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Obsoleted by: 765
Updated by: 614, 640 Errata Exist
                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


Nancy J. Neigus                            See Also:  RFCs 354, 454, 495
Bolt Beranek and Newman, Inc.
Cambridge, Mass.























              File Transfer Protocol for the ARPA Network

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


                                PREFACE

This document is the result of several months discussion via RFC
(relevant numbers are 430, 448, 454, 463, 468, 478, 480), followed by a
meeting of the FTP committee at BBN on March 16, followed by further
communication among committee members.  There are a considerable number
of changes for the last "official" version, see RFCs 354, 385, but the
gross structure remains the same.  The places to look for differences
are (1) in the definitions pf types and modes, (2) in the specification
of the data connection and data sockets, (3) in the command-reply
sequences, (4) in the functions dependent on the TELNET protocol (FTP
has been altered to correspond to the new TELNET spec).  The model has
been clarified and enlarged to allow inter-server file transfer, and
several new commands have been added to accommodate more specialized (or
site-specific) functions.  It is my belief that this new specificiation
reflects the views expressed by the committee at the above-mentioned
meeting and in subsequent conversations.

The large number of incompatibilities would complicate a phased
implementation schedule, such as is in effect for the TELNET protocol.
Therefore we have assigned a new socket, decimal 21, as a temporary
logger socket for the new version and a change-over date of 1 February
1974.  Until that date the old (354, 385) version of FTP will be
available on Socket 3 and the new version (attached) should be
implemented on Socket 21.  On 1 February the new version will shift to
Socket 3 and the old disappear from view.

The File Transfer protocol should be considered stable at least until
February, though one should feel free to propose further changes via
RFC.  (Implementation of new commands on an experimental basis is
encouraged and should also be reported by RFC.)  In addition, members of
the FTP committee may be contacted directly about changes.  Based on
attendance at the March 16 meeting, they are:

   Abhay Bhushan MIT-DMCG
   Bob Braden UCLA-CCN
   Bob Bressler BBN-NET
   Bob Clements BBN-TENEX
   John Day ILL-ANTS
   Peter Deutsch PARC-MAXC
   Wayne Hathaway AMES-67
   Mike Kudlick SRI-ARC
   Alex McKenzie BBN-NET
   Bob Merryman UCSD-CC
   Nancy Neigus BBN-NET
   Mike Padlipsky MIT-Multics
   Jim Pepin USC-44
   Ken Pogran MIT-Multics
   Jon Postel UCLA-NMC


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                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


   Milton Reese FNWC
   Brad Reussow HARV-10
   Marc Seriff MIT-DMCG
   Ed Taft HARV-10
   Bob Thomas BBN-TENEX
   Ric Werme CMU-10
   Jim White SRI-ARC

I would especially like to thank Bob Braden, Ken Pogran, Wayne Hathaway,
Jon Postel, Ed Taft and Alex McKenzie for their help in preparing this
document.

NJN/jm






































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                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


                         FILE TRANSFER PROTOCOL

INTRODUCTION

   The File Transfer Protocol (FTP) is a protocol for file transfer
   between Hosts (including Terminal Interface Message Processors
   (TIPs)) 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, and easily implemented protocol design.

   This paper assumes knowledge of the following protocols described in
   NIC #7104:

      The Host-Host Protocol

      The Initial Connection Protocol

      The TELNET Protocol

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.

   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.


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         It is the prerogative of a server-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.  The data connection
         byte size is not necessarily the byte size in which data is to
         be stored in a system, nor the logical byte size for
         interpretation of the structure of the data.

      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
         path may be between a server-DTP and a user-DTP, or between two
         server-DTPs.

      data socket

         The passive data transfer process "listens" on the data socket
         for an RFC from the active transfer process (server) in order
         to open the data connection.  The server has fixed data
         sockets; the passive process may or may not.

      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.




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      file

         An ordered set of computer data (including programs), of
         arbitrary length, uniquely identified by a pathname.

      mode

         The mode in which data is to be transferred via the data
         connection. The mode defines the data format during transfer
         including EOR and EOF.  The transfer modes defined in FTP are
         described in the Section on Transmission Modes.

      NVT

         The Network Virtual Terminal as defined in the 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 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 a file need not have record structure.

      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 a text string.  The codes
         are for use by programs and the text is usually intended for
         human users.




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                                                       RFC 542 NIC 17759


      server-DTP

         The data transfer process, in its normal "active" state,
         establishes the data connection by RFC to the "listening" data
         socket, sets up parameters for transfer and storage, and
         tranfers data on command from its PI.  The DTP can be placed in
         a "passive" state to listen for, rather than initiate, an RFC
         on the data socket.

      server-FTP process

         A process or set of processes which perform the function of
         file transfer in cooperation with a user-FTP process and,
         possibly, another server.  The functions consist of a protocol
         interpreter (PI) and a data transfer process (DTP).

      server-PI

         The protocol interpreter "listens" on Socket 3 for an ICP from
         a user-PI and establishes a TELNET communication connection.
         It receives standard FTP commands from the user-PI, sends
         replies, and governs the server-DTP.

      TELNET connections

         The full-duplex communication path between a user-PI and a
         server-PI.  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 the Section on Establishing
         Data Connections.

      user

         A human being or a process on behalf of a human being wishing
         to obtain file transfer service.  The human user may interact
         directly with a server-FTP process, but use of a user-FTP
         process is preferred since the protocol design is weighted
         towards automata.







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      user-DTP

         The data transfer process "listens" on the data socket for an
         RFC from a server-FTP process.  If two servers are transferring
         data between them, the user-DTP is inactive.

      user-FTP process

         A set of functions including a protocol interpreter, a data
         transfer process and a user interface which together perform
         the function of file transfer in cooperation with one or more
         server-FTP processes.  The user interface allows a local
         language to be used in the command-reply dialogue with the
         user.

      user-PI

         The protocol interpreter initiates the ICP to the server-FTP
         process, initiates FTP commands, and governs the user-DTP if
         that process is part of the file transfer.

   THE FTP MODEL

   With the above definitions in mind, the following model (shown in
   Figure 1) may be diagrammed for an FTP service.

                                         -------------
                                         !/---------\!
                                         !!   User  !!    --------
                                         !!Interface!<--->! User !
                                         !\----:----/!    --------
               ----------                !     V     !
               !/------\!  FTP Commands  !/---------\!
               !!Server!<-----------------!   User  !!
               !!  PI  !----------------->!    PI   !!
               !\--:---/!   FTP Replies  !\----:----/!
               !   V    !                !     V     !
   --------    !/------\!      Data      !/---------\!    --------
   ! File !<--->!Server!<---------------->!  User   !<--->! File !
   !System!    !! DTP  !!   Connections  !!   DTP   !!    !System!
   --------    !\------/!                !\---------/!    --------
               ----------                -------------

               Server-FTP                   User-FTP

   NOTES: 1. The data connection may be in either direction.
          2. The data connection need not exist all of the time.

