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Network Working Group                                          B. ThomasRequest for Comments: 535                                      BBN-TENEXNIC: 17454                                                     July 1973Categories: Protocols, FTPReferences:RFC 520                    Comments on File Access Protocol   A file access protocol (FAP) of the sort proposed by John Day inRFC520 is a good idea.  The following comments suggest improvements   (mostly additions) to the protocol described inRFC 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:         - 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         - 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.,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 ofRFC 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]