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Network Working Group                                          R. BradenRequest for Comments: 430                                       CCN/UCLANIC: 13299                                               7 February 1973COMMENTS ON FILE TRANSFER PROTOCOL   On January 23, 1973, Jon Postel (NMC), Eric Harslem (RAND), Stephen   Wolfe (CCN), and Robert Braden (CCN), held and informal meeting at   UCLA on FTP.  This RFC generally reports the consensus of that   meeting on the following issues: server-server transfers (ref.RFC438 by Thomas and Clements); site-dependent information; and   miscellaneous questions/disagreements withRFC 354, 385, and 414.   There was also a discussion of the print file muddle, but that   subject is addressed in a separate RFC, No. 448.Miscellaneous Comments on FTP   1.RFC 385, P. 1 (3)      The question of print files will be discussed at length in another      RFC.  However, we did feel that the word "still" on the second      line from the bottom of Page 1 is gratuitous.   2.RFC 385, P. 2 (5.)RFC 385, P. 3 (8.)RFC 414, P. 4 (11.i)      To the extent that we understand these items, they seem to be      unnecessary and probably undesirable concessions to particular bad      implementations ("hacks").  In reference to the second item, No. 8      inRFC 385, one should note that in an asynchronous multi-process      system like the ARPA Network, the phrase "immediately after" has      little meaning.  An implementation which depends upon "immediately      after" is erroneous and should be fixed.  If the protocol as      defined has an intrinsic race condition, of course, the protocol      should be fixed, but we don't believe such a problem exists.  It      would help if definitions of command-response sequences in the FTP      document were tightened up considerably.  As for the last item, we      don't understand why Wayne Hathaway is so strongly opposed to      "implied eor".   3.RFC 354, P. 13, Format Definitions for Block Mode      (a) The definition of the header length presumably is meant to be          the "smallest integral number of bytes whose length is greater          or equal to 24 bits".Braden                                                          [Page 1]

RFC 430            COMMENTS ON FILE TRANSFER PROTOCOL      FEBRUARY 1973      (b) The same definitional problem occurs for restart markers.      (c) Why does the restart marker have to be greater than 8 bits?      (d) Note that changing the Descriptor coding to bit flags would          abolish the implied eor as well as the problem ofRFC 385, P.          2, #6.   4.RFC 414, P. 5 (11.iii)          Note that text mode is not possible for any EBCDIC coded file.          Since EBCDIC is an 8-bit code, Telnet control characters          (128-255) cannot be used to distinguish either eor or eof.          Stream and block modes will work, however.  We have found the          diagram on the last page to be useful for keeping track of the          three-dimensional space of FTP parameters.   5.RFC 354, P. 17, PASS Command          There is no mechanism within FTP for changing a password.  A          user shouldn't have to use a different protocol (e.g., log          into a time sharing system) to merely change his password.   6.RFC 385, P. 3 (9.), TYPE Before BYTE      This admonition (to send TYPE before BYTE) should be clearly      labeled as a recommended procedure for user FTP, not a restriction      on a server FTP.   7.RFC 385, P. 2-3 (7) Order of 255 Reply      Some of the participants felt (strongly) that the timing problem      dealt with in this item is the result of bad NCP implementations      and ought not be dignified in the protocol.  The issue here is the      old, familiar, and touchy one of queueing RFC's or not. (My own      view is that the protocol asymmetry forced by NCP's which can't      queue RFC's is at least unaesthetic, and makes some elegant      solutions impossible.  For examples, seeRFC 414 and the comments      below on server-server interaction, andRFC 438 on Reconnection      Protocol).   8.RFC 354, P. 15, Restart      Following a RESTart command, APPend and STORe presumably have      identical meanings.Braden                                                          [Page 2]

RFC 430            COMMENTS ON FILE TRANSFER PROTOCOL      FEBRUARY 1973B. FTP Parameter EncodingRFC 448, which discusses print files, points out that the print file   attribute is logically independent of the character code attribute   (ASCII vs. EBCDIC) in the type dimension; the set of allowable types   in FTP is the outer product of the individual attributes.  Thus FTP   has (at least) four character types, summarized by the following two   x two matrix:                  |  ASCII  |   EBCDIC   ---------------+---------+------------   Not Print File |         |   ---------------+---------+------------   Print File     |         |   ---------------+---------+------------   I propose that the encoding in the TYPE command model this   interdependence of the types.  Instead of using a distinct single   ASCII character for each type, we should use multiple ASCII   characters---qualifiers, if you wish.  For example:         A represents ASCII code         E represents EBCDIC code         P represents print file         I represents image         L represents local byte   Then the legal types according toRFC 385 would be:         A         AP         E         EP         I         L   Note that the attributes under consideration here are type-like; they   are not (logically) concerned with the structure or the transmission   mode, only the internal encoding of the file.   At present, this would be a trivial change.  However, I foresee the   file transfer protocol expanding significantly over the next several   years as new types are added.  Some servers will want to add server-   specific type variations, and the NWG will want to add some.  How   about an APL character set?  Or the multiple-overlay 256 character   ASCII which has been proposed?  Multiple qualifiers (and later   perhaps more structure) in the type seems to be the cleanest escape   mechanism for future growth.Braden                                                          [Page 3]