                     Figure 1  Model for FTP Use


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   In the model described in Figure 1, the user-protocol interpreter
   initiates the TELNET connections. At the initiation of the user,
   standard FTP commands are generated by the user-PI and transmitted to
   the server process via the TELNET connections.  (The user may
   establish a direct TELNET connection to the server-FTP, from a TIP
   terminal for example, and generate standard FTP commands himself,
   by-passing the user-FTP process.) Standard replies are sent from the
   server-PI to the user-PI over the TELNET connections in response to
   the commands.

   The FTP commands specify the parameters for the data connection (data
   socket, byte size, transfer mode, representation type, and structure)
   and the nature of file system operation (store, retrieve, append,
   delete, etc.).  The user-DTP or its designate should "listen" on the
   specified data socket, and the server initiate the data connection
   and data transfer in accordance with the specified 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.  It should also be noted that two data connections, one
   for send and the other for receive, may exist simultaneously.

   In another situation a user might wish to transfer files between two
   Hosts, neither of which is his local Host. He sets up TELNET
   connections to the two servers and then arranges for a data
   connection between them.  In this manner control information is
   passed to the user-PI but data is transferred between he server data
   transfer processes.  Following is a model of this server-server
   interaction.


                 TELNET     ------------    TELNET
                 -----------! User-FTP !------------
                 ! -------->! User-PI  !<--------- !
                 ! !        !   "C"    !         ! !
                 V !        ------------         ! V
         --------------                        --------------
         ! Server-FTP !   Data Connection      ! Server-FTP !
         !    "A"     !<-----------------------!    "B"     !
         -------------- Socket(A)    Socket(B) --------------



                             Figure 2







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                                                       RFC 542 NIC 17759


   The protocol requires that the TELNET connections be open while data
   transfer is in progress.  It is the responsibility of the user to
   request the closing of the TELNET connections when finished using the
   FTP service, while it is the server who takes the action.  The server
   may abort data transfer if the TELNET connections are closed without
   command.

DATA TRANSFER FUNCTIONS

   Files are transferred only via the data connection(s).  The TELNET
   connection is used for the transfer of commands, which describe the
   functions to be performed, and the replies to these commands (see the
   Section on FTP Replies).  Several commands are concerned with the
   transfer of data between Hosts.  These data transfer commands include
   the BYTE, MODE, and SOCKet commands which specify how the bits of the
   data are to be transmitted, and the STRUcture and TYPE commands,
   which are used to define the way in which the data are to be
   represented. The transmission and representation are basically
   independent but "Stream" transmission mode is dependent on the file
   structure attribute and if "Compressed" transmission mode is used the
   nature of the filler byte depends on the representation type.

   DATA REPRESENTATION AND STORAGE

   Data is transferred from a storage device in the sending Host to a
   storage device in the receiving Host.  Often it is necessary to
   perform certain transformations on the data because data storage
   representations in the two systems are different.  For example,
   NVT-ASCII has different data storage representations in diffeent
   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
   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 type representations.
   Transformations desired beyond this limited capability should be


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   performed by the user directly or via the use of the Data
   Reconfiguration Sevice (DRS, RFC #138, NIC #6715).  Additonal
   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.  This type may implicitly (as in ASCII or
   EBCDIC) or explicitly (as in Local byte) define a byte size for
   interpretation which is referred to as the "logical byte size."  This
   has nothing to do with the byte size used for transmission over the
   data connection(s) (called the "transfer byte size") and the two
   should not be confused.  For example, NVT-ASCII has a logical byte
   size of 8 bits but an ASCII file might be transferred using a
   transfer byte size of 32.  If the type is Local byte, then the TYPE
   command has an obligatory second parameter specifying the logical
   byte size.

   The types ASCII and EBCDIC also take a second (optional) parameter;
   this is to indicate what kind of vertical format control, if any, is
   associated with a file.  The following data representation types are
   defined in FTP:

      ASCII Format

         This is the default type and must be accepted by all FTP
         implementations.  It is intended primarily for the transfer of
         text files, except when both Hosts would find the EBCDIC type
         more convenient.

         The sender converts the data from his internal character
         representation to the standard 8-bit NVT-ASCII representation
         (see the TELNET specification).  The receiver will convert the
         data from the standard form to his own internal form.

         In accordance with the NVT standard, the <CRLF> sequence should
         be used, where necessary, to denote the end of a line of text.
         (See the discussion of file structure at the end of the Section
         on Data Representation and Storage).

         Using the standard NVT-ASCII representation means that data
         must be interpreted as 8-bit bytes.  If the BYTE command (see
         the Section on Transfer Parameter Commands) specifies a
         transfer byte size different from 8 bits, the 8-bit ASCII
         characters should be packed contiguously without regard for
         transfer byte boundaries.

         The Format parameter for ASCII and EBCDIC types is discussed
         below.



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      EBCDIC Format

         This type is intended for efficient transfer between Hosts
         which use EBCDIC for their internal character representation.

         For transmission the data are represented as 8-bit EBCDIC
         characters.  The character code is the only difference between
         the functional specifications of EBCDIC and ASCII types.

         End-of-line (as opposed to end-of-record--see the discussion of
         structure) will probably be rarely used with EBCDIC type for
         purposes of denoting structure, but where it is necessary the
         <NL> character should be used.

   A character file may be transferred to a Host for one of three
   purposes: for printing, for storage and later retrieval, or for
   processing.  If a file is sent for printing, the receiving Host must
   know how the vertical format control is represented.  In the second
   case, it must be possible to store a file at a Host and then retrieve
   it later in exactly the same form.  Finally, it ought to be possible
   to move a file from one Host to another and process the file at the
   second Host without undue trouble.  A single ASCII or EBCDIC format
   does not satisfy all these conditions and so these types have a
   second parameter specifying one of the following three formats:

      Non-print

         This is the default format to be used if the second (format)
         parameter is omitted.  Non-print format must be accepted by all
         FTP implementations.

         The file need contain no vertical format information.  If it is
         passed to a printer process, this process may assume standard
         values for spacing and margins.

         Normally, this format will be used with files destined for
         processing or just storage.

      TELNET Format Controls

         The file contains ASCII/EBCDIC vertical format controls (i.e.,
         <CR>, <LF>, <NL>, <VT>, <FF>) which the printer process will
         interpret appropriately.  <CRLF>, in exactly this sequence,
         also denotes end-of-line.

