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rfc:rfc354

Network Working Group Abhay Bhushan Request for Comments: 354 MIT-MAC NIC: 10596 July 8, 1972 Categories D.4, D.5, D.7 Obsoletes: RFC 264 and 265

                     THE FILE TRANSFER PROTOCOL

I. INTRODUCTION

  The File Transfer Protocol (FTP) is a protocol for file

transfer betweet 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) fo 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 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.

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The File Transfer Protocol July 8, 1972

11.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 es 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 user-FTP process to provide
                  access controls.

byte size The byte size specified for the transfer of

                  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 conidition 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

                  form 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.
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The File Transfer Protocol July 8, 1972

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.A.

NVT The Network Virtual Terminal as defined in

                  the ARPANET TELNET Protocol.

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
                  NFS 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 hte 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 acknowledgment (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.
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The File Transfer Protocol July 8, 1972

server site A HOST site wich 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 perform 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).

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 precesses 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.
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The File Transfer Protocol July 8, 1972

II.B. The FTP Model

  With the above definitions in mind, the following model

(shown in Figure 1) may be diagrammed for an FTP service.

                          TELNET
                        connections

File Server Server←———– User User File Systems↔ FTP ↔TELNET FTP Commands TELNET↔FTP ↔System

        Process         ------------>          Process
                                                Data
               <------------------------------>Socket
                      Data Connection(s)         |
                                                 |
                                                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
  In the model described in Figure 1, the user-TELNET

initiates the TELNET connection. 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.

  The FTP commands specify the parameters for teh data

connection (data socket, byte size, transfer mode, and representation type), and the nature of file system operation (store, retrieve, append, delete, etc.). The user-FTP process or its designate should "listen" on

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The File Transfer Protocol July 8, 1972

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 connection, 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) whishing 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.

  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 data 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.

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 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 of after initiating the data connection.

  The byte size for the data connection is specified by the

TYPE (ASCII is 8 bits), or TYPE and BYTE commands. It is not required by the protocol that servers accept all possible byte size. The user of various byte size is for efficiency in data transfer and servers may implement only those byte size for which their data transfer is efficient. It is however recommended that servers implement at least the byte size of 8 bits.

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The File Transfer Protocol July 8, 1972

  After the data transfer is completed, it is the server's

responsibility to close the data connection except when the user is sender of data. The data connection shall be closed under any of the following conditions:

  1) server receives an abort command form user.
  2) EOF in stream mode indicated by closing data connection.
  3) the socket or byte size specification is changed.
  4) any of the TELNET connections are closed.
  5) an irrecoverable error condition.
  It should be noted that two simultaneous data connections

(for send and receive) may exist. It is a server option, however, to close the data connection after each instance of file transfer.

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 representations 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 desirable to convert characters into the standard NVT-ASCII representation when transmitting text between disimilar systems. The sending and receiving site 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 length. 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) o 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 that FTP provides for very limited data types reprentations. Transformations desired beyond this limited capability should be performed by the user directly or via the use of the Data Reconfiguration Service (DRS, RFC #138, NIC #6715). Additional representation types may be defined later if there is a demonstrable need.

                                                              [Page 7]

The File Transfer Protocol July 8, 1972

  Data representations are handled in FTP by a user specifying

a representation type. The type may also specify a fixed transfer byte size. For example in ASCII and Print File representations, the transfer byte size must be 8 bits. Only in the Image and Local Byte representations the byte size specified by the BYTE command is to be used. The following data representation types are currently defined in FTP:

1. ASCII The sender converts data form its internal

                  character representation to the standard
                  ARPANET 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 be default
                  type, and it is recommended that this type be
                  implemented by all.

2. Image The sender transforms data from contiguous

                  bits to bytes for transfer. The receiver
                  transforms the bytes into bits, storing them
                  contiguously independent of the byte size
                  chosen for data transfer. 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.

3. Local Byte This representation allows for efficient

                  storage, use, and retrieval of data. The
                  mann 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 b 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 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
                                                              [Page 8]

The File Transfer Protocol July 8, 1972

                  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-

4. Print File- The server site will transform the ASCII

 ASCII            file in a form suitable for printing at the
                  server site. The byte size must be 8 bits.
                  The transformation may not be invertible.
                  This type is different from ASCII in that
                  TABs, FFs and other ASCII format effector
                  characters may be replaced by SPs, LFs, and
                  other substitute characters. The print file
                  conversions are to be well publicized by all
                  server sites. This type would be used when
                  the file is destined for an ASCII printer.
                  This type in some systems may be identical to
                  the ASCII type. It is recommended that this
                  type be implemented by all.

