path: root/doc/rfc/rfc686.txt

Network Working Group                                       Brian Harvey
Request for Comments: 686                                          SU-AI
NIC 32481                                                    10 May 1975
References: 354, 385, 630, 542, 640.


                       Leaving Well Enough Alone

   I recently decided it was time for an overhaul of our FTP user and
   server programs.  This was my first venture into the world of network
   protocols, and I soon discovered that there was a lot we were doing
   wrong -- and a few things that everyone seemed to be doing
   differently from each other.  When I enquired about this, the
   response from some quarters was "Oh, you're running version 1!"

   Since, as far as I can tell, all but one network host are running
   version 1, and basically transferring files OK, it seems to me that
   the existence on paper of an unused protocol should not stand in the
   way of maintaining the current one unless there is a good reason to
   believe that the new one is either imminent or strongly superior or
   both. (I understand, by the way, that FTP-2 represents a lot of
   thought and effort by several people who are greater network experts
   than I, and that it isn't nice of me to propose junking all that
   work, and I hereby apologize for it.)  Let me list what strike me as
   the main differences in FTP-2 and examine their potential impact on
   the world.

      1. FTP-2 uses TELNET-2.  The main advantage of the new Telnet
      protocol is that it allows flexible negotiation about things like
      echoing.  But the communicators in the case of FTP are computer
      programs, not people, and don't want any echoing anyway.  The
      argument that new hosts might not know about old Telnet seems an
      unlikely one for quite some time to come if TELNET-2 ever does
      really take over the world, FTP-1 could be implemented in it.

      2. FTP-2 straightens out the "print file" mess.  This is more of a
      mess on paper than in practice, I think.  Although the protocol
      document is confusing on the subject, I think it is perfectly
      obvious what to do:  if the user specifies, and the server
      accepts, TYPE P (ASCII print file) or TYPE F (EBCDIC print file),
      then the data sent over the network should contain Fortran control
      characters.  That is, the source file should contain Fortran
      controls, and should be sent over the net as is, and reformatted
      if necessary not by the SERVER as the protocol says but by the
      RECIPIENT (server for STOR, user for RETR).  As a non-Fortran-user
      I may be missing something here but I don't think so; it is just
      like the well-understood TYPE E in which the data is sent in
      EBCDIC and the recipient can format it for local use as desired.



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      One never reformats a file from ASCII to EBCDIC at the sending
      end.  Perhaps the confusion happened because the protocol authors
      had in mind using these types to send files directly to a line
      printer at the server end, and indeed maybe that's all it's good
      for and nobody's user program will implement TYPE P RETR.  In any
      event, using a two-dimensional scheme to specify the combinations
      of ASCII/EBCDIC and ASA/normal conveys no more information than
      the present A-P-E-F scheme.  If there is any straightening out of
      FTP-2, it could only be in the handling of these files once the
      negotiation is settled, not in the negotiation process.

      3. FTP-2 approves of the Network Virtual File System concept even
      though it doesn't actually implement it.  It seems to me that the
      NVFS notion is full of pitfalls, the least of which is the problem
      of incompatibilities in filename syntax. (For example, one would
      like to be able to do random access over the network, which
      requires that different systems find a way to accommodate each
      other's rules about record sizes and so on.)  In any case, FTP-2
      doesn't really use NVFS and I mention it here only because RFC 542
      does.

      4. FTP-2 reshuffles reply codes somewhat.  The reply codes in the
      original FTP-2 document, RFC 542, don't address what I see as the
      real reply code problems.  The increased specificity of reply
      codes doesn't seem to be much of a virtue; if, say, a rename
      operation fails, it is the human user, not the FTP user program,
      who needs to know that it was because of a name conflict rather
      than some other file system error.  I am all for putting such
      information in the text part of FTP replies.  Some real problems
      are actually addressed in the reply code revision of RFC 640, in
      which the basic scheme for assigning reply code numbers is more
      rational than either the FTP-1 scheme or the original FTP-2
      scheme.  However, I think that most of the benefits of RFC 640 can
      be obtained in a way which does not require cataclysmic
      reprogramming.  More on this below.

      5. FTP-2 was established by a duly constituted ARPAnet committee
      and we are duty-bound to implement it.  I don't suppose anyone
      would actually put it that baldly, but I've heard things which
      amounted to that.  It's silly.