      Carriage Control (ASA)

         The file contains ASA (FORTRAN) vertical format control
         characters.  (See NWG/RFC #189 Appendix C and Communications of


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         the ACM, Vol. 7, No. 10, 606 (Oct. 1964)).  In a line or a
         record, formatted according to the ASA Standard, the first
         character is not to be printed.  Instead it should be used to
         determine the vertical movement of the paper which should take
         place before the rest of the record is printed.  The ASA
         Standard specifies the following control characters:

            Character     Vertical Spacing

            blank         Move paper up one line
            0             Move paper up two lines
            1             Move paper to top of next page
            +             No movement, i.e., overprint

         Clearly there must be some way for a printer process to
         distinguish the end of the structural entity.  If a file has
         record structure (see below) this is no problem; records will
         be explicitly marked during transfer and storage.  If the file
         has no record structure, the <CRLF> end-of-line sequence is
         used to separate printing lines, but these format effectors are
         overridden by the ASA controls.

      Image

         The data are sent as contiguous bits which, for transfer, are
         packed into transfer bytes of the size specified in the BYTE
         command.  The receiving site must store the data as contiguous
         bits.  The structure of the storage system might necessitate
         the padding of the file (or of each record, for a
         record-structured file) to some convenient boundary (byte, word
         or block).  This padding, which must be all zeroes, may occur
         only at the end of the file (or at the end of each record) and
         there must be a way of identifying the padding bits so that
         they may be stripped off if the file is retrieved.  The padding
         transformation should be well publicized to enable a user to
         process a file at the storage site.

         Image type is intended for the efficient storage and retrieval
         of files and for the transfer of binary data.  It is
         recommended that this type be accepted by all FTP
         implementations.

      Local byte Byte size

         The data is transferred in logical bytes of the size specified
         by the obligatory second parameter, Byte size.  The value of
         Byte size must be a decimal integer; there is no default value.
         The logical byte size is not necessarily the same as the
         transfer byte size.  If there is a difference in byte sizes,


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         then the logical bytes should be packed contiguously,
         disregarding transfer byte boundaries and with any necessary
         padding at the end.

         When the data reaches the receiving Host it will be transformed
         in a manner dependent on the logical byte size and the
         particular Host.  This transformation must be invertible (that
         is an identical file can be retrieved if the same parameters
         are used) and should be well publicized by the FTP
         implementors.

         This type is intended for the transfer of structured data.  For
         example, a user sending 36-bit floating-point numbers to a Host
         with a 32-bit word could send his data as Local byte with a
         logical byte size of 36.  The receiving Host would then be
         expected to store the logical bytes so that they could be
         easily manipulated; in this example putting the 36-bit logical
         bytes into 64-bit double words should suffice.

   A note of caution about parameters:  a file must be stored and
   retrieved with the same parameters if the retrieved version is to be
   identical to the version originally transmitted.  Conversely, FTP
   implementations must return a file identical to the original if the
   parameters used to store and retrieve a file are the same.

   In addition to different representation types, FTP allows the
   structure of a file to be specified.  Currently two file structures
   are recognized in FTP: file-structure, where there is no internal
   structure, and record-structure, where the file is made up of
   records.  File-structure is the default, to be assumed if the
   STRUcture command has not been used but both structures must be
   accepted for "text" files (i.e., files with TYPE ASCII or EBCDIC) by
   all FTP implementations.  The structure of a file will affect both
   the transfer mode of a file (see the Section on Transmission Modes)
   and the interpretation and storage of the file.

   The "natural" structure of a file will depend on which Host stores
   the file.  A source-code file will usually be stored on an IBM 360 in
   fixed length records but on a PDP-10 as a stream of characters
   partitioned into lines, for example by <CRLF>.  If the transfer of
   files between such disparate sites is to be useful, there must be
   some way for one site to recognize the other's assumptions about the
   file.

   With some sites being naturally file-oriented and others naturally
   record-oriented there may be problems if a file with one structure is
   sent to a Host oriented to the other.  If a text file is sent with
   record-structure to a Host which is file oriented, then that Host
   should apply an internal transformation to the file based on the


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   record structure.  Obviously this transformation should be useful but
   it must also be invertible so that an identical file may be
   retreieved using record structure.

   In the case of a file being sent with file-structure to a
   record-oriented Host, there exists the question of what criteria the
   Host should use to divide the file into records which can be
   processed locally.  If this division is necessary the FTP
   implementation should use the end-of-line sequence, <CRLF> for ASCII,
   or <NL> for EBCDIC text files, as the delimiter.  If an FTP
   implementation adopts this technique, it must be prepared to reverse
   the transformation if the file is retrieved with file-structure.

   ESTABLISHING DATA CONNECTIONS

   The mechanics of transferring data consists of setting up the data
   connection to the appropriate sockets and choosing the parameters for
   transfer--byte size and mode. Both the user and the server-DTPs have
   default data sockets; these are the two sockets (for send and
   receive) immediately following the standard ICP TELNET socket ,i.e.,
   (U+4) and (U+5) for the user-process and (S+2), (S+3) for the server.
   The use of default sockets will ensure the security of the data
   transfer, without requiring the socket information to be explicitly
   exchanged.

   The byte size for the data connection is specified by the BYTE
   command, or, if left unspecified, defaults to 8-bit bytes.  This byte
   size is relevant only for the actual transfer of the data; it has no
   bearing on representation of the data within a Host's file system.
   The protocol does not require servers to accept all possible byte
   sizes. Since the use of various byte sizes is intended for efficiency
   of transfer, servers may implement only those sizes for which their
   data transfer is efficient including the default byte size of 8 bits.

   The passive data transfer process (this may be a user-DTP or a second
   server-DTP) shall "listen" on the data socket prior to sending a
   transfer request command.  The FTP request command determines the
   direction of the data transfer and thus which data socket (odd or
   even) is to be used in establishing the connection.  The server, upon
   receiving the transfer request, will initiate the data connection by
   RFC to the appropriate socket using the specified (or default) byte
   size.  When the connection is opened, the data transfer begins
   between DTP's, and the server-PI sends a confirming reply to the
   user-PI.

   It is possible for the user to specify an alternate data socket by
   use of the SOCK command.  He might want a file dumped on a TIP line
   printer or retrieved from a third party Host.  In the latter case the
   user-PI sets up TELNET connections with both server-PI's and sends


                                   14

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   each a SOCK command indicating the fixed data sockets of the other.
   One server is then told (by an FTP command) to "listen" for an RFC
   which the other will initiate and finally both are sent the
   appropriate transfer commands.  The exact sequence of commands and
   replies sent between the user-controller and the servers is defined
   in the Section on FTP Replies.

   In general it is the server's responsibility to maintain the data
   connection--to initiate the RFC's and the closes.  The exception to
   this is when the user-DTP is sending the data in a transfer mode that
   requires the connection to be closed to indicate EOF.  The server
   MUST close the data connection under the following conditions:

      1. The server has completed sending data in a transfer mode that
         requires a close to indicate EOF.

      2. The server receives an ABORT command from the user.

      3. The socket or byte size specification is changed by a command
         from the user.

      4. The TELNET connections are closed legally or otherwise.

      5. An irrecoverable error condition occurs.

   Otherwise the close is a server option, the exercise of which he must
   indicate to the user-process by an appropriate reply.

