wdiff rfc9738.original rfc9738.txt

Network Working Group

Internet Engineering Task Force (IETF) A. Melnikov
Internet-Draft
Request for Comments: 9738 Isode
Intended status:
Category: Experimental A. P. Achuthan
Expires: 30 January 2025
ISSN: 2070-1721 V. Nagulakonda
Yahoo!
L. Alves
29 July 2024
February 2025

IMAP MESSAGELIMIT Extension
draft-ietf-extra-imap-messagelimit-10

Abstract

The MESSAGELIMIT extension of the Internet Message Access Protocol
(RFC 3501/RFC 9051) allows servers to announce a limit on the number
of messages that can be processed in a single
FETCH/SEARCH/STORE/COPY/MOVE FETCH, SEARCH, STORE,
COPY, or MOVE command (or their UID variants), or in a single APPEND
or UID EXPUNGE command. This helps servers to control resource usage
when performing various IMAP operations. This helps clients to know
the message limit enforced by the corresponding IMAP server and avoid
issuing commands that would exceed such limit.

Status of This Memo

This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 not an Internet Standards Track specification; it is
published for examination, experimental implementation, and BCP 79.

Internet-Drafts are working documents
evaluation.

This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents valid approved by the IESG are candidates for a maximum any level of
Internet Standard; see Section 2 of RFC 7841.

Information about the current status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on 30 January 2025.
https://www.rfc-editor.org/info/rfc9738.

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Table of Contents

1. Introduction and Overview . . . . . . . . . . . . . . . . . . 3
2. Document Conventions . . . . . . . . . . . . . . . . . . . . 3
3. The MESSAGELIMIT extension . . . . . . . . . . . . . . . . . 3 Extension
3.1. Returning limits Limits on the number Number of messages processed Messages Processed in a
single
Single SEARCH/FETCH/STORE/COPY/MOVE/APPEND/EXPUNGE
command . . . . . . . . . . . . . . . . . . . . . . . . . 4 Command
3.2. UIDAFTER and UIDBEFORE SEARCH criteria . . . . . . . . . 7 Criteria
3.3. Interaction with SORT and THREAD extensions . . . . . . . 8 Extensions
3.4. Interaction with SEARCHRES extension Extension and IMAP4rev2 . . . 8
4. Interoperability Considerations . . . . . . . . . . . . . . . 8
4.1. Effects of MESSAGELIMIT/SAVELIMIT extensions MESSAGELIMIT and SAVELIMIT Extensions on non
compliant clients . . . . . . . . . . . . . . . . . . . . 8
Noncompliant Clients
4.2. Maintaining Compatibility . . . . . . . . . . . . . . . . 9
5. Formal syntax . . . . . . . . . . . . . . . . . . . . . . . . 10 Syntax
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7.1. Changes/additions Additions to the IMAP4 capabilities registry . . 10 IMAP Capabilities Registry
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1.
8.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2.
8.2. Informative References . . . . . . . . . . . . . . . . . 12
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Acknowledgments
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12

1. Introduction and Overview

This document defines an extension to the Internet Message Access
Protocol [RFC3501] for announcing a server limit on the number of
messages that can be processed in a single FETCH/SEARCH/STORE/COPY/ FETCH, SEARCH, STORE,
COPY, or MOVE command (or their UID variants), or a single APPEND or
UID EXPUNGE command. This extension is compatible with both
IMAP4rev1 [RFC3501] and IMAP4rev2 [RFC9051].

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 capitalised capitalized words are IMAP key words [RFC3501][RFC9051] or key
words from this document.

3. The MESSAGELIMIT extension Extension

An IMAP server advertises support for the MESSAGELIMIT extension by
including "MESSAGELIMIT=<limit>" capability in the CAPABILITY
response/response
response or response code, where "<limit>" is a positive integer that
conveys the maximum number of messages that can be processed in a
single [UID] SEARCH/FETCH/STORE/COPY/MOVE, SEARCH, FETCH, STORE, COPY, MOVE command (or their UID
variants), or in a single APPEND or UID EXPUNGE command.

An IMAP server that only enforces the message limit on [UID] COPY/APPEND COPY and
APPEND commands (and their UID variants) would include the
"SAVELIMIT=<limit>" capability (instead of the
"MESSAGELIMIT=<limit>") in the CAPABILITY response/response response or response code.

The limit advertised in the MESSAGELIMIT or SAVELIMIT capability
SHOULD NOT be lower than 1000 messages.

3.1. Returning limits Limits on the number Number of messages processed Messages Processed in a single Single
SEARCH/FETCH/STORE/COPY/MOVE/APPEND/EXPUNGE command Command

If a server implementation doesn't allow more than <N> messages to be
operated on by a single COPY/UID COPY or UID COPY command, it MUST fail the
command by returning a tagged NO response with the MESSAGELIMIT
response code defined below. No messages are copied in this case.
If a server implementation doesn't allow more than <N> messages to be
operated on by a single SEARCH/FETCH/STORE/MOVE SEARCH, FETCH, STORE, or MOVE command (or
their UID variants), or an APPEND or UID EXPUNGE command, it MUST
return the MESSAGELIMIT response code defined below:

MESSAGELIMIT
The server doesn't allow more than <N> messages to be operated on
by a single SEARCH/FETCH/STORE/COPY/MOVE SEARCH, FETCH, STORE, COPY, or MOVE command (or their
UID variants). The lowest processed UID is <LastUID>. The client
needs to repeat the operation for remaining messages, if required.

The server doesn't allow more than <N> \Deleted messages to be
operated on by a single UID EXPUNGE command. The lowest processed
UID is <LastUID>. The client needs to repeat the operation for
remaining messages, if required.

Note that when the MESSAGELIMIT response code is returned, the
server is REQUIRED to process messages from highest to lowest
UIDs. UID.

Note that when the MESSAGELIMIT response code is similar to the LIMIT ([RFC9051])
response code, code [RFC9051], but it provides more details on the exact
type of the limit and how to resume the command once the limit is
exceeded.

In the following example example, the <N> value is 1000 1000, and the lowest
processed UID <LastUID> is 23221.

C: 03 FETCH 10000:14589 (UID FLAGS)
S: * 14589 FETCH (FLAGS (\Seen) UID 25000)
S: * 14588 FETCH (FLAGS (\Answered) UID 24998)
S: ... further 997 fetch responses
S: * 13590 FETCH (FLAGS () UID 23221)
S: 03 OK [MESSAGELIMIT 1000 23221] FETCH completed with 1000
partial results

In the following example the client searches for UNDELETED UIDs
between 22000:25000. The total number of searched messages (note,
NOT the number of matched messages) exceeds the server's published 1000 messages
1000-message limit.

C: 04 UID SEARCH UID 22000:25000 UNDELETED
S: * SEARCH 25000 24998 (... UIDs ...) 23221
S: 04 OK [MESSAGELIMIT 1000 23221] SEARCH completed with 1000
partial results

The following example demonstrates the copy of messages with UIDs
between 18000:21000. The total message count exceeds the server's
published 1000 messages 1000-message limit. As COPY/UID COPY or UID COPY needs to be
atomic (as per [RFC3501]/[RFC9051]), no messages are copied.

C: 05 UID COPY 18000:21000 "Trash"
S: 05 NO [MESSAGELIMIT 1000 20001] Too many messages to copy,
try a smaller subset

The following example shows MOVE the move of messages with UIDs between
18000:21000. The total message count exceeds the server's
published 1000 messages 1000-message limit. (Unlike COPY/UID COPY or UID COPY, MOVE/UID MOVE or
UID MOVE don't need to be atomic.) The client that wants to move
all messages in the range and observes a MESSAGELIMIT response
code,
code can repeat the UID MOVE command with the same parameter.
(For the MOVE command, the message set parameter need needs to be
updated before repeating the command.) The client needs to keep
doing this until the MESSAGELIMIT response is not returned (or
until a tagged NO/BAD NO or BAD is returned).

C: 06 UID MOVE 18000:21000 "Archive/2021/2021-12"
S: * OK [COPYUID 1397597919 20001:21000 22363:23362] Some
messages were not moved
S: * 12336 EXPUNGE
S: * 12335 EXPUNGE
...
S: * 11337 EXPUNGE
S: 06 OK [MESSAGELIMIT 1000 20001] MOVE completed for the last
1000 messages

The following example shows the update of flags for messages with
UIDs between 18000:20000. The total number of existing messages
in the UID range exceeds the server's published 1000
messages 1000-message
limit. The client that wants to change flags for all messages in
the range and observes a MESSAGELIMIT response
code, code can repeat the
UID STORE command with the updated UID range that doesn't include
the UID returned in the MESSAGELIMIT response code. (For the
STORE command, the message set parameter also need needs to be updated
before repeating the command.) The client needs to keep doing
this until the MESSAGELIMIT response is not returned (or until a
tagged NO/BAD NO or BAD is returned).

C: 07 UID STORE 18000:20000 +FLAGS (\Seen)
S: * 11215 FETCH (FLAGS (\Seen \Deleted) UID 20000)
S: * 11214 FETCH (FLAGS (\Seen \Answered \Deleted) UID 19998)
...
S: * 10216 FETCH (FLAGS (\Seen) UID 19578)
S: 07 OK [MESSAGELIMIT 1000 19578] STORE completed for the last
1000 messages

The following example shows the removal of messages (using UID
EXPUNGE) that have the \Deleted flag set with UIDs between
11000:13000. The total message count of messages with the
\Deleted flag set exceeds the server's published 1000 messages 1000-message
limit. The client that wants to remove all messages marked as
\Deleted in the range and observes a MESSAGELIMIT response code, code
can repeat the UID EXPUNGE command with the same parameter. The
client needs to keep doing this until the MESSAGELIMIT response is
not returned (or until a tagged NO/BAD NO or BAD is returned).

C: 08 UID EXPUNGE 11000:13000
S: * 4306 EXPUNGE
S: * 4305 EXPUNGE
...
S: * 3307 EXPUNGE
S: 08 OK [MESSAGELIMIT 1000 11627] UID EXPUNGE completed for
the last 1000 messages

The following example shows removal of messages (using EXPUNGE)
that have the \Deleted flag set. Unlike UID EXPUNGE, the server
MUST NOT impose any message limit when processing EXPUNGE.

C: 09 EXPUNGE
S: * 4306 EXPUNGE
S: * 4305 EXPUNGE
...
S: * 3307 EXPUNGE
S: * 112 EXPUNGE
S: 09 OK EXPUNGE completed

Similarly, the server MUST NOT impose any message limit when
processing a "CLOSE" or a "STATUS UNSEEN" command.

The following example shows use of the MESSAGELIMIT response code
together with the PARTIAL [RFC9394] extension. The total message
count (as specified by the PARTIAL range) exceeds the server's
published 1000 messages 1000-message limit, so the server refuses to do any work
in this case.

C: 10 UID FETCH 22000:25000 (UID FLAGS MODSEQ)
(PARTIAL -1:-1500)
S: 10 NO [MESSAGELIMIT 1000] FETCH exceeds the maximum 1000 1000-
message limit

Without the PARTIAL parameter parameter, the above UID FETCH can look like
this:

C: 10 UID FETCH 22000:25000 (UID FLAGS MODSEQ)
S: * 12367 FETCH (FLAGS (\Seen \Deleted) UID 23007)
S: * 12366 FETCH (FLAGS (\Seen \Answered \Deleted) UID 23114)
...
S: * 13366 FETCH (FLAGS (\Seen) UID 24598)
S: 10 OK [MESSAGELIMIT 1000 23007] FETCH exceeds the maximum 1000 message
1000-message limit

Note that when the server needs to return both EXPUNGEISSUED
([RFC9051])
[RFC9051] and MESSAGELIMIT response codes, the former MUST be
returned in the tagged OK response, while the latter MUST be returned
in an untagged NO response. The following example demonstrates that:

C: 11 FETCH 10000:14589 (UID FLAGS)
S: * 14589 FETCH (FLAGS (\Seen) UID 25000)
S: * 14588 FETCH (FLAGS (\Answered) UID 24998)
S: ... further 997 fetch responses
S: * 13590 FETCH (FLAGS () UID 23221)
S: * NO [MESSAGELIMIT 1000 23221] FETCH completed with 1000 partial
results
S: 11 OK [EXPUNGEISSUED] Some messages were also expunged

When the IMAP MULTIAPPEND [RFC3502] extension [RFC3502] is also supported by
the server, the message limit also applies to the APPEND command. As
MULTIAPPEND APPEND needs to atomic (as per [RFC3502]), no messages
are appended when the server MESSAGELIMIT is exceeded.

3.2. UIDAFTER and UIDBEFORE SEARCH criteria Criteria

The MESSAGELIMIT extension also defines 2 two extra SEARCH keys: keys,
UIDAFTER and UIDBEFORE, which make it easier to convert a single UID
to a range of UIDs.

"UIDAFTER <uid>" - Messages that have a UID greater than the specified
UID. This is semantically the same as "UID <uid>+1:*".

"UIDBEFORE <uid>" - Messages that have a UID less than the specified
UID. This is semantically the same as "UID 1:<uid>-1" (or if
<uid> has the value 1, then the empty set).

These 2 two SEARCH keys are particularly useful when the SEARCHRES
[RFC5182]
extension [RFC5182] is also supported, but they can be used without
it. For example, this allows a SEARCH that sets the "$" marker to be
converted to a range of messages in a subsequent SEARCH, and both
SEARCH requests can be pipelined.

C: 12 UID SEARCH UIDAFTER 25000 UNDELETED
S: * SEARCH 27800 27798 (... 250 UIDs ...) 25001
S: 12 OK SEARCH completed

3.3. Interaction with SORT and THREAD extensions Extensions

Servers that advertise MESSAGELIMIT N will be unable to execute a
THREAD [RFC5256] command [RFC5256] in a mailbox with more than N messages.

Servers that advertise MESSAGELIMIT N might be unable to execute a
SORT [RFC5256] command [RFC5256] in a mailbox with more than N messages, unless
they maintain indices for different SORT orders they support. In
absence of such indeces indices, server implementors will need to decide
whether their server advertises SORT or MESSAGELIMIT capability.

3.4. Interaction with SEARCHRES extension Extension and IMAP4rev2

Servers that support both MESSAGELIMIT and SEARCHRES [RFC5182] extensions
[RFC5182] MUST truncate SEARCH SAVE result stored in the $ variable
when the SEARCH command succeeds, but the MESSAGELIMIT response code
is returned. For example, if the following SEARCH would have
returned 1200 results in absence of MESSAGELIMIT, and the
MESSAGELIMIT is 1000, only 1000 matching results will be saved