Comments on the File Transfer Protocol

This document replaces RFC 607, which was inadvertently released

while still in rough draft form. It would be appreciated if RFC 607

were disregarded, and this document considered the accurate statement

of the authors' opinions.

There are several aspects of the File Transfer Protocol of RFC

542 that constitute serious drawbacks. Some of these are quite basic

in nature, and imply substantial design changes; these will be

discussed in a later RFC. Others could be remedied with very little

effort, and this should be done as soon as possible.

Following is a list of those problems that can be easily solved,

together with their proposed solutions:

1 Once a server has been set to the state where he is "passive"

with regard to establishment of data connections, there is no

convenient way for the user to make him "active" again. The

"REIN" command accomplishes this, but affects more than just the

desired active/passive state. SOLUTION: define a new command,

with a command verb of "ACTV", to mean that the server is to issue

a CONNECT rather than a LISTEN on the data socket. If the server

is already "active", the command is a no op. "ACTV" is to have

the same reply codes as "PASV".

2 Design of an FTP server or user would be simpler if all

command verbs were the same length. While it is certainly

possible to handle varying length verbs, fixed length string

manipulation is in general easier to write and faster to run than

varying length string manipulation, and it would seem that nothing

is to be gained in this application by allowing varying length

strings. SOLUTION: replace the only three-letter verb, "BYE",

with a four-letter one, such as "QUIT", and constrain future

command verbs to be four letters long.

3 The order of the handshaking elements following a file transfer

command is left unspecified. After sending a STOR command, for

example, a user process has no way of knowing which to wait for

first, the "250 FILE TRANSFER STARTED" reply, or establishment of

the data connection. SOLUTION: specify that the server is to

send a "250" reply before attempting to establish the data

connection. If it is desired to check if the user is logged in,

if the file exists, or if the user is to be allowed access to the

file, these checks must be made before any reply is sent. The

text of the "250" reply would perhaps be more appropriate as "250

OPENING DATA CONNECTION", since it comes before actual data

transfer begins. If the server wishes to send an error reply in

the event that the data connection cannot be opened, it is to be

sent in lieu of the "252 TRANSFER COMPLETE" reply.

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4 Some hosts currently send an error reply on receipt of a

command that is unimplemented because it is hot needed (e.g.,

"ACCT" or "ALLO"). Even though the text of the reply indicates

that the command has been ignored, it is obviously impossible for

a user process to know that there is no real "error". SOLUTION:

require that any server that does not support a particular command

because it is not needed in that system must return the success

reply for that command.

5 There is no specified maximum length of a TELNET command line,

TELNET reply line, user name, password, account, or pathname. It

is true that every system implementing an FTP server likely has

different maxima for its own parameters, but it is inconvenient,

at least in some systems, for the writer of an FTP user (which

must converse with many FTP servers) to construct an indefinite

length buffer. Similar difficulties confront the writer of a

server FTP. SOLUTION: specify a maximum length for TELNET

command lines, TELNET replies, user names, passwords, account

numbers, and pathnames. This is to be done after conducting a

Poll of serving sites concerning their individual maxima. If

Network mail is to be included in FTP, the mail text, if sent over

the TELNET connection, is to be subject to the same line length

maximum.

6 The notion of allowing continuation lines to start with

arbitrary text solves a minor problem for a few server FTP

implementors at the expense of creating a major problem for all

user FTP implementors. The logic needed to decode a multi-line

reply is unnecessarily complex, and made an order of magnitude

more so by the fact that multi-line replies arc allowed to be

nested. SOLUTION: assign a unique (numeric) reply code, such as

"009", to be used on all lines of a multi-line reply after the

first. The reply code used for this purpose must begin with "0"

(it cannot be three blanks, for example), so that it will appear

as extraneous to a user process by virtue of the already existing

rules concerning reply code groupings.

7 If it is the case that the above solution to (6) is not

accepted, the fact that the maximum allowed level of nesting is

left unspecified creates a hardship for implementors of user FTPs.

