Comments on the File Transfer Protocol (RFC0607)
Original Publication Date: 1974-Jan-07
Included in the Prior Art Database: 2000-Sep-13
Internet Society Requests For Comment (RFCs)
M. Krilanovich: AUTHOR [+2]
Request for Comments: 607 Mark Krilanovich
NIC # 21255 George Gregg
references: RFC #542 UCSB Jan 1974
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"
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
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