RFC385 - Comments on the File Transfer Protocol

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　　NWG/RFC385 Abhay K. Bhushan
NIC 11357 MIT-MAC
Updates: RFC354 August 18, 1972
RFC354

COMMENTS ON THE FILE TRANSFER PROTOCOL (RFC354)
------------------------------------------------

The following comments pertain to the File Transfer Protocol, NWG/RFC
354. The comments include errata, further discussion, emphasis
points, and additions to the protocol. I shall incorporate these
comments into the main protocol document after we have had sufficient
experience.

1. Please note the following corrections:
(i) Page 2, line 15: replace user-FTP by server-FTP.
(ii) Page 3, line 12: replace III.A by III.C.
(iii) Page 15, last para, line 1: replace user s by user is.
(iv) Page 28, line 21: replace _CRCRLF_ by _CRLF_.
(v) Page 27, line 10: replace 451,451 by 451.
(vi) Note that on Page 26, line 15 mode code is S|B|T|H.

2. The language of RFC354 reads that it is recommended for
hosts to implement the default parameters. The sense of the
word recommended should be taken as required. Thus the
required minimum implementations for FTP servers is:

Type - ASCII (8-bit bytes)
Mode - Stream
Structure - File
Commands - RETR, STOR, USER (and PASS), SOCK and BYE

3. The "Print File-ASCII" and "EBCIDIC Print File" types are
incorrectly specified (pages 10 and 11, RFC354). The real
problem with print files is of ASA (Fortran) vertical format
control. Instead of the two print file types, there should
really be three types as described below:

BCDIC - The sender transfers data using the EBCDIC
character code and 8-bit transfer byte size.
The _CRLF_ convention is used for vertical format
control. This type will be used for efficient
transfer of EBCDIC files between systems which
use EBCDIC for their internal character
representation.

ASCII with ASA vertical format Control - This is the
"Print file-ASCII" defined in RFC354. The
server is to transform the data in accordance
with ASA (Fortran) vertical format control
procedures for printing on printers that
still use this standard. The data is to be
transferred as 8-bit bytes.

EBCDIC with ASA vertical format control - This is the
EBCDIC Print File defined in RFC354. The
server is to transform the data in accordance
with ASA (Fortran) vertical format control
standards but using the EBCDIC character code.
The data is to be transferred in 8-bit bytes.

The new types are to be denoted by symbols E for EBCDIC, P
for Print file-ASCII and F for Formatted (ASA standard)
EBCDIC print file. A discussion of the ASA vertical format
control appears in NWG/RFC189, Appendix C, and in
Communications of the ACM, Vol 7, No. 10, p. 606, October
1964. According to the ASA vertical format control
standards, the first character of a formatted record is not
printed but determines vertical spacing as follows:

Character Vertical Spacing before printing
--------- --------------------------------
Blank One line
0 Two lines
1 To first line of next page
+ No advance

In addition to the above four, there are more characters
(defined in Appendix C, RFC189) which represent an IBM
extension to the ASA standard.

4. A comparison of "stream" and "text" modes is in order. The
advantages of "stream" mode are:
1) The receiver need not scan the incoming bytes.
2) It is usable with all data types.

The disadvantages are:
1) The EOF by closing the connection is not reliable.

2) The EOR by ASCII _CRLF_ is unreliable as the _CRLF_
really may be valid data rather than an EOR. It is
an EOR only if the sender and receiver have a _prior_
agreement to that effect.

5. In the Block mode the protocol states that left-most bits not
containing information should be zero. It appears that some
sites have difficulty sending null bytes in the beginning of
a block. Since it is really not necessary for these bytes to
be zero, these bits are now defined to be "don't care" bits.

6. In the use of block mode it is possible for two or more
conditions requiring different descriptor codes (suspected
errors and either end of record or end of file) to exist
simultaneously. Such a possibility may be handled by sending
a separate EOR or EOF block with a zero byte count (this is
allowed by the protocol). Also it should be noted that an
EOF is an implicit EOR.

7. It needs to be emphasized again that the user-FTP must
"listen" on the data socket prior to sending a command
requiring a file transfer. Specifically the user-FTP should
not wait for a 255 reply (server data socket) before doing
the "listen". (The security check may be come later, as the
data connection can be closed if connection is to a socket
other than that specified by the 255 reply). Although the
protocol suggests that the 255 reply would be sent before
making the connection, it does not guarantee that the 255
reply would arrive before the initiating RFCat the user
site. The above argument also applies to receiving a a close
(NCP-CLS) on the data connection before receiving a reply
indicating the reason for the close (note assertion on page
24, paragraph 3, RFC354).

