in accordance with NVT conventions and the
 TELNET protocol.  Note that commands may be initiated by the user
 directly through the user-TELNET or via a user-FTP process.  Standard
 replies are sent from the server to the user in response to the
 commands over the TELNET connections.

McKenzie [Page 6] RFC 454 File Transfer Protocol July 1972

 The FTP commands specify the parameters for the data connection (data
 socket, byte size, transfer mode, representation type, and format)
 and the nature of file system operation (store, retrieve, append,
 delete, etc.). The user-FTP process or its designate should "listen"
 on the specified data socket, and it is the server's responsibility
 to initiate the data connection and data transfer in accordance with
 the specified data connection parameters.  It should be noted that
 the data socket need not be in the same HOST that initiates the FTP
 commands via the TELNET connections, but the user or his user-FTP
 process must ensure a "listen" on the specified data socket.  A
 practical example of such file transfer to third HOSTs is a maxi-HOST
 user (who may actually be a TIP user) wishing to transmit a file to
 or from an I/O device attached to a TIP.  It should also be noted
 that two data connections, one for send and the other for receive,
 may exist simultaneously.
                              TELNET
                            Connections

+—–+ +——-+ +——+ +——+ +——-+ +—–+

FileServer-Server←———User User- File
Sys FTP TELNET FTP Cmds TELNET FTP Sys-
-tem Process ———→ Process tem

+—–+ | | +——+FTP Replies+——+ | | +—–+

        |       |                                 |       |
        |       |<------------------------------->|Data   |
        |       |         Data Connection(s)      |Socket |
        +-------+                                 +-------+
                                                      |
                                                      |
                                                  +------+
                                                  |      |
                                                  | USER |
                                                  |      |
                                                  +------+
 Notes:  1.  The data connection may be in either direction.
         2. The data connection need not exist all of the time.
         3. The distinctions between user-FTP and user-TELNET, and
             between server-FTP and server-TELNET may not be as
             clear-cut as shown above.  For example, a user-TELNET may
             be directly driven by the user.
             FIGURE 1  Model for FTP Use

McKenzie [Page 7] RFC 454 File Transfer Protocol July 1972

 The protocol requires that the TELNET connections be open while data
 transfer is in progress.  It is the responsibility of the user to
 close the TELNET connections when finished using the FTP service.
 The server may abort data transfer if the TELNET connections are
 closed.

III. DATA TRANSFER FUNCTIONS

 Data and files are transferred only via the data connection.  The
 transfer of data is governed by FTP data transfer commands received
 on the TELNET connections.  The data transfer functions include
 establishing the data connection to the specified data socket in the
 specified HOST (using the specified byte size), transmitting and
 receiving data in the specified representation type and transfer
 mode, handling EOR and EOF conditions, and error recovery (where
 applicable).

III.A Establishing Data Connection

 The user site shall "listen" on the specified data socket, prior to
 sending a transfer request command.  The FTP request command
 determines the direction of data transfer, and the socket number (odd
 or even) which is to be used in establishing the data connection.
 The server on receiving the appropriate store or retrieve request
 shall initiate the data connection to the specified user data socket
 in the specified byte size (default byte size is 8 bits), and send a
 reply indicating that file transfer may proceed.  Prior to this
 reply, the server should send a reply indicating the server socket
 for the data connection.  The user may use this server socket
 information to ensure the security of his data transfer.  The server
 may send this reply either before or after initiating the data
 connection.
 The byte size for the data connection is specified by the BYTE
 command.  It is not required by the protocol that servers accept all
 possible byte sizes.  The use of various byte sizes is for efficiency
 in data transfer and servers may implement only those byte sizes for
 which their data transfer is efficient.  It is, however, required
 that servers implement at least the byte size of 8 bits.
 After the data transfer is completed, it is the server's
 responsibility to close the data connection, except when the user is
 sending the data.  In stream mode the sender must close the data
 connection to indicate EOF, i.e., completion of the transfer.
 Closing the connection is a server option except under the following
 conditions:

McKenzie [Page 8] RFC 454 File Transfer Protocol July 1972

 1)       The server receives an abort command from the user.
 2)       The socket or the byte size specification is changed by the
          user.
 3)       The TELNET connections are closed.
 4)       An irrecoverable error condition occurs.
 It should be noted that if none of the above conditions occur it is
 possible to maintain two simultaneous data connections, for send and
 receive.

III.B Data Representation and Storage

 Data is transferred from a storage device in sending HOST to a
 storage device in receiving HOST.  Often it is necessary to perform
 certain transformations on the data because data storage representa-
 tions in the two systems are different.  For example, NVT-ASCII has
 different data storage representations in different systems.  PDP-10'
 s generally store NVT-ASCII as five 7-bit ASCII characters, left-
 justified in a 36-bit word. 360's store NVT-ASCII as 8-bit EBCDIC
 codes.  Multics stores NVT-ASCII as four 9-bit characters in a 36-bit
 word.  It may be 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 sys-
 tem.  In any case, the user should have the option of specifying data
 representation and transformation functions.  It should be noted that
 FTP provides for very limited data type representations.  Transforma-
 tions desired beyond this limited capability should be performed by
 the user directly or via the use of the Data Reconfiguration (DRS,
 RFC #138, NIC #6715).  Additional representation types may be defined
 later if there is a demonstrable need.
 Data representations are handled in FTP by a user specifying a
 representation type.  The type may also imply a transfer byte size.
 For example, in ASCII representation, the transfer byte size should
 be 8 bits, and any other byte size specification will result in

