NWG/RFC 385 Abhay K. Bhushan
NIC 11357 MIT-MAC
Updates: RFC 354 August 18, 1972
RFC 354
 COMMENTS ON THE FILE TRANSFER PROTOCOL (RFC 354)
 ------------------------------------------------
 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 RFC 354 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, RFC 354). 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 RFC 354. 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 RFC 354. 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/RFC 189, 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, RFC 189) 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 RFC at 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, RFC 354).
 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 RFC was put into machine readable form for entry ]
 [ into the online RFC archives by BBN Corp. under the ]
 [ direction of Alex McKenzie. 1/97 ]