   TRANSMISSION MODES

   The next consideration in transferring data is choosing the
   appropriate transmission mode.  There are three modes: one which
   formats the data and allows for restart procedures; one which also
   compresses the data for efficient transfer; and one which passes the
   data with little or no processing.  In this last case the mode
   interacts with the structure attribute to determine the type of
   processing.  In the compressed mode the representation type
   determines the filler byte.

   All data transfers must be completed with an end-of-file (EOF) which
   may be explicitly stated or implied by the closing of the data
   connection.  For files with record structure, all the end-of-record
   markers (EOR) are explicit, including the final one.

   Note:  In the rest of this section, byte means "transfer byte" except
   where explicitly stated otherwise.





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   The following transmission modes are defined in FTP:

      Stream

         The data is transmitted as a stream of bytes.  There is no
         restriction on the representation type used; record structures
         are allowed, in which case the transfer byte size must be at
         least 3 bits!

         In a record structured file EOR and EOF will each be indicated
         by a two-byte control code of whatever byte size is used for
         the transfer.  The first byte of the control code will be all
         ones, the escape character.  The second byte will have the low
         order bit on and zeroes elsewhere for EOR and the second low
         order bit on for EOF; that is, the byte will have value 1 for
         EOR and value 2 for EOF.  EOR and EOF may be indicated together
         on the last byte transmitted by turning both low order bits on,
         i.e., the value 3.  If a byte of all ones was intended to be
         sent as data, it should be repeated in the second byte of the
         control code.

         If the file does not have record structure, the EOF is
         indicated by the sending Host closing the data connection and
         all bytes are data bytes.

   For the purpose of standardized transfer, the sending Host will
   translate his internal end of line or end of record denotation into
   the representation prescribed by the transfer mode and file
   structure, and the receiving Host will perform the inverse
   translation to his internal denotation.  An IBM 360 record count
   field may not be recognized at another Host, so the end of record
   information may be transferred as a two byte control code in Stream
   mode or as a flagged bit in a Block or Compressed mode descriptor.
   End of line in an ASCII or EBCDIC file with no record structure
   should be indicated by <CRLF> or <NL>, respectively.  Since these
   transformations imply extra work for some systems, identical systems
   transferring non-record structured text files might wish to use a
   binary representation and stream mode for the transfer.

      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 block in the file (EOF) last
         block in the record (EOR), restart marker (see the Section on
         Error Recovery and Restart) or suspect data (i.e., the data


                                   16

                                                  File Transfer Protocol
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                                                       RFC 542 NIC 17759


         being transferred is suspected of errors and is not reliable).
         This last code is NOT intended for error control within FTP.
         It is motivated by the desire of sites exchanging certain types
         of data (e.g., seismic or weather data) to send and receive all
         the data despite local errors (such as "magnetic tape read
         errors"), but to indicate in the transmission that certain
         portions are suspect).  Record structures are allowed in this
         mode, and any representation type may be used.  There is no
         restriction on the transfer byte size.

         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 number of bytes greater than or equal to 24 bits
         --------------------------------------------------------
         !    Don't care    !    Descriptor    !    Byte Count  !
         !  0 to 231 bits   !      8 bits      !     16 bits    !
         --------------------------------------------------------


         The descriptor codes are indicated by bit flags in the
         descriptor byte.  Four codes have been assigned, where each
         code number is the decimal value of the corresponding bit in
         the byte.

            Code     Meaning

             128     End of data block is EOR
              64     End of data block is EOF
              32     Suspected errors in data block
              16     Data block is a restart marker


         With this encoding more than one descriptor coded condition may
         exist for a particular block.  As many bits as necessary may be
         flagged.

         The restart marker is embedded in the data stream as an
         integral number of 8-bit bytes representing printable
         characters in the language being used over the TELNET
         connection (e.g., default--NVT-ASCII).  These marker bytes are
         right-justified in the smallest integral number of transfer
         bytes greater than or equal to 8 bits.  For example, if the



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         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   !
            -------------------------

         If the transfer byte size is 16 or more bits, the maximum
         possible number of complete marker bytes should be packed,
         right-justified, into each transfer byte.  The restart marker
         should begin in the first marker byte.  If there are any unused
         marker bytes, these should be filled with the character <SP>
         (Space, in the appropriate language).  <SP> must not be used
         WITHIN a restart marker.  For example, to transmit a
         six-character marker with a 36-bit transfer byte size, the
         following three 36-bit bytes would be sent:

            ------------------------------------------
            ! Don't care !Descriptor! Byte count = 2 !
            !   12 bits  ! code = 16!                !
            ------------------------------------------

            ------------------------------------------
            !    ! Marker ! Marker ! Marker ! Marker !
            !    ! 8 bits ! 8 bits ! 8 bits ! 8 bits !
            ------------------------------------------

            ------------------------------------------
            !    ! Marker ! Marker ! Space  ! Space  !
            !    ! 8 bits ! 8 bits ! 8 bits ! 8 bits !
            ------------------------------------------


      Compressed

         The file is transmitted as series of bytes of the size
         specified by the BYTE command.  There are three kinds of
         information to be sent:  regular data, sent in a byte string;
         compressed data, consisting of replications or filler; and
         control information, sent in a two-byte escape sequence.  If
         the byte size is B bits and n>0 bytes of regular data are sent,
         these n bytes are preceded by a byte with the left-most bit set
         to 0 and the right-most B-1 bits containing the number n.






                                   18

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                                                       RFC 542 NIC 17759


                        1  B-1    B        B
                       -------  ------   ------
         Byte string:  !0! n !  !d(1)!...!d(n)!
                       -------  ------   ------
                                ^             ^
                                !---n bytes---!
                                    of data

            String of n data bytes d(1),..., d(n)
            Count n must be positive

         To compress a string of n replications of the data byte d, the
         following 2 bytes are sent:


                              2     B-2       B
                           ---------------  ------
         Replicated Byte:  ! 1 0 !   n   !  ! d  !
                           ---------------  ------

         A string of n filler bytes can be compressed into a single
         byte, where the filler byte varies with the representation
         type.  If the type is ASCII or EBCDIC the filler byte is <SP>
         (Space, ASCII code 32., EBCDIC code 64).  If the transfer byte
         size is not 8, the expanded byte string should be filled with
         8-bit <SP> characters in the manner described in the definition
         of ASCII representation type (see the Section on Data
         Representation and Storage).  If the type is Image or Local
         byte the filler is a zero byte.

                            2     B-2
                         ---------------
         Filler String:  ! 1 1 !   n   !
                         ---------------

         The escape sequence is a double byte, the first of which is the
         escape byte (all zeroes) and the second of which contains
         descriptor codes as defined in Block mode.  This implies that
         the byte size must be at least 8 bits, which is not much of a
         restriction for efficiency in this mode.  The descriptor codes
         have the same meaning as in Block mode and apply to the
         succeeding string of bytes.

