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IETF RFC 686
Leaving well enough alone
Last modified on Tuesday, December 5th, 2000
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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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RFC 686 Leaving Well Enough Alone May 1975
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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RFC 686 Leaving Well Enough Alone May 1975
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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RFC 686 Leaving Well Enough Alone May 1975
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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RFC 686 Leaving Well Enough Alone May 1975
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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RFC 686 Leaving Well Enough Alone May 1975
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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RFC 686 Leaving Well Enough Alone May 1975
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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RFC 686 Leaving Well Enough Alone May 1975
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.
[ This RFC was put into machine readable form for entry ]
[ into the online RFC archives by Via Genie 3/00 ]
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Leaving well enough alone
RFC TOTAL SIZE: 24605 bytes
PUBLICATION DATE: Tuesday, December 5th, 2000
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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