5. EBCDIC Print The server site will transform the EBCDIC

 File             file into a form suitable for printing at the
                  server site. The byte size must be 8 bits.
                  the transformation may not be invertible.
                  This type would be used when the file is
                  destined for an EBCDIC printer. Only systems
                  which use EBCDIC for their internal character
                  representation need accept this type.
  It should be noted that a serving HOST need not accept all

representation types and/or byte size, but it must inform the user of the fact by sending an appropriate 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 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 discarded by the receiver.

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The File Transfer Protocol July 8, 1972

  All data transfer 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 or files with record structures. Records may be of zero length but they must be contained in file boundaries. The relationship between files and records is heirarchical and an EOF implies 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 signalled by
                  closing the data connection. Any representation
                  type and byte size may be used in the stream mode
                  but record structures are possible only with the
                  ASCII representation type. The convention is that
                  the ASCII character CR (Carriage Return, Code 13.)
                  followed by LF (Line Feed, Code 10.) Indicates an
                  EOR in stream mode and ASCII representation 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 sequence of

                  8-bit bytes in the ASCII representation type.
                  Record structures are allowed in this mode. The
                  EOR and EOF are defined by the presence of special
                  "TELNET-control" codes (most significant bit set
                  of one) in the data stream. The EOR code is 192
                  (octal 300, hex C0). The EOF code os 193 (octal
                  301, hex C1). The byte size for transfer is 8
                  bits.

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 reliable). Record structures are allowed in
                  this mode, and any representation type or byte
                  size may be used. The header consists of integral
                  number of bytes whose length is greater than or
                  equal to 24 bits. Only the least significant 24
                  bits (right-jusified) of header shall have
                                                             [Page 10]

The File Transfer Protocol July 8, 1972

                  information, other must significant bits must be
                  zero. 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
                  | Must be Zero  | Descriptor    | Byte Count    |
                  | 0 to 231 bits |   8 bits      |    16 bits    |
                  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.
                  The restart marker is imbedded in the data stream
                  as integral number of 8-bit bytes (representing
                  printable ASCII characters) right-justified in
                  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 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
                  no exactly fit an integral byte, the unused
                  character slots should contain 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:
                  |   Zero    |  Descriptor  |               |
                  |   12 bits |  code=4      | Byte count=2  |
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The File Transfer Protocol July 8, 1972

                  |    | 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.

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 teh 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 codes 33. through 126. (i.e., not including codes 0. through 31. and the characters SP and DEL). The marker could represent a bit-count,a record-count, or any other information by wich 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 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 coressponding 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

                                                             [Page 12]

The File Transfer Protocol July 8, 1972

last server marker by sending a restart command with the server's marker code at its argument. The restart command 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 occured.

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

inserts the appropriate marker block in the data stream. The server-FTP process receiving the data, marks the corresponding data point in its file system. The server does not store this marker but conveys 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.

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 invoive sending the FTP commands, interpreting the replies received and transferring data over the data connection 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.

IV.A FTP Commands

  FTP commands are ASCII terminated by the ASCII

character sequence CRLF (Carriage Return follow by Line Feed). The command codes themselves are ASCII alpabetic characters terminated by the ASCII character 'space' (code = 32.). For convenience, the command codes are defined to be four (or less) ASCII alphanumeric characters (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 sequence 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.

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The File Transfer Protocol July 8, 1972

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
     identifying the user. The user identification is that wich 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 pasword command may also be required by some servers.
     Password (PASS) - The argument field is an ASCII string
     identifying the user's password. This command must be immediately
     preceded by the user name command, and together it completes the
     user's identifecation for access control.

IV.A.2 Data Transfer 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 teh last specified value, or if no value has been
     specified, the standard default value specified 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 for local byte and image representation types. The
     default byte size is 8 bits.  The byte size is always 8 bits in
     the ASCII and Print file representation types. A server may
     reject specific byte size/type combinations by sending an
     appropriate reply.
     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
     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).
                                                             [Page 14]

The File Transfer Protocol July 8, 1972

     Representation Type (TYPE) - The argument is a single ASCII
     character code specifying the representation types described in
     section III.B. The following codes are assigned for type:
        A - ASCII
        I - Image
        L - Local Byte
        P - Print file in ASCII
        E - EBCDIC print file
     The default representation type is ASCII
     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 record structure)
        R - Record structure
     The default structure is File (i.e., 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:
        S - Stream (bytes, close is EOF)
        B - Block (Header with descriptor and count)
        T - Text (TELNET control mode 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 command will normally be a pathname.  the syntax of
     pathnames must conform to server site conventions (with standard
     defaults applicable), except that ASCII characters must be used
     (in conformance with the TELNET Protool). 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 some servers may
     require an "allocate" command before a "store" command. The data
     when transferred in response to FTP service commands shall always
     be over the data connection. The following commands specify FTP
     service requests:
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The File Transfer Protocol July 8, 1972