      6. FTP-2 specifies default sockets for the data connection.  Most
      places use the default sockets already anyway, and it is easy
      enough to ignore the 255 message if you want to.  This is a
      security issue, of course, and I'm afraid that I can't work up
      much excitement about helping the CIA keep track of what anti-war
      demonstrations I attended in 1968 and which Vietnamese hamlets to
      bomb for the greatest strategic effect even if they do pay my



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      salary indirectly.  I could rave about this subject for pages, and
      probably will if I ever get around to writing an argument against
      MAIL-2, but for now let me just get one anecdote off my chest: I
      have access to an account at an ARPAnet host because I am
      responsible at my own site for local maintenance of a program
      which was written by, and is maintained by, someone at the other
      site.  However, the other site doesn't really trust us outsiders
      (the account is shared by people in my position at several other
      hosts) to protect their vital system security, so every week they
      run a computer program to generate a new random password for the
      account (last week's was HRHPUK) and notify us all by network
      mail.  Well, on my system and at least one of the others, that
      mail isn't read protected.  I delete my mail when I read it, but
      since it is hard enough remembering HRHPUK without them changing
      it every week, I naturally write it in a file on our system.  That
      file could in principle be read protected but it isn't, since
      sometimes I'm in someone else's office when I want to use it, and
      the other passwords in it are for open guest accounts which are
      widely known.  Moral #1: Security freaks are pretty wierd.  Moral
      #2: If you have a secret don't keep it on the ARPAnet.  (In the
      past week I have heard about two newly discovered holes in Tenex
      security.)

      7. FTP-2 is available online and FTP-1 isn't, so new hosts can't
      find out how to do it.  Aargh!!!  What a reason for doing
      anything!  Surely it would be less costly for someone to type it
      in again than for everyone to reprogram.  Meanwhile these new
      hosts can ask Jon or Geoff or Bobby or even me for help in getting
      FTP up.

      8. FTP-2 has some changes to the strange MODEs and STRUs.  This is
      another thing I can't get too excited about.  We support only MODE
      S and STR F and that will probably still be true even if we are
      forced into FTP-2.  If the relatively few people who do very large
      file transfers need to improve the restart capability, they can do
      so within FTP-1 without impacting the rest of us.  The recent
      implementation of paged file transfers by TENEX shows that
      problems of individual systems can be solved within the FTP-1
      framework.  If the IBM people have some problem about record
      structure in FTP-1, for example, let them solve it in FTP-1, and
      whatever the solution is, nobody who isn't affected has to
      reprogram.

   Well, to sum up, I am pretty happy with the success I've had
   transferring files around the network the way things are.  When I do
   run into trouble it's generally because some particular host hasn't
   implemented some particular feature of FTP-1, and there's no reason
   to suppose they'll do it any faster if they also have to convert to



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   FTP-2 at the same time.  The main thing about FTP-2, as I said at the
   beginning, is that its existence is an excuse for not solving
   problems in FTP-1.  Some such problems are quite trivial except for
   the fact that people are reluctant to go against anything in the
   protocol document, as if the latter were the Holy writ.  A few
   actually require some coordinated effort.  Here is my problem list:

      1.  It is almost true that an FTP user program can understand
      reply codes by the following simple algorithm:

         a. Replies starting with 0 or 1 should be typed out and
         otherwise ignored.

         b. Replies starting with 2 indicate success (of this step or of
         the whole operation, depending on the command).

         c. Replies starting with 4 or 5 indicate failure of the
         command.

         d. Replies starting with 3 are only recognized in three cases:
         the initial 300 message, the 330 password request, and the 350
         MAIL response.  (Note that the user program need not
         distinguish which 300 message it got, merely whether or not it
         is expecting one right now.)

      The only real problem with this, aside from bugs in a few servers
      whose maintainers tell me they're working on it, is the HELP
      command, which is not in the original protocol and which returns
      0xx, 1xx, or 2xx depending on the server. (Sometimes more than one
      message is returned.)  The word from one network protocol expert
      at BBN is that (a) 050 or 030 is the correct response to HELP, and
      (b) there is a perfectly good mechanism in the protocol for
      multi-line responses.  Unfortunately this does not do much good in
      dealing with reality.  There seems to be a uniform, albeit
      contra-protocol, procedure for handling the STAT command:

         151 information
         151 information
         151 ...
         151 information
         200 END OF STATUS

      which fits right in with the above algorithm.  This is despite the
      fact that 1xx is supposed to constitute a positive response to a
      command like STAT, so that according to the protocol it ought to
      be





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         151-information
         information
         ...
         151 information

      instead.  (It seems to me, by the way, that 050 and 030 aren't
      good enough as response to HELP since they "constitute neither a
      positive nor a negative acknowledgment" of the HELP command and
      thus don't tell the user program when it ought to ask the human
      user what to do next.)  I suggest that despite the protocol, a 200
      response be given by all servers at the end of whatever other HELP
      it gives as of, let's say, June 1.  The alternatives are either to
      let the current rather chaotic situation continue forever while
      waiting for FTP-2, or to try to standardize everyone on a multi-
      line 1xx for both HELP and STAT.  I'm against changing STAT, which
      works perfectly for everyone as far as I can tell, and it should
      be clear that I'm against waiting for FTP-2.  Unfortunately there
      is no real mechanism for "officially" adopting my plan, but I bet
      if TENEX does it on June 1 the rest of the world will come along.

