RFC765

From RFC-Wiki


IEN 149 J. Postel

                                                           June 1980
                     FILE TRANSFER PROTOCOL


INTRODUCTION

The objectives of FTP are 1) to promote sharing of files (computer programs and/or data), 2) to encourage indirect or implicit (via programs) use of remote computers, 3) to shield a user from variations in file storage systems among Hosts, and 4) to transfer data reliably and efficiently. FTP, though usable directly by a user at a terminal, is designed mainly for use by programs.

The attempt in this specification is to satisfy the diverse needs of users of maxi-Hosts, mini-Hosts, and TIPs, with a simple, and easily implemented protocol design.

This paper assumes knowledge of the following protocols described in the ARPA Internet Protocol Handbook.

  The Transmission Control 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. Some of the terminology is very specific to the FTP model; some readers may wish to turn to the section on the FTP model while reviewing the terminology.

TERMINOLOGY

  ASCII
     The ASCII character set as defined in the ARPA Internet
     Protocol Handbook.  In FTP, ASCII characters are defined to be
     the lower half of an eight-bit code set (i.e., the most
     significant bit is zero).
  access controls
     Access controls define users' access privileges to the use of a
     system, and to the files in that system.  Access controls are
     necessary to prevent unauthorized or accidental use of files.
     It is the prerogative of a server-FTP process to invoke access
     controls.



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June 1980 IEN 149 File Transfer Protocol RFC 765


  byte size
     There are two byte sizes of interest in FTP:  the logical byte
     size of the file, and the transfer byte size used for the
     transmission of the data.  The transfer byte size is always 8
     bits.  The transfer byte size is not necessarily the byte size
     in which data is to be stored in a system, nor the logical byte
     size for interpretation of the structure of the data.
  data connection
     A simplex connection over which data is transferred, in a
     specified mode and type. The data transferred may be a part of
     a file, an entire file or a number of files.  The path may be
     between a server-DTP and a user-DTP, or between two
     server-DTPs.
  data port
     The passive data transfer process "listens" on the data port
     for a connection from the active transfer process in order to
     open the data connection.
  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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IEN 149 June 1980


  file
     An ordered set of computer data (including programs), of
     arbitrary length, uniquely identified by a pathname.
  mode
     The mode in which data is to be transferred via the data
     connection. The mode defines the data format during transfer
     including EOR and EOF.  The transfer modes defined in FTP are
     described in the Section on Transmission Modes.
  NVT
     The Network Virtual Terminal as defined in the TELNET Protocol.
  NVFS
     The Network Virtual File System.  A concept which defines a
     standard network file system with standard commands and
     pathname conventions.  FTP only partially implements the NVFS
     concept at this time.
  page
     A file may be structured as a set of independent parts called
     pages.  FTP supports the transmission of discontinuous files as
     independent indexed pages.
  pathname
     Pathname is defined to be the character string which must be
     input to a file system by a user in order to identify a file.
     Pathname normally contains device and/or directory names, and
     file name specification.  FTP does not yet specify a standard
     pathname convention.  Each user must follow the file naming
     conventions of the file systems involved in the transfer.
  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.




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June 1980 IEN 149 File Transfer Protocol RFC 765


  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.
  server-DTP
     The data transfer process, in its normal "active" state,
     establishes the data connection with the "listening" data port,
     sets up parameters for transfer and storage, and transfers data
     on command from its PI.  The DTP can be placed in a "passive"
     state to listen for, rather than initiate a, connection on the
     data port.
  server-FTP process
     A process or set of processes which perform the function of
     file transfer in cooperation with a user-FTP process and,
     possibly, another server.  The functions consist of a protocol
     interpreter (PI) and a data transfer process (DTP).
  server-PI
     The protocol interpreter "listens" on Port L for a connection
     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, operating according to the TELNET Protocol.
  type
     The data representation type used for data transfer and
     storage.  Type implies certain transformations between the time
     of data storage and data transfer.  The representation types
     defined in FTP are described in the Section on Establishing
     Data Connections.




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IEN 149 June 1980


  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.
  user-DTP
     The data transfer process "listens" on the data port for a
     connection from a server-FTP process.  If two servers are
     transferring data between them, the user-DTP is inactive.
  user-FTP process
     A set of functions including a protocol interpreter, a data
     transfer process and a user interface which together perform
     the function of file transfer in cooperation with one or more
     server-FTP processes.  The user interface allows a local
     language to be used in the command-reply dialogue with the
     user.
  user-PI
     The protocol interpreter initiates the TELNET connection from
     its port U to the server-FTP process, initiates FTP commands,
     and governs the user-DTP if that process is part of the file
     transfer.











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June 1980 IEN 149 File Transfer Protocol RFC 765


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  ||   FTP Replies  ||    PI   ||
              |\--:---/|                |\----:----/|
              |   V    |                |     V     |
  --------    |/------\|      Data      |/---------\|    --------
  | File |<--->|Server|<---------------->|  User   |<--->| File |
  |System|    || DTP  ||   Connection   ||   DTP   ||    |System|
  --------    |\------/|                |\---------/|    --------
              ----------                -------------
              Server-FTP                   User-FTP
  NOTES: 1. The data connection may be used in either direction.
         2. The data connection need not exist all of the time.
                  Figure 1  Model for FTP Use
  In the model described in Figure 1, the user-protocol interpreter
  initiates the TELNET connection. At the initiation of the user,
  standard FTP commands are generated by the user-PI and transmitted
  to the server process via the TELNET connection.  (The user may
  establish a direct TELNET connection to the server-FTP, from a TIP
  terminal for example, and generate standard FTP commands himself,
  bypassing the user-FTP process.) Standard replies are sent from
  the server-PI to the user-PI over the TELNET connection in
  response to the commands.
  The FTP commands specify the parameters for the data connection
  (data port, transfer mode, representation type, and structure) and
  the nature of file system operation (store, retrieve, append,
  delete, etc.).  The user-DTP or its designate should "listen" on
  the specified data port, and the server initiate the data
  connection and data transfer in accordance with the specified
  parameters.  It should be noted that the data port need not be in



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IEN 149 June 1980


  the same Host that initiates the FTP commands via the TELNET
  connection, but the user or his user-FTP process must ensure a
  "listen" on the specified data port.  It should also be noted that
  the data connection may be used for simultaneous sending and
  receiving.
  In another situation a user might wish to transfer files between
  two Hosts, neither of which is 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 the 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"     |
        --------------  Port (A)     Port (B) --------------
  
                             Figure 2
  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. 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 MODE command which specify how the bits of the data are to be transmitted, and the STRUcture and TYPE commands, which are used to define the way in which the data are to be represented. The transmission and representation are basically independent but



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"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 different
  systems.  PDP-10's generally store NVT-ASCII as five 7-bit ASCII
  characters, left-justified in a 36-bit word. 360's store NVT-ASCII
  as 8-bit EBCDIC codes. Multics stores NVT-ASCII as four 9-bit
  characters in a 36-bit word.  It may be desirable to convert
  characters into the standard NVT-ASCII representation when
  transmitting text between dissimilar systems.  The sending and
  receiving sites would have to perform the necessary
  transformations between the standard representation and their
  internal representations.
  A different problem in representation arises when transmitting
  binary data (not character codes) between Host systems with
  different word lengths.  It is not always clear how the sender
  should send data, and the receiver store it.  For example, when
  transmitting 32-bit bytes from a 32-bit word-length system to a
  36-bit word-length system, it may be desirable (for reasons of
  efficiency and usefulness) to store the 32-bit bytes
  right-justified in a 36-bit word in the latter 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
  performed by the user directly.
  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, called the "transfer byte size", and the
  two should not be confused.  For example, NVT-ASCII has a logical
  byte size of 8 bits.  If the type is Local byte, then the TYPE
  command has an obligatory second parameter specifying the logical
  byte size.  The transfer byte size is always 8 bits.



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IEN 149 June 1980


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




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  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 RFC 740 Appendix C and Communications of
        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.





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IEN 149 June 1980


        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 the 8-bit transfer bytes.  The receiving
        site must store the data as contiguous bits.  The structure
        of the storage system might necessitate the padding of the
        file (or of each record, for a record-structured file) to
        some convenient boundary (byte, word or block).  This
        padding, which must be all zeros, may occur only at the end
        of the file (or at the end of each record) and there must be
        a way of identifying the padding bits so that they may be
        stripped off if the file is retrieved.  The padding
        transformation should be well publicized to enable a user to
        process a file at the storage site.
        Image type is intended for the efficient storage and
        retrieval of files and for the transfer of binary data.  It
        is recommended that this type be accepted by all FTP
        implementations.
     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




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        difference in byte sizes, 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.
        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.
        Another example, a pair of hosts with a 36-bit word size may
        send data to one another in words by using TYPE L 36.  The
        data would be sent in the 8-bit transmission bytes packed so
        that 9 transmission bytes carried two host words.
  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.  Three file structures are
  defined in FTP:
     file-structure, where there is no internal structure and the
                       file is considered to be a continuous
                       sequence of data bytes,
     record-structure, where the file is made up of sequential
                       records,
     and page-structure, where the file is made up of independent
                       indexed pages.
  File-structure is the default, to be assumed if the STRUcture
  command has not been used but both file and record structures must



