RFC 959 File Transfer Protocol

       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.

RFC 959    File Transfer Protocol

         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.

 3.  DATA TRANSFER FUNCTIONS

    Files are transferred only via the data connection.  The control 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 the "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.

   3.1.  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.  DEC TOPS-20s's generally store NVT-ASCII as five 7-bit ASCII characters, left-justified in a 36-bit word. IBM Mainframe's

      store NVT-ASCII as 8-bit EBCDIC codes.  Multics stores NVT-ASCII  as four 9-bit characters in a 36-bit word.  It is 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.