      2.  Another reply code problem is the use of 9xx for
      "experimental" replies not in the protocol.  This includes the BBN
      mail-forwarding message and one other that I know of.  This
      procedure is sanctioned by RFC 385, but it seems like a bad idea
      to me.  For one thing, the user program has no way of knowing
      whether the reply is positive, negative, or irrelevant.  The
      examples I've been burned by all should have been 0xx messages.  I
      propose that all such messages be given codes in the 000-599
      range, chosen to fit the scheme given above for interpreting reply
      codes. x9x or xx9 could be used to indicate experiments.

      3.  One more on reply: RFC 630 (the one about the TENEX mod to the
      reply codes for MAIL and MLFL) raises the issue of "temporary"
      versus "permanent" failures within the 4xx category.  RFC 640
      deals with this question in the FTP-2 context by changing the
      meaning of 4xx and 5xx so that the former are for temporary errors
      and the latter are for permanent errors.  I like this idea, and I
      think it could easily be adapted for FTP-1 use in a way which
      would allow people to ignore the change and still win.  At
      present, I believe that the only program which attempts to
      distinguish between temporary and permanent errors is the TENEX
      mailer.  For other programs, no distinction is currently made
      between 4xx and 5xx responses; both indicate failure, and any
      retrials are done by the human user based on the text part of the
      message.  A specific set of changes to the reply codes codes is
      proposed below.





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RFC 686                Leaving Well Enough Alone                May 1975


      Perhaps I should make a few more points about RFC 640, since it's
      the best thing about FTP-2 and the only argument for it I find at
      all convincing.  Let me try to pick out the virtues of 640 and
      indicate how they might be achieved in FTP-1.

         a. The 3xx category is used uniformly for "positive
         intermediate replies" where further negotiation in the Telnet
         connection is required, as for RNFR.  I'm afraid this one can't
         be changed without affecting existing user programs.  (One of
         my goals here is to enable exiting user programs to work while
         some servers continue as now and others adopt the suggestions I
         make below.)  However, although this 3xx idea is logically
         pleasing, it is not really necessary for a simple-minded user
         program to be able to interpret replies.  The only really new
         3xx in RFC 640 is the 350 code for RNFR.  But this would only
         be a real improvement for the user program if there were also a
         2xx code which might be returned after RNFR, which is not the
         case.  640 also abolishes the 300 initial connection message
         with 220, but again there is clearly no conflict here.

         b. The use of 1xx is expanded to include what is now the 250
         code for the beginning of a file transfer.  The idea is that a
         1xx message doesn't affect the state of the user process, but
         this is not really true.  Consider the file transfer commands.
         The state diagram on page 13 of RFC 640 is slightly misleading.
         It appears as if 1xx replies are simply ignored by the user
         program.  In reality, that little loop hides a lot of work: the
         file transfer itself!  If the server replied to the file
         transfer command immediately with a 2xx message, it would be a
         bug in the server, not a successful transfer.  The real state
         diagram is more like

            B --> cmd --> W --> 1 --> W --> 2 --> S

         (with branches out from the "W"s for bad replies).  It should
         be clear from this diagram that the user program, if it trusts
         the server to know what it's doing, can expect a 2xx instead of
         the 1xx without getting confused, since it knows which of the W
         states it's in.  In fact, the use of 1xx in file transfer is
         very different from its other uses, which are indeed more like
         the 0xx and 1xx replies in FTP-1.  I'd call this particular
         point a bug in RFC 640.

         c.  Automatic programs which use FTP (like mailers) can decide
         whether to queue or abandon an unsuccessful transfer based on
         the distinction between 4xx and 5xx codes.  I like this idea,
         although those temporary errors virtually never happen in real
         life.  This could be accomplished in FTP-1 by moving many of



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         the 4xx replies to 5xx.  Mailers would be modified to use the
         first digit to decide whether or not to retry.  This scheme
         does not cause any catastrophes; if some server is slow in
         converting it merely leads to unnecessary retries.  A few CPU
         cycles would be wasted in the month following the official
         switch.  Thus, this feature is very different from (a) and (b),
         which could lead to catastrophic failures if not implemented
         all at once.  (Yes, I know that FTP-2 is supposed to be done on
         a different ICP socket.  I am not discussing FTP-2 but whether
         its virtues can be transferred to FTP-1.)  The specific codes
         involved are listed below.

