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Network Working Group                                          B. Billon
Internet-Draft                                                     Splio
Intended status: Standards Track                               J. Levine
Expires: 16 October 2022                                   Standcore LLC
                                                           14 April 2022


            Updated Use of the Expires Message Header Field
                        draft-billon-expires-03

Abstract

   This document allows broader use of the Expires message header field
   for SMTP.  Senders can then indicate when a message sent becomes
   valueless and can safely be deleted, while recipients would use the
   information to delete these valueless messages.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 16 October 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   provided without warranty as described in the Revised BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Header Field example  . . . . . . . . . . . . . . . . . . . .   2
   3.  Security considerations . . . . . . . . . . . . . . . . . . .   3
   4.  Advice to Senders . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Advice to Receivers (Mailbox providers, Webmails and MUAs)  .   3
   6.  Past History of the Expires: header . . . . . . . . . . . . .   4
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   4
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   10. Informative References  . . . . . . . . . . . . . . . . . . .   4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   The date and time of expiration can be used by the mailbox provider
   or the MUA to indicate to the user that certain messages could be
   deleted, in an attempt to unclutter the user's mailbox and spare
   storage resources.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Header Field example

   The field definition and syntax remain the same.

   expires = "Expires" ":" date-time

   Example:

   Expires: Wed, 1 Dec 2021 17:22:57 +0000

   Senders MUST NOT include more than one Expires header in the message
   they send.

   If there is more than one Expires header then receivers SHOULD treat
   this as if no Expires header is present.









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3.  Security considerations

   Dates in this header can be set a long way in the past or in the
   future, including outside the range of internal time representations
   in some programming environments - all software which processes the
   Expires header MUST be made safe against this possibility.

4.  Advice to Senders

   Senders SHOULD add the header field along with a relevant date and
   time when they know that the content of the message has no value
   after a given point of time (e.g.  Commercial newsletters
   --especially when including time-limited offers, Event announcements,
   Social notifications, Time-limited access codes ...).

   In all other cases, senders SHOULD NOT set an Expires header.

5.  Advice to Receivers (Mailbox providers, Webmails and MUAs)

   The expiration of a message's validity would logically lead to the
   deletion of the message.  However, users on most systems do not
   expect their emails to disappear, and may not be aware that any
   particular email has an Expires header.  Therefore, no email should
   be silently and automatically deleted solely based on the value of
   the Expires header field.

   Mailbox providers SHOULD explain to users how the information
   provided in the Expires header are processed, SHOULD indicate when
   viewing an expired message, and SHOULD give users control over the
   actions to take for expired messages.

   The information provided in the header should be used as a signal
   that could be used to provide a feature or improved experience to the
   end-user.  For instance, systems may allow users to set up an
   automatic rule to clean up expired email from specific senders or
   with specific characteristics, or provide a mode to quickly view and
   process all expired email.

   In certain cases, email messages can be used as proof or element of
   investigation.  As an early deletion may compromise the intended
   investigation, mailbox providers can ignore the Expires information
   in such cases.

   Presence of the Expires header field MUST NOT be interpreted as a
   sign of legitimacy.






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6.  Past History of the Expires: header

   [RFC4021] defines a number of header fields that can be added to
   Internet messages such as those used for mapping between X.400 and
   RFC822/MIME [RFC2156].  One of them is the Expires header field that
   provides the date and time at which a message is considered to lose
   its validity.

   The same principle can be applied to the Expires header field in a
   SMTP context, whether the message comes from a X.400 gateway as
   initially intended in [RFC2156], or from a RFC821/SMTP MTA.

7.  Acknowledgements

   This document was informed by discussions with and/or contributions
   from Jonathan Loriaux, Charles Sauthier and Simon Bressier.

8.  IANA Considerations

   IANA is requested to update an existing entry in the Permanent
   Message Headers Field Names registry
   (https://www.iana.org/assignments/message-headers/message-
   headers.xhtml)

   Header field name: Expires

   Applicable protocol: mail

   Status: standard

   Author/Change controller: IETF

   Specification document: this document

9.  Normative References

   [RFC2156]  Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay):
              Mapping between X.400 and RFC 822/MIME", RFC 2156,
              DOI 10.17487/RFC2156, January 1998,
              <https://www.rfc-editor.org/info/rfc2156>.

10.  Informative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.




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   [RFC4021]  Klyne, G. and J. Palme, "Registration of Mail and MIME
              Header Fields", RFC 4021, DOI 10.17487/RFC4021, March
              2005, <https://www.rfc-editor.org/info/rfc4021>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Authors' Addresses

   Benjamin Billon
   Splio
   Email: bbillon@splio.com


   John Levine
   Standcore LLC
   Email: standards@standcore.com

































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