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IEN 149 June 1980


  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 record structure.  Obviously this transformation should be
  useful but it must also be invertible so that an identical file
  may be retrieved using record structure.
  In the case of a file being sent with file-structure to a
  record-oriented Host, there exists the question of what criteria
  the Host should use to divide the file into records which can be
  processed locally.  If this division is necessary the FTP
  implementation should use the end-of-line sequence, <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.
  Page Structure
     To transmit files that are discontinuous FTP defines a page
     structure.  Files of this type are sometimes know as "random
     access files" or even as "holey files".  In these files there
     is sometimes other information associated with the file as a
     whole (e.g., a file descriptor), or with a section of the file
     (e.g., page access controls), or both.  In FTP, the sections of
     the file are called pages.
     To provide for various page sizes and associated information
     each page is sent with a page header.  The page header has the
     following defined fields:



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        Header Length
           The number of logical bytes in the page header including
           this byte.  The minimum header length is 4.
        Page Index
           The logical page number of this section of the file.
           This is not the transmission sequence number of this
           page, but the index used to identify this page of the
           file.
        Data Length
           The number of logical bytes in the page data.  The
           minimum data length is 0.
        Page Type
           The type of page this is.  The following page types are
           defined:
              0 = Last Page
                 This is used to indicate the end of a paged
                 structured transmission.  The header length must be
                 4, and the data length must be 0.
              1 = Simple Page
                 This is the normal type for simple paged files with
                 no page level associated control information.  The
                 header length must be 4.
              2 = Descriptor Page
                 This type is used to transmit the descriptive
                 information for the file as a whole.
              3 = Access Controled Page
                 This is type includes an additional header field
                 for paged files with page level access control
                 information.  The header length must be 5.




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IEN 149 June 1980


        Optional Fields
           Further header fields may be used to supply per page
           control information, for example, per page access
           control.
     All fields are one logical byte in length.  The logical byte
     size is specified by the TYPE command.

ESTABLISHING DATA CONNECTIONS

  The mechanics of transferring data consists of setting up the data
  connection to the appropriate ports and choosing the parameters
  for transfer.  Both the user and the server-DTPs have a default
  data port.  The user-process default data port is the same as the
  control connection port, i.e., U.  The server-process default data
  port is the port adjacent to the control connection port, i.e.,
  L-1.
  The transfer byte size is 8-bit bytes.  This byte size is relevant
  only for the actual transfer of the data; it has no bearing on
  representation of the data within a Host's file system.
  The passive data transfer process (this may be a user-DTP or a
  second server-DTP) shall "listen" on the data port prior to
  sending a transfer request command.  The FTP request command
  determines the direction of the data transfer.  The server, upon
  receiving the transfer request, will initiate the data connection
  to the port.  When the connection is established, the data
  transfer begins between DTP's, and the server-PI sends a
  confirming reply to the user-PI.
  It is possible for the user to specify an alternate data port by
  use of the PORT 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.
  One server is then told (by an FTP command) to "listen" for a
  connection which the other will initiate.  The user-PI sends one
  server-PI a PORT command indicating the data port of the other.
  Finally both are sent the appropriate transfer commands.  The
  exact sequence of commands and replies sent between the
  user-controller and the servers is defined in the Section on FTP
  Replies.
  In general it is the server's responsibility to maintain the data
  connection--to initiate it and to close it.  The exception to this



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  is when the user-DTP is sending the data in a transfer mode that
  requires the connection to be closed to indicate EOF.  The server
  MUST close the data connection under the following conditions:
     1. The server has completed sending data in a transfer mode
        that requires a close to indicate EOF.
     2. The server receives an ABORT command from the user.
     3. The port specification is changed by a command from the
        user.
     4. The TELNET connection is 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.
  For files transmitted in page structure a "last-page" page type is
  used.
  NOTE:  In the rest of this section, byte means "transfer byte"
  except where explicitly stated otherwise.
  For the purpose of standardized transfer, the sending Host will
  translate 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



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IEN 149 June 1980


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



                               17


June 1980 IEN 149 File Transfer Protocol RFC 765


        exchanging certain types of data (e.g., seismic or weather
        data) to send and receive all the data despite local errors
        (such as "magnetic tape read errors"), but to indicate in
        the transmission that certain portions are suspect).  Record
        structures are allowed in this mode, and any representation
        type may be used.
        The header consists of the three bytes.  Of the 24 bits of
        header information, the 16 low order bits shall represent
        byte count, and the 8 high order bits shall represent
        descriptor codes as shown below.
        Block Header
           +----------------+----------------+----------------+
           | Descriptor     |    Byte Count                   |
           |         8 bits |                      16 bits    |
           +----------------+----------------+----------------+
           
        The descriptor codes are indicated by bit flags in the
        descriptor byte.  Four codes have been assigned, where each
        code number is the decimal value of the corresponding bit in
        the byte.
           Code     Meaning
           
            128     End of data block is EOR
             64     End of data block is EOF
             32     Suspected errors in data block
             16     Data block is a restart marker


        With this encoding more than one descriptor coded condition
        may exist for a particular block.  As many bits as necessary
        may be flagged.
        The restart marker is embedded in the data stream as an
        integral number of 8-bit bytes representing printable
        characters in the language being used over the TELNET
        connection (e.g., default--NVT-ASCII).  <SP> (Space, in the
        appropriate language) must not be used WITHIN a restart
        marker.




                               18


IEN 149 June 1980


        For example, to transmit a six-character marker, the
        following would be sent:
           +--------+--------+--------+
           |Descrptr|  Byte count     |
           |code= 16|             = 6 |
           +--------+--------+--------+
           
           +--------+--------+--------+
           | Marker | Marker | Marker |
           | 8 bits | 8 bits | 8 bits |
           +--------+--------+--------+
           
           +--------+--------+--------+
           | Marker | Marker | Marker |
           | 8 bits | 8 bits | 8 bits |
           +--------+--------+--------+
           
     COMPRESSED
        There are three kinds of information to be sent:  regular
        data, sent in a byte string; compressed data, consisting of
        replications or filler; and control information, sent in a
        two-byte escape sequence.  If n>0 bytes (up to 127) of
        regular data are sent, these n bytes are preceded by a byte
        with the left-most bit set to 0 and the right-most 7 bits
        containing the number n.
        Byte string:
            1       7                8                     8
           +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
           |0|       n     | |    d(1)       | ... |      d(n)     |
           +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
                                         ^             ^
                                         |---n bytes---|
                                             of data
           String of n data bytes d(1),..., d(n)
           Count n must be positive.
        To compress a string of n replications of the data byte d,
        the following 2 bytes are sent:




                               19


June 1980 IEN 149 File Transfer Protocol RFC 765


        Replicated Byte:
             2       6               8
           +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
           |1 0|     n     | |       d       |
           +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
        A string of n filler bytes can be compressed into a single
        byte, where the filler byte varies with the representation
        type.  If the type is ASCII or EBCDIC the filler byte is
        <SP> (Space, ASCII code 32., EBCDIC code 64).  If the type
        is Image or Local byte the filler is a zero byte.
        Filler String:
             2       6
           +-+-+-+-+-+-+-+-+
           |1 1|     n     |
           +-+-+-+-+-+-+-+-+
        The escape sequence is a double byte, the first of which is
        the escape byte (all zeros) and the second of which contains
        descriptor codes as defined in Block mode.  The descriptor
        codes have the same meaning as in Block mode and apply to
        the succeeding string of bytes.
        Compressed mode is useful for obtaining increased bandwidth
        on very large network transmissions at a little extra CPU
        cost.  It can be most effectively used to reduce the size of
        printer files such as those generated by RJE Hosts.

ERROR RECOVERY AND RESTART

  There is no provision for detecting bits lost or scrambled in data
  transfer; this level of error control is handled by the TCP.
  However, a restart procedure is provided to protect users from
  gross system failures (including failures of a Host, an
  FTP-process, or the underlying network).
  The restart procedure is defined only for the block and compressed
  modes of data transfer.  It requires the sender of data to insert
  a special marker code in the data stream with some marker
  information.  The marker information has meaning only to the
  sender, but must consist of printable characters in the default or
  negotiated language of the TELNET connection (ASCII or EBCDIC).
  The marker could represent a bit-count, a record-count, or any



                               20


IEN 149 June 1980


  other information by which a system may identify a data
  checkpoint.  The receiver of data, if it implements the restart
  procedure, would then mark the corresponding position of this
  marker in the receiving system, and return this information to the
  user.
  In the event of a system failure, the user can restart the data
  transfer by identifying the marker point with the FTP restart
  procedure.  The following example illustrates the use of the
  restart procedure.
  The sender of the data inserts an appropriate marker block in the
  data stream at a convenient point.  The receiving Host marks the
  corresponding data point in its file system and conveys the last
  known sender and receiver marker information to the user, either
  directly or over the TELNET connection in a 110 reply (depending
  on who is the sender).  In the event of a system failure, the user
  or controller process restarts the server at the last server
  marker by sending a restart command with server's marker code as
  its argument.  The restart command is transmitted over the 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 a TCP connection from the user to a standard server port. The user protocol interpreter is responsible for sending FTP commands and interpreting the replies received; the server-PI interprets commands, sends replies and directs its DTP to set up the data connection and transfer the data. If the second party to the data transfer (the passive transfer process) is the user-DTP then it is governed through the internal protocol of the user-FTP Host; if it is a second server-DTP then it is governed by its PI on command from the user-PI. The FTP replies are discussed in the next section. In the description of a few of the commands in this section it is helpful to be explicit about the possible replies.

