Updated Use of the Expires Message Header Field
draft-billon-expires-08

Network Working Group                                          B. Billon
Internet-Draft                                                     Splio
Intended status: Standards Track                               J. Levine
Expires: 14 June 2023                                      Standcore LLC
                                                        11 December 2022

            Updated Use of the Expires Message Header Field
                        draft-billon-expires-08

Abstract

   This document allows broader use of the Expires message header field
   for mail messages.  Message creators can then indicate when a message
   loses its validity, while recipients would use the information to
   ignore or change the display of these messages.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Header Field definition . . . . . . . . . . . . . . . . . . .   2
   3.  Advice to Message Creators  . . . . . . . . . . . . . . . . .   3
   4.  Advice to Message Readers . . . . . . . . . . . . . . . . . .   3
   5.  Security considerations . . . . . . . . . . . . . . . . . . .   3
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   4
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   4
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   [RFC2156] defined a mapping of header fields between X.400 and
   RFC822/MIME.  One of the mapped fields is the "Expires" header field,
   which provides a date and time at which a message is considered to
   lose its validity.

   Netnews articles [RFC5536] have an Expires header with a similar
   slightly more strict syntax and similar meaning.

   This document extends the use of the "Expires" header field to
   Internet email in general, whether the message comes from an X.400
   gateway or elsewhere.

   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 de-
   emphasized or not shown to the user, to unclutter the user's mailbox.

   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 definition

   The header field definition remains the same as in [RFC2156] and
   [RFC4021].  It indicates the time at which a message loses its
   validity.  Using the ABNF from [RFC5322], its syntax is:

   expires = "Expires:" date-time CRLF

   Example:

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

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   Message creators MUST NOT include more than one Expires header field
   in the message they send.

   If there is more than one Expires header field then message readers
   SHOULD act as if no Expires header field is present.

3.  Advice to Message Creators

   Message creators add the header field along with a relevant date and
   time when they know that the message loses its validity.  This could
   apply to commercial newsletters that include time-limited offers,
   event announcements, social notifications, and periodic announcement
   messages.

4.  Advice to Message Readers

   Message readers, such as mailbox providers, web mail and MUAs could
   de-emphasize the display of expired messages or determine not do
   display them.  They could allow users to control the actions to take
   for expired messages.

   The information provided in the header field is intended to be used
   as a signal to provide an improved experience to the end-user.  For
   instance, systems might allow automatic rules to clean up expired
   email from specific message creators or with defined characteristics,
   or to provide a mode to quickly handle all expired email.

5.  Security considerations

   A message creator can put any date in an Expires header field,
   including dates in the distant past or future.  Without further
   knowledge about the message creator, recipient systems and message
   readers cannot assume that the contents of the header are accurate or
   benign.

   For example, a malicious message creator might send spam mail that
   includes a expiry date in the past, in the hope that recipients will
   not see or report the mail, and then adaptive spam filters would use
   it as non-spam training material.  A creator might include a date in
   the immediate future in the hope that a recipient would see and act
   on a message, but could not find it later to complain about it.  Or a
   creator might include a date in the distant future in the hope that
   the message would stay in a recipient's inbox and would be more
   likely to be read.

   While the header field can be useful to determine how to display a
   message to a user, it is unlikely to be useful to determine whether
   or not the message is wanted or is fraudulent.

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6.  Acknowledgements

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

7.  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

8.  Normative 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>.

   [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>.

   [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>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [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>.

9.  Informative References

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   [RFC5536]  Murchison, K., Ed., Lindsey, C., and D. Kohn, "Netnews
              Article Format", RFC 5536, DOI 10.17487/RFC5536, November
              2009, <https://www.rfc-editor.org/info/rfc5536>.

Authors' Addresses

   Benjamin Billon
   Splio
   Email: bbillon@splio.com

   John Levine
   Standcore LLC
   Email: standards@standcore.com

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