RFC 430            COMMENTS ON FILE TRANSFER PROTOCOL      FEBRUARY 1973C. Server-Server Interaction   The FTP changes proposed by Thomas and Clements inRFC 438 are a   particular solution to a general problem inherent in the current   host-host protocol and higher-level protocols like FTP.  There seems   to be a need for a secure and simple way for two (server) processes   in different hosts to exchange socket names (i.e., 40-bit numbers) so   they can subsequently exchange RFC's and establish a connection.   Current second-level (host-host) protocol provides exactly one   (secure) mechanism by which one host can learn a socket name of a   process at another host in order to establish a connection: ICP.  The   ICP mechanism by itself is not adequate for server-server connection   in FTP.  Therefore, Thomas and Clements have proposed an extension to   the FTP protocol, roughly as follows:      (1) A controller ("user") process at Host A uses ICP to invoke and          establish Telnet control connections to two automata          ("server") processes at two other hosts.  An automaton process          invoked in this manner then executes controller commands sent          in a standard command language over the Telnet control          connection.      (2) The controller process commands each automaton to reserve a          suitable data transfer socket and to return the socket name to          the controller over the control connection.  An automaton          presumably negotiates with his own NCP in a host-dependent          manner to obtain the socket reservation.      (3) The controller now knows both data transfer socket names; he          will send them in subsequent commands to the automata so each          automaton will know the foreign socket name to which he is to          connect.  Later commands cause the automata to issue RFC's and          open the data connection as needed.   This appears to be useful general model for process-process   interaction over the Network.  Personally, I believe this symmetrical   model should be the basis of all FTP the controller and one of the   automata could be in the same host.  Then the user/server problem   (for any pair of hosts to transfer files, one must have a server FTP   and the other a user FTP) would vanish.  At least one host somewhere   in the Network would need a controller process; all other hosts would   need only an automaton process.   Perhaps at a future time the NWG should consider whether a socket-   reservation-and-passing mechanism ought to be incorporated into   second-level protocol rather than duplicated in a number of third-   level protocols.  We should note that this model provides secureBraden                                                          [Page 4]

RFC 430            COMMENTS ON FILE TRANSFER PROTOCOL      FEBRUARY 1973   sockets only if both user and server processes "release" the socket   reservations when the Telnet control connection breaks.  The same   problem seems to occur with Thomas' Reconnection Protocol (426).   In any case, for the present we would endorse the general third-level   model ofRFC 438.  However, we would propose a slightly different,   and more symmetrical, approach.      1. The requirement in FTP that the FTP user listen on the data         socket before issuing a data transfer command should be         removed.  The beauty of host-host protocol is that it doesn't         matter which host sends the first RFC, as long as they both         send matching RFC's "eventually".  (Timeouts, of course, are         annoying, but I believe they are workable and ultimately         unavoidable); queueing RFC's is also necessary).      2. We propose, instead of LSTN, a new command GETSocket.  The         controller (i.e., user FTP) process would send a GETSocket to         each automaton, probably after a successful login.  Upon         receiving GETSocket, an automaton would assign a (send,         receive) pair of data transfer sockets and return the numbers         over the Telnet connection. (Alternatively, FTP could specify         that a (send, receive) pair of sockets always be assigned when         the server is first entered, and the numbers returned to the         user process via unsolicited 255 replies).      3. Then the user process would send the socket numbers to the         opposite hosts by sending SOCK commands to both.      4. When it receives a data transfer command, the automaton         (server) process would issue an RFC containing the two socket         numbers.  When both servers are fired up, RFC's are exchanged         and data transfer starts.D. Site-Dependent FTP Parameters   Some hosts will have a problem with the current FTP because their   file system needs additional host-specific parameters in certain   cases.  As an example, the IBM operating systems tend to give the   programmer a number of options on the logical and physical mapping of   a file onto the disk.   This is true both of TSS/360 (see Wayne Hathaway's discussion of his   STOR command implementation, Page 5 ofRFC 418), and OS/360.  The   large set of options and parameters to the OS/360 file system is, in   fact, the (legitimate) origin of most complaints about OS Job Control   Language (JCL).Braden                                                          [Page 5]

