RFC 630 (rfc630) - Page 1 of 3

FTP error code usage for more reliable mail service

Network Working Group                                     Julie Sussman
RFC # 630                                                 BBN
NIC # 30237                                               April 10, 1974


          FTP Error Code Usage for More Reliable Mail Service


                                PURPOSE

    A major hindrance to providing reliable mail service is the lack of
well-defined FTP error replies that would enable a mailing process to
decide how to handle a failure.  New FTP error codes are currently in
the design stage, and a proposal will be announced soon.  In the
interim, we can get some improvement by simply defining how we intend to
use the current FTP codes.  The purpose of this RFC is to inform all
sites of how TENEX sites will use and interpret the codes starting in
the near future.


                        CURRENT CODE DEFINITIONS

    The FTP error codes defined for failure to perform a file action
(including mail) are:

450 File not found
451 File access denied to you
452 Data connection closed
453 Insufficient storage
454 Cannot connect to your data socket

450, 451, and 453 are applicable to both the MAIL and MLFL commands,
while 452 and 454 are only meaningful for MLFL.


                   SHORTCOMING OF CURRENT DEFINITIONS

    There are more possible causes of failure to deliver mail than the
ones defined above.  Implementors of FTP servers thus had to make
arbitrary assignments of error conditions to defined codes.  As a
result, although the text of the reply might distinguish these
conditions for the benefit of human users, the code doesn't distinguish
them for the benefit of processes.

    The minimum distinction needed by the TENEX mail-sending processes
is between permanent and non-permanent failures.  In the latter case,
the process will repeatedly try to deliver the mail for several days.




Sussman