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NWG/RFC 385                                       Abhay K. BhushanNIC 11357                                                  MIT-MACUpdates: RFC354                                  August 18, 1972RFC 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 ofRFC 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 inRFC 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 inRFC 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 inAppendix 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]