McKenzie [Page 9] RFC 454 File Transfer Protocol July 1972

 cancellation of the transfer request.  In image and Local Byte
 representations any byte size is possible.  The following data
 representation types are currently defined in FTP:
 1. ASCII        The sender converts data from its internal character
                 representation to the standard NVT ASCII form.  The
                 receiver converts the data from the standard form to
                 its own internal form.  The data is transferred in
                 the standard form.  The transfer byte size must be 8
                 bits.  This type would be used for transfer of text
                 files.  This is the default type, and it is recom-
                 mended that this type be implemented by all.
 2. EBCDIC       The sender transfers data using the EBCDIC character
                 code and 8-bit transfer byte size.  This type may be
                 used for efficient transfer of EBCDIC files between
                 systems which use EBCDIC for their internal character
                 representation.
 3. Image        The sender transforms data from contiguous bits to
                 bytes for transfer.  The receiver transforms the
                 bytes into bits, storing them contiguously indepen-
                 dent of the byte size chosen for data transfer.  With
                 record structure and block mode, the server might
                 need to pad each record for convenient storage.  This
                 padding is allowed at the end of a record, and should
                 be remembered by the server so it will be stripped
                 off when the file is retrieved by the user.  The pad-
                 ding transformation should be well publicized by the
                 server in case the user processes his file at the
                 server site.  Typical uses for the Image type are
                 transfer of executable programs between like
                 machines, and transfer of binary (non-text) data.  It
                 is recommended that this type be implemented by all
                 for some byte size, preferably including the 8 bit
                 byte size.
 4. Local Byte   This representation allows for efficient storage,
                 use, and retrieval of data.  The manner in which data
                 is to be transformed depends on the byte size for
                 data transfer, and the particular HOST being used.
                 The transformation scheme for different byte size is
                 to be well publicized by all server sites.  This
                 transformation shall be invertible (i.e., if a file
                 is stored using a certain transfer byte size, an
                 identical file must be retrievable using the same
                 byte size and representation type).  It is the user's
                 responsibility to keep track of the representation

McKenzie [Page 10] RFC 454 File Transfer Protocol July 1972

                 type and byte size used for his transfer.  Typical
                 uses of the Local Byte type are in efficient storage
                 and retrieval of files, and transfer of structured
                 binary data.  This type may be identical to the Image
                 type for byte size which are integral multiples of or
                 factors of the computer word length.
 Representation type may also be affected by another attribute, the
 format.  For example, some printers can use ASA (Fortran) vertical
 format control procedures to transform printed data of type ASCII or
 EBCDIC.  Currently format may take one of two values.
 1. Unformatted  The representation type as specified is unaffected by
                 any format transformations.  This is the default
                 value.
 2. Printfile    The server is to transform data of either ASCII or
                 EBCDIC type in accordance with ASA (Fortran) vertical
                 format control standards.  The data is to be
                 transferred in 8-bit bytes.
 A discussion of the ASA vertical format control appears in NWG/RFC
 189, Appendix C, and in Communications of the ACM, Vol. 7, No. 10, p.
 606, October 1964.  According to the ASA vertical format control
 standards, the first character of a formatted record is not printed
 but determines vertical spacing as follow:
    Character                  Vertical Spacing before printing
     Blank                       One line
       0                         Two lines
       1                         To first line of the next page
       +                         No advance
 In addition to the above four, there are more characters (defined in
 Appendix C, RFC 189) which represent an IBM extension to the ASA
 standard.
 It should be noted that a serving host need not accept all represen-
 tation types and/or byte sizes, but it must inform the user request-
 ing an unacceptable type or size of this fact by sending an appropri-
 ate reply.

III.C. File Structure and Transfer Modes

 The only file structures supported directly in FTP at the present
 time are record structures.  However, the use of record structures is
 not mandatory.  A user with no record structure in his file should be

McKenzie [Page 11] RFC 454 File Transfer Protocol July 1972

 able to store and retrieve his file at any HOST.  A user wishing to
 transmit a record structured file must send the appropriate FTP
 'STRU' command (the default assumption is no record structure).  A
 serving HOST need not accept record structures, but it must inform
 the user of this fact by sending an appropriate reply.  Any record
 structure information in the data stream may subsequently be dis-
 carded by the receiver.
 All data transfers must end with an EOF.  The EOF is defined by the
 data transfer mode.  For files that have record structures, an EOR is
 also defined by the transfer mode.  Only the transfer modes and
 representation type combinations that have EOR defined may be used
 for transfer of files with record structures.  Records may be of zero
 length but they must be contained in file boundaries.  The relation-
 ship between files and records is hierarchical but an EOF does not
 imply an EOR.
 The following data transfer modes are defined in FTP:
 1. Stream       The file is transmitted as a stream of bytes of the
                 specified byte size.  The EOF is signaled by closing
                 the data connection.  Any representation type and
                 byte size may be used in the stream mode with file
                 structure, but use of record structure limits the
                 type to ASCII or EBCDIC with or without Printfile
                 format.  The convention is that the ASCII character
                 CR (Carriage Return, Code 15 (octal)) followed by LF
                 (Line Feed, Code 12 (octal)) indicates an EOR for
                 ASCII representation type, and the EBCDIC character
                 NL (New Line, Code 15 (hex)) indicates an EOR for
                 EBCDIC type.  This is the default mode, and it is
                 recommended that this mode be implemented by all.
 2. Text         The file is ASCII text transmitted as a sequence of
                 8-bit bytes in the ASCII representation type, and
                 optional Printfile format.  Record structures are
                 allowed in this mode.  The EOR and EOF are defined by
                 the presence of special "TELNET-control" codes (,ost
                 significant bit set to one) in the data stream.  The
                 EOR code is 192 (octal 300, hex CO).  The EOF code is
                 193 (octal 301, hex C1).  The byte size for transfer
                 is 8 bits.
 (For ASCII type, text and stream modes are almost identical.)

