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 Only 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.
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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 for
maintenance of message number to UID map.
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]
[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], [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