     Retrieve (RETR) - This command achieves the transfer of a copy of
     file specified in pathname, from server to user site. The status
     and contents of a file at server site shall be unaffected.
     Store (STOR) - This command achieves the transfer of a copy of
     file from user to server site. If file specified in pathname
     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 pathname does not
     already exist.
     Append (with create) (APPE) - This command achieves the transfer
     of data from using to serving site. If file specified in pathname
     exists at the server site, then the data transferred shall be
     appended to that file, otherwise the file specified in pathname
     shall be created at the server site.
     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.
     Delete (DELE) - This command causes teh file specified in
     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 pathname specifies a directory, the server
     should transfer a list of files in the specified directory. If
     pathname specifies a file then server should send current
     information on the file. This command may be used to obtain the
     contents of a file directory (the response should be sent in
     ASCII type) or test the existence of a file and its current
     status.
     Allocate (ALLO) - This command my be required by some servers to
     reserve sufficient storage to accomodate the new file to be
     transferred. The command 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
     command shall be followed by a store or append command. The ALLO
     command should be treated as a NO-OP (no operation) by thuse
     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
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     data checkpoint. This command shall be immediately followed by
     the appropriate FTP service command which shall cause file
     transfer to resume.
     Status (STAT) - This command shall cause a status response to be
     sent over the TELNET connection in form of a reply.  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 no argument is specified, the server should return
     general status information about the server FTP process. This may
     include service availability, the current settings for the
     relevant FTP parameters (including default settings), and the
     status of command execution and connections.
     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 is
     not to be closed by the server, but the data connection may be
     closed. An appropriate reply should be sent by the server.
     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.  During the interim a new
     USER command (and no other command) is acceptable.
     An unexpected close on TELNET connection will cause the server to
     take the effective action of an abort (ABOR) and a logout (BYE).

IV.B FTP Replies

  The server sends FTP replies to user over the TELNET

connections in response to FTP commands. The FTP replies constitute the acknowledgement or completion code (including errors). The FTP-server replies are formatted for human or program interpretation. The replies consist of a leading three digit numeric code followed by a space followed by a text explanation of the code. The numeric codes are assigned by groups and for ease of interpretation by programs in a manner consistent with other protocols such as the RJE protocol. The three digits of the code are to be interpredet as follows:

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The File Transfer Protocol July 8, 1972

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 acknowledgement.
 1xx informative replies to status inrequiries. These constitute
     a positive acknowledgment to the status command.
 2xx Positive acknowledgment of previous command or other
     successful 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 corretly fulfilling
     it.
 5xx Incorrect or illegal command. The command or its
     parameters were invalid or incomplete from a syntactic
     viewpoint, or the command its inconsistent with a previous
     command. The command in question has been completely
     ignored.
 6xx - 9xx Reserved for future expansion.

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 resutls.
 x7x-x9x Reserved for future expansion.
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The File Transfer Protocol July 8, 1972

c) the final digit specifies a particular message type. Since the code is designed for an automaton 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 (ended by CRLF) from the server is intended

to be a complete reply message. if it is necessary to continue the text of a reply onto following lines, then those continuation replies contain the special reply code of three spaces. It should be noted that text of replies are 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.) 030 Server availibility information. 050 FTP commentary or user information. 100 System status reply. 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. 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 decimal integer representing
        the server socket for data connection.

300 Connection greeting message, awaiting input. 301 Current command incomplete (no CRLF for long time). 330 Enter password (may be sent with hide-your-input). 400 This service not implemented. 401 This service not accepting users now, goodbye. 430 Log-on time or tries exceeded, goodbye. 431 Log-on unsuccessful. User and/or password invalid. 432 User not valid for this service.

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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. 500 Last command line completely unrecognized. 501 Syntax of last command in incorrect. 502 Last command incomplete, parameters missing. 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). 506 Requested action not implemented by the server.

V. DECLARATIVE SPECIFICATIONS

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. The TELNET connections shall be closed by the user site upon completion of use.

  The user site shall "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 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 ensure the security of data transfer. This can be done at any time prior to the first transfer of data over a data connection.

  The data connection shall be closed by the server site under

the conditions described is Section III.A. The server should in general send a reply before closing the data connection to avoid problems at the user end.

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.