McKenzie [Page 12] RFC 454 File Transfer Protocol July 1972

 Comparing the two, the advantages of "stream" mode are:
    1) The receiver need not scan the incoming bytes.
    2) It is usable with all data types.
 and the disadvantages are:
    1) Closing the data connection under error conditions can be
       misconstrued as an EOF in stream mode when in fact the data
       transfer was interrupted.  In text mode the EOF is sent expli-
       citly.
    2) If record structure is specified in stream mode then CRLF
       implies EOR, and in order for CRLF to be sent as valid data it
       must be transformed, e.g., into CR NUL LF or LF CR.
 3. Block        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 file block (EOF), last
                 record block (EOR), restart marker (see Section
                 III.D), or suspect data (i.e., the data being
                 transferred is suspected of errors and is not reli-
                 able).  Record structures are allowed in this mode,
                 and any representation type or byte size may be used.
                 The header consists of the smallest integral number
                 of bytes whose length is greater than or equal to 24
                 bits.  Only the _least_ significant 24 bits (right-
                 justified) of header shall have information; the
                 remaining most significant bits are "don't care"
                 bits.  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.
                          Integral data bytes >= 24
                 +---------------+---------------+--------------+
                 | Don't care    |   Descriptor  |  Byte Count  |
                 | 0 to 231 bits |     8 bits    |    16 bits   |
                 +---------------+---------------+--------------+

McKenzie [Page 13] RFC 454 File Transfer Protocol July 1972

                 The following descriptor codes are assigned:
                 Code       Meaning
                 ----       -------
                  0         An ordinary block of data.
                  1         End of data block is EOR.
                  2         End of data block is EOF.
                  3         Suspected errors in data block.
                  4         Data block is a restart marker.
                 In the use of block mode it is possible for two or
                 more conditions requiring different descriptor codes
                 (suspected errors and either end of record or end of
                 file) to exist simultaneously.  Such a possibility
                 may be handled by sending a separate EOR or EOF block
                 with a zero byte count. (This is allowed by the pro-
                 tocol.)
                 The restart marker is embedded in the data stream as
                 an integral number of 8-bit bytes (representing
                 printable ASCII characters) right-justified in an
                 integral number of data bytes greater than 8 bits.
                 For example if the byte size is 7 bits, the restart
                 marker byte would be one byte right-justified per two
                 7-bit bytes as shown below:
                      Two 7-bit bytes
                 +----------+------------+
                 |          | Marker Char|
                 |          |    8 bits  |
                 +----------+------------+
                 For byte size of 16 bits or more, two or more marker
                 bytes shall be packed right-justified.  The end of
                 the marker may be delimited by the character SP (code
                 32.).  If marker characters do not exactly fit an
                 integral byte, the unused character slots should con-
                 tain the ASCII character SP (code 32.).  For example,
                 to transmit a six character marker in a 36-bit byte
                 size, the following three 36-bit bytes would be sent:

McKenzie [Page 14] RFC 454 File Transfer Protocol July 1972

                 +-------------+-------------+---------------+
                 | Don't care  | Descriptor  |               |
                 |    12 bits  |  code=4     | Byte count=2  |
                 +-------------+-------------+---------------+
                 +----+---------+---------+--------+---------+
                 |    | Marker  | Marker  | Marker | Marker  |
                 |    | 8 bits  | 8 bits  | 8 bits | 8 bits  |
                 +----+---------+---------+--------+---------+
                 +----+---------+---------+--------+---------+
                 |    | Marker  | Marker  | SP     | SP      |
                 |    | 8 bits  | 8 bits  | 8 bits | 8 bits  |
                 +----+---------+---------+--------+---------+
 4. Hasp
                 The file is transmitted as a sequence of 8-bit bytes
                 in the standard Hasp-compressed data format (document
                 to be issued by Bob Braden, UCLA).  This mode
                 achieves considerable compression of data for print
                 files.  Record structures are allowed in the Hasp
                 mode.
 The following matrix summarizes the legal combinations of file
 transfer parameters.  The decimal integers represent legal byte sizes
 for each particular STRU-MODE-TYPE-FORM grouping absence of a number
 implies illegality.  Note that HASP mode is not included since it has
 never been defined.
         STRU           F               |        R
        +-------------------------------+-----+-----+------+
 TYPE   |\ MODE                         |     |     |      |
        |  \                            |     |     |      |
        |    \     S       T       B    |  S  |   T |   B  |
        | FORM +--------+-----+---------+-----+-----+------+
     A  |   U  |   8    |  8  |    8    |  8  |   8 |   8  |
        |      +--------+-----+---------+-----+-----+------+
        |   P  |   8    |  8  |    8    |  8  |   8 |   8  |
    ----+------+--------+-----+---------+-----+-----+------+
    E   |   U  |   8    |     |    8    |  8  |     |   8  |
        |      +--------+-----+---------+-----+-----+------+
        |   P  |   8    |     |    8    |  8  |     |   8  |
    ----+------+--------+-----+---------+-----+-----+------+
    I   |   U  | 1-255  |     | 1-255   |     |     |1-255 |
    ----+------+--------+-----+---------+-----+-----+------+
    L   |   U  | 1-255  |     | 1-255   |     |     |1-255 |
    ----+------+--------+-----+---------+-----+-----+------+

McKenzie [Page 15] RFC 454 File Transfer Protocol July 1972

III.D Error Recovery and Restart

 There is no provision for detecting bits lost or scrambled in data
 transfer.  This issue is perhaps handled best at the NCP level where
 it benefits most users.  However, a restart procedure is provided to
 protect user from system failures (such as failure of either HOST,
 FTP-process, or the IMP subnet).
 The restart procedure is defined only for the block mode 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 ASCII characters.  The printable ASCII characters are
 defined to be octal codes 41 through 176 (i.e., not including codes 0
 through 37 and the characters SP and DEL).  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 pro-
 cedure.  The following examples illustrate the use of the restart
 procedure.