FTP COMMANDS

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



                               21


June 1980 IEN 149 File Transfer Protocol RFC 765


     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 account.
     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 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
        332.  On the other hand, if account information is NOT
        required for login, the reply to a successful PASSword
        command is 230; and if the account information is needed for
        a command issued later in the dialogue, the server should




                               22


IEN 149 June 1980


        return a 332 or 532 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 TELNET connection is opened.  A USER command may be
        expected to follow.
     LOGOUT (QUIT)
        This command terminates a USER and if file transfer is not
        in progress, the server closes the 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 QUIT.
        An unexpected close on the TELNET connection will cause the
        server to take the effective action of an abort (ABOR) and a
        logout (QUIT).
  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.
     DATA PORT (PORT)
        The argument is a HOST-PORT specification for the data port
        to be used in data connection.  There defaults for both the
        user and server data ports, and under normal circumstances
        this command and its reply are not needed.  If this command



                               23


June 1980 IEN 149 File Transfer Protocol RFC 765


        is used  the argument is the concatenation of a 32-bit
        internet host address and a 16-bit TCP port address.  This
        address information is broken into 8-bit fields and the
        value of each field is transmitted as a decimal number (in
        character string representation).  The fields are separated
        by commas.  A port command would be:
           PORT h1,h2,h3,h4,p1,p2
        where, h1 is the high order 8 bits of the internet host
        address.
     PASSIVE (PASV)
        This command requests the server-DTP to "listen" on a data
        port (which is not its default data port) and to wait for a
        connection rather than initiate one upon receipt of a
        transfer command.  The response to this command includes the
        host and port address this server is listening on.
     REPRESENTATION TYPE (TYPE)
        The argument specifies the representation type as described
        in the Section on Data Representation and Storage.  Several
        types take a second parameter.  The first parameter is
        denoted by a single TELNET character, as is the second
        Format parameter for ASCII and EBCDIC; the second parameter
        for local byte is a decimal integer to indicate Bytesize.
        The parameters are separated by a <SP> (Space, ASCII code
        32.).
        The following codes are assigned for type:
                     \    /
           A - ASCII |    | N - Non-print
                     |-><-| T - TELNET format effectors
           E - EBCDIC|    | C - Carriage Control (ASA)
                     /    \
           I - Image
           
           L <byte size> - Local byte Byte size
        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


IEN 149 June 1980


     FILE STRUCTURE (STRU)
        The argument is a single TELNET character code specifying
        file structure described in the Section on Data
        Representation and Storage.
        The following codes are assigned for structure:
           F - File (no record structure)
           R - Record structure
           P - Page structure
        The default structure is File.
     TRANSFER MODE (MODE)
        The argument is a single TELNET character code specifying
        the data transfer modes described in the Section on
        Transmission Modes.
        The following codes are assigned for transfer modes:
           S - Stream
           B - Block
           C - Compressed
        The default transfer mode is Stream.
  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:



                               25


June 1980 IEN 149 File Transfer Protocol RFC 765


     RETRIEVE (RETR)
        This command causes the server-DTP to transfer a copy of the
        file, specified in the pathname, to the server- or user-DTP
        at the other end of the data connection.  The status and
        contents of the file at the server site shall be unaffected.
     STORE (STOR)
        This command causes the server-DTP to accept the data
        transferred via the data connection and to store the data as
        a file at the server site.  If the file specified in the
        pathname exists at the server site then its contents shall
        be replaced by the data being transferred.  A new file is
        created at the server site if the file specified in the
        pathname does not already exist.
     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.
     MAIL FILE (MLFL)
        The intent of this command is to enable a user at the user
        site to mail data (in form of a file) to another user at the
        server site.  It should be noted that the files to be mailed
        are transmitted via the data connection in ASCII or EBCDIC
        type.  (It is the user's responsibility to ensure that the
        type is correct.)  These files should be inserted into the
        destination user's mailbox by the server in accordance with
        serving Host mail conventions.  The mail may be marked as
        sent from the particular user HOST and the user specified by
        the 'USER' command.  The argument field may contain a Host
        system ident, or it may be empty.  If the argument field is
        empty or blank (one or more spaces), then the mail is
        destined for a printer or other designated place for general
        delivery site mail.





                               26


IEN 149 June 1980


     MAIL (MAIL)
        This command allows a user to send mail that is NOT in a
        file over the TELNET connection.  The argument field may
        contain system ident, or it may be empty.  The ident is
        defined as above for the MLFL command.  After the 'MAIL'
        command is received, the server is to treat the following
        lines as text of the mail sent by the user.  The mail text
        is to be terminated by a line containing only a single
        period, that is, the character sequence "CRLF.CRLF".  It is
        suggested that a modest volume of mail service should be
        free; i.e., it may be entered before a USER command.
     MAIL SEND TO TERMINAL (MSND)
        This command is like the MAIL command, except that the data
        is displayed on the addressed user's terminal, if such
        access is currently allowed, otherwise an error is returned.
     MAIL SEND TO TERMINAL OR MAILBOX (MSOM)
        This command is like the MAIL command, except that the data
        is displayed on the addressed user's terminal, if such
        access is currently allowed, otherwise the data is placed in
        the user's mailbox.
     MAIL SEND TO TERMINAL AND MAILBOX (MSAM)
        This command is like the MAIL command, except that the data
        is displayed on the addressed user's terminal, if such
        access is currently allowed, and, in any case, the data is
        placed in the user's mailbox.
     MAIL RECIPIENT SCHEME QUESTION (MRSQ)
        This FTP command is used to select a scheme for the
        transmission of mail to several users at the same host.  The
        schemes are to list the recipients first, or to send the
        mail first.
     MAIL RECIPIENT (MRCP)
        This command is used to identify the individual recipients
        of the mail in the transmission of mail for multiple users
        at one host.



                               27


June 1980 IEN 149 File Transfer Protocol RFC 765


     ALLOCATE (ALLO)
        This command may be required by some servers to reserve
        sufficient storage to accommodate the new file to be
        transferred.  The argument shall be a decimal integer
        representing the number of bytes (using the logical byte
        size) of storage to be reserved for the file.  For files
        sent with record or page structure a maximum record or page
        size (in logical bytes) might also be necessary; this is
        indicated by a decimal integer in a second argument field of
        the command.  This second argument is optional, but when
        present should be separated from the first by the three
        TELNET characters <SP> R <SP>.  This command shall be
        followed by a STORe or APPEnd command.  The ALLO command
        should be treated as a NOOP (no operation) by those servers
        which do not require that the maximum size of the file be
        declared beforehand, and those servers interested in only
        the maximum record or page size should accept a dummy value
        in the first argument and ignore it.
     RESTART (REST)
        The argument field represents the server marker at which
        file transfer is to be restarted.  This command does not
        cause file transfer but "spaces" over the file to the
        specified data checkpoint.  This command shall be
        immediately followed by the appropriate FTP service command
        which shall cause file transfer to resume.
     RENAME FROM (RNFR)
        This command specifies the file which is to be renamed.
        This command must be immediately followed by a "rename to"
        command specifying the new file pathname.
     RENAME TO (RNTO)
        This command specifies the new pathname of the file
        specified in the immediately preceding "rename from"
        command.  Together the two commands cause a file to be
        renamed.
     ABORT (ABOR)
        This command tells the server to abort the previous FTP
        service command and any associated transfer of data.  The



                               28


IEN 149 June 1980


        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
        connection is not to be closed by the server, but the data
        connection must be closed.
        There are two cases for the server upon receipt of this
        command: (1) the FTP service command was already completed,
        or (2) the FTP service command is still in progress.
           In the first case, the server closes the data connection
           (if it is open) and responds with a 226 reply, indicating
           that the abort command was successfully processed.
           In the second case, the server aborts the FTP service in
           progress and closes the data connection, returning a 426
           reply to indicate that the service request terminated in
           abnormally.  The server then sends a 226 reply,
           indicating that the abort command was successfully
           processed.
     DELETE (DELE)
        This command causes the file specified in the pathname to be
        deleted at the server site.  If an extra level of protection
        is desired (such as the query, "DO you really wish to
        delete?"), it should be provided by the user-FTP process.
     CHANGE WORKING DIRECTORY (CWD)
        This command allows the user to work with a different
        directory or dataset for file storage or retrieval without
        altering his login or accounting information.  Transfer
        parameters are similarly unchanged.  The argument is a
        pathname specifying a directory or other system dependent
        file group designator.
     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



                               29


June 1980 IEN 149 File Transfer Protocol RFC 765


        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 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
        transferred 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.




