"TYPE L m" is sometimes specified to the FTP programs
                 on two m-bit word machines to ensure the correct
                 transfer of a native-mode binary file from one machine
                 to the other.  However, this command should have the
                 same effect on these machines as "TYPE I".





 



RFC1123                   FILE TRANSFER -- FTP              October 1989


         4.1.2.2  Telnet Format Control: RFC-959 Section 3.1.1.5.2

            A host that makes no distinction between TYPE N and TYPE T
            SHOULD implement TYPE T to be identical to TYPE N.

            DISCUSSION:
                 This provision should ease interoperation with hosts
                 that do make this distinction.

                 Many hosts represent text files internally as strings
                 of ASCII characters, using the embedded ASCII format
                 effector characters (LF, BS, FF, ...) to control the
                 format when a file is printed.  For such hosts, there
                 is no distinction between "print" files and other
                 files.  However, systems that use record structured
                 files typically need a special format for printable
                 files (e.g., ASA carriage control).   For the latter
                 hosts, FTP allows a choice of TYPE N or TYPE T.

         4.1.2.3  Page Structure: RFC-959 Section 3.1.2.3 and Appendix I

            Implementation of page structure is NOT RECOMMENDED in
            general. However, if a host system does need to implement
            FTP for "random access" or "holey" files, it MUST use the
            defined page structure format rather than define a new
            private FTP format.

         4.1.2.4  Data Structure Transformations: RFC-959 Section 3.1.2

            An FTP transformation between record-structure and file-
            structure SHOULD be invertible, to the extent possible while
            making the result useful on the target host.

            DISCUSSION:
                 RFC-959 required strict invertibility between record-
                 structure and file-structure, but in practice,
                 efficiency and convenience often preclude it.
                 Therefore, the requirement is being relaxed.  There are
                 two different objectives for transferring a file:
                 processing it on the target host, or just storage.  For
                 storage, strict invertibility is important.  For
                 processing, the file created on the target host needs
                 to be in the format expected by application programs on
                 that host.

                 As an example of the conflict, imagine a record-
                 oriented operating system that requires some data files
                 to have exactly 80 bytes in each record.  While STORing




 



RFC1123                   FILE TRANSFER -- FTP              October 1989


                 a file on such a host, an FTP Server must be able to
                 pad each line or record to 80 bytes; a later retrieval
                 of such a file cannot be strictly invertible.

         4.1.2.5  Data Connection Management: RFC-959 Section 3.3

            A User-FTP that uses STREAM mode SHOULD send a PORT command
            to assign a non-default data port before each transfer
            command is issued.

            DISCUSSION:
                 This is required because of the long delay after a TCP
                 connection is closed until its socket pair can be
                 reused, to allow multiple transfers during a single FTP
                 session.  Sending a port command can avoided if a
                 transfer mode other than stream is used, by leaving the
                 data transfer connection open between transfers.

         4.1.2.6  PASV Command: RFC-959 Section 4.1.2

            A server-FTP MUST implement the PASV command.

            If multiple third-party transfers are to be executed during
            the same session, a new PASV command MUST be issued before