"to the user".  This should not be taken literally.  In
                 general, the User-FTP should save the restart
                 information (ssss,rrrr) in stable storage, e.g., append
                 it to a restart control file.  An empty restart control
                 file should be created when the transfer first starts
                 and deleted automatically when the transfer completes
                 successfully.  It is suggested that this file have a
                 name derived in an easily-identifiable manner from the
                 name of the file being transferred and the remote host
                 name; this is analogous to the means used by many text
                 editors for naming "backup" files.

                 There are three cases for FTP restart.

                 (1)  User-to-Server Transfer

                      The User-FTP puts Restart Markers  at
                      convenient places in the data stream.  When the
                      Server-FTP receives a Marker, it writes all prior
                      data to disk, encodes its file system position and
                      transformation state as rrrr, and returns a "110
                      MARK ssss = rrrr" reply over the control
                      connection.  The User-FTP appends the pair
                      (ssss,rrrr) to its restart control file.

                      To restart the transfer, the User-FTP fetches the
                      last (ssss,rrrr) pair from the restart control
                      file, repositions its local file system and
                      transformation state using ssss, and sends the
                      command "REST rrrr" to the Server-FTP.

                 (2)  Server-to-User Transfer

                      The Server-FTP puts Restart Markers  at
                      convenient places in the data stream.  When the
                      User-FTP receives a Marker, it writes all prior
                      data to disk, encodes its file system position and




 



RFC1123                   FILE TRANSFER -- FTP              October 1989


                      transformation state as rrrr, and appends the pair
                      (rrrr,ssss) to its restart control file.

                      To restart the transfer, the User-FTP fetches the
                      last (rrrr,ssss) pair from the restart control
                      file, repositions its local file system and
                      transformation state using rrrr, and sends the
                      command "REST ssss" to the Server-FTP.

                 (3)  Server-to-Server ("Third-Party") Transfer

                      The sending Server-FTP puts Restart Markers 
                      at convenient places in the data stream.  When it
                      receives a Marker, the receiving Server-FTP writes
                      all prior data to disk, encodes its file system
                      position and transformation state as rrrr, and
                      sends a "110 MARK ssss = rrrr" reply over the
                      control connection to the User.  The User-FTP
                      appends the pair (ssss,rrrr) to its restart
                      control file.

                      To restart the transfer, the User-FTP fetches the
                      last (ssss,rrrr) pair from the restart control
                      file, sends "REST ssss" to the sending Server-FTP,
                      and sends "REST rrrr" to the receiving Server-FTP.


      4.1.4  FTP/USER INTERFACE

         This section discusses the user interface for a User-FTP
         program.

         4.1.4.1  Pathname Specification

            Since FTP is intended for use in a heterogeneous
            environment, User-FTP implementations MUST support remote
            pathnames as arbitrary character strings, so that their form
            and content are not limited by the conventions of the local
            operating system.

            DISCUSSION:
                 In particular, remote pathnames can be of arbitrary
                 length, and all the printing ASCII characters as well
                 as space (0x20) must be allowed.  RFC-959 allows a
                 pathname to contain any 7-bit ASCII character except CR
                 or LF.