1. When server is the sender of data, the server-FTP process inserts

     an appropriate marker block in the data stream at a convenient
     data point.  The user-FTP process, receiving the data, marks the
     corresponding data point in its file system and conveys the last
     known sender and receiver marker information to the user.  In the
     event of system failure, the user or user-FTP process restarts
     the server at the last server marker by sending a restart command
     with the server's marker code as its argument.  The restart com-
     mand is transmitted over the TELNET connection and is immediately
     followed by the command (such as store or retrieve) which was
     being executed when the system failure occurred.

2. When user is the sender of data, the user-FTP process inserts the

     appropriate marker block in the data stream.  The server-FTP pro-
     cess, receiving the data, marks the corresponding data point in
     its file system.  The server does not store this marker but con-
     veys the last known sender and receiver marker information to the
     user over the TELNET connections by appropriate reply codes.  The
     user or the user-FTP process then restarts transfer in a manner
     identical to that described in the first example.

McKenzie [Page 16] RFC 454 File Transfer Protocol July 1972

IV. FILE TRANSFER FUNCTIONS

 The TELNET connections on which FTP commands and replies are
 transmitted are initiated by the user-FTP process via an ICP to a
 standard server socket.  FTP commands are then transmitted from user
 to server, and replies are transmitted from server to user.  The user
 file transfer functions involve sending the FTP commands, interpret-
 ing the replies received and transferring data over the data connec-
 tion in the specified manner.  The server file transfer functions
 involve accepting and interpreting FTP commands, sending replies,
 setting up the data connection, and transferring data.

McKenzie [Page 17] RFC 454 File Transfer Protocol July 1972

IV.A FTP Commands

 FTP commands are ASCII strings terminated by the ASCII character
 sequence CRLF (Carriage Return followed by Line Feed).  The command
 codes themselves are ASCII alphabetic characters terminated by the
 ASCII character 'space' (octal code 40).  For convenience, the com-
 mand codes are defined to be four (or less) ASCII alphanumeric char-
 acters (including both upper and lower case alphabetic characters).
 The command codes and the semantics of commands are described in this
 section, but the detailed syntax of commands is specified in Section
 V.B, the reply sequences are discussed in Section V.C, and scenarios
 illustrating the use of commands are provided in Section V.D.
 FTP commands may be partitioned as those specifying access-control
 identifiers, data transfer parameters, or FTP service requests.  Cer-
 tain commands (such as ABOR, STAT, BYE) may be sent over the TELNET
 connections while a data transfer is in progress.  Some servers may
 not be able to monitor the TELNET and data connections simultane-
 ously, in which case these commands should be preceded by a TELNET
 SYNC to awaken the server. (For other servers this may not be neces-
 sary and the SYNC will be ignored.)

IV.A.1 Access Control Commands

 The following commands specify access control identifiers (command
 codes are shown in parentheses).
    User name (USER) - The argument field is an ASCII string identify-
    ing 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
    TELNET 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 accounting information.  All parameters are
    unchanged and any file transfer in progress is completed under the
    old account.
    Password (PASS) - The argument field is an ASCII string identify-
    ing the user's password.  This command must be immediatly 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 type out.  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.

McKenzie [Page 18] RFC 454 File Transfer Protocol July 1972

    Account (ACCT) - The argument field is an ASCII 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 two
    reply codes to differentiate these cases for the automaton: When
    account information is required for login and the server receives
    another command which he buffers, the legal response is reply code
    331 when an account is required for a specific transfer requested,
    the reply code 433 is returned and the request command is flushed.
    Reinitialize (REIN) - This command terminates a USER, flushing all
    I/O and account information, except to allow any transfer in pro-
    gress to be completed.  All parameters are reset to the default
    setting and the TELNET connection is left open.  A USER command is
    expected to follow.
    Logout (BYE) - This command terminates a USER and if file transfer
    is not in progress, closes the TELNET connection.  If file
    transfer is in progress, the connection will remain open for
    result response and will then close.  For "hot card-reader" mode
    the REIN command should be used instead.
    An unexpected close on the TELNET connection will cause the server
    to take the effective action of an abort (ABOR) and a logout
    (BYE).

IV.A.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 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
    Byte size (BYTE) - The argument is an ASCII-represented decimal
    integer (1 through 255), specifying the byte size for the data
    connection.  The default byte size is 8 bits.  The byte size is
    always 8 bits in the ASCII and EBCDIC representation types.  A
    server may reject specific byte size/type combinations by sending
    an error reply code in response to a transfer request command.
    Data socket (SOCK) - The argument is a HOST-socket specification
    for the data socket to be used in data connection.  There may be
    two data sockets, one from server to user and the other for user

