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rfc:rfc430

Network Working Group R. Braden Request for Comments: 430 CCN/UCLA NIC: 13299 7 February 1973

                 COMMENTS 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.  RFC
 438 by Thomas and Clements); site-dependent information; and
 miscellaneous questions/disagreements with RFC 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
    in RFC 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 of RFC 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, see RFC 414 and the comments
    below on server-server interaction, and RFC 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 1973

B. FTP Parameter Encoding

 RFC 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 to RFC 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 1973

C. Server-Server Interaction

 The FTP changes proposed by Thomas and Clements in RFC 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 secure

Braden [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 of RFC 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 of RFC 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 (see RFC 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 in RFC 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: +—+ |/

+—+ 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]

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