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rfc: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.
                                                              [Page 1]

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.
                                                              [Page 2]

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.
                                                              [Page 3]

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
                                                              [Page 4]

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.
                                                              [Page 5]

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 ]
                                                              [Page 6]
/data/webs/external/dokuwiki/data/pages/rfc/rfc385.txt · Last modified: 1997/03/05 20:02 (external edit)