                               30


IEN 149 June 1980


     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 211 or 214.
        It is suggested that HELP be allowed before entering a USER
        command. The server may use this reply to specify
        site-dependent parameters, e.g., in response to HELP SITE.
     NOOP (NOOP)
        This command does not affect any parameters or previously
        entered commands. It specifies no action other than that the
        server send an OK reply.
  The File Transfer Protocol follows the specifications of the
  TELNET protocol for all communications over the TELNET connection.
  Since, the language used for TELNET communication may be a
  negotiated option, all references in the next two sections will be
  to the "TELNET language" and the corresponding "TELNET end of line
  code".  Currently one may take these to mean NVT-ASCII and <CRLF>.
  No other specifications of the TELNET protocol will be cited.
  FTP commands are "TELNET strings" terminated by the "TELNET end of
  line code".  The command codes themselves are alphabetic
  characters terminated by the character <SP> (Space) if parameters
  follow and TELNET-EOL otherwise.  The command codes and the
  semantics of commands are described in this section; the detailed
  syntax of commands is specified in the Section on Commands, the
  reply sequences are discussed in the Section on Sequencing of
  Commands and Replies, and scenarios illustrating the use of
  commands are provided in the Section on Typical FTP Scenarios.
  FTP commands may be partitioned as those specifying access-control
  identifiers, data transfer parameters, or FTP service requests.
  Certain commands (such as ABOR, STAT, QUIT) may be sent over the
  TELNET connection 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 undefined; but the following ordered format is
  tentatively recommended:




                               31


June 1980 IEN 149 File Transfer Protocol RFC 765


     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.)

FTP REPLIES

  Replies to File Transfer Protocol commands are devised to ensure
  the synchronization of requests and actions in the process of file
  transfer, and to guarantee that the user process always knows the
  state of the Server. Every command must generate at least one
  reply, although there may be more than one; in the latter case,
  the multiple replies must be easily distinguished.  In addition,
  some commands occur in sequential groups, such as USER, PASS and
  ACCT, or RNFR and RNTO.  The replies show the existence of an
  intermediate state if all preceding commands have been successful.
  A failure at any point in the sequence necessitates the repetition
  of the entire sequence from the beginning.
     The details of the command-reply sequence are made explicit in
     a set of state diagrams below.
  An FTP reply consists of a three digit number (transmitted as
  three alphanumeric characters) followed by some text.  The number
  is intended for use by automata to determine what state to enter
  next; the text is intended for the human user.  It is intended
  that the three digits contain enough encoded information that the
  user-process (the User-PI) will not need to examine the text and
  may either discard it or pass it on to the user, as appropriate.
  In particular, the text may be server-dependent, so there are
  likely to be varying texts for each reply code.
  Formally, a reply is defined to contain the 3-digit code, followed
  by Space <SP>, followed by one line of text (where some maximum
  line length has been specified), and terminated by the TELNET
  end-of-line code.  There will be cases, however, where the text is
  longer than a single line.  In these cases the complete text must



                               32


IEN 149 June 1980


  be bracketed so the User-process knows when it may stop reading
  the reply (i.e. stop processing input on the TELNET connection)
  and go do other things.  This requires a special format on the
  first line to indicate that more than one line is coming, and
  another on the last line to designate it as the last.  At least
  one of these must contain the appropriate reply code to indicate
  the state of the transaction.  To satisfy all factions it was
  decided that both the first and last line codes should be the
  same.
     Thus the format for multi-line replies is that the first line
     will begin with the exact required reply code, followed
     immediately by a Hyphen, "-" (also known as Minus), followed by
     text.  The last line will begin with the same code, followed
     immediately by Space <SP>, optionally some text, and the TELNET
     end-of-line code.
        For example:
                            123-First line
                            Second line
                              234 A line beginning with numbers
                            123 The last line
     The user-process then simply needs to search for the second
     occurrence of the same reply code, followed by <SP> (Space), at
     the beginning of a line, and ignore all intermediary lines.  If
     an intermediary line begins with a 3-digit number, the Server
     must pad the front to avoid confusion.
        This scheme allows standard system routines to be used for
        reply information (such as for the STAT reply), with
        "artificial" first and last lines tacked on.  In the rare
        cases where these routines are able to generate three digits
        and a Space at the beginning of any line, the beginning of
        each text line should be offset by some neutral text, like
        Space.
     This scheme assumes that multi-line replies may not be nested.
     We  have found that, in general, nesting of replies will not
     occur, except for random system messages (also called
     spontaneous replies) which may interrupt another reply.  System
     messages (i.e. those not processed by the FTP server) will NOT
     carry reply codes and may occur anywhere in the command-reply
     sequence.  They may be ignored by the User-process as they are
     only information for the human user.



                               33


June 1980 IEN 149 File Transfer Protocol RFC 765


  The three digits of the reply each have a special significance.
  This is intended to allow a range of very simple to very
  sophisticated response by the user-process.  The first digit
  denotes whether the response is good, bad or incomplete.
  (Referring to the state diagram) an unsophisticated user-process
  will be able to determine its next action (proceed as planned,
  redo, retrench, etc.) by simply examining this first digit.  A
  user-process that wants to know approximately what kind of error
  occurred (e.g. file system error, command syntax error) may
  examine the second digit, reserving the third digit for the finest
  gradation of information (e.g. RNTO command without a preceding
  RNFR.)
     There are five values for the first digit of the reply code:
        1yz   Positive Preliminary reply
           The requested action is being initiated; expect another
           reply before proceeding with a new command.  (The
           user-process sending another command before the
           completion reply would be in violation of protocol; but
           server-FTP processes should queue any commands that
           arrive while a preceding command is in progress.)  This
           type of reply can be used to indicate that the command
           was accepted and the user-process may now pay attention
           to the data connections, for implementations where
           simultaneous monitoring is difficult.
        2yz   Positive Completion reply
           The requested action has been successfully completed.  A
           new request may be initiated.
        3yz   Positive Intermediate reply
           The command has been accepted, but the requested action
           is being held in abeyance, pending receipt of further
           information.  The user should send another command
           specifying this information.  This reply is used in
           command sequence groups.
        4yz   Transient Negative Completion reply
           The command was not accepted and the requested action did
           not take place, but the error condition is temporary and
           the action may be requested again.  The user should



                               34


IEN 149 June 1980


           return to the beginning of the command sequence, if any.
           It is difficult to assign a meaning to "transient",
           particularly when two distinct sites (Server and
           User-processes) have to agree on the interpretation.
           Each reply in the 4yz category might have a slightly
           different time value, but the intent is that the
           user-process is encouraged to try again.  A rule of thumb
           in determining if a reply fits into the 4yz or the 5yz
           (Permanent Negative) category is that replies are 4yz if
           the commands can be repeated without any change in
           command form or in properties of the User or Server (e.g.
           the command is spelled the same with the same arguments
           used; the user does not change his file access or user
           name; the server does not put up a new implementation.)
        5yz   Permanent Negative Completion reply
           The command was not accepted and the requested action did
           not take place.  The User-process is discouraged from
           repeating the exact request (in the same sequence).  Even
           some "permanent" error conditions can be corrected, so
           the human user may want to direct his User-process to
           reinitiate the command sequence by direct action at some
           point in the future (e.g. after the spelling has been
           changed, or the user has altered his directory status.)
     The following function groupings are encoded in the second
     digit:
        x0z   Syntax - These replies refer to syntax errors,
              syntactically correct  commands that don't fit any
              functional category, unimplemented or superfluous
              commands.
        x1z   Information -  These are replies to requests for
              information, such as status or help.
        x2z   Connections - Replies referring to the TELNET and data
              connections.
        x3z   Authentication and accounting - Replies for the login
              process and accounting procedures.
        x4z   Unspecified as yet




                               35


June 1980 IEN 149 File Transfer Protocol RFC 765


        x5z   File system - These replies indicate the status of the
              Server file system vis-a-vis the requested transfer or
              other file system action.
     The third digit gives a finer gradation of meaning in each of
     the function categories, specified by the second digit.  The
     list of replies below will illustrate this.  Note that the text
     associated with each reply is recommended, rather than
     mandatory, and may even change according to the command with
     which it is associated.  The reply codes, on the other hand,
     must strictly follow the specifications in the last section;
     that is, Server implementations should not invent new codes for
     situations that are only slightly different from the ones
     described here, but rather should adapt codes already defined.
        A command such as TYPE or ALLO whose successful execution
        does not offer the user-process any new information will
        cause a 200 reply to be returned.  If the command is not
        implemented by a particular Server-FTP process because it
        has no relevance to that computer system, for example ALLO
        at a TOPS20 site, a Positive Completion reply is still
        desired so that the simple User-process knows it can proceed
        with its course of action.  A 202 reply is used in this case
        with, for example, the reply text:  "No storage allocation
        necessary."  If, on the other hand, the command requests a
        non-site-specific action and is unimplemented, the response
        is 502.  A refinement of that is the 504 reply for a command
        that IS implemented, but that requests an unimplemented
        parameter.
  Reply Codes by Function Groups
     200 Command okay
     500 Syntax error, command unrecognized
        [This may include errors such as command line too long.]
     501 Syntax error in parameters or arguments
     202 Command not implemented, superfluous at this site.
     502 Command not implemented
     503 Bad sequence of commands
     504 Command not implemented for that parameter
      
     110 Restart marker reply.





                               36


IEN 149 June 1980


        In this case the text is exact and not left to the
        particular implementation; it must read:
             MARK yyyy = mmmm
        where yyyy is User-process data stream marker, and mmmm
        server's equivalent marker.  (note the spaces between
        markers and "=".)
     119 Terminal not available, will try mailbox.
     211 System status, or system help reply
     212 Directory status
     213 File status
     214 Help message
        (on how to use the server or the meaning of a particular
        non-standard command.  This reply is useful only to the
        human user.)
     215 <scheme> is the preferred scheme.
      
