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

        FTP Error Code Usage for More Reliable Mail Service
  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.

  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 [Page 1] RFC 630 FTP Error Code Usage for Mail Service April 1974

  The following changes will be installed at TENEX sites over the next

couple of months.


  The TENEX FTP server will continue to use 452 and 454 as specified

for the MLFL command.

  For MAIL and MLFL, it will send the other codes as follows:

450 Permanent failures due to the user addressed in the Mail or MLFL

  Examples:  No such user; No mailbox for that user; Can't access file
  (because net users can't write in that mailbox).

451 Permanent failures due to the message itself.

  Example:  Line sent over TELNET connection is too long (MAIL command

453 Temporary failures

  Examples:  TELNET connection unexpectedly closed;  Mailbox busy;
  Unexpected local errors (such as failure to create scratch file).


  TENEX mailing processes currently interpret all the codes 450-454 as

meaning permanent failure. They will be changed to interpret 452, 453, and 454 as temporary while leaving 450 and 451 permanent.

  These interpretations should not adversely affect the interaction of

TENEX and non-TENEX mail processes, since we are simply changing from one arbitrary set of interpretations to another. Moreover:

–Our interpretation of 450-451 as permanent and 452-454 as temporary is consistent with their original meanings.

–Our new choice of what codes to use for what failure is no farther from the original meanings than our old choice was, and conveys more information.

Sussman [Page 2] RFC 630 FTP Error Code Usage for Mail Service April 1974

     [ This RFC was put into machine readable form for entry ]
     [ into the online RFC archives by Alex McKenzie with    ]
     [ support from GTE, formerly BBN Corp.            10/99 ]

Sussman [Page 3]

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