McKenzie [Page 19] RFC 454 File Transfer Protocol July 1972

    to server data transfer.  An odd socket number defines a send
    socket and an even socket number defines a receive socket.  The
    default HOST is the user HOST to which TELNET connections are
    made.  The default data sockets are (U+4) and (U+5) where U is the
    socket number used in the TELNET ICP and the TELNET connections
    are on sockets (U+2) and (U+3).
    Listen (LSTN) - The argument is a single ASCII character code to
    specify the direction of the socket that the server must allocate
    for use as a data connection.  The server is to "listen" on the
    allocated socket when an appropriate transfer command is given.
    The following codes are assigned:
          S - send
          R - receive
    Representation Type (TYPE) - The argument is a single ASCII char-
    acter code specifying the representation types described in Sec-
    tion III.B.  The following codes are assigned for type:
          A - ASCII
          I - Image
          L - Local Byte
          E - EBCDIC
    The default representation type is ASCII.
    Format (FORM) - The argument is a single ASCII character code
    specifying the formats described in Section III.B. The following
    codes are assigned for format:
          U - Unformatted
          P - Printfile
    The default format is Unformatted.
    File Structure (STRU) - The argument is a single ASCII character
    code specifying file structure described in Section III.C.  The
    following codes are assigned for structure:
          F - File (no ecord structure)
          R - Record structure
    The default structure is File (ie. no records).
    Transfer Mode (MODE) - The argument is a single ASCII character
    code specifying the data transfer modes described in Section
    III.C.  The following codes are assigned for transfer modes:

McKenzie [Page 20] RFC 454 File Transfer Protocol July 1972

          S - Stream (bytes, close is EOF)
          B - Block (header with descriptor and count)
          T - Text (TELNET control code for EOR, EOF)
          H - Hasp (specially formatted compressed data)
    The default transfer mode is Stream.

IV.A.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 com-
 mand will normally be a pathname.  The syntax of pathnames must con-
 form to server site conventions (with standard defaults applicable),
 except that ASCII characters must be used (in conformance with the
 TELNET Protocol).  The suggested default handling is to use the last
 specified device, directory or file name, or the standard default
 defined for local users.  The command may be in any order except that
 a "rename from" command, must be followed by a "rename to" command,
 and some servers may require an "allocate" command before a "store"
 command.  The data, when transferred in response to FTP service
 commands, shall always be sent over the data connection.  The follow-
 ing commands specify FTP service requests:
    Retrieve (RETR) - This command achieves the transfer of a copy of
    the file specified in the pathname, from server to user site.  The
    status and contents of the file at the server site shall be unaf-
    fected.
    Store (STOR) - This command achieves the transfer of a copy of a
    file from user to server site.  If the file specified in the path-
    name exists at the server site, then its contents shall be
    replaced by the contents of the file being transferred.  A new
    file is created at the server site if the file specified in the
    pathname does not already exist.
    Append (with create) (APPE) - This command achieves the transfer
    of data from using to serving site.  If the file specified in the
    pathname exists at the server site, then the data transferred
    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 required by some servers to
    reserve sufficient storage to accommodate the new file to be
    transferred.  The argument field shall be a decimal integer
    representing the number of bytes (of size specified by the byte
    size command) of storage to be reserved for the file.  This

McKenzie [Page 21] RFC 454 File Transfer Protocol July 1972

    command shall be followed by a store or append command.  The ALLO
    command should be treated as a NO-OP (no operation) by those
    servers which do not require that the maximum size of the file be
    declared beforehand.
    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 "spaces" 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 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 indicates to the server to abort the
    previous FTP service command and any associated transfer of data.
    The abort command should be preceded by the TELNET SYNCH condition
    (indicated by the combination of the DATA MARK and the INS).  No
    action is to be taken if the previous command has been completed
    (including data transfer).  The TELNET connections are not to be
    closed by the server, but the data connection may be closed.  An
    appropriate reply should be sent by the server.
    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.
    List (LIST) - This command causes a list to be sent from server to
    user site.  If the pathname specifies a directory, the server
    should transfer a list of files in the specified directory.  If
    the pathname specifies a file then 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.  (It is the user
    's responsibility to ensure the correct parameters.)
    NList (NLST) - This command causes a directory listing to be sent
    from server to user site.  The pathname should specify a directory
    and 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

McKenzie [Page 22] RFC 454 File Transfer Protocol July 1972

    CRLF.  This command will allow automatic copying of an entire
    directory when used with the appropriate transfer commands.
    Status (STAT) - This command shall cause a status response to be
    sent over the TELNET connection in form of a reply.  The command
    may be sent during a file transfer (preceded by a TELNET SYNC) in
    which case the server will respond with the status of the opera-
    tion in progress, or it may be sent between file transfers.  In
    the latter case the command may have an argument field such as a
    pathname.  If the argument is a pathname, the command is analogous
    to the "list" command except that data shall be transferred in
    ASCII on the TELNET 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 TELNET
    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 100, general system status.  It is
    suggested that HELP be allowed before entering a USER command.
    Mail File (MLFL) - The intent of this command is to enable a user
    site to mail data (in form of a file) to another user at the
    server site.  It should be noted that the files to be mailed are
    transmitted via the data connection in ASCII or EBCDIC type. (It
    is the user's responsibility to ensure that the type is correct.)
    These files should be appended to the destination user's mail by
    the server in accordance with serving HOST mail conventions.  The
    mail may be marked as sent from the particular using HOST and the
    user specified by the 'USER' command.  The argument field may con-
    tain one or more system or NIC idents (it is recommended that mul-
    tiple ident be allowed so the same mail can easily be sent to
    several users), or it may be empty.  If the argument field is
    empty or blank (one or more spaces), then the mail is destined for
    a printer or other designated place for site mail.  A NIC ident
    refers to the standard identification described in the NIC Direc-
    tory of Network Participants.  A serving host may keep a table
    mapping NIC indents into system idents, although NIC idents are
    not required in the implementation.  A system ident is the user's
    normal identification at the serving host.  The use of system
    idents would allow a network user to send mail to other users who
    do not have NIC identification but whose system ident is known.