RFC 430            COMMENTS ON FILE TRANSFER PROTOCOL      FEBRUARY 1973   If the FTP user merely wants to store data without using it at one of   these sites, he has no problem; defaults can be chosen to handle any   reasonable FTP request.  However, the FTP user who sends a file to an   IBM/360 for use there may need to specify local file system   parameters which are not derivable from any of the existing FTP   commands.   In designing an FTP server implementation for CCN, for example, we   first tried to handle the mapping problem by choosing a (possibly   different) default mapping for each combination of FTP parameters--   type, mode, and structure.  We hoped that if a user chose   "reasonable" or "suitable" FTP parameters for a particular case   (e.g., "ASCII, stream, record" for source programs, and "image,   block, record" for load modules), then the right OS/360 file mapping   would result.  We were forced to abandon this approach, however,   because of the following arguments:      1. Some user FTP's probably may not implement all FTP         type/mode/structure combinations (though they ought to!).      2. Some user FTP's may not give the user full or convenient         control over his type/mode/structure.  Indeed, the mode should         be chosen on grounds of efficiency, not end use.      3. There weren't enough logically distinct combinations of FTP         parameters.      4. The result would have been a set of hard-to-remember rules for         sending files to CCN for use here.      5. Some common cases require non-invertible transformations on the         data.  For example, most IBM language processors (i.e.,         compilers) accept only fixed length records of (surprise!) 80         bytes each, i.e., literal card images.  Such ugly (and         logically unnecessary) implementation stupidities in OS/360 are         a fact of life.  Now if a FTP user innocently sent a data file         to CCN with the particular type/mode combination which         defaulted to card images, he would find his records truncated         to 80 bytes.  That would be downright unfriendly.   Thus, the CCN server FTP would have to choose between being useful or   being friendly.  We decided upon the following strategy:      1. The defaults will be friendly; we will accept any FTP         type/mode/structure and store it invertibly (except print         files).  However, the user who uses only these defaults will         probably find he has to later run a utility under TSO to         reformat the data.Braden                                                          [Page 6]

RFC 430            COMMENTS ON FILE TRANSFER PROTOCOL      FEBRUARY 1973      2. We will provide some mnmonic keywords associated with STOR         commands to choose the proper disk mapping.  For example, if he         wants to STORe a Fortran source file for compilation at CCN,         the user will need only to specify "SOURCE" or "FORT" to get         reasonable and workable OS/360 file system parameters.  In         addition, we will provide fairly complete "DD" parameters for         the sophisticated user.  The syntax and semantics of these         keywords and parameters will be as close as possible to the         corresponding TSO commands.  Full details will be published as         soon as the implementation is working.   All of this discussion leads to a general protocol question: how   should such host-dependent information appear within FTP? Hathaway   used the ALLO command (seeRFC 418, P. 6).  CCN, on the other hand,   feels that such information belongs in the only part of FTP syntax   which is already host-dependent: the pathname.  So CCN plans to allow   a "generalized" pathname in a STOR command, a (full or partial) file   name optionally followed by one or keywords or keyword parameters   separated by commas.   A third possible solution might be for the user to precede his STORe   command by a server-dependent data set creation command, using   Hathaway's proposed SRVR command.  The data set creation command   could then have all the parameters necessary for the server file   system.  CCN might change to this approach if SRVR is adopted and if   people find the generalized pathname objectionable or unworkable.   For another interesting example of host-dependent problems, see   Hathaway's discussion of his DELE command inRFC 418 (pp.6-7).Braden                                                          [Page 7]

RFC 430            COMMENTS ON FILE TRANSFER PROTOCOL      FEBRUARY 1973+-------++-------+-------+-------++-------+-------+-------++| \ MODE||       |       |       ||       |       |       |||   \   ||STREAM | TEXT  | BLOCK ||STREAM | TEXT  | BLOCK |||TYPE \ ||       |       |       ||       |       |       ||+-------++-------+-------+-------++-------+-------+-------++|       ||       |       |       ||       |       |       ||| ASCII ||       |       |       ||       |       |       |||       ||       |       |       ||       |       |       ||+-------++-------+-------+-------++-------+-------+-------++|       ||       |///////|       ||///////|///////|       ||| IMAGE ||       |///////|       ||///////|///////|       |||       ||       |///////|       ||///////|///////|       ||+-------++-------+-------+-------++-------+-------+-------++| LOCAL ||       |///////|       ||///////|///////|       ||| BYTE  ||       |///////|       ||///////|///////|       |||       ||       |///////|       ||///////|///////|       ||+-------++-------+-------+-------++-------+-------+-------++|       ||       |///////|       ||       |///////|       ||| EBCDI ||       |///////|       ||       |///////|       |||       ||       |///////|       ||       |///////|       ||+-------++-------+-------+-------++-------+-------+-------++| ASCII/||///////|///////|///////||       |       |       ||| ASA   ||///////|///////|///////||       |       |       ||| VRC   ||///////|///////|///////||       |       |       ||+-------++-------+-------+-------++-------+-------+-------++|EBCDIC/||///////|///////|///////||       |///////|       ||| ASA   ||///////|///////|///////||       |///////|       ||| VRC   ||///////|///////|///////||       |///////|       |||       ||///////|///////|///////||       |///////|       ||+-------++-------+-------+-------++-------+-------+-------++ KEY: +---+ |///| Excluded +---+  case        [This RFC was put into machine readable form for entry]    [into the online RFC archives by Helene Morin, Via Genie, 12/99]Braden                                                          [Page 8]