     120 Service ready in nnn minutes
     220 Service ready for new user
     221 Service closing TELNET connection
        (logged out if appropriate)
     421 Service not available, closing TELNET connection.
        This may be a reply to any command if the service knows it
        must shut down.]
     125 Data connection already open; transfer starting
     225 Data connection open; no transfer in progress
     425 Can't open data connection
     226 Closing data connection;
        requested file action successful (for example, file transfer
        or file abort.)
     426 Connection closed; transfer aborted.
     227 Entering Passive Mode.  h1,h2,h3,h4,p1,p2
      
     230 User logged in, proceed
     530 Not logged in
     331 User name okay, need password
     332 Need account for login
     532 Need account for storing files
      
     150 File status okay; about to open data connection.
     151 User not local; Will forward to <user>@<host>.
     152 User Unknown; Mail will be forwarded by the operator.
     250 Requested file action okay, completed.
     350 Requested file action pending further information
     450 Requested file action not taken:
        file unavailable (e.g. file busy)
     550 Requested action not taken:



                               37


June 1980 IEN 149 File Transfer Protocol RFC 765


        file unavailable (e.g. file not found, no access)
     451 Requested action aborted: local error in processing
     551 Requested action aborted: page type unknown
     452 Requested action not taken:
        insufficient storage space in system
     552 Requested file action aborted:
        exceeded storage allocation (for current directory or
        dataset)
     553 Requested action not taken:
        file name not allowed
     354 Start mail input; end with <CR><LF>.<CR><LF>
     
  Numeric Order List of Reply Codes
     110 Restart marker reply.
        In this case the text is exact and not left to the
        particular implementation; it must read:
             MARK yyyy = mmmm
        where yyyy is User-process data stream marker, and mmmm
        server's equivalent marker.  (note the spaces between
        markers and "=".)
     119 Terminal not available, will try mailbox.
     120 Service ready in nnn minutes
     125 Data connection already open; transfer starting
     150 File status okay; about to open data connection.
     151 User not local; Will forward to <user>@<host>.
     152 User Unknown; Mail will be forwarded by the operator.
     200 Command okay
     202 Command not implemented, superfluous at this site.
     211 System status, or system help reply
     212 Directory status
     213 File status
     214 Help message
        (on how to use the server or the meaning of a particular
        non-standard command.  This reply is useful only to the
        human user.)
     215 <scheme> is the preferred scheme.
     220 Service ready for new user
     221 Service closing TELNET connection
        (logged out if appropriate)
     225 Data connection open; no transfer in progress
     226 Closing data connection;
        requested file action successful (for example, file transfer
        or file abort.)
     227 Entering Passive Mode.  h1,h2,h3,h4,p1,p2



                               38


IEN 149 June 1980


     230 User logged in, proceed
     250 Requested file action okay, completed.
     331 User name okay, need password
     332 Need account for login
     350 Requested file action pending further information
     354 Start mail input; end with <CR><LF>.<CR><LF>
     421 Service not available, closing TELNET connection.
        This may be a reply to any command if the service knows it
        must shut down.]
     425 Can't open data connection
     426 Connection closed; transfer aborted.
     450 Requested file action not taken:
        file unavailable (e.g. file busy)
     451 Requested action aborted: local error in processing
     452 Requested action not taken:
        insufficient storage space in system
     500 Syntax error, command unrecognized
        [This may include errors such as command line too long.]
     501 Syntax error in parameters or arguments
     502 Command not implemented
     503 Bad sequence of commands
     504 Command not implemented for that parameter
     530 Not logged in
     532 Need account for storing files
     550 Requested action not taken:
        file unavailable (e.g. file not found, no access)
     551 Requested action aborted: page type unknown
     552 Requested file action aborted:
        exceeded storage allocation (for current directory or
        dataset)
     553 Requested action not taken:
        file name not allowed
     









                               39


June 1980 IEN 149 File Transfer Protocol RFC 765


DECLARATIVE SPECIFICATIONS

MINIMUM IMPLEMENTATION

  In order to make FTP workable without needless error messages, the
  following minimum implementation is required for all servers:
     TYPE - ASCII Non-print
     MODE - Stream
     STRUCTURE - File, Record
     COMMANDS - USER, QUIT, PORT,
                TYPE, MODE, STRU,
                  for the default values
                RETR, STOR,
                NOOP.
  The default values for transfer parameters are:


     TYPE - ASCII Non-print
     MODE - Stream
     STRU - File
  All Hosts must accept the above as the standard defaults.

CONNECTIONS

  The server protocol interpreter shall "listen" on Port L.  The
  user or user protocol interpreter shall initiate the full-duplex
  TELNET connection.  Server- and user- processes should follow the
  conventions of the TELNET protocol as specified in the ARPA
  Internet Protocol Handbook.  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 connection shall be closed by
  the server at the user's request after all transfers and replies
  are completed.
  The user-DTP must "listen" on the specified data port; this may be
  the default user port (U) or a port specified in the PORT command.
  The server shall initiate the data connection from his own default
  data port (L-1) using the specified user data port.  The direction
  of the transfer and the port used will be determined by the FTP
  service command.




                               40


IEN 149 June 1980


  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.  One of the servers, say A, is then sent a PASV
  command telling him to "listen" on his data port rather than
  initiate a connection when he receives a transfer service command.
  When the user-PI receives an acknowledgment to the PASV command,
  which includes the identity of the host and port being listened
  on, the user-PI then sends A's port, a, to B in a PORT command; a
  reply is returned.  The user-PI may then send the corresponding
  service commands to A and B.  Server B initiates the connection
  and the transfer proceeds.  The command-reply sequence is listed
  below where the messages are vertically synchronous but
  horizontally asynchronous:
     User-PI - Server A                User-PI - Server B
     ------------------                ------------------
     
     C->A : Connect                    C->B : Connect
     C->A : PASV
     A->C : 227 Entering Passive Mode. A1,A2,A3,A4,a1,a2
                                       C->B : PORT A1,A2,A3,A4,a1,a2
                                       B->C : 200 Okay
     C->A : STOR                       C->B : RETR
                B->A : Connect to HOST-A, PORT-a
  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 port BEFORE sending the transfer request).  To prevent a race
  condition here, the server sends a reply (226) after closing the
  data connection (or if the connection is left open, a "file
  transfer completed" reply (250) and the user-PI should wait for
  one of these replies before issuing a new transfer command.







                               41


June 1980 IEN 149 File Transfer Protocol RFC 765


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.  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.









                               42


IEN 149 June 1980


  The following are the FTP commands:
     USER <SP> <username> <CRLF>
     PASS <SP> <password> <CRLF>
     ACCT <SP> <account information> <CRLF>
     REIN <CRLF>
     QUIT <CRLF>
     PORT <SP> <Host-port> <CRLF>
     PASV <CRLF>
     TYPE <SP> <type code> <CRLF>
     STRU <SP> <structure code> <CRLF>
     MODE <SP> <mode code> <CRLF>
     RETR <SP> <pathname> <CRLF>
     STOR <SP> <pathname> <CRLF>
     APPE <SP> <pathname> <CRLF>
     MLFL [<SP> <ident>] <CRLF>
     MAIL [<SP> <ident>] <CRLF>
     MSND [<SP> <ident>] <CRLF>
     MSOM [<SP> <ident>] <CRLF>
     MSAM [<SP> <ident>] <CRLF>
     MRSQ [<SP> <scheme>] <CRLF>
     MRCP <SP> <ident> <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>
     CWD <SP> <pathname> <CRLF>
     LIST [<SP> <pathname>] <CRLF>
     NLST [<SP> <pathname>] <CRLF>
     SITE <SP> <string> <CRLF>
     STAT [<SP> <pathname>] <CRLF>
     HELP [<SP> <string>] <CRLF>
     NOOP <CRLF>








                               43


June 1980 IEN 149 File Transfer Protocol RFC 765


  The syntax of the above argument fields (using BNF notation where
  applicable ) is:
     <username> ::= <string>
     <password> ::= <string>
     <account information> ::= <string>
     <string> ::= <char> | <char><string>
     <char> ::= any of the 128 ASCII characters except <CR> and <LF>
     <marker> ::= <pr string>
     <pr string> ::= <pr char> | <pr char><pr string>
     <pr char> ::= printable characters, any
                   ASCII code 33 through 126
     <byte size> ::= any decimal integer 1 through 255
     <Host-port> ::= <Host-number>,<Port-number>
     <Host-number> ::= <number>,<number>,<number>,<number>
     <Port-number> ::= <number>,<number>
     <number> ::= any decimal integer 0 through 255
     <ident> ::= <string>
     <scheme> ::= R | T | ?
     <form code> ::= N | T | C
     <type code> ::= A [<SP> <form code>]
                   | E [<SP> <form code>]
                   | I
                   | L <SP> <byte size>
     <structure code> ::= F | R | P
     <mode code> ::= S | B | C
     <pathname> ::= <string>












                               44


IEN 149 June 1980


SEQUENCING OF COMMANDS AND REPLIES

  The communication between the user and server is intended to be an
  alternating dialogue.  As such, the user issues an FTP command and
  the server responds with a prompt primary reply.  The user should
  wait for this initial primary success or failure response before
  sending further commands.
  Certain commands require a second reply for which the user should
  also wait.  These replies may, for example, report on the progress
  or completion of file transfer or the closing of the data
  connection.  They are secondary replies to file transfer commands.
  One important group of informational replies is the connection
  greetings.  Under normal circumstances, a server will send a 220
  reply, "awaiting input", when the connection is completed.  The
  user should wait for this greeting message before sending any
  commands.  If the server is unable to accept input right away, he
  should send a 120 "expected delay" reply immediately and a 220
  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 Sequences
     In this section, the command-reply sequence is presented.  Each
     command is listed with its possible replies; command groups are
     listed together.  Preliminary replies are listed first (with
     their succeeding replies indented and under them), then
     positive and negative completion, and finally intermediary
     replies with the remaining commands from the sequence
     following.  This listing forms the basis for the state
     diagrams, which will be presented separately.
        Connection Establishment
           120
              220
           220
           421




