IMAP UIDONLY: IMAP Extension for Using and Returning Unique Identifiers (UIDs) Only

Internet Engineering Task Force (IETF)                       A. Melnikov
Request for Comments: 9586                                         Isode
Category: Experimental                                    A. P. Achuthan
ISSN: 2070-1721                                           V. Nagulakonda
                                                                A. Singh
                                                                  Yahoo!
                                                                L. Alves
                                                                May 2024


 IMAP Extension for Using and Returning Unique Identifiers (UIDs) Only

Abstract

   The UIDONLY extension to the Internet Message Access Protocol (RFCs
   3501 and 9051) allows clients to enable a mode in which information
   about mailbox changes is returned using only Unique Identifiers
   (UIDs).  Message numbers are not returned in responses and cannot be
   used in requests once this extension is enabled.  This helps both
   clients and servers to reduce resource usage required to maintain a
   map between message numbers and UIDs.

   This document defines an experimental IMAP extension.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  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).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see 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/rfc9586.

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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction and Overview
   2.  Document Conventions
   3.  The UIDONLY Extension
     3.1.  Enabling the UIDONLY Extension
     3.2.  Changes to FETCH/STORE/SEARCH/COPY/MOVE
     3.3.  Changes to UID FETCH / UID STORE
     3.4.  Changes to EXPUNGE / UID EXPUNGE
     3.5.  Changes to UID SEARCH
     3.6.  Changes to How Other Mailbox Changes Are Announced
     3.7.  Interaction with the CONDSTORE and QRESYNC Extensions
     3.8.  Interaction with Other Extensions
   4.  Formal Syntax
   5.  Security Considerations
   6.  IANA Considerations
   7.  Alternative Solutions Not Taken
   8.  Normative References
   9.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction and Overview

   This document defines an extension to the Internet Message Access
   Protocol [RFC3501] [RFC9051] for eliminating the use of message
   numbers.  This extension is compatible with both IMAP4rev1 [RFC3501]
   and IMAP4rev2 [RFC9051].

   The UIDONLY extension of the Internet Message Access Protocol allows
   clients to request that servers only use and return UIDs.  This helps
   both clients and servers to reduce resource usage required to
   maintain a map between message numbers and UIDs.

2.  Document Conventions

   In protocol examples, this document uses a prefix of "C:" to denote
   lines sent by the client to the server and "S:" for lines sent by the
   server to the client.  Lines prefixed with "//" are comments
   explaining the previous protocol line.  These prefixes and comments
   are not part of the protocol.  Lines without any of these prefixes
   are continuations of the previous line, and no line break is present
   in the protocol unless specifically mentioned.

   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.

   Other capitalized words are names of IMAP commands or responses
   [RFC3501] [RFC9051] or keywords from this document.

3.  The UIDONLY Extension

   An IMAP server advertises support for the UIDONLY extension by
   including the "UIDONLY" capability in the CAPABILITY response/
   response code.

   Once the UIDONLY extension is enabled (see Section 3.1), the client
   MUST NOT use message sequence numbers (including the special marker
   "*") in any arguments to IMAP commands, and the server MUST return a
   tagged BAD response if the client uses message sequence numbers.  The
   server MUST include the UIDREQUIRED response code in such BAD
   responses (see below).  Additionally, once the UIDONLY extension is
   enabled, the server MUST NOT return message sequence numbers in any
   response.

   The UIDREQUIRED response code is defined as follows:

   UIDREQUIRED:  Once the UIDONLY extension is enabled, the server
      returns the UIDREQUIRED response code when the client issues a
      command that includes message numbers instead of UIDs.

        C: 07 FETCH 10000:14589 (UID FLAGS)
        S: 07 BAD [UIDREQUIRED] Message numbers are not allowed
            once UIDONLY is enabled

   The UIDONLY extension affects how information about new, expunged, or
   changed messages is returned in unsolicited responses.  In
   particular, it affects responses to UID FETCH/UID STORE/EXPUNGE/UID
   EXPUNGE, as well as how unsolicited mailbox changes are announced.

   The following subsections describe changes introduced by this
   extension in more detail.

3.1.  Enabling the UIDONLY Extension

   As the UIDONLY extension affects how information about new, expunged,
   or changed messages is returned in unsolicited responses, it has to
   be enabled by the client first using the ENABLE command.

     S: * OK [CAPABILITY IMAP4rev1 ENABLE CONDSTORE QRESYNC UIDONLY
         AUTH=SCRAM-SHA-256]
     C: 01 ENABLE UIDONLY
     S: * ENABLED UIDONLY
     S: 01 OK ENABLE completed

3.2.  Changes to FETCH/STORE/SEARCH/COPY/MOVE

   When UIDONLY is enabled, the FETCH, STORE, SEARCH, COPY, and MOVE
   commands are prohibited and MUST result in a tagged BAD response.
   Clients should instead use UID FETCH, UID STORE, UID SEARCH, UID
   COPY, or UID MOVE, respectively.

3.3.  Changes to UID FETCH / UID STORE

   When UIDONLY is enabled, all FETCH responses that would be returned
   by UID FETCH / UID STORE are replaced by UIDFETCH responses.

   Note that the UIDFETCH response contains the same response data items
   as specified for the FETCH response.  The UID is always returned at
   the beginning of a UIDFETCH response, and it can also appear in the
   UID response data item, if requested by the client.  While the UID
   response data item is redundant when requested, it can simplify the
   updating of existing (non-UIDONLY) implementations to support
   UIDONLY.

     C: 10 UID FETCH 25900:26600 (FLAGS)
     [...]
     S: * 25996 UIDFETCH (FLAGS (\Seen))
     S: * 25997 UIDFETCH (FLAGS (\Flagged \Answered))
     S: * 26600 UIDFETCH (FLAGS ())
     S: 10 OK FETCH completed

     C: 11 UID FETCH 25900:26600 (UID FLAGS)
     S: * 25900 UIDFETCH (FLAGS (\Seen) UID 25900)
     S: * 25902 UIDFETCH (FLAGS (\Flagged) UID 25902)
     S: * 26310 UIDFETCH (FLAGS (\Answered) UID 26310)
     S: * 26311 UIDFETCH (FLAGS () UID 26311)
     S: * 26498 UIDFETCH (FLAGS (\Answered) UID 26498)
     [...]
     S: 11 OK FETCH completed

3.4.  Changes to EXPUNGE / UID EXPUNGE

   When UIDONLY is enabled, all EXPUNGED responses that would be
   returned by EXPUNGE / UID EXPUNGE are replaced by VANISHED responses,
   as defined in [RFC7162].  Note that a server that implements the
   UIDONLY extension is not required (but allowed) to also implement the
   CONDSTORE and/or QRESYNC extensions.

     C: 12 EXPUNGE
     S: * VANISHED 405,407,410,425
     S: 12 OK expunged

3.5.  Changes to UID SEARCH

   The "<sequence set>" UID SEARCH criterion is prohibited (and results
   in a tagged BAD response) once UIDONLY is enabled.  Clients should
   use ALL or "UID <sequence set>" UID SEARCH criterion instead.

3.6.  Changes to How Other Mailbox Changes Are Announced

   When UIDONLY is enabled, all changes to flags (and other dynamic
   message attributes) are returned using UIDFETCH responses instead of
   FETCH responses.

   Similarly, all expunged messages are announced using VANISHED
   responses instead of EXPUNGE responses.

   This extension doesn't affect EXISTS or RECENT responses.

   The UID MOVE / UID COPY commands SHOULD return the COPYUID response
   code, as specified in [RFC4315].

   Use of the UIDNOTSTICKY response code (see [RFC4315]) is not
   compatible with the UIDONLY extension, i.e., a server that advertises
   the UIDONLY extension MUST NOT return a UIDNOTSTICKY response code.

     C: 15 UID move 597 "Archives/2023/2023-05"
     S: * OK [COPYUID 1685977201 597 2] UID MOVE
     S: * VANISHED 597
     S: 15 OK UID MOVE Completed

3.7.  Interaction with the CONDSTORE and QRESYNC Extensions

   The CONDSTORE extension is compatible with the UIDONLY extension.
   The MODSEQ message data item is returned in UIDFETCH responses
   instead of FETCH responses.

   The QRESYNC extension is compatible with the UIDONLY extension, but
   once UIDONLY is enabled, the fourth SELECT QRESYNC parameter (see
   Section 3.2.5.2 ("Message Sequence Match Data") of [RFC7162]) MUST
   NOT be used.  The server MUST return a tagged BAD response if such a
   parameter is observed once UIDONLY is enabled.

3.8.  Interaction with Other Extensions

   IMAP extensions might define other commands that accept message
   sequence numbers ("sequence-set" ABNF non-terminal; see Section 9 of
   [RFC9051]).  Once UIDONLY is enabled, the server MUST reject such
   commands with a tagged BAD response.  For example, the SORT and
   THREAD [RFC5256] commands are prohibited, similarly to the SEARCH
   command.  However, UID SORT and UID THREAD can be used instead.

4.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [ABNF].

   Non-terminals referenced but not defined below are as defined in
   Section 9 of IMAP4 [RFC9051].

   Except as noted otherwise, all alphabetic characters are case
   insensitive.  The use of uppercase or lowercase characters to define
   token strings is for editorial clarity only.  Implementations MUST
   accept these strings in a case-insensitive fashion.

   SP                  = <Defined in RFC 5234>

   capability          =/ "UIDONLY"
                          ;; <capability>; see RFC 9051

   message-data        =/ uidfetch-resp

   uidfetch-resp       = uniqueid SP "UIDFETCH" SP msg-att
                         ;; The uniqueid is the UID of
                         ;; the corresponding message

   message-data        =/ expunged-resp

   expunged-resp       = <defines VANISHED response; see RFC 7162>

   resp-text-code      =/ "UIDREQUIRED"

5.  Security Considerations

   This IMAP extension is not believed to add any additional Security
   Considerations beyond the ones that are generally applicable to
   IMAP4rev1 [RFC3501] and IMAP4rev2 [RFC9051].

6.  IANA Considerations

   IMAP4 capabilities are registered by publishing a Standards Track or
   IESG-approved Informational or Experimental RFC.

   IANA has added the UIDONLY extension to the "IMAP Capabilities"
   registry with RFC 9586 as the reference.  The registry is located at
   <https://www.iana.org/assignments/imap4-capabilities/>.

   IANA has also added the UIDREQUIRED response code to the "IMAP
   Response Codes" registry with RFC 9586 as the reference.  The
   registry is located at <https://www.iana.org/assignments/imap-
   response-codes/>.

7.  Alternative Solutions Not Taken

   An earlier draft version of this document proposed use of FETCH
   responses with the message number parameter always set to 0.  This
   was considered to be too risky as it could cause unexpected side
   effects and cache corruptions in client code that was not properly
   updated to handle a lack of message numbers.

8.  Normative References

   [ABNF]     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>.

   [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>.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <https://www.rfc-editor.org/info/rfc3501>.

   [RFC4315]  Crispin, M., "Internet Message Access Protocol (IMAP) -
              UIDPLUS extension", RFC 4315, DOI 10.17487/RFC4315,
              December 2005, <https://www.rfc-editor.org/info/rfc4315>.

   [RFC5256]  Crispin, M. and K. Murchison, "Internet Message Access
              Protocol - SORT and THREAD Extensions", RFC 5256,
              DOI 10.17487/RFC5256, June 2008,
              <https://www.rfc-editor.org/info/rfc5256>.

   [RFC7162]  Melnikov, A. and D. Cridland, "IMAP Extensions: Quick Flag
              Changes Resynchronization (CONDSTORE) and Quick Mailbox
              Resynchronization (QRESYNC)", RFC 7162,
              DOI 10.17487/RFC7162, May 2014,
              <https://www.rfc-editor.org/info/rfc7162>.

   [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.  Informative References

   [IMAP-UIDONLY-ORIG]
              Gulbrandsen, A., "The IMAP UIDONLY Extension", Work in
              Progress, Internet-Draft, draft-gulbrandsen-imap-uidonly-
              00, 25 April 2014, <https://datatracker.ietf.org/doc/html/
              draft-gulbrandsen-imap-uidonly-00>.

Acknowledgments

   The editors of this document would like to thank the following people
   who provided useful comments and/or participated in discussions that
   lead to this document: Arnt Gulbrandsen, Ken Murchison, Bron
   Gondwana, Barry Leiba, and Elwyn Davis.

   This document is similar to [IMAP-UIDONLY-ORIG], but some different
   syntactic choices were made in the end.

Authors' Addresses

   Alexey Melnikov
   Isode Limited
   Email: alexey.melnikov@isode.com
   URI:   https://www.isode.com


   Arun Prakash Achuthan
   Yahoo Inc.
   Email: arunprakash@myyahoo.com


   Vikram Nagulakonda
   Yahoo Inc.
   Email: nvikram_imap@yahoo.com


   Ashutosh Singh
   Yahoo Inc.
   Email: ashutoshvsingh@yahoo.com


   Luis Alves
   Email: luis.alves@lafaspot.com