         d.  The use of the second digit to indicate the type of
         message. (The proposed division is not totally clean; for
         example, why is 150 ("file status okay; about to open data
         connection") considered to be more about the file system than
         about data connection?)  This can easily be done, since the
         second digit is not currently important to any user process--
         the TENEX mailer is, in this plan, already due for modification
         because of (c).  Since this is mostly an aesthetic point, I'm
         hesitant to do it if it would be difficult for anyone.  In
         particular, I would want to leave the 25x messages alone, in
         case some user programs distinguish these.  This is especially
         likely for the ones which are entirely meant for the program:
         251 and 255.  Therefore I propose that if this idea is adopted
         in FTP-1 the meanings of x2x and x5x be interchanged.  This
         proposal is reflected in the specific list below.

      4.  The print file thing again.  Let's get it made "official" that
      it is the recipient, not the server, who is responsible for any
      reformatting which is to be done on these files.  After all, the
      recipient knows what his own print programs want.

   Let me summarize the specific changes to FTP-1 I'd like to see made,
   most of which are merely documentation changes to reflect reality:

      1. HELP should return 200.  All commands should return 2xx if
      successful, and I believe all do except HELP.

      2. The definition of 1xx messages should be changed to read:
      "Informative replies to status inquiries.  These constitute
      neither a positive nor negative acknowledgment."

      3. Experimental reply codes should be of the form x9x or xx9,
      where the first digit is chosen to reflect the significance of the
      reply to automated user programs.  Reply codes greater than 599
      are not permitted.  The xx9 form should be used if the reply falls
      into one of the existing categories for the second digit.  User



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      programs are encouraged to determine the significance of the reply
      from the first digit, rather than requiring a specific reply code,
      when possible.

      4. The STAT command with no argument is considered a request for a
      directory listing for the current working directory, except that
      it may be given along with TELNET SYNCH while a transfer is in
      progress, in which case it is a request for the status of that
      transfer. (Everyone seems to do the first part of this.  I'm not
      sure if anyone actually implements the second.  This is just
      getting the protocol to agree with reality.) The reply to a STAT
      command should be zero or more 1xx messages followed by a 200.

      5. TYPEs P and F mean that the source file contains ASA control
      characters and that the recipient program should reformat it if
      necessary.

   Here is a list of the current FTP-1 replies, and how they should be
   renumbered for the new scheme.  The changes from 4xx to 5xx should be
   REQUIRED as of June 1; changes in the second or third digit are not
   so important. (As explained above, it will not be catastrophic even
   if some hosts do not meet the requirement.)  The list also contains
   one new possible reply adapted from RFC 640.

   OLD    NEW     TEXT
   0x0    0x0     (These messages are not very well defined nor
       very important.  Servers should use their judgment.)
   100    110     System status reply.  (Since nobody does STAT
       as in the protocol, this may be a moot point.)
   150    150     "File status reply."  (If this were really that,
       it would be switched to 120, but I believe what is meant is
       the response to a bare STAT in mid-transfer, which is more
       a connection status reply than a file status reply.
   151    121     Directory listing reply.
   200    200     Last command ok.
   201    251     ABOR ok.
   202    252     ABOR ignored, no transfer in progress.
   new    206     Command ignored, superfluous here.
   230    230     Login complete.
   231    231     Logout complete.
   232    232     Logout command will be processed when
       transfer is complete.
   250    250     Transfer started correctly.
   251    251     MARK yyyy = mmmm
   252    252     Transfer completed ok.
   253    223     Rename ok.
   254    224     Delete ok.
   255    255     SOCK nnnn



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   256    256     Mail completed ok.
   300    300     Connection greeting
   301    301     Command incomplete (no crlf)
   330    330     Enter password
   350    350     Enter mail.
   400    huh?    "This service not implemented." I don't
       understand this; how does it differ from 506?  If it means
       no FTP at all, who gave the message?  Flush.
   401    451     Service not accepting users now, goodbye.
   430    430     Foo, you are a password hacker!
   431    531     Invalid user or password.
   432    532     User invalid for this service.
   434    454     Logout by operator.
   435    455     Logout by system.
   436    456     Service shutting down.
   450    520     File not found.
   451    521     Access denied.
   452    452     Transfer incomplete, connection closed.
   453    423     Transfer incomplete, insufficient storage space.
   454    454     Can't connect to your socket.
   500    500     Command gibberish.
   501    501     Argument gibberish.
   502    502     Argument missing.
   503    503     Arguments conflict.
   504    504     You can't get there from here.
   505    505     Command conflicts with previous command.
   506    506     Action not implemented.


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Harvey                                                          [Page 9]
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