                               45


June 1980 IEN 149 File Transfer Protocol RFC 765


        Login
           USER
              230
              530
              500, 501, 421
              331, 332
           PASS
              230
              202
              530
              500, 501, 503, 421
              332
           ACCT
              230
              202
              530
              500, 501, 503, 421
        Logout
           QUIT
              221
              500
           REIN
              120
                 220
              220
              421
              500, 502
        Transfer parameters
           PORT
              200
              500, 501, 421, 530
           PASV
              227
              500, 501, 502, 421, 530
           MODE, TYPE, STRU
              200
              500, 501, 504, 421, 530
        File action commands
           ALLO
              200
              202
              500, 501, 504, 421, 530
           REST
              500, 501, 502, 421, 530
              350



                               46


IEN 149 June 1980


           STOR
              125, 150
                 (110)
                 226, 250
                 425, 426, 451, 551, 552
              532, 450, 452, 553
              500, 501, 421, 530
           RETR
              125, 150
                 (110)
                 226, 250
                 425, 426, 451
              450, 550
              500, 501, 421, 530
           LIST, NLST
              125, 150
                 226, 250
                 425, 426, 451
              450
              500, 501, 502, 421, 530
           APPE
              125, 150
                 (110)
                 226, 250
                 425, 426, 451, 551, 552
              532, 450, 550, 452, 553
              500, 501, 502, 421, 530
           MLFL
              125, 150, 151, 152
                 226, 250
                 425, 426, 451, 552
              532, 450, 550, 452, 553
              500, 501, 502, 421, 530
           RNFR
              450, 550
              500, 501, 502, 421, 530
              350
           RNTO
              250
              532, 553
              500, 501, 502, 503, 421, 530
           DELE, CWD
              250
              450, 550
              500, 501, 502, 421, 530



                               47


June 1980 IEN 149 File Transfer Protocol RFC 765


           ABOR
              225, 226
              500, 501, 502, 421
           MAIL, MSND
              151, 152
                 354
                    250
                    451, 552
              354
                 250
                 451, 552
              450, 550, 452, 553
              500, 501, 502, 421, 530
           MSOM, MSAM
              119, 151, 152
                 354
                    250
                    451, 552
              354
                 250
                 451, 552
              450, 550, 452, 553
              500, 501, 502, 421, 530
           MRSQ
              200, 215
              500, 501, 502, 421, 530
           MRCP
              151, 152
                 200
              200
              450, 550, 452, 553
              500, 501, 502, 503, 421
        Informational commands
           STAT
              211, 212, 213
              450
              500, 501, 502, 421, 530
           HELP
              211, 214
              500, 501, 502, 421
        Miscellaneous commands
           SITE
              200
              202
              500, 501, 530



                               48


IEN 149 June 1980


           NOOP
              200
              500 421
























                               49


June 1980 IEN 149 File Transfer Protocol RFC 765


STATE DIAGRAMS

Here we present state diagrams for a very simple minded FTP implementation. Only the first digit of the reply codes is used. There is one state diagram for each group of FTP commands or command sequences.

The command groupings were determined by constructing a model for each command then collecting together the commands with structurally identical models.

For each command or command sequence there are three possible outcomes: success (S), failure (F), and error (E). In the state diagrams below we use the symbol B for "begin", and the symbol W for "wait for reply".

We first present the diagram that represents the largest group of FTP commands:


                           1,3    +---+
                      ----------->| E |
                     |            +---+
                     |
  +---+    cmd    +---+    2      +---+
  | B |---------->| W |---------->| S |
  +---+           +---+           +---+
                     |
                     |     4,5    +---+
                      ----------->| F |
                                  +---+
  
  This diagram models the commands:
     ABOR, ALLO, DELE, CWD, HELP, MODE, MRCP, MRSQ, NOOP, PASV,
     QUIT, SITE, PORT, STAT, STRU, TYPE.







                               50


IEN 149 June 1980


The other large group of commands is represented by a very similar diagram:


                           3      +---+
                      ----------->| E |
                     |            +---+
                     |
  +---+    cmd    +---+    2      +---+
  | B |---------->| W |---------->| S |
  +---+       --->+---+           +---+
             |     | |
             |     | |     4,5    +---+
             |  1  |  ----------->| F |
              -----               +---+
  
  This diagram models the commands:
     APPE, LIST, MLFL, NLST, REIN, RETR, STOR.

Note that this second model could also be used to represent the first group of commands, the only difference being that in the first group the 100 series replies are unexpected and therefore treated as error, while the second group expects (some may require) 100 series replies.

The remaining diagrams model command sequences, perhaps the simplest of these is the rename sequence:


  +---+   RNFR    +---+    1,2    +---+
  | B |---------->| W |---------->| E |
  +---+           +---+        -->+---+
                   | |        |
            3      | | 4,5    |
     --------------  ------   |
    |                      |  |   +---+
    |               ------------->| S |
    |              |   1,3 |  |   +---+
    |             2|  --------
    |              | |     |
    V              | |     |
  +---+   RNTO    +---+ 4,5 ----->+---+
  |   |---------->| W |---------->| F |
  +---+           +---+           +---+
  



                               51


June 1980 IEN 149 File Transfer Protocol RFC 765


A very similar diagram models the Mail and Send commands:


               ----  1
              |    |
  +---+  cmd   -->+---+     2     +---+
  | B |---------->| W |---------->| E |
  +---+           +---+        -->+---+
                   | |        |
            3      | | 4,5    |
     --------------  ------   |
    |                      |  |   +---+
    |               ------------->| S |
    |              |   1,3 |  |   +---+
    |             2|  --------
    |              | |     |
    V              | |     |
  +---+   text    +---+ 4,5 ----->+---+
  |   |---------->| W |---------->| F |
  +---+           +---+           +---+
  
     This diagram models the commands:
        MAIL, MSND, MSOM, MSAM.
  Note that the "text" here is a series of lines sent from the user
  to the server with no response expected until the last line is
  sent, recall that the last line must consist only of a single
  period.











                               52


IEN 149 June 1980


The next diagram is a simple model of the Restart command:


  +---+   REST    +---+    1,2    +---+
  | B |---------->| W |---------->| E |
  +---+           +---+        -->+---+
                   | |        |
            3      | | 4,5    |
     --------------  ------   |
    |                      |  |   +---+
    |               ------------->| S |
    |              |   3   |  |   +---+
    |             2|  --------
    |              | |     |
    V              | |     |
  +---+   cmd     +---+ 4,5 ----->+---+
  |   |---------->| W |---------->| F |
  +---+        -->+---+           +---+
              |      |
              |  1   |
               ------


     Where "cmd" is APPE, STOR, RETR, or MLFL.

We note that the above three models are similar, in fact the Mail diagram and the Rename diagram are structurally identical. The Restart differs from the other two only in the treatment of 100 series replies at the second stage.











                               53


June 1980 IEN 149 File Transfer Protocol RFC 765


The most complicated diagram is for the Login sequence:


                        1
  +---+   USER    +---+------------->+---+
  | B |---------->| W | 2       ---->| E |
  +---+           +---+------  |  -->+---+
                   | |       | | |
                 3 | | 4,5   | | |
     --------------   -----  | | |
    |                      | | | |
    |                      | | | |
    |                 ---------  |
    |               1|     | |   |
    V                |     | |   |
  +---+   PASS    +---+ 2  |  ------>+---+
  |   |---------->| W |------------->| S |
  +---+           +---+   ---------->+---+
                   | |   | |     |
                 3 | |4,5| |     |
     --------------   --------   |
    |                    | |  |  |
    |                    | |  |  |
    |                 -----------
    |             1,3|   | |  |
    V                |  2| |  |
  +---+   ACCT    +---+--  |   ----->+---+
  |   |---------->| W | 4,5 -------->| F |
  +---+           +---+------------->+---+











                               54


IEN 149 June 1980


Finally we present a generalized diagram that could be used to model the command and reply interchange:


           ------------------------------------
          |                                    |
  Begin   |                                    |
    |     V                                    |
    |   +---+  cmd   +---+ 2         +---+     |
     -->|   |------->|   |---------->|   |     |
        |   |        | W |           | S |-----|
     -->|   |     -->|   |-----      |   |     |
    |   +---+    |   +---+ 4,5 |     +---+     |
    |     |      |    | |      |               |
    |     |      |   1| |3     |     +---+     |
    |     |      |    | |      |     |   |     |
    |     |       ----  |       ---->| F |-----
    |     |             |            |   |
    |     |             |            +---+
     -------------------
          |
          |
          V
         End
  













                               55


June 1980 IEN 149 File Transfer Protocol RFC 765


TYPICAL FTP SCENARIO

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

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

  LOCAL COMMANDS BY USER              ACTION INVOLVED
  ftp (host) multics<CR>         Connect to Host S, port L,
                                 establishing TELNET connections
                                 <---- 220 Service ready <CRLF>
  username Doe <CR>              USER Doe<CRLF>---->
                                 <---- 331 User name ok,
                                           need password<CRLF>
  password mumble <CR>           PASS mumble<CRLF>---->
                                 <---- 230 User logged in.<CRLF>
  retrieve (local type) ASCII<CR>
  (local pathname) test 1 <CR>   User-FTP opens local file in ASCII.
  (for.pathname) test.pl1<CR>    RETR test.pl1<CRLF> ---->
                                 <---- 150 File status okay;
                                       about to open data connection
                                 Server makes data connection
                                 to port U
  <CRLF>
                                 <---- 226 Closing data connection,
                                     file transfer successful<CRLF>
  type Image<CR>                 TYPE I<CRLF> ---->
                                 <---- 200 Command OK<CRLF>
  store (local type) image<CR>
  (local pathname) file dump<CR> User-FTP opens local file in Image.
  (for.pathname) >udd>cn>fd<CR>  STOR >udd>cn>fd<CRLF> ---->
                                 <---- 450 Access denied<CRLF>
  terminate                      QUIT <CRLF> ---->
                                 Server closes all
                                 connections.






