FTP: Comments on File Access Protocol

Network Working Group                                          B. Thomas
Request for Comments: 535                                      BBN-TENEX
NIC: 17454                                                     July 1973
Categories: Protocols, FTP
References: RFC 520


                    Comments on File Access Protocol

   A file access protocol (FAP) of the sort proposed by John Day in RFC
   520 is a good idea.  The following comments suggest improvements
   (mostly additions) to the protocol described in RFC 520.

   1.  (Philosophical comment)  The intent of both FTP and FAP is to
       make it possible for a user to remotely access files.  In effect,
       FTP provides means for a user to have (parts of) file activity of
       the sort typically initiated at the command language level
       "slaved" across the network to the site where the file resides.
       In a similar way the intent of FAP is to provide a mechanism
       which allows activity of the sort typically initiated by programs
       at the operating system or monitor level to be "slaved" across
       the network to the site where the file resides.  The OPEN, CLOS,
       SETP, etc.  commands of FAP can be viewed as attempts to define
       "generic" file system monitor calls.  The suggestions made below
       are further attempts to make features typically available to
       local users also available to remote users via FAP.

   2.  The OPEN command should allow for a third OPEN mode called A for
       append.  In terms of its action with respect to a file and file
       pointer, the command

          OPEN A FOO

       would be equivalent to the sequence:

          OPEN W FOO
          SETP E

       The difference would be with respect to access control.  Many
       systems allow a user to control separately write and append
       access to a file (e.g., on TENEX a user usually sets the
       protection on his MESSAGE.TXT file such that anyone can append to
       it but only he can write it).  For such systems the append OPEN
       would succeed in many cases in which the write OPEN would fail.
       The principle here is that FAP (to as large as degree as is
       practical) should allow remote users to access files in the same
       way as local users may.

   3.  The protocol as proposed allows for the creation of non-
       sequential files but provides no convenient way for remotely
       accessing them after they are created.  For example if sent to a
       TENEX server, the sequence:

               OPEN W FOO     //byte size assumed = 36
               SETP B
               WRITE 512
               SETP 1024
               WRITE 512
               CLOS

       would create a file FOO with two pages (on TENEX a page = 512 36
       bit words).  The two pages would be page #0 and page #2; because
       page #1 does not exist the file is said to have a "hole" in it.
       Access to FOO via FAP would be difficult unless the remote user
       knew its (page) structure prior to access.  To support remote
       access to files such as FOO, FAP should have means for a user to
       determine a file's structure.  Consider a value-returning command
       that returns the value the file pointer should be set to in order
       to point to the first byte of the next used page (block or
       record) beyond the current position of the file pointer.  With
       such a command, call it FNUB (Find Next Used Block), the
       following sequence could be used to retrieve a holey file such as
       FOO:

               OPEN R FILE
               SETP B
       a:      FNUB               //let x=the value returned
               if x=null
                  then CLOS
                  else ( SETP x
                         READ 512   //page size=512
                         goto a )

       This presumes that the remote user knows the block (page) size so
       that he can properly access the file.  One can imagine files
       having blocks of variable size; perhaps FNUB should return two
       values: the file pointer position of the next block and the size
       of that block in bytes.

   4.  FAP should provide means for a remote user to acquire certain
       status and "descriptor" information about a given file.  The
       following is a (non-exhaustive) list of information which would
       be useful to a user remotely accessing TENEX files:

         - user's access to file; can he read, write, execute or append
           the file?

         - size information; byte size used in last write access (OPEN
           W) of the file; file size in bytes (of that size).

         - file access dates; date of create, last read, last write.

         - on TENEX a user can specify different access control for
           different pages within the same file; a remote user should be
           able to acquire such access control information about files
           (and be able to specify such access control when he creates
           them).

   5.  There are many applications in which a remote user would like to
       access several files simultaneously in much the same way as a
       local user can.  FAP as proposed can not support such multiple
       file access (of course, the user always has the option of going
       through an ICP to establish another connection with the server).
       FAP can be extended in a simple way to support multiple file
       access by including the notion of a "file handle" which is used
       to specify which file a given FAP command refers to.  When the
       user does:

               OPEN R FOO

       the server's response would include a handle for FOO which the
       user would use in subsequent references to FOO.  The handle
       returned would be a string of the server's choice; it might be
       the file's name (FOO), a small integer, etc.  Use of a (server
       chosen) file handle rather than the complete file name enables
       the server to respond to FAP commands without incurring the
       overhead of re-parsing the file name for each command.  To
       illustrate, consider the following sequence which opens a file
       for reading and one for writing, reads 3 bytes from the first
       file as data, computes using the data and writes a 2 byte result
       to the second file:

               OPEN R FOO   //server returns FH as handle
               OPEN W MOO   //server returns MH as handle
               READ 3 FH    //user reads data
               //User does some computation on the 3 bytes
               WRIT 2  MH   //user writes the result
               CLOS MH
               CLOS FH

       Reasonable defaults could be provided with handles: e.g., a FAP
       command without a handle refers to the same file as the previous
       command; etc.  (The association of a handle with a file is
       probably better achieved via a separate FAP command rather than
       as a side effect of the OPEN command; e.g.,

               HNDL FOO   )

   6.  It is important to take local transformations into account (page
       3 of RFC 520).  However, it is equally important to allow a
       remote user to suppress local transformations, if he wishes, so
       that he can access the file as it is stored.  This would enable a
       program that manipulates a file to work equally well whether the
       file is local (and accessed "directly" via system calls) or
       remote (and accessed "indirectly" via system calls that are
       "trapped" and transformed into FAP commands which are sent to the
       remote site).










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