McKenzie [Page 23] RFC 454 File Transfer Protocol July 1972

    Mail (MAIL) - This command allows a user to send mail that is not
    in a file over the TELNET connection.  The argument field may con-
    tain one or more system or NIC idents, or it may be empty.  The
    idents are defined as above for the MLFL command.  After the
    'MAIL' command is received, the server is to treat the following
    lines as text of the mail sent by the user.  The mail text is to
    be terminated by a line containing only a single period, that is,
    the character sequence ".CRLF" in a new line.  It is suggested
    that a modest volume of mail service should be free; i.e., it may
    be entered before a USER command.

IV.A.4 Miscellaneous Commands

    NoOP (NOOP) - This command does not affect any parameters or pre-
    viously entered command.  The server simply sends a no-op reply.
    Quote (QUOT) - This command allows the user to talk directly to
    the FTP-server.  After parsing this command, the user-FTP process
    will pass without examination all succeeding liners until the NQUO
    command is received.  Between these two commands the server will
    respond appropriately to his implementation and the user's
    requests.
    NoQuote (NQUO) - This command returns the user and server
    processes to normal interactive mode.  Both QUOT and NQUO have
    reply codes to be sent by th server process to the user process to
    ensure agreement on the current mode.
 The quote commands provide a convenient method of testing server-
 implemented experimental commands.  The names of the latter should
 begin with an X, and can be listed in the system HELP reply.  It
 should be noted that the official command set is expandable; sugges-
 tions should go first to Alexander A. McKenzie (BBN).

IV.B FTP Replies

 The server sends FTP replies over the TELNET connection in response
 to user FTP commands.  The FTP replies constitute the acknowledgment
 or completion code (including errors).  The FTP-server replies are
 formatted for human or program interpretation.  Single line replies
 consist of a leading three-digit numeric code followed by a space,
 followed by a one-line text explanation of the code.  For replies
 that contain several lines of text, the first line will have a lead-
 ing three-digit numeric code followed immediately by the ASCII char-
 acter "-" (Hyphen, Code 55 (octal)) and possibly some text.  All
 succeeding continuation lines except the last are constrained not to
 begin with three digits; the last line must repeat the numeric code
 of the first line and be followed immediately by a space.

McKenzie [Page 24] RFC 454 File Transfer Protocol July 1972

 For example:
         100-First Line
         Continuation Line
         Another Line
         100 Last Line
 The numeric codes are assigned by groups and for ease of interpreta-
 tion by programs in a manner consistent with other protocols such as
 the RJE protocol.  The three digits of the code are to be interpreted
 as follows:
 a) The first digit specifies type of response as indicated below:
     000 These replies are purely informative and constitute neither a
         positive nor a negative acknowledgment.
     1xx Informative replies to status inquiries.  These constitute a
         positive acknowledgment to the status command.
     2xx Positive acknowledgment of previous command or other success-
         ful action.
     3xx Incomplete information.  Activity cannot proceed without
         further specification and input.
     4xx Unsuccessful reply.  The request is correctly specified but
         the server is unsuccessful in correctly fulfilling it.
     5xx Incorrect or illegal command.  The command or its parameters
         were invalid or incomplete from a syntactic viewpoint, or the
         command is inconsistent with a previous command.  The command
         in question has been completely ignored.
     6xx-9xx Reserved for future expansion.

McKenzie [Page 25] RFC 454 File Transfer Protocol July 1972

 b) The second digit specifies the general category to which the
     response refers:
     x00-x29 General purpose replies, not assignable to other
     categories.
     x30 Primary access.  Informative replies to the "log-on" attempt.
     x40 Secondary access.  The primary server is commenting on its
     ability to access a secondary service.
     x5x FTP results
     x6x RJE results.
     x7x-x9x Reserved for future expansion.
 c) The final digit specifies a particular message type.  Since the
     code is designed for an automation process to interpret, it is
     not necessary for every variation of a reply to have a unique
     number.  Only the basic meaning of replies need have unique
     numbers.  The text of a reply can explain the specific reason for
     that reply to a human user.
     Each TELNET line delimited by a numeric code and CRLF (or group
     of text lines bounded by coded lines) that is sent by the server
     is intended to be a complete reply message.  It should be noted
     that the text of replies is intended for a human user.  Only the
     reply codes and in some instances the first line of text are
     intended for programs.

The assigned reply codes relating to FTP are:

000 General information message (site, time of day, etc.). 010 Message from system operator. 030 Server availability information. 050 FTP commentary or user information. 100 System status reply. 110 System busy doing… 150 File status reply 151 Directory listing reply. 200 Last command received correctly. 201 An ABORT has terminated activity, as requested. 202 Abort request ignored, no activity in progress. 230 User is "logged in". May proceed. 231 User is "logged out". Service terminated. 232 Logout command noted, will complete when transfer done. 233 User is "logged out". Parameters reinitialized.

McKenzie [Page 26] RFC 454 File Transfer Protocol July 1972

250 FTP file transfer started correctly. 251 FTP Restart-marker reply.

      Text is : MARK yyyy = mmmm
      where yyyy is user's data stream marker (yours)
      and mmmm is server's equivalent marker (mine)
      (Note the spaces between the markers and '=')

252 FTP transfer completed correctly. 253 Rename completed. 254 Delete completed. 255 FTP server data socket reply

      Text is: SOCK nnnn
      where nnnn is a decimal integer representing
      the server socket for data connection

256 Mail completed. 300 Connection greeting message, awaiting input. 301 Current command incompleted (no CRLF for long time). 330 Enter password 331 Enter account (if account required as part of login

  sequence).