         Compressed mode is useful for obtaining increased bandwidth on
         very large network transmissions at a little extra CPU cost.
         It is most efficient when the byte size chosen is that of the
         word size of the transmitting Host, and can be most effectively
         used to reduce the size of printer files such as those
         generated by RJE Hosts.


                                   19

                                                  File Transfer Protocol
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   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 users from gross system failures (including failures of a
   Host, an FTP-process, or the IMP subnet).

   The restart procedure is defined only for the block and compressed
   modes of data transfer.  It requires the sender of data to insert a
   special marker code in the data stream with some marker information.
   The marker information has meaning only to the sender, but must
   consist of printable characters in the default or negotiated language
   of the TELNET connection.  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 recieving system, and return this information to the
   user.

   In the event of a system failure, the user can restart the data
   transfer by identifying the marker point with the FTP restart
   procedure.  The following example illustrates the use of the restart
   procedure.

   The sender of the data inserts an appropriate marker block in the
   data stream at a convenient point.  The receiving Host marks the
   corresponding data point in its file system and conveys the last
   known sender and receiver marker information to the user, either
   directly or over the TELNET connection in a 251 reply (depending on
   who is the sender).  In the event of a system failure, the user or
   controller process restarts the server at the last server marker by
   sending a restart command with server's marker code as its argument.
   The restrart command is transmitted over the TELNET connection and is
   immediately followed by the command (such as RETR, STOR or LIST)
   which was being executed when the system failure occurred.

FILE TRANSFER FUNCTIONS

   The communication channel from the user-PI to the server-PI is
   established by ICP from the user to a standard server socket.  The
   user protocol interpreter is responsible for sending FTP commands and
   interpreting the replies received; the server-PI interprets commands,
   sends replies and directs its DTP to set up the data connection and
   transfer the data.  If the second party to the data transfer (the
   passive transfer process) is the user-DTP then it is governed through
   the internal protocol of the user-FTP Host; if it is a second
   server-DTP then it is governed by its PI on command from the user-PI.



                                   20

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


   FTP COMMANDS

   The File Transfer Protocol follows the specifications of the TELNET
   protocol for all communications over the TELNET connection - see NIC
   #7104.  Since, in the future, the language used for TELNET
   communication may be a negotiated option, all references in the next
   two sections will be to the "TELNET language" and the corresponding
   "TELNET end of line code".  Currently one may take these to mean
   NVT-ASCII and <CRLF>.  No other specifications of the TELNET protocol
   will be cited.

   FTP commands are "TELNET strings" terminated by the "TELNET end of
   line code".  The command codes themselves are alphabetic characters
   terminated by the character <SP> (Space) if parameters follow and
   TELNET-EOL otherwise.  The command codes and the semantics of
   commands are described in this section; the detailed syntax of
   commands is specified in the Section on Commands, the reply sequences
   are discussed in the Section on Sequencing of Commands and Replies,
   and scenarios illustrating the use of commands are provided in the
   Section on Typical FTP Scenarios.

   FTP commands may be partitioned as those specifying access-control
   identifiers, data transfer parameters, or FTP service requests.
   Certain commands (such as ABOR, STAT, 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
   simultaneously, in which case some special action will be necessary
   to get the server's attention.  The exact form of the "special
   action" is related to decisions currently under review by the TELNET
   committee; but the following ordered format is tentatively
   recommended:

      1. User system inserts the TELNET "Interrupt Process" (IP) signal
         in the TELNET stream.

      2. User system sends the TELNET "Synch" signal

      3. User system inserts the command (e.g., ABOR) in the TELNET
         stream.

      4. Server PI,, after receiving "IP", scans the TELNET stream for
         EXACTLY ONE FTP command.

   (For other servers this may not be necessary but the actions listed
   above should have no unusual effect.)






                                   21

                                                  File Transfer Protocol
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   ACCESS CONTROL COMMANDS

   The following commands specify access control identifiers (command
   codes are shown in parentheses).

      USER NAME (USER)

         The argument field is a TELNET string identifying the user.
         The user identification is that which is required by the server
         for access to its file system.  This command will normally be
         the first command transmitted by the user after the 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 access control and/or accounting
         information.  This has the effect of flushing any user,
         password, and account information already supplied and
         beginning the login sequence again.  All transfer parameters
         are unchanged and any file transfer in progress is completed
         under the old acccount.

      PASSWORD (PASS)

         The argument field is a TELNET string identifying the user's
         password.  This command must be immediately preceded by the
         user name command, and, for some sites, completes the user's
         identification for access control.  Since password information
         is quite sensitive, it is desirable in general to "mask" it or
         suppress typeout.  It appears that the server has no foolproof
         way to achieve this.  It is therefore the responsibility of the
         user-FTP process to hide the sensitive password information.

      ACCOUNT (ACCT)

         The argument field is a TELNET string identifying the user's
         account.  The command is not necessarily related to the USER
         command, as some sites may require an account for login and
         others only for specific access, such as storing files.  In the
         latter case the command may arrive at any time.  There are two
         reply codes to differentiate these cases for the automaton:
         when account information is required for login, the response to
         a successful PASSword command is reply code 331; then if a
         command other than ACCounT is sent, the server may remember it
         and return a 331 reply, prepared to act on the command after
         the account information is received; or he may flush the
         command and return a 433 reply asking for the account.  On the
         other hand, if account information is NOT required for login,
         the reply to a successful PASSword command is 230; and if the


                                   22

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


         information is needed for a command issued later in the
         dialogue, the server should return a 331 or 433 reply depending
         on whether he stores (pending receipt of the ACCounT command)
         or discards the command, respectively.

      REINITIALIZE (REIN)

         This command terminates a USER, flushing all I/O and account
         information, except to allow any transfer in progress to be
         completed.  All parameters are reset to the default settings
         and the TELNET connection is left open.  This is identical to
         the state in which a user finds himself immediately after the
         ICP is completed and the TELNET connections are opened.  A USER
         command may be expected to follow.

      LOGOUT (BYE)

         This command terminates a USER and if file transfer is not in
         progress, the server closes the TELNET connection.  If file
         transfer is in progress, the connection will remain open for
         result response and the server will then close it.  If the
         user-process is transferring files for several USERs but does
         not wish to close and then reopen connections for each, then
         the REIN command should be used instead of BYE.

         An unexpected close on the TELNET connection will cause the
         server to take the effective action of an abort (ABOR) and a
         logout (BYE).

   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 a decimal integer (1 through 255) specifying
         the byte size for the data connection.  The default byte size
         is 8 bits.  A server may reject certain byte sizes that he has
         not implemented.





                                   23

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


      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 for transfer from the "active" DTP to the "passive" DTP and
         one for "passive" to "active".  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).  The server has
         fixed data sockets (S+2) and (S+3) as well, and under normal
         circimstances this command and its reply are not needed.