8. Although the protocol does not restrict closing or leaving
open the data connection in Block and Text modes, it should
be emphasized that the closing of the data connection, if it
is to be done at all, should be done immediately after the
file transfer rather than just after a new transfer command
is received. This is because the server and user may have to
test whether the data connection is open or not before doing
a "listen" or an "init" respectively.

9. It should be emphasized again that 'Type' supersedes 'Byte',
and that the TYPE command should be sent before the BYTE
command.

10. It should be noted that both upper and lower case alphabetic
characters are to be treated identically in the command
syntax. This applies also to the symbols for type, mode,
and structure. For example, 'A' and 'a' both indicate ASCII
type.

11. It should be noted that in the 'LIST' command, the data
transfer is over the data connection in type ASCII.

12. The following reply code is to be added:

454 FTP: Cannot connect to your data socket.

This is a fail response any of the commands requiring data
transfer (including RETR, STOR, APPE, and LIST)

13. Rather than use the append command for sending mail files, a
new command 'MLFL' (for mail file) is defined. The syntax
of the mail file command is:

MLFL <user>CRLF
where
<user> ::= <empty>| <NIC ident>| <sys ident>

If the user field is empty or blank (one or more spaces),
then the mail is destined for a printer or other designated
place for site mail. <NIC ident> refers to the standard
identification described in the NIC Directory of Network
Participant. A serving host may keep a table mapping <NIC
ident> into <sys ident>. This would provide for uniform
convenient usage. <sys ident> is the user's normal
identification at the serving HOST. The use of <sys ident>
would allow a network user to send mail to other users who
do not have NIC identification but whose <sys ident> is
known.

The intent of this command is to enable a user at the user
site to mail data (in form of a file) to another user at the
server site. It should be noted that the files to be mailed
are transmitted via the data connection in ASCII type.
These files should be appended to the destination user's
mail by the server in accordance with serving Host mail
conventions. The mail my be marked as sent from the
particular using HOST and the user specified by the 'USER'
command. The reply codes for the "MLFL" command are
identical to that in the "APPE" command, as shown below:

COMMAND SUCCESS FAIL
------- ------- ----
MLFL 250 451,454,500-506
Sec. reply 252 452,453

14. The 'MLFL' command for network mail, though a useful and
essential addition to the FTP command repertoire, does not

allow TIP users to send mail conveniently without using
third hosts. It would be more convenient for TIP users to
send mail over the TELNET connection instead of the data
connection as provided by the 'MLFL' command. The following
'MAIL' command is therefore defined to send mail via the
TELNET connection:

MAIL <user>CRLF

the syntax of <user> is identical to that in the MLFL
command described above. After the 'MAIL' command is
received, the server is to treat the following lines as text
of the mail sent by the user. The mail text is to be
terminated by a line containing only a single period, that
is the character sequence ".CRLF" in a new line. The
following new reply codes are defined to handle the mail
command:

350 Enter mail, terminate by a line with only a '.'
256 Mail completed.

The reply codes are:

COMMAND SUCCESS FAIL
------- ------- ----
MAIL 350 450,451,500-506
Sec Reply 256

15. An additional access control command called account (ACCT)
is now defined to facilitate accounting in systems such as
TENEX which require in addition to user and password, a
separate account specification. The 'ACCT' command is
different from the 'PASS' command in that it is not
necessarily related to the 'USER' command and may arrive at
any time. For example, a user may transfer different files
using different accounts. The 'ACCT' command has the same
reply codes as the 'PASS' command (230 for success and 430-
432,500-506 for fail). Some servers may require that an
account command must be sent before the user is "logged in".
For suchcases the success reply to the 'PASS' command could
be '330 Enter account'.

16. Since password information is quite sensitive, it is
desirable in general to "mask" it or suppress type out. It
appears that the server has really no fool-proof effective
way to achieve this. It is therefore the user-FTP process
responsibility to hide the sensitive password information.

17. The FTP is an open-ended protocol designed for easy
expandability. Experimental commands may be defined by
sites wishing to implement such commands. These
experimental commands should begin with the alphabetic
character 'X'. Standard reply codes may be used with these
commands. If new reply codes need to assigned, these
should be chosen between 900 and 999. If the experimental
command is useful and of general interest, it shall be
included in the FTP command repertoire.

[ This RFCwas put into machine readable form for entry ]
[ into the online RFCarchives by BBN Corp. under the ]
[ direction of Alex McKenzie. 1/97 ]

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