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The File Transfer Protocol July 8, 1972

  The commands begin with a command code followed by an

argument field. The command codes are four of less ASCII alphabetic characters. Upper and lower case alphabetic characters are to be treated identically. Thus any of the following may represent the retrieve command:

  RETR Retr retr ReTr rETr

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 syntax 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.

  The following are all the currently defined FTP commands:
       USER <user name> CRLF
       PASS <password> CRLF
       BYTE <byte size> CRLF
       SOCK <HOST-socket> CRLF
       TYPE <type code> CRLF
       STRU <structure code> CRLF
       MODE <mode code> CRLF
       RETR <pathname> CRLF
       STOR <pathname> CRLF
       APPE <pathname> CRLF
       RNFR <pathname> CRLF
       RNTO <pathname> CRLF
       DELE <pathname> CRLF
       LIST <pathname> CRLF
       ALLO <decimal integer> CRLF
       REST <marker> CRLF
       STAT <pathname> CRLF
       ABOR <empty> CRLF
        Bye <empty> CRLF
  The syntax of the above argument fields (using BNF notation

where aplicable) is:

   <username> ::= <string>
   <password> ::= <string>
   <string> ::= <empty> | <char> | <char><string>
   <char> ::= any of the 128 ASCII characters except CR and LF.
   <marker> ::= <pr string>
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The File Transfer Protocol July 8, 1972

   <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 o and (2**32)-1
   <type code> ::= A|I|L|P|E
   <structure code> ::= F|R
   <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 of default).

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 of 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.

             COMMAND-REPLY CORRESPONDENCE TABLE

COMMAND SUCCESS FAIL

USER 230,330 430-432,500-505 PASS 230 430-432,500-505 BYE 231,232 430-432,500-505 BYTE 200 500-506 SOCK 200 500-506 TYPE 200 500-506 MODE 200 500-506 RETR 250 450,451,500-506

Secondary Reply   252                  452

STOR 250 451,451,500-506

Secondary Reply   252                  452,453

APPE 250 451,451,500-506

Secondary Reply   252                  452,453
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The File Transfer Protocol July 8, 1972

RNFR 200 450,451,500-506 RNTO 253 450,451,500-505 DELE 254 450,451,500-506 LIST 250 450,453,500-506

Secondary Reply   252                  452

ALLO 200 500-506 STAT 100,150,151 450,451,500-506 REST 200 500-506 ABOR 201,202 500-505

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

V.D. Tyical FTP Scenarious

1. TIP User wanting o transfer file from FOST 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 and can thus use
       local TIP editing such as character delete.
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The File Transfer Protocol July 8, 1972

2. User at Host U wanting to transfer files to/from HOST S:

   In general the user would communicate to the server via a
 mediating user-FTP process. The following may be a typical
 scenario. The user-FTP prompts are shown in parenthesis,
 '---->' 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 ICP to HOST S, socket 3,

                            establishing TELNET connections.

username Doe CR USER DoeCRLF —→

                            <---- 330 passwordCRLF

password mumble CR PASS mumbleCRLF —→

                            <---- 230 Doe logged in.CRLF

retrieve (local type ASCIICR (local pathname) test 1 CR USER-FTP open local file in ASCII. (for. pathname) test.pl1CR RETR test.pl1 CRLF —→

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

type imageCR TYPE |CRLF —→

                            <---- 200 Command OKCRLF

byte 36CR BYTE 36CRLF —→

                            <---- 200 Command OKCRLF

store (local type) ImageCR (local pathname) file dumpCR User-FTP opens local file in Image. (for. pathname) >udd>cn>fdCR STOR >udd>cn>fdCRLF —→

                            <---- 451 Access deniedCRLF

terminate BYECRLF

                            <---- 231 Doe logged outCRLF
                            Server closes all connections.
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The File Transfer Protocol July 8, 1972

ACKNOWLEDGEMENTS

  The work on file transfer protocol has involved many people.

This document reports the work of a group rather than the author alone. The author gratefully acknowledges the conributions of the following:

        Bob Braden         UCLA-CCCN
        Arvola Chan        MIT-MAC
        Bill Crowther      BBN-TIP
        Eric Harslem       RAND
        John Heafner       RAND
        Chuck Holland      UCSD
        Alex McKenzie      BBN (NET)
        Bob Metcalfe       XPARC
        Jon Postel         UCLA
        Neal Ryan          MIT-MAC
        Bob Sundberg       HARVARD
        Ray Tomlinson      BBN (TENEX)
        Dick Watson        SRI-ARC
        Jim White          SRI-ARC
        Richard Winter     CCA
     [ This RFC was put into machine readable form for entry ]
     [ into the online RFC archives by Gottfried Janik 9/97  ]
                                                             [Page 25]
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