                               56


IEN 149 June 1980


CONNECTION ESTABLISHMENT

The FTP control connection is established via TCP between the user process port U and the server process port L. This protocol is assigned the service port 21 (25 octal), that is L=21.























                               57


June 1980 IEN 149 File Transfer Protocol RFC 765


APPENDIX ON MAIL

The basic commands transmitting mail are the MAIL and the MLFL commands. These commands cause the transmitted data to be entered into the recipients mailbox.

  MAIL <SP> <recipient name> <CRLF>
     If accepted, returns 354 reply and considers all succeeding
     lines to be the message text, terminated by a line containing
     only a period, upon which a 250 completion reply is returned.
     Various errors are possible.
  MLFL <SP> <recipient name> <CRLF>
     If accepted, acts like a STOR command, except that the data is
     considered to be the message text.  Various errors are
     possible.

There are two possible preliminary replies that a server may use to indicate that it is accepting mail for a user whose mailbox is not at that server.

  151 User not local; Will forward to <user>@<host>.
     This reply indicates that the server knows the user's mailbox
     is on another host and will take responsibility for forwarding
     the mail to that host.  For example, at BBN (or ISI) there are
     several host which each have a list of many of the users on
     several of the host.  These hosts then can accept mail for any
     user on their list and forward it to the correct host.
  152 User Unknown; Mail will be forwarded by the operator.
     This reply indicates that the host does not recognize the user
     name, but that it will accept the mail and have the operator
     attempt to deliver it.  This is useful if the user name is
     misspelled, but may be a disservice if the mail is really
     undeliverable.

Three FTP commands provide for "sending" a message to a logged-in user's terminal, as well as variants for mailing it normally whether the user is logged in or not.




                               58


IEN 149 June 1980


  MSND -- SeND to terminal.
     Returns 450 failure reply if the addressee is refusing or not
     logged in.
  MSOM -- Send to terminal Or Mailbox.
     Returns 119 notification reply if terminal is not accessible.
  MSAM -- Send to terminal And Mailbox.
     Returns 119 notification reply if terminal is not accessible.

Note that for MSOM and MSAM, it is the mailing which determines success, not the sending, although MSOM as implemented uses a 119 reply (in addition to the normal success/failure code) to indicate that because the SEND failed, an attempt is being made to mail the message instead. There are no corresponding variants for MLFL, since messages transmitted in this way are generally short.

There are two FTP commands which allow one to mail the text of a message to several recipients simultaneously; such message transmission is far more efficient than the practice of sending the text again and again for each additional recipient at a site.

There are two basic ways of sending a single text to several recipients. In one, all recipients are specified first, and then the text is sent; in the other, the order is reversed and the text is sent first, followed by the recipients. Both schemes are necessary because neither by itself is optimal for all systems, as will be explained later. To select a particular scheme, the MRSQ command is used; to specify recipients after a scheme is chosen, MRCP commands are given; and to furnish text, the MAIL or MLFL commands are used.

Scheme Selection: MRSQ

  MRSQ is the means by which a user program can test for
  implementation of MRSQ/MRCP, select a particular scheme, reset its
  state thereof, and even do some rudimentary negotiation.  Its
  format is like that of the TYPE command, as follows:






                               59


June 1980 IEN 149 File Transfer Protocol RFC 765


     MRSQ [<SP> <scheme>] <CRLF>
     <scheme> = a single character.  The following are defined:
        R  Recipients first.  If not implemented, T must be.
        T  Text first.  If this is not implemented, R must be.
        ?  Request for preference.  Must always be implemented.
        No argument means a "selection" of none of the schemes (the
        default).
     Replies:
        200 OK, we'll use specified scheme.
        215 <scheme> This is the scheme I prefer.
        501 I understand MRSQ but can't use that scheme.
        5xx Command unrecognized or unimplemented.
  Three aspects of MRSQ need to be pointed out here.  The first is
  that an MRSQ with no argument must always return a 200 reply and
  restore the default state of having no scheme selected.  Any other
  reply implies that MRSQ and hence MRCP are not understood or
  cannot be performed correctly.
  The second is that the use of "?" as a <scheme> asks the FTP
  server to return a 215 reply in which the server specifies a
  "preferred" scheme.  The format of this reply is simple:
     215 <SP> <scheme> [<SP> <arbitrary text>] <CRLF>
     Any other reply (e.g. 4xx or 5xx) implies that MRSQ and MRCP
     are not implemented, because "?" must always be implemented if
     MRSQ is.
  The third important thing about MRSQ is that it always has the
  side effect of resetting all schemes to their initial state.  This
  reset must be done no matter what the reply will be - 200, 215, or
  501.  The actions necessary for a reset will be explained when
  discussing how each scheme actually works.

Message Text Specification: MAIL/MLFL

  Regardless of which scheme (if any) has been selected, a MAIL or
  MLFL with a non-null argument will behave exactly as before; the
  MRSQ/MRCP commands have no effect on them.  However, such normal
  MAIL/MLFL commands do have the same side effect as MRSQ; they
  "reset" the current scheme to its initial state.



                               60


IEN 149 June 1980


  It is only when the argument is null (e.g. MAIL<CRLF> or
  MLFL<CRLF>) that the particular scheme being used is important,
  because rather than producing an error (as most servers currently
  do), the server will accept message text for this "null"
  specification; what it does with it depends on which scheme is in
  effect, and will be described in "Scheme Mechanics".

Recipient specification: MRCP

  In order to specify recipient names  (i.e., idents) and receive
  some acknowledgment (or refusal) for each name, the following
  command is used:
     MRCP <SP> <ident> <CRLF>
     Reply for no scheme:
        503 No scheme specified yet; use MRSQ.
     Replies for scheme T are identical to those for MAIL/MLFL.
     Replies for scheme R (recipients first):
        200 OK, name stored.
        452 Recipient table full, this name not stored.
        553 Recipient name rejected.
        4xx Temporary error, try this name again later.
        5xx Permanent error, report to sender.
  Note that use of this command is an error if no scheme has been
  selected yet; an MRSQ <scheme> must have been given if MRCP is to
  be used.

Scheme mechanics: MRSQ R (Recipients first)

  In the recipients-first scheme, MRCP is used to specify names
  which the FTP server stores in a list or table.  Normally the
  reply for each MRCP will be either a 200 for acceptance, or a
  4xx/5xx code for rejection; all 5xx codes are permanent rejections
  (e.g. user not known) which should be reported to the human
  sender, whereas 4xx codes in general connote some temporary error
  that may be rectified later.  None of the 4xx/5xx replies impinge
  on previous or succeeding MRCP commands, except for 452 which
  indicates that no further MRCP's will succeed unless a message is
  sent to the already stored recipients or a reset is done.
  Sending message text to stored recipients is done by giving a MAIL
  or MLFL command with no argument; that is, just MAIL<CRLF> or
  MLFL<CRLF>.  Transmission of the message text is exactly the same
  as for normal MAIL/MLFL; however, a positive acknowledgment at the



                               61


June 1980 IEN 149 File Transfer Protocol RFC 765


  end of transmission means that the message has been sent to ALL
  recipients that were remembered with MRCP, and a failure code
  means that it should be considered to have failed for ALL of these
  specified recipients.  This applies regardless of the actual error
  code; and whether the reply signifies success or failure, all
  stored recipient names are flushed and forgotten - in other words,
  things are reset to their initial state.  This purging of the
  recipient name list must also be done as the "reset" side effect
  of any use of MRSQ.
  A 452 reply to an MRCP can thus be handled by using a MAIL/MLFL to
  specify the message for currently stored recipients, and then
  sending more MRCP's and another MAIL/MLFL, as many times as
  necessary; for example, if a server only had room for 10 names
  this would result in a 50-recipient message being sent 5 times, to
  10 different recipients each time.
  If a user attempts to specify message text (MAIL/MLFL with no
  argument) before any successful MRCP's have been given, this
  should be treated exactly as a "normal" MAIL/MLFL with a null
  recipient would be; some servers will return an error of some
  type, such as "550 Null recipient".
  See Example 1 for an example using MRSQ R.