This hardship is somewhat easily solved on a machine that has

hardware stacks, but not so for other machines. SOLUTION: either

disallow nested replies (preferred), or specify a maximum level of

nesting of multi-line replies.

8 The prose descriptions of the meanings of the various reply

codes are in several cases unclear or ambiguous. For example, the

code "020" is explained only as "announcing FTP". It is given as

a reply that can be issued when a server cannot accept input

immediately after an ICP, but its exact meaning is not obvious.

Also. the code "331" is said to mean "ENTER ACCOUNT (if required

as part of login sequence)", but is listed as a possible success

reply for most of the commands. The explanation indicates that it

is only valid in the login sequence, but the command-reply

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correspondence table implies that it also means, "I can't do that

without an account". SOLUTION: an expanded effort should be made

by those who originated the reply codes to define them more

completely.

A major complaint about the protocol concerns the fact that the

writer of an FTP user process must handle a considerable number of

special cases merely to determine Whether or not the last command

sent was successful. It is admitted that the protocol is

well-defined in all the following areas, but it is important to

realize that the characteristic "well-defined" is necessary, but hot

sufficient; for many reasons, it is very desirable to employ the

simplest mechanism that satisfies all the needs. Following is a list

of those drawbacks that unduly complicate the flow chart of an FTP

user process:

9 Different commands have different success reply codes. A

successful "USER" command, for example, returns a "230", whereas a

successful "BYTE" command returns a "200". The stated concept

that the first digit would carry this information does not apply,

as "100" means success for "STAT", and "200" means success for

"SOCK". SOLUTION: specify that any command must return a reply

code beginning with some unique digit, such as "2", if successful,

and anything other than that digit if not successful. For

example this includes changing the success reply for STAT,

Perhaps to "200".

10 Some commands have multiple possible success reply codes,

e.g., "USER" and "REIN". It is undesirable for ah FTP user to be

required to keep a list of reply codes for each command, all of

which mean "command accepted, continue". Again, the stated

concept concerning the first digit fails, as "230" and "330" are

in truth both acknowledgments to a successful "USER" command.

SOLUTION: same as for (9) above. The desire to communicate more

specific information than simply "yes" or "no", such as the

difficulty that some servers do not need all the login parameters,

may be solved by having, for example, "230" mean "PASSWORD

ACCEPTED, YOU ARE NOW LOGGED IN", and "237" mean "PASSWORD

ACCEPTED, ACCOUNT NOW NEEDED". Given the solution to (4) above, a

user process becomes much less interested in the difference

between "YOU ARE NOW LOGGED IN" and "ACCOUNT NOW NEEDED". The

important point is that the idea of "command accepted" is conveyed

by the initial "2, and that finer gradations of meaning can be

deduced by the user process, if desired.

11 The meanings of the various connection greeting reply codes

are somewhat inconsistent. "300 connection greeting, awaiting

input", if intended as a positive acknowledgments to the ICP,

should be a 200-series reply, or if intended to be purely

informative, a 000-series reply. If the former, then clearly "020

expected delay" is the corresponding negative acknowledgments, and

should be a 400-series reply. It is however unlikely that

notification of an expected delay would be of importance to a user

Process without knowledge of the length of the delay. SOLUTION.:

change "300 connection greeting" to a 000-series reply, perhaps

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"011" (preferred), or change "300 connection greeting" to a

200-series reply, perhaps "211", and "020 expected delay" to a

400-series reply, perhaps "411".

In addition to the above mentioned weaknesses in the protocol,

the following is believed to be a typographical error:

12 Reply code "332 LOGIN PLEASE" is not listed anywhere in the

command-reply correspondence table. It Would seem that this would

be a more-information-needed (success) reply for all those

commands which require the user to be logged in. It should also

be stressed that the "332" code is to be used for this purpose, as

many servers currently use other codes, such as "451" and "504",

to mean "LOGIN PLEASE".

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