350 Enter mail, terminate by a line with only a '.' 400 This service not implemented. 401 This service not accepting user now, goodbye. 430 Log-on time or tries exceeded, goodbye. 431 Log-on unsuccessful. Usre and/or password invalid. 432 User not valid for this service. 433 Cannot transfer files without valid account. Enter account. 434 Log-out forced by operator action. Phone site. 435 Log-out forced by system problem. 436 Service shutting down, goodbye. 450 FTP: File not found. 451 FTP: File access denied to you. 452 FTP: File transfer incomplete, data connection closed. 453 FTP: File transfer incomplete, insufficient storage space. 454 FTP: Cannot connect to your data socket. 455 FTP: File system error not covered by other reply codes. 456 FTP: Name duplication rename failed. 457 FTP: Transfer parameters in error. 500 Last command line completely unrecognized. 501 Syntax of last command is incorrect. 502 Last command incomplete, parameters missing. 123456789012345678901234567890123456789012345678901234567890123456789012 503 Last command invalid (ignored), illegal parameter combination. 504 Last command invalid, action not possible at this time. 505 Last command conflicts illegally with previous command(s).

McKenzie [Page 27] RFC 454 File Transfer Protocol July 1972

506 Requested action not implemented by the server. 507 Catchall error reply. 550 Bad pathname specification (e.g., syntax error).

V. DECLARATIVE SPECIFICATIONS

 In order to make FTP workable without needless error messages, the
 following minimum implementation is required for servers:

TYPE – ASCII (with 8-bit bytes)

               MODE -- Stream
               STRUCTURE -- File
                            Record (with ASCII type and CRLF for EOR)
               FORM -- Unformatted
               COMMANDS -- USER, BYE, SOCK
                           TYPE, BYTE, MODE, STRU, FORM
                               for the default values
                           RETR, STOR
                           NOOP
 The initial default values for transfer parameters are:
    TYPE -- ASCII
                    BYTE -- 8
                    MODE -- Stream
                    STRU -- File
                    FORM -- Unformatted

V.A Connections

 The server-FTP process at the server site shall "listen" on Socket 3,
 via its server-TELNET.  The user or user-FTP process at the user site
 shall initiate the full-duplex TELNET connections via its user-TELNET
 performing the ARPANET standard initial connection protocol (ICP) to
 server socket 3.  Servers may specify that interaction over the TEL-
 NET connections be line-at-a-time with local echo.  The server is not
 obliged to provide remote echo and may ignore TELNET control charac-
 ters; he should not, however, return error response to the latter.
 All editing of command lines similarly must be local.  The TELNET
 connections shall be closed by the user site upon completion of use
 and receipt of the last server reply.
 The user site must "listen" on the specified data socket or sockets
 (a send and/or a receive socket).  The server site shall initiate the
 data connection using the specified data socket and byte size.  The
 direction of data connection and the data socket used shall be

McKenzie [Page 28] RFC 454 File Transfer Protocol July 1972

 determined by the FTP service command.  The server shall send a reply
 to the user indicating the server data socket so that the user may
 ensue the security of data transfer.  This can be done at any time
 prior to the first transfer of data over a data connection.  It
 should be emphasized that the user-FTP should not wait for a 255
 (server data socket) reply before doing the "listen", since there is
 no guarantee that the reply will arrive before the user site receives
 the initiating RFC.  The security check can be done when the reply
 arrives and the data connection closed if it was made to a socket
 other than the one specified.
 The data connection shall be closed by the server site under the con-
 ditions described in Section III.A.  If the server wishes to close
 the connection in modes where that is not required, it is recommended
 that the close be sent immediately after the file transfer is com-
 pleted rather than after a new transfer command is received, because
 the user or server may have to test the state of the socket before
 doing a "listen" or "init".  The server should in general send a
 reply before closing the data connection to avoid problems at the
 user end, though, for reasons stated above, the user-FTP should not
 wait for the reply before doing his close.

V.B Commands

 The commands are ASCII character strings transmitted over the TELNET
 connections as described in section IV.A.  The command functions and
 semantics are described in sections IV.A.1, IV.A.2, IV.A.3, and
 IV.A.4.  The command syntax is specified here.
 The commands begin with a command code followed by an argument field.
 The command codes are four or less ASCII alphabetic characters.
 Upper and lower case alphabetic characters are to be treated identi-
 cally.  Thus any of the following may represent the retrieve command:
 RETR    Retr    retr   ReTr     rETr
 This also applies to any symbols representing parameters 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 ASCII character
 string ending with the character sequence CRLF (Carriage Return
 immediately followed by Line Feed).  In the following section on syn-
 tax it should be stressed that all characters in the argument field
 are ASCII characters.  Thus a decimal integer shall mean an ASCII
 represented decimal integer.

McKenzie [Page 29] RFC 454 File Transfer Protocol July 1972

 The following are all the currently defined FTP commands:
    USER <username> CRLF
    PASS <password> CRLF
    ACCT <acctno> CRLF
    REIN CRLF
    BYE CRLF
    BYTE <byte size> CRLF
    SOCK <HOST-socket> CRLF
    LSTN <direction> CRLF
    TYPE <type code> CRLF
    FORM <form code> CRLF
    STRU <structure code> CRLF
    MODE <mode code> CRLF
    RETR <pathname> CRLF
    STOR <pathname> CRLF
    APPE <pathname> CRLF
    ALLO <decimal integer> CRLF
    REST <marker> CRLF
    RNFR <pathname> CRLF
    RNTO <pathname> CRLF
    ABOR CRLF
    DELE <pathname> CRLF
    LIST <pathname> CRLF
    NLST <pathname> CRLF

