RFC385

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.

NWG/RFC 385 Page 2

       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.

NWG/RFC 385 Page 2

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.

NWG/RFC 385 Page 2

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

NWG/RFC 385 Page 2

   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.

NWG/RFC 385 Page 2

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 ]