      PASSIVE (PASV)

         This command requests the server-DTP to "listen" on both of his
         data sockets and to wait for an RFC to arrive for one socket
         rather than initiate one upon receipt of a transfer command.
         It is assumed the server has already received a SOCK command to
         indicate the foreign socket from which the RFC will arrive to
         ensure the security of the transfer.

      REPRESENTATION TYPE (TYPE)

         The argument specifies the representation type as described in
         the Section on Data Representation and Storage.  Several types
         take a second parameter.  The first parameter is denoted by a
         single TELNET character, as is the second Format parameter for
         ASCII and EBCDIC; the second parameter for local byte is a
         decimal integer to indicate Bytesize.  The parameters are
         separated by a <SP> (Space, ASCII code 32.).  The following
         codes are assigned for type:


                      \    /
            A - ASCII !    ! N - Non-print
                      !-><-! T - TELNET format effectors
            E - EBCDIC!    ! C - Carriage Control (ASA)
                      /    \
            I - Image

            L # - Local byte Bytesize



         The default representation type is ASCII Non-print.  If the
         Format parameter is changed, and later just the first argument
         is changed, Format then returns to the Non-print default.


                                   24

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


      FILE STRUCTURE (STRU)

         The argument is a single TELNET character code specifying file
         structure described in the Section on Data Representation and
         Storage.  The following codes are assigned for structure:

            F - File (no record structure)
            R - Record structure

         The default structure is File  (i.e., no records).

      TRANSFER MODE (MODE)

         The argument is a single TELNET character code specifying the
         data transfer modes described in the Section on Transmission
         Modes.  The following codes are assigned for transfer modes:

            S - Stream
            B - Block
            C - Compressed

         The default transfer mode is Stream.

   FTP SERVICE COMMANDS

   The FTP service commands define the file transfer or the file system
   function requested by the user.  The argument of an FTP service
   command will normally be a pathname.  The syntax of pathnames must
   conform to server site conventions (with standard defaults
   applicable), and the language conventions of the TELNET connection.
   The suggested default handling is to use the last specified device,
   directory or file name, or the standard default defined for local
   users.  The commands may be in any order except that a "rename from"
   command must be followed by a "rename to" command and the restart
   command must be followed by the interrupted service command.  The
   data, when transferred in response to FTP service commands, shall
   always be sent over the data connection, except for certain
   informative replies.  The following commands specify FTP service
   requests:

      RETRIEVE (RETR)

         This command causes the server-DTP to transfer a copy of the
         file, specified in the pathname, to the server- or user-DTP at
         the other end of the data connection.  The status and contents
         of the file at the server site shall be unaffected.





                                   25

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


      STORE (STOR)

         This command causes the server-DTP to accept the data
         transferred via the data connection and to store the data as a
         file at the server site.  If the file specified in the pathname
         exists at the server site then its contents shall be replaced
         by the data being transferred.  A new file is created at the
         server site if the file specified in the pathname does not
         already exist.

      APPEND (with create) (APPE)

         This command causes the server-DTP to accept the data
         transferred via the data connection and to store the data in a
         file at the server site.  If the file specified in the pathname
         exists at the server site, then the data shall be appended to
         that file; otherwise the file specified in the pathname shall
         be created at the server site.

      ALLOCATE (ALLO)

         This command may be required by some servers to reserve
         sufficient storage to accommodate the new file to be
         transferred.  The argument shall be a decimal integer
         representing the number of bytes (using the logical byte size)
         of storage to be reserved for the file.  For files sent with
         record structure a maximum record size (in logical bytes) might
         also be necessary; this is indicated by a decimal integer in a
         second argument field of the command.  This second argument is
         optional, but when present should be separated from the first
         by the three TELNET characters <SP> R <SP>.  This command shall
         be followed by a STORe or APPEnd command.  The ALLO command
         should be treated as a NOOP (no operation) by those servers
         which do not require that the maximum size of the file be
         declared beforehand, and those servers interested in only the
         maximum record size should accept a dummy value in the first
         argument and ignore it.

      RESTART (REST)

         The argument field represents the server marker at which file
         transfer is to be restarted.  This command does not cause file
         transfer but "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.





                                   26

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


      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 may require "special action", as discussed in the
         Section on FTP Commands, to force recognition by the server.
         No action is to be taken if the previous command has been
         completed (including data transfer).  The TELNET connections
         are not to be closed by the server, but the data connection
         must be closed.  An appropriate reply should be sent by the
         server in all cases.

      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 the server to the
         passive DTP.  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 the server should send
         current information on the file.  A null argument implies the
         user's current working or default directory.  The data transfer
         is over the data connection in type ASCII or type EBCDIC.  (The
         user must ensure that the TYPE is appropriately ASCII or
         EBCDIC).

      NAME-LIST (NLST)

         This command causes a directory listing to be sent from server
         to user site.  The pathname should specify a directory or other
         system-specific file group descriptor; a null argument implies
         the current directory.  The server will return a stream of


                                   27

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


         names of files and no other information.  The data will be
         transferred in ASCII or EBCDIC type over the data connection as
         valid pathname strings separated by <CRLF> or <NL>.  (Again the
         user must ensure that the TYPE is correct.)

      SITE PARAMETERS (SITE)

         This command is used by the server to provide services specific
         to his system that are essential to file transfer but not
         sufficiently universal to be included as commands in the
         protocol.  The nature of these services and the specification
         of their syntax can be stated in a reply to the HELP SITE
         command.

      STATUS (STAT)

         This command shall cause a status response to be sent over the
         TELNET connection in the form of a reply.  The command may be
         sent during a file transfer (along with the TELNET IP and Synch
         signals--see the Section on FTP Commands) in which case the
         server will respond with the status of the operation in
         progress, or it may be sent between file transfers.  In the
         latter case the command may have an argument field.  If the
         argument is a pathname, the command is analogous to the "list"
         command except that data shall be trasferred over 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 Oxx, general system status.  It is
         suggested that HELP be allowed before entering a USER command.
         The server may use this reply to specify site-dependent
         parameters, e.g., in response to HELP SITE.

      NOOP (NOOP)

         This command does not affect any parameters or previously
         entered commands. It specifies no action other than that the
         server send a 200 reply.



                                   28

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


   MISCELLANEOUS COMMANDS

   There are several functions that utilize the services of file
   transfer but go beyond it in scope.  These are the Mail and Remote
   Job Entry functions.  It is suggested that these become auxiliary
   protocols that can assume recognition of file transfer commands on
   the part of the server, i.e., they may depend on the core of FTP
   commands.  The command sets specific to Mail and RJE will be given in
   separate documents.

