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+Network Working Group                                  Mark Krilanovich (UCSB)
+Request for Comments: 624                                  George Gregg (UCSB)
+NIC #22054                                            Wayne Hathaway (AMES-67)
+references: RFC 542                                        Jim White (SRI-ARC)
+obsoletes: RFC 607                                                   Feb  1974
+      
+                                                           
+              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.
+
+                                 -1-
+
+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
+
+                                 -2-
+
+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
+
+                                 -3-
+
+"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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+                                   -4-
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