McKenzie [Page 30] RFC 454 File Transfer Protocol July 1972

    STAT <pathname> CRLF
    HELP <string> CRLF
    MLFL <users> CRLF
    MAIL <users> CRLF
    NOOP CRLF
    QUOT CRLF
    NQUO CRLF
 The syntax of the above argument fields (using BNF notation where
 applicable) is:
    <username> ::= <string>
    <password> ::= <string>
    <acctno> ::= <string>
    <string> ::= <empty>/<char>/<char><string>
    <char> ::= any of the 128 ASCII characters except CR and LF.
    <marker> ::= <pr string>
    <pr string> ::= <empty>/<pr char>/<pr char> <pr string>
    <pr char> ::= any ASCII code 33 through 126
    <byte size> ::= any decimal integer 1 through 255.
    <HOST-socket> ::= <socket>/HOST number>,<socket>
    <HOST number> ::= a decimal integer specifying an ARPANET HOST
    <socket> ::= decimal integer between 0 and (2**32)-1
    <direction> ::= S/R
    <form code> ::= U/P
    <type code> ::= A/E/I/L
    <structure code> ::= F/R

McKenzie [Page 31] RFC 454 File Transfer Protocol July 1972

    <mode code> ::= S/B/T/H
    <pathname> ::= <string>
    <decimal integer> ::= <digit>/<digit><decimal integer>
    <digit> ::= 0|1|2|3|4|5|6|7|8|9
    <empty> ::= the null string (specifies use the default).
    <users> ::= <user>|<user,<users>
    <user> ::= <empty>|<NIC ident>|<sys ident>
    <NIC ident> ::= <string>
    <sys ident> ::= <string>

V.C 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.
 A second type of reply is sent asynchronously with respect to user
 commands.  These replies may, for example, report on the progress or
 completion of file transfer and as such are secondary replies to file
 transfer commands.
 The third class of replies are informational and spontaneous replies
 which may arrive at any time.  These replies are listed below as
 spontaneous.

McKenzie [Page 32] RFC 454 File Transfer Protocol July 1972

COMMAND-REPLY CORRESPONDENCE TABLE

COMMAND SUCCESS FAIL ——- ——- —- USER 230,330 430-432,500-505,507 PASS 230,331 430-432,500-507 ACCT 230 430-432,500-507 REIN 232,233 401,436,500-507

 Secondary Reply   300

BYE 231,232 430-432,500-505,507 BYTE 200,331 500-507 SOCK 200,331 500-505,507 LSTN 255,331 500-507 TYPE 200,331 500-507 FORM 200,331 500-507 STRU 200,331 500-507 MODE 200,331 500-507

RETR 250,331 433,450,451,454,455,500-505,507,550

 Secondary Reply   252           452

STOR 250,331 433,451,454,455,457,500-505,507,550

 Secondary Reply   252           452,453

APPE 250,331 433,451,454,455,457,500-507,550

 Secondary Reply   252           452,453

ALLO 200,331 500-507 REST 200,331 500-507 RNFR 200,331 433,450,451,455,500-507,550 RNTO 253,331 433,450,451,455,456,500-505,507,550 ABOR 201,202,331 500-507 DELE 254,331 433,450,451,455,500-507,550 LIST 250,331 433,450,451,454,455,457,500-507,550

 Secondary Reply   252           452

NLST 250,331 433,450,451,454,455,457,500-507

 Secondary Reply   252           452

STAT 100,110,150, 450,451,454,455,500-507,550

                       151,331

HELP 000,030,050, 500-507

                       331

MLFL 250,331 433,450,451,454,455,457,500-507

 Secondary Reply   252           452,453

MAIL 331,350 433,450,451,455,500-507

 Secondary Reply   256

NOOP 200 500-505,507 QUOT 200,331 500-507 NQUO 200 500-505,507

Spontaneous 0xx,300,301 400,401,434-436 Replies 251,255

McKenzie [Page 33] RFC 454 File Transfer Protocol July 1972

V.D Typical FTP Scenarios

 1. TIP User wanting to transfer file from HOST X to local printer:
    a) TIP user opens TELNET connections by ICP to HOST X, socket 3.
    b) The following commands and replies are exchanged:
       TIP                            HOST X
       ---                            ------
       USER username CRLF ---------->
       <----------330 Enter Password CRLF
       PASS password CRLF ---------->
       <----------230 User logged in CRLF
       SOCK 65538 CRLF    ---------->
       <----------200 Command received OK CRLF
       RETR this.file CRLF ---------->
       <----------255 SOCK 5533 CRLF
       (HOST X initiates data connection to
        TIP socket 65538, i.e., PORT 1 receive)
       <----------250 File transfer started
       BYE CRLF   ----------------->
       <----------252 File transfer completed
    c) HOST X closes the TELNET and data connections.
    Note: The TIP user should be in line mode.
 2. User at HOST U wanting to transfer files to/from HOST S:
    In general the user would communicate to the server via a mediat-
    ing 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.

McKenzie [Page 34] RFC 454 File Transfer Protocol July 1972

Local Commands by User Action Involved ———————- —————

ftp (host) multics CR ICP to HOST S, socket 3,

                                establishing TELNET connections.

username Doe CR USER Doe CRLF —→

                                <---- 330 password CRLF

password mumble CR PASS mumble CRLF —→

                                <---- 230 Doe logged in. CRLF

retrieve (local type) ASCII CR (local pathname) test 1 CR User-FTP opens local file in ASCII. (for.pathname) test.p11 CR RETR test.p11 CRLF

                                <---- 255 SOCK 1233 CRLF
                                Server makes data connection to (U+4).
                                <---- 250 File transfer starts CRLF
                                <---- 252 File transfer complete CRLF

type ImageCR TYPE I CRLF —→

                                <---- 200 Command OK CRLF

byte 36CR BYTE 36 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 —→

                                <---- 451 Access denied CRLF

terminate ←— 231 Doe logged out CRLF

                                Server closes all connections.
     [ This RFC was put into machine readable form for entry ]
        [ into the online RFC archives by Via Genie 03/00 ]

McKenzie [Page 35]

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