   Commands that are closely related to file transfer but not proven
   essential to the protocol may be implemented by servers on an
   experimental basis.  The command name should begin with an X and may
   be listed in the HELP command.  The official command set is
   expandable from these experiments; all experimental commands or
   proposals for expanding the official command set should be announced
   via RFC.  An example of a current experimental command is:

      Change Working Directory (XCWD)

         This command allows the user to work with a different directory
         or dataset for file storage or retrieval without altering his
         login or accounting information.  Transfer parameters are
         similarly unchanged.  The argument is a pathname specifying a
         directory or other system dependent file group designator.

   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
   leading three-digit numeric code followed immediately by the
   character "-" (Hyphen, ASCII code 45), 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.  For
   example:

      100-First Line
      Continuation Line
      Another Line
      100 Last Line

   It is possible to nest (but not overlap) a reply withiin a multi-line



                                   29

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


   reply.  The same format for matched number-coded first and last lines
   holds.

   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 interpreted as follows:

      1. The first digit specifies type of response as indicated below:

         0xx  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
              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 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.

      2. The second digit specifies the general category to which the
         response refers:

         x00-x29  General purpose replies, not assignable to other
              categories.

         x3x  Primary access.  Informative replies to the "log-on"
              attempt.

         x4x  Secondary access.  The primary server is commenting on its
              ability to access a secondary service.

         x5x  FTP results.

         x6x  RJE results.



                                   30

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


         x7x  Mail Portocol results.

         x8x-x9x  Reserved for future expansion.

      3. 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 delimited by a numeric code and the TELNET EOL (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  Announcing FTP.
   010  Message from system operator.
   020  Exected delay.
   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.
   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.
   257  Closing the data connection, transfer completed.
   300  Connection greeting message, awaiting input.
   301  Current command incomplete (no <CRLF> for long time).
   330  Enter password.


                                   31

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


   331  Enter account (if account required as part of login sequence).
   332  Login first, please.
   400  This service not implemented.
   401  This service not accepting users now, goodbye.
   402  Command not implemented for requested value or action.
   430  Log-on time or tries exceeded, goodbye.
   431  Log-on unsuccessful.  User and/or password invalid.
   432  User not valid for this service.
   433  Cannot transfer files without valid account.  Enter account and
        resend command.
   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.
   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  Last command not implemented by the server.
   507  Catchall error reply.
   550  Bad pathname specification (e.g., syntax error).





















                                   32

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


DECLARATIVE SPECIFICATIONS

   MINIMUM IMPLEMENTATION

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


      TYPE - ASCII Non-print
      MODE - Stream
      STRUCTURE - File
                  Record
      BYTE - 8
      COMMANDS - USER, BYE, SOCK,
                 TYPE, BYTE, MODE, STRU,
                   for the default values
                 RETR, STOR,
                 NOOP.


   The initial default values for transfer parameters are:


      TYPE - ASCII Non-print
      BYTE - 8
      MODE - Stream
      STRU - File


   All Hosts must accept the above as the standard defaults.

   CONNECTIONS

   The server protocol interpreter shall "listen" on Socket 3.  The user
   or user protocol interpreter shall initiate the full-duplex TELNET
   connections performing the ARPANET standard initial connection
   protocol (ICP) to server Socket 3.  Server- and user- processes
   should follow the conventions of the TELNET protocol as specified in
   NIC #7104.  Servers are under no obligation to provide for editing of
   command lines and may specify that it be done in the user Host.  The
   TELNET connections shall be closed by the server at the user's
   request after all transfers and replies are completed.

   The user-DTP must "listen" on the specified data sockets (send and/or
   receive); these may be the default user sockets (U+4) and (U+5) or a
   socket specified in the SOCK command.  The server shall initiate the
   data connection from his own fixed sockets (S+2) and (S+3) using the
   specified user data socket and byte size (default - 8 bits).  The



                                   33

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


   direction of the transfer and the sockets used will be determined by
   the FTP service command.

   When data is to be transferred between two servers, A and B (refer to
   Figure 2), the user-PI, C, sets up TELNET connections with both
   server-PI's.  He then sends A's fixed sockets, S(A), to B in a SOCK
   command and B's to A; replies are returned.  One of the servers, say
   A, is then sent a PASV command telling him to "listen" on his data
   sockets rather than initiate an RFC when he receives a transfer
   service command.  When the user-PI receives an acknowledgment to the
   PASV command, he may send (in either order) the corresponding service
   commands to A and B.  Server B initiates the RFC and the transfer
   proceeds.  The command-reply sequence is listed below where the
   messages are vertically synchronous but horizontally asynchronous:

      User-PI - Server A                User-PI - Server B
      ------------------                ------------------

      C->A : ICP                        C->B : ICP
      C->A : SOCK HOST-B, SKT-S(B)      C->B : SOCK HOST-A, SKT-S(A)
      A->C : 200 Okay                   B->C : 200 Okay
      C->A : PASV
      A->C : 200 Okay
      C->A : STOR                       C->B : RETR


   The data connection shall be closed by the server under the
   conditions described in the Section on Establishing Data Connections.
   If the server wishes to close the connection after a transfer where
   it is not required, he should do so immediately after the file
   transfer is completed.  He should not wait until after a new transfer
   command is received because the user-process will have already tested
   the data connection to see if it needs to do a "listen"; (recall that
   the user must "listen" on a closed data socket BEFORE sending the
   transfer request).  To prevent a race condition here, the server
   sends a secondary reply (257) after closing the data connection (or
   if the connection is left open, a "file transfer completed" reply
   (252) and the user-PI should wait for one of these replies before
   issuing a new transfer command.

   COMMANDS

   The commands are TELNET character string transmitted over the TELNET
   connections as described in the Section on FTP Commands.  The command
   functions and semantics are described in the Section on Access
   Control Commands, Transfer Parameter Commands, FTP Service Commands,
   and Miscellaneous Commands.  The command syntax is specified here.

   The commands begin with a command code followed by an argument field.


                                   34

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


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

      RETR    Retr    retr    ReTr    rETr

   This also applies to any symbols representing parameter values, such
   as A or a for ASCII TYPE.  The command codes and the argument fields
   are separated by one or more spaces.

   The argument field consists of a variable length character string
   ending with the character sequence <CRLF> (Carriage Return, Linefeed)
   for NVT-ASCII representation; for other negotiated languages a
   different end of line character might be used.  It should be noted
   that the server is to take NO action until the end of line code is
   received.

   The syntax is specified below in NVT-ASCII.  All characters in the
   argument field are ASCII characters including any ASCII represented
   decimal integers.  Square brackets denote an optional argument field.
   If the option is not taken, the appropriate default is implied.