Scheme mechanics: MRSQ T (Text first)

  In the text-first scheme, MAIL/MLFL with no argument is used to
  specify message text, which the server stores away.  Succeeding
  MRCP's are then treated as if they were MAIL/MLFL commands, except
  that none of the text transfer manipulations are done; the stored
  message text is sent to the specified recipient, and a reply code
  is returned identical to that which an actual MAIL/MLFL would
  invoke. (Note ANY 2xx code indicates success.)
  The stored message text is not forgotten until the next MAIL/MLFL
  or MRSQ, which will either replace it with new text or flush it
  entirely.  Any use of MRSQ will reset this scheme by flushing
  stored text, as will any use of MAIL/MLFL with a non-null
  argument.
  If an MRCP is seen before any message text has been stored, the
  user in effect is trying to send a null message; some servers
  might allow this, others would return an error code.
  See Example 2 for an example using MRSQ T.



                               62


IEN 149 June 1980


Why two schemes anyway?

  Because neither by itself is optimal for all systems.  MRSQ R
  allows more of a "bulk" mailing, because everything is saved up
  and then mailed simultaneously; this is very useful for systems
  such as ITS where the FTP server does not itself write mail
  directly, but hands it on to a central mailer demon of great
  power; the more information (e.g. recipients) associated with a
  single "hand-off", the more efficiently mail can be delivered.
  By contrast, MRSQ T is geared to FTP servers which want to deliver
  mail directly, in one-by-one incremental fashion.  This way they
  can return an individual success/failure reply code for each
  recipient given which may depend on variable file system factors
  such as exceeding disk allocation, mailbox access conflicts, and
  so forth; if they tried to emulate MRSQ R's bulk mailing, they
  would have to ensure that a success reply to the MAIL/MLFL indeed
  meant that it had been delivered to ALL recipients specified - not
  just some.

Notes:

  * Because these commands are not required in the minimum
    implementation of FTP, one must be prepared to deal with sites
    which don't recognize either MRSQ or MRCP.  "MRSQ" and "MRSQ ?"
    are explicitly designed as tests to see whether either scheme is
    implemented; MRCP is not, and a failure return of the
    "unimplemented" variety could be confused with "No scheme
    selected yet", or even with "Recipient unknown".  Be safe, be
    sure, use MRSQ!
  * There is no way to indicate in a positive response to "MRSQ ?"
    that the preferred "scheme" for a server is that of the default
    state; i.e. none of the multi-recipient schemes.  The rationale
    is that in this case, it would be pointless to implement
    MRSQ/MRCP at all, and the response would therefore be negative.
  * One reason that the use of MAIL/MLFL is restricted to null
    arguments with this multi-recipient extension is the ambiguity
    that would result if a non-null argument were allowed; for
    example, if MRSQ R was in effect and some MRCP's had been given,
    and a MAIL FOO<CRLF> was done, there would be no way to
    distinguish a failure reply for mailbox "FOO" from a global
    failure for all recipients specified.  A similar situation
    exists for MRSQ T; it would not be clear whether the text was
    stored and the mailbox failed, or vice versa, or both.



                               63


June 1980 IEN 149 File Transfer Protocol RFC 765


  * "Resets" are done by all MRSQ's and "normal" MAIL/MLFL's to
    avoid confusion and overly complicated implementation.  The MRSQ
    command implies a change or uncertainty of status, and the
    latter commands would otherwise have to use some independent
    mechanisms to avoid clobbering the data bases (e.g., message
    text storage area) used by the T/R schemes.  However, once a
    scheme is selected, it remains "in effect" just as a "TYPE A"
    remains selected.  The recommended way for doing a reset,
    without changing the current selection, is with "MRSQ ?".
    Remember that "MRSQ" alone reverts to the no-scheme state.
  * It is permissible to intersperse other FTP commands among the
    MRSQ/MRCP/MAIL sequences.



















                               64


IEN 149 June 1980


Example 1

              Example of MRSQ R (Recipients first)
  This is an example of how MRSQ R is used; first the user must
  establish that the server in fact implements MRSQ:
     U: MRSQ
     S: 200 OK, no scheme selected.
  An MRSQ with a null argument always returns a 200 if implemented,
  selecting the "scheme" of null, i.e. none of them.  If MRSQ were
  not implemented, a code of 4xx or 5xx would be returned.
     U: MRSQ R
     S: 200 OK, using that scheme
  All's well; now the recipients can be specified.
     U: MRCP Foo
     S: 200 OK
     U: MRCP Raboof
     S: 553 Who's that?  No such user here.
     U: MRCP bar
     S: 200 OK
  Well, two out of three ain't bad.  Note that the demise of
  "Raboof" has no effect on the storage of "Foo" or "bar".  Now to
  furnish the message text, by giving a MAIL or MLFL with no
  argument:
     U: MAIL
     S: 354 Type mail, ended by <CRLF>.<CRLF>
     U: Blah blah blah blah....etc etc etc
     U: .
     S: 250 Mail sent.
  The text has now been sent to both "Foo" and "bar".






                               65


June 1980 IEN 149 File Transfer Protocol RFC 765


Example 2

                 Example of MRSQ T (Text first)
  Using the same message as the previous example:
     U: MRSQ ?
     S: 215 T Text first, please.
  MRSQ is indeed implemented, and the server says that it prefers
  "T", but that needn't stop the user from trying something else:
     U: MRSQ R
     S: 501 Sorry, I really can't do that.
  It's possible that it could have understood "R" also, but in
  general it's best to use the "preferred" scheme, since the server
  knows which is most efficient for its particular site.  Anyway:
     U: MRSQ T
     S: 200 OK, using that scheme.
  Scheme "T" is now selected, and the text must be sent:
     U: MAIL
     S: 354 Type mail, ended by <CRLF>.<CRLF>
     U: Blah blah blah blah....etc etc etc
     U: .
     S: 250 Mail stored.
  Now recipients can be specified:
     U: MRCP Foo
     S: 250 Stored mail sent.
     U: MRCP Raboof
     S: 553 Who's that?  No such user here.
     U: MRCP bar
     S: 250 Stored mail sent.






                               66


IEN 149 June 1980


  Again, the text has now been sent to both "Foo" and "bar", and
  still remains stored.  A new message can be sent with another
  MAIL/MRCP... sequence, but the fastidious or paranoid could chose
  to do:
     U: MRSQ ?
     S: 215 T Text first, please.
  Which resets things without altering the scheme in effect.





















                               67


June 1980 IEN 149 File Transfer Protocol RFC 765


APPENDIX ON PAGE STRUCTURE

The need for FTP to support page structure derives principally from the need to support efficient transmission of files between TOPS20 systems, particularly the files used by NLS.

The file system of TOPS20 is based on the concept of pages. The system level is most efficient at manipulating files as pages. System level programs provide an interface to the file system so that many applications view files as sequential streams of characters. However, a few applications use the underlying page structures directly, and some of these create holey files.

A TOPS20 file is just a bunch of words pointed to by a page table. If those words contain CRLF's, fine -- but that doesn't mean "record" to TOPS20.

A TOPS20 disk file consists of four things: a pathname, a page table, a (possibly empty) set of pages, and a set of attributes.

The pathname is specified in the RETR or STOR command. It includes the directory name, file name, file name extension, and version number.

The page table contains up to 2**18 entries. Each entry may be EMPTY, or may point to a page. If it is not empty, there are also some page-specific access bits; not all pages of a file need have the same access protection.

  A page is a contiguous set of 512 words of 36 bits each.

The attributes of the file, in the File Descriptor Block (FDB), contain such things as creation time, write time, read time, writer's byte-size, end of file pointer, count of reads and writes, backup system tape numbers, etc.

Note that there is NO requirement that pages in the page table be contiguous. There may be empty page table slots between occupied ones. Also, the end of file pointer is simply a number. There is no requirement that it in fact point at the "last" datum in the file. Ordinary sequential I/O calls in TOPS20 will cause the end of file pointer to be left after the last datum written, but other operations may cause it not to be so, if a particular programming system so requires.




                               68


IEN 149 June 1980


In fact both of these special cases, "holey" files and end-of-file pointers not at the end of the file, occur with NLS data files.

The TOPS20 paged files can be sent with the FTP transfer parameters: TYPE L 36, STRU P, and MODE S (in fact any mode could be used).

Each page of information has a header. Each header field, which is a logical byte, is a TOPS20 word, since the TYPE is L 36.

The header fields are:

  Word 0: Header Length.
     The header length is 5.
  Word 1: Page Index.
     If the data is a disk file page, this is the number of that
     page in the file's page map.  Empty pages (holes) in the file
     are simply not sent.  Note that a hole is NOT the same as a
     page of zeros.
  Word 2: Data Length.
     The number of data words in this page, following the header.
     Thus the total length of the transmission unit is the Header
     Length plus the Data Length.
  Word 3: Page Type.
     A code for what type of chunk this is. A data page is type 3,
     the FDB page is type 2.
  Word 4: Page Access Control.
     The access bits associated with the page in the file's page
     map.  (This full word quantity is put into AC2 of an SPACS by
     the program reading from net to disk.)

After the header are Data Length data words. Data Length is currently either 512 for a data page or 21 for an FDB. Trailing zeros in a disk file page may be discarded, making Data Length less than 512 in that case.




                               69


June 1980 IEN 149 File Transfer Protocol RFC 765


Data transfers are implemented like the layers of an onion: some characters are packaged into a line. Some lines are packaged into a file. The file is broken into other manageable units for transmission. Those units have compression applied to them. The units may be flagged by restart markers. On the other end, the process is reversed.























                               70