rfc959.txt

RFC 相关的技术文档

                                                                        
RFC 959                                                     October 1985
File Transfer Protocol


         3.1.1.2.  EBCDIC TYPE

            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.

         3.1.1.3.  IMAGE TYPE

            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.

         3.1.1.4.  LOCAL TYPE

            The data is transferred in logical bytes of the size
            specified by the obligatory second parameter, Byte size.
            The value of Byte size must be a decimal integer; there is
            no default value.  The logical byte size is not necessarily
            the same as the transfer byte size.  If there is a
            difference in byte sizes, then the logical bytes should be
            packed contiguously, disregarding transfer byte boundaries
            and with any necessary padding at the end.



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RFC 959                                                     October 1985
File Transfer Protocol


            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 (i.e., 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 that 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.

            In 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.

         3.1.1.5.  FORMAT CONTROL

            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:

            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 should 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.  Therefore, these types have a second parameter
            specifying one of the following three formats:

            3.1.1.5.1.  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.




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RFC 959                                                     October 1985
File Transfer Protocol


               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.

            3.1.1.5.2.  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.

            3.1.1.5.2.  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, p. 606, October 1964.)  In a
               line or a record formatted according to the ASA Standard,
               the first character is not to be printed.  Instead, it
               should be used to determine the vertical movement of the
               paper which should take place before the rest of the
               record is printed.

               The ASA Standard specifies the following control
               characters:

                  Character     Vertical Spacing

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

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






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RFC 959                                                     October 1985
File Transfer Protocol


      3.1.2.  DATA STRUCTURES

         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 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 Mainframe in fixed length records but on a DEC TOPS-20
         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,


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RFC 959                                                     October 1985
File Transfer Protocol


         <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.

         3.1.2.1.  FILE STRUCTURE

            File structure is the default to be assumed if the STRUcture
            command has not been used.

            In file-structure there is no internal structure and the
            file is considered to be a continuous sequence of data
            bytes.

         3.1.2.2.  RECORD STRUCTURE

            Record structures must be accepted for "text" files (i.e.,
            files with TYPE ASCII or EBCDIC) by all FTP implementations.

            In record-structure the file is made up of sequential
            records.

         3.1.2.3.  PAGE STRUCTURE

            To transmit files that are discontinuous, FTP defines a page
            structure.  Files of this type are sometimes known 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:

               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.


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RFC 959                                                     October 1985
File Transfer Protocol


               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