   The following are all the currently defined FTP commmands:

      USER <SP> <username> <CRLF>
      PASS <SP> <password> <CRLF>
      ACCT <SP> <acctno> <CRLF>
      REIN <CRLF>
      BYE <CRLF>
      BYTE <SP> <byte size> <CRLF>
      SOCK <SP> <Host-socket> <CRLF>
      PASV <CRLF>
      TYPE <SP> <type code> <CRLF>
      STRU <SP> <structure code> <CRLF>
      MODE <SP> <mode code> <CRLF>
      RETR <SP> <pathname> <CRLF>
      STOR <SP> <pathname> <CRLF>
      APPE <SP> <pathname> <CRLF>
      ALLO <SP> <decimal integer> [<SP> R <SP> <decimal integer>] <CRLF>
      REST <SP> <marker> <CRLF>
      RNFR <SP> <pathname> <CRLF>
      RNTO <SP> <pathname> <CRLF>
      ABOR <CRLF>
      DELE <SP> <pathname> <CRLF>
      LIST [<SP> <pathname>] <CRLF>
      NLST  [<SP> <pathname>] <CRLF>
      SITE <SP> <string> <CRLF>
      STAT [<SP> <pathname>] <CRLF>
      HELP [<SP> <string>] <CRLF>


                                   35

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


      NOOP <CRLF>

   The syntax of the above argument fields (using BNF notation where
   applicable ) is:

      <username> ::= <string>
      <password> ::= <string>
      <acctno> ::= <string>
      <string> ::= <char>|<char><string>
      <char> ::= any of the 128 ASCII characters except <CR> and <LF>
      <marker> ::= <pr string>
      <pr string> ::= <pr char>|<pr char><pr string>
      <pr char> ::= any ASCII code 33. through 126., printable
         characters
      <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
      <form code> ::= N|T|C
      <type code> ::= A[<SP> <form code>]|E [SP> <form code>]|I|
      L <SP> <byte size>
      <structure code> ::= F|R
      <mode code> ::= S|B|C
      <pathname> ::= <string>

   SEQUENCING OF COMMANDS AND REPLIES

   The communication between the user and server is intended to be an
   alternating dialogue.  As such, the user issues an FTP command and
   the server responds with a prompt primary reply.  The user should
   wait for this initial primary success or failure response before
   sending further commands.

   Certain commands require a second reply for which the user should
   also wait.  These replies may, for example, report on the progress or
   completion of file transfer or the closing of the data connection.
   They are secondary replies to file transfer commands.

   The third class of replies are informational and spontaneous replies
   which may arrive at any time.  The user-PI should be prepared to
   receive them.  These replies are listed below as sponteneous.

   One important group of spontaneous replies is the connection
   greetings.  Under normal circumstances, a server will send a 300
   reply, "awaiting input", when the ICP is completed.  The user should
   wait for this greeting message before sending any commands.  If the
   server is unable to accept input right away, he should send a 000
   "announcing FTP" or a 020 "expected delay" reply immediately and a



                                   36

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


   300 reply when ready.  The user will then know not to hang up if
   there is a delay.

   The table below lists alternative success and failure replies for
   each command.  These must be strictly adhered to; a server may
   substitute text in the replies, but the meaning and action implied by
   the code numbers and by the specific command reply sequence cannot be
   altered.

                   COMMAND-REPLY CORRESPONDENCE TABLE

   COMMAND             SUCCESS         FAILURE

   USER                230,330         430-432,500-505,507
   PASS                230,330         430-432,500-507
   ACCT                230             430-432,500-507
   REIN                232,233         401,436,500-507
      Secondary Reply  300
   BYE                 231,232         500-505,507
   BYTE                200,331         402,500-505,507
   SOCK                200,331         500-505,507
   PASV                200,331         500-507
   TYPE                200,331         402,500-505,507
   STRU                200,331         500-505,507
   MODE                200,331         402,500-505,507
   RETR                250             402,433,450,451,454,455,457,
                                         500-505,507,550
      Secondary Reply  252,257         452
   STOR                250             402,433,451,454,455,457,
                                         500-505,507,550
      Secondary Reply  252,257         452,453
   APPE                250             402,433,451,454,455,457,500-507,
                                         550
      Secondary Reply  252,257         452,453
   ALLO                200,331         402,500-507
   REST                200,331         500-507
   RNFR                200             402,433,450,451,455,500-507,550
   RNTO                253             402,433,450,451,455,456,500-507,
                                         550
   ABOR                201,202,331     500-507
   DELE                254             402,433,450,451,455,500-507,550
   LIST                250             402,433,450,451,454,455,457,
                                         500-507,550
      Secondary Reply  252,257         452
   NLST                250             402,433,450,451,454,455,457,
                                         500-507,550
      Secondary Reply  252,257         452
   SITE                200,331         402,500-507


                                   37

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


   STAT                100,110,        450,451,455,500-507,550
                         150,151,331
   HELP                030,050         500-507
   NOOP                200             500-505,507
   Spontaneous Replies 000,010,020,    400,401,434-436
                         300,301,251,255

   TYPICAL FTP SCENARIOS

   TIP User wanting to transfer file from Host X to local printer:

      1. TIP user opens TELNET connections by ICP to Host X socket 3.

      2. The following commands and replies are exchanged:

         TIP                       HOST X

         <---------- 300 Awaiting input <CRLF>
         USER username <CRLF> ---------->
         <---------- 330 Enter Password <CRLF>
         PASS password <CRLF> ---------->
         <---------- 230 User logged in <CRLF>
         SOCK 65538 <CRLF> ---------->
         <---------- 200 Commmand received OK<CRLF>
         RETR this.file <CRLF> ---------->
            (Host X initiates data connection to TIP socket 65538,
             i.e., PORT 1 receive)
         <---------- 250 File transfer started <CRLF>
         <---------- 252 File transfer completed <CRLF>
         BYE<CRLF>         ---------->
         <---------- 231 User logged out <CRLF>

      3. Host X closes the TELNET and data connections.

         Note: The TIP user should be in line mode.

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

   In general the user will communicate to the server via a mediating
   user-FTP process.  The following may be a typical scenario.  The
   user-FTP prompts are shown in parentheses, '---->' represents
   commands from Host U to Host S, and '<----' represents replies from
   Host S to Host U.







                                   38

                                                  File Transfer Protocol
                                                         (Aug. 12, 1973)
                                                       RFC 542 NIC 17759


      LOCAL COMMANDS BY USER              ACTION INVOLVED

      ftp (host) multics<CR>         ICP to Host S, socket 3,
                                     establishing TELNET connections
                                     <---- 330 Awaiting input <CRLF>
      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) testp11<CR>     RETR test.p11<CRLF> ---->
                                     Server makes data connection to
      (U+4)
                                     <---- 250 File transfer starts
      <CRLF>
                                     <---- 252 File transfer
      complete<CRLF>
      type Image<CR>                 TYPE I<CRLF> ---->
                                     <---- 200 Command OK<CRLF>
      byte 36<CR>                    BYTE 36<CR>LF ---->
                                     <---- 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                      BYE <CRLF> ---->
                                     Server closes all connections.






















                                   39



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