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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.

Thomas [Page 1]  RFC 535 Comments on File Access Protocol July 1973

 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:

1. user's access to file; can he read, write, execute or append

the file?

Thomas [Page 2]  RFC 535 Comments on File Access Protocol July 1973

1. size information; byte size used in last write access (OPEN

W) of the file; file size in bytes (of that size).

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

1. 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.,

Thomas [Page 3]  RFC 535 Comments on File Access Protocol July 1973

             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).

     [ This RFC was put into machine readable form for entry ]
     [ into the online RFC archives by Alex McKenzie with    ]
     [ support from GTE, formerly BBN Corp.            10/99 ]

Thomas [Page 4]