Comments on the File Transfer Protocol

There are several aspects of the File Transfer Protocol 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 told to be "passive" with regard to establishment

of data connections, there is no way for the user to make him "active"

again. 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 would be simpler if all command verbs were the

same length, and design of an FTP user would be simpler if either all

command verbs were the same length, or if multiple blanks were allowed

following the verb. 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.

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

that is unimplemented because it is not 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 a

success reply.

5 There is no specified maximum length of a TELNET 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

nearly impossible for the writer of an FTP user (which must converse with

many FTP servers) to construct an indefinite length buffer. Typically some

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arbitrary maximum must be chosen. SOLUTION: specify a maximum length for

TELNET lines, user names, passwords, account numbers, and pathnames. This

is to be done after conducting a poll of serving sites concerning their

individual maxima.

6 The notion of allowing continuation lines to start with arbitrary text

solves a minor problem for a few server FTP implementers at the expense of

creating a major problem for all user FTP implementers. 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 are 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.

7 Given that multi-line replies are allowed to be nested, the fact that

the maximum allowed level of nesting is left unspecified creates a hardship

for implementers of user FTPs. This hardship is somewhat easily solved on a

machine that has hardware stacks, but not so for other machines. SOLUTION:

specify a maximum level of nesting of multi-line replies.

8 In blocked mode, the protocol states that "all end-of-record markers

(EOR) are explicit, including the final one." This prohibits sending data

between the final end of record and the end of file, but does not specify

what the receiver of data is to do if this rule is broken. That is, should

the intervening data be discarded or treated as a new (final) record?

SOLUTION: specify that if an end-of-file marker is not immediately preceded

by an end-of-record marker, the intervening data is to be discarded.

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 not 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 concept that success replies should have an

even first digit and failure replies an odd first digit does not apply, as

"100, means success for "STAT", and "402" means failure for "BYTE".

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.

10 Some commands have multiple possible success reply codes, e.g., "USER",

"REIN", and "BYE". It is undesirable for an FTP user to be required to keep

a list of reply codes for each command, all of which mean "command

accepted, continue". SOLUTION: same as for (9.) above. The desire to

communicate more specific information than simply "yes" or "no", such as

the difficulty in the login procedure that some sites do not need all the

parameters, may be solved by having, for example, "238" mean "PASSWORD

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

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.

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11 There are several types of replies that are extraneous from the point

of view of a user FTP process, and their reply codes have no characteristic

that makes them easily distinguishable. For example, "010 message from

operator" and "050 FTP commentary" are superfluous to a user process, and

"000 announcing FTP" (in place of "300" greeting) is not. SOLUTION: specify

that any reply that has meaning only to a human user and not to a user

process must have a reply code beginning with a unique digit, such as "0".

The continuation line reply code proposed in (8.) above falls into this

category, and therefore must start with the same unique digit.

12 The notion of a server sending a "000 announcing FTP" or a "020

expected delay" immediately after completion of the ICP if input cannot be

accepted right away is another instance of multiple reply codes having the

same meaning to a user process. SOLUTION: require that the server send a

reply with a "020" reply code in the situation cited. If it is desired to

communicate more detailed information, the text of the reply may used for

this purpose.

In addition to the above mentioned weaknesses in the protocol, the

following is believed to be a typographical error:

13 Reply code "331" is cited as a possible success reply code for the

commands "BYTE", "SOCK", "PASV", "TYPE", "STRU", "MODE", "ALLO", "REST",

"SITE", AND "STAT". This reply code means "ENTER ACCOUNT" (if required as

part of login sequence), and perhaps should be "332 LOGIN FIRST, PLEASE".

This is especially indicated by the fact that "332" is not listed anywhere

in the command-reply correspondence table.

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