Section 2. OVERVIEW


   In this section, the history, the terminology, and the FTP model are
   discussed.  The terms defined in this section are only those that
   have special significance in FTP.  Some of the terminology is very
   specific to the FTP model; some readers may wish to turn to the
   section on the FTP model while reviewing the terminology.








   2.1.  HISTORY

      FTP has had a long evolution over the years.  Appendix III is a
      chronological compilation of Request for Comments documents
      relating to FTP.  These include the first proposed file transfer
      mechanisms in 1971 that were developed for implementation on hosts
      at M.I.T. (RFC 114), plus comments and discussion in RFC 141.

      RFC 172 provided a user-level oriented protocol for file transfer
      between host computers (including terminal IMPs).  A revision of
      this as RFC 265, restated FTP for additional review, while RFC 281
      suggested further changes.  The use of a "Set Data Type"
      transaction was proposed in RFC 294 in January 1982.

      RFC 354 obsoleted RFCs 264 and 265.  The File Transfer Protocol
      was now defined as a protocol for file transfer between HOSTs on
      the ARPANET, with the primary function of FTP defined as
      transfering files efficiently and reliably among hosts and
      allowing the convenient use of remote file storage capabilities.
      RFC 385 further commented on errors, emphasis points, and
      additions to the protocol, while RFC 414 provided a status report
      on the working server and user FTPs.  RFC 430, issued in 1973,
      (among other RFCs too numerous to mention) presented further
      comments on FTP.  Finally, an "official" FTP document was
      published as RFC 454.

      By July 1973, considerable changes from the last versions of FTP
      were made, but the general structure remained the same.  RFC 542
      was published as a new "official" specification to reflect these
      changes.  However, many implementations based on the older
      specification were not updated.

      In 1974, RFCs 607 and 614 continued comments on FTP.  RFC 624
      proposed further design changes and minor modifications.  In 1975,
      RFC 686 entitled, "Leaving Well Enough Alone", discussed the
      differences between all of the early and later versions of FTP.
      RFC 691 presented a minor revision of RFC 686, regarding the
      subject of print files.

      Motivated by the transition from the NCP to the TCP as the
      underlying protocol, a phoenix was born out of all of the above
      efforts in RFC 765 as the specification of FTP for use on TCP.

      This current edition of the FTP specification is intended to
      correct some minor documentation errors, to improve the
      explanation of some protocol features, and to add some new
      optional commands.



      In particular, the following new optional commands are included in
      this edition of the specification:

         CDUP - Change to Parent Directory

         SMNT - Structure Mount

         STOU - Store Unique

         RMD - Remove Directory

         MKD - Make Directory

         PWD - Print Directory

         SYST - System

      This specification is compatible with the previous edition.  A
      program implemented in conformance to the previous specification
      should automatically be in conformance to this specification.

   2.2.  TERMINOLOGY

      ASCII

         The ASCII character set is as defined in the ARPA-Internet
         Protocol Handbook.  In FTP, ASCII characters are defined to be
         the lower half of an eight-bit code set (i.e., the most
         significant bit is zero).

      access controls

         Access controls define users' access privileges to the use of a
         system, and to the files in that system.  Access controls are
         necessary to prevent unauthorized or accidental use of files.
         It is the prerogative of a server-FTP process to invoke access
         controls.

      byte size

         There are two byte sizes of interest in FTP:  the logical byte
         size of the file, and the transfer byte size used for the
         transmission of the data.  The transfer byte size is always 8
         bits.  The transfer byte size is not necessarily the byte size
         in which data is to be stored in a system, nor the logical byte
         size for interpretation of the structure of the data.




      control connection

         The communication path between the USER-PI and SERVER-PI for
         the exchange of commands and replies.  This connection follows
         the Telnet Protocol.

      data connection

         A full duplex connection over which data is transferred, in a
         specified mode and type. The data transferred may be a part of
         a file, an entire file or a number of files.  The path may be
         between a server-DTP and a user-DTP, or between two
         server-DTPs.

      data port

         The passive data transfer process "listens" on the data port
         for a connection from the active transfer process in order to
         open the data connection.

      DTP

         The data transfer process establishes and manages the data
         connection.  The DTP can be passive or active.

      End-of-Line

         The end-of-line sequence defines the separation of printing
         lines.  The sequence is Carriage Return, followed by Line Feed.

      EOF

         The end-of-file condition that defines the end of a file being
         transferred.

      EOR

         The end-of-record condition that defines the end of a record
         being transferred.

      error recovery

         A procedure that allows a user to recover from certain errors
         such as failure of either host system or transfer process.  In
         FTP, error recovery may involve restarting a file transfer at a
         given checkpoint.




      FTP commands

         A set of commands that comprise the control information flowing
         from the user-FTP to the server-FTP process.

      file

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

      mode

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

      NVT

         The Network Virtual Terminal as defined in the Telnet Protocol.

      NVFS

         The Network Virtual File System.  A concept which defines a
         standard network file system with standard commands and
         pathname conventions.

      page

         A file may be structured as a set of independent parts called
         pages.  FTP supports the transmission of discontinuous files as
         independent indexed pages.

      pathname

         Pathname is defined to be the character string which must be
         input to a file system by a user in order to identify a file.
         Pathname normally contains device and/or directory names, and
         file name specification.  FTP does not yet specify a standard
         pathname convention.  Each user must follow the file naming
         conventions of the file systems involved in the transfer.

      PI

         The protocol interpreter.  The user and server sides of the
         protocol have distinct roles implemented in a user-PI and a
         server-PI.



      record

         A sequential file may be structured as a number of contiguous
         parts called records.  Record structures are supported by FTP
         but a file need not have record structure.

      reply

         A reply is an acknowledgment (positive or negative) sent from
         server to user via the control connection in response to FTP
         commands.  The general form of a reply is a completion code
         (including error codes) followed by a text string.  The codes
         are for use by programs and the text is usually intended for
         human users.

      server-DTP

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

      server-FTP process

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

      server-PI

         The server protocol interpreter "listens" on Port L for a
         connection from a user-PI and establishes a control
         communication connection.  It receives standard FTP commands
         from the user-PI, sends replies, and governs the server-DTP.

      type

         The data representation type used for data transfer and
         storage.  Type implies certain transformations between the time
         of data storage and data transfer.  The representation types
         defined in FTP are described in the Section on Establishing
         Data Connections.





      user

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

      user-DTP

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

      user-FTP process

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

      user-PI

         The user protocol interpreter initiates the control connection
         from its port U to the server-FTP process, initiates FTP
         commands, and governs the user-DTP if that process is part of
         the file transfer.





















   2.3.  THE FTP MODEL

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

                                            -------------
                                            |/---------\|
                                            ||   User  ||    --------
                                            ||Interface|<--->| User |
                                            |\----^----/|    --------
                  ----------                |     |     |
                  |/------\|  FTP Commands  |/----V----\|
                  ||Server|<---------------->|   User  ||
                  ||  PI  ||   FTP Replies  ||    PI   ||
                  |\--^---/|                |\----^----/|
                  |   |    |                |     |     |
      --------    |/--V---\|      Data      |/----V----\|    --------
      | File |<--->|Server|<---------------->|  User   |<--->| File |
      |System|    || DTP  ||   Connection   ||   DTP   ||    |System|
      --------    |\------/|                |\---------/|    --------
                  ----------                -------------

                  Server-FTP                   USER-FTP

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

                      Figure 1  Model for FTP Use

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

      The FTP commands specify the parameters for the data connection
      (data port, transfer mode, representation type, and structure) and
      the nature of file system operation (store, retrieve, append,
      delete, etc.).  The user-DTP or its designate should "listen" on
      the specified data port, and the server initiate the data
      connection and data transfer in accordance with the specified
      parameters.  It should be noted that the data port need not be in



      the same host that initiates the FTP commands via the control
      connection, but the user or the user-FTP process must ensure a
      "listen" on the specified data port.  It ought to also be noted
      that the data connection may be used for simultaneous sending and
      receiving.

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

      
                    Control     ------------   Control
                    ---------->| User-FTP |<-----------
                    |          | User-PI  |           |
                    |          |   "C"    |           |
                    V          ------------           V
            --------------                        --------------
            | Server-FTP |   Data Connection      | Server-FTP |
            |    "A"     |<---------------------->|    "B"     |
            -------------- Port (A)      Port (B) --------------
      

                                 Figure 2

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

      The Relationship between FTP and Telnet:

         The FTP uses the Telnet protocol on the control connection.
         This can be achieved in two ways: first, the user-PI or the
         server-PI may implement the rules of the Telnet Protocol
         directly in their own procedures; or, second, the user-PI or
         the server-PI may make use of the existing Telnet module in the
         system.

         Ease of implementaion, sharing code, and modular programming
         argue for the second approach.  Efficiency and independence




         argue for the first approach.  In practice, FTP relies on very
         little of the Telnet Protocol, so the first approach does not
         necessarily involve a large amount of code.