



Internet Engineering Task Force (IETF)                      K. Murchison
Request for Comments: 9590                                   B. Gondwana
Category: Standards Track                                       Fastmail
ISSN: 2070-1721                                                 May 2024


     IMAP Extension for Returning Mailbox METADATA in Extended LIST

Abstract

   This document defines an extension to the Internet Message Access
   Protocol (IMAP) LIST command that allows the client to request
   mailbox annotations (metadata), along with other information
   typically returned by the LIST command.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9590.

Copyright Notice

   Copyright (c) 2024 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
   Provisions Relating to IETF Documents
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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions Used in This Document
   3.  METADATA Return Option to LIST Command
   4.  Examples
   5.  Formal Syntax
   6.  Security Considerations
   7.  Privacy Considerations
   8.  IANA Considerations
     8.1.  Registration of IMAP Capability LIST-METADATA
     8.2.  Registration of LIST-EXTENDED Option METADATA
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Authors' Addresses

1.  Introduction

   IMAP clients sometimes fetch mailbox metadata (e.g., color) to
   augment the display of mailboxes for the logged-in user.  In order to
   do that, the client is forced to issue a LIST or LSUB command to list
   all available mailboxes, followed by a GETMETADATA command for each
   mailbox found.  This document defines an extension to the IMAP LIST
   command that is identified by the capability string "LIST-METADATA".
   The LIST-METADATA extension allows the client to request annotations
   on available mailboxes, along with other information typically
   returned by the LIST command.

2.  Conventions Used in This Document

   In examples, "C:" indicates lines sent by a client that is connected
   to a server.  "S:" indicates lines sent by the server to the client.

   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.

   Long lines in examples are wrapped using "The Single Backslash
   Strategy" described in [RFC8792].

3.  METADATA Return Option to LIST Command

   [RFC5464] defines the GETMETADATA command that is used by an IMAP
   client to retrieve mailbox annotations.  Sometimes, a client will
   have to look up the metadata for some or all of the mailboxes
   returned by the LIST command.  Doing so in multiple GETMETADATA
   commands wastes bandwidth and can degrade performance if the client
   does not pipeline the requests.

   This document extends the LIST command with a new return option,
   "METADATA", which allows the client to request all of the desired
   information in a single command.  For each listable mailbox matching
   the list pattern and selection options, the server MUST return an
   untagged LIST response, followed by one or more untagged METADATA
   responses containing the mailbox annotations requested by the client.
   The untagged METADATA responses to an extended LIST command have the
   same syntax and semantics as those that would be returned by
   GETMETADATA commands on the same set of listable mailboxes (see
   Section 4.4.1 of [RFC5464]).  As per Section 4.4 of [RFC5464], the
   server may return all requested annotations in a single METADATA
   response for each mailbox, or it may split the requested annotations
   into multiple METADATA responses for each mailbox.

   If the server is unable to look up the annotations for given mailbox,
   it MAY drop the corresponding METADATA response.  In such a
   situation, the LIST command would still return a tagged OK reply.

4.  Examples

   The following are examples of fetching metadata from only the top-
   level hierarchies of the mailbox using different sets of selection
   criteria (see Section 6.3.9 of [RFC9051]).

   In this example:

   *  The "color" annotation for the "foo" mailbox has not been set, so
      the METADATA response has a value of "NIL" (i.e., has no value).

   *  "bar" has children, but isn't an actual mailbox itself, so it has
      no METADATA response.

  ========== NOTE: '\' line wrapping per RFC 8792 ===========

  C: A00 CAPABILITY
  S: * CAPABILITY IMAP4rev1 IMAP4rev2 \
                  LIST-EXTENDED LIST-METADATA METADATA
  S: A00 OK Completed.
  C: A01 LIST "" % \
              RETURN (METADATA ("/shared/vendor/cmu/cyrus-imapd/color"))
  S: * LIST () "." "INBOX"
  S: * METADATA INBOX ("/shared/vendor/cmu/cyrus-imapd/color" "#b71c1c")
  S: * LIST () "." "foo"
  S: * METADATA "foo" ("/shared/vendor/cmu/cyrus-imapd/color" NIL)
  S: * LIST (\NonExistent) "." "bar"
  S: A01 OK List completed.

   In this example, the LIST response for the "foo" mailbox is returned
   because it has matching children, but no METADATA response is
   returned because "foo" itself doesn't match the selection criteria.

  ========== NOTE: '\' line wrapping per RFC 8792 ===========

  C: A02 LIST (SUBSCRIBED RECURSIVEMATCH) "" % \
              RETURN (METADATA ("/shared/vendor/cmu/cyrus-imapd/color"))
  S: * LIST (\Subscribed) "." "INBOX"
  S: * METADATA INBOX ("/shared/vendor/cmu/cyrus-imapd/color" "#b71c1c")
  S: * LIST () "." "foo" (CHILDINFO ("SUBSCRIBED"))
  S: A02 OK List completed.

5.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) as described in [RFC5234].  Note that "return-option" is
   defined in [RFC5258] and "entry" is defined in [RFC5464].

   return-option =/ "METADATA" SP "(" entry *(SP entry) ")"

6.  Security Considerations

   This specification does not introduce any additional security
   concerns beyond those described in [RFC5258] and [RFC5464].

7.  Privacy Considerations

   This specification does not introduce any additional privacy concerns
   beyond those described in [RFC5464].

8.  IANA Considerations

8.1.  Registration of IMAP Capability LIST-METADATA

   Per this document, IANA has added the "LIST-METADATA" IMAP capability
   to the "IMAP Capabilities" registry located at
   <https://www.iana.org/assignments/imap4-capabilities/>.

8.2.  Registration of LIST-EXTENDED Option METADATA

   Per this document, IANA has registered the "METADATA" LIST-EXTENDED
   option in the "LIST-EXTENDED options" registry located at
   <https://www.iana.org/assignments/imap-list-extended/>.

   LIST-EXTENDED option name:
      METADATA

   LIST-EXTENDED option type:
      RETURN

   LIST-EXTENDED option description:
      Causes the LIST command to return METADATA responses in addition
      to LIST responses.

   Published specification:
      RFC 9590, Section 3

   Security considerations:
      RFC 9590, Section 6

   Intended usage:
      COMMON

   Person and email address to contact for further information:
      Kenneth Murchison <murch@fastmailteam.com> and
      Bron Gondwana <brong@fastmailteam.com>

   Owner/Change controller:
      IESG <iesg@ietf.org>

9.  References

9.1.  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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5258]  Leiba, B. and A. Melnikov, "Internet Message Access
              Protocol version 4 - LIST Command Extensions", RFC 5258,
              DOI 10.17487/RFC5258, June 2008,
              <https://www.rfc-editor.org/info/rfc5258>.

   [RFC5464]  Daboo, C., "The IMAP METADATA Extension", RFC 5464,
              DOI 10.17487/RFC5464, February 2009,
              <https://www.rfc-editor.org/info/rfc5464>.

   [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>.

   [RFC9051]  Melnikov, A., Ed. and B. Leiba, Ed., "Internet Message
              Access Protocol (IMAP) - Version 4rev2", RFC 9051,
              DOI 10.17487/RFC9051, August 2021,
              <https://www.rfc-editor.org/info/rfc9051>.

9.2.  Informative References

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
              <https://www.rfc-editor.org/info/rfc8792>.

Authors' Addresses

   Kenneth Murchison
   Fastmail US LLC
   1429 Walnut Street
   Suite 1201
   Philadelphia, PA 19102
   United States of America
   Email: murch@fastmailteam.com


   Bron Gondwana
   Fastmail Pty Ltd
   Level 2, 114 William Street
   Melbourne VIC 3000
   Australia
   Email: brong@fastmailteam.com
