rfc959.txt

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                  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 Controlled Page

                        This type includes an additional header field
                        for paged files with page level access control
                        information.  The header length must be 5.

               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.  See Appendix I for
            further details and a specific case at the page structure.

      A note of caution about parameters:  a file must be stored and
      retrieved with the same parameters if the retrieved version is to




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      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.

   3.2.  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.

      Every FTP implementation must support the use of the default data
      ports, and only the USER-PI can initiate a change to non-default
      ports.

      It is possible for the user to specify an alternate data port by
      use of the PORT command.  The user may want a file dumped on a TAC
      line printer or retrieved from a third party host.  In the latter
      case, the user-PI sets up control 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 control connection is closed legally or otherwise.

         5. An irrecoverable error condition occurs.

      Otherwise the close is a server option, the exercise of which the
      server must indicate to the user-process by either a 250 or 226
      reply only.

   3.3.  DATA CONNECTION MANAGEMENT

      Default Data Connection Ports:  All FTP implementations must
      support use of the default data connection ports, and only the
      User-PI may initiate the use of non-default ports.

      Negotiating Non-Default Data Ports:   The User-PI may specify a
      non-default user side data port with the PORT command.  The
      User-PI may request the server side to identify a non-default
      server side data port with the PASV command.  Since a connection
      is defined by the pair of addresses, either of these actions is
      enough to get a different data connection, still it is permitted
      to do both commands to use new ports on both ends of the data
      connection.

      Reuse of the Data Connection:  When using the stream mode of data
      transfer the end of the file must be indicated by closing the
      connection.  This causes a problem if multiple files are to be
      transfered in the session, due to need for TCP to hold the
      connection record for a time out period to guarantee the reliable
      communication.  Thus the connection can not be reopened at once.

         There are two solutions to this problem.  The first is to
         negotiate a non-default port.  The second is to use another
         transfer mode.

         A comment on transfer modes.  The stream transfer mode is


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         inherently unreliable, since one can not determine if the
         connection closed prematurely or not.  The other transfer modes
         (Block, Compressed) do not close the connection to indicate the
         end of file.  They have enough FTP encoding that the data
         connection can be parsed to determine the end of the file.
         Thus using these modes one can leave the data connection open
         for multiple file transfers.

   3.4.  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 its 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 its internal denotation.  An IBM Mainframe record
      count field may not be recognized at another host, so the
      end-of-record 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.






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      The following transmission modes are defined in FTP:

      3.4.1.  STREAM MODE

         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 a file structure, the EOF is indicated by
         the sending host closing the data connection and all bytes are
         data bytes.

      3.4.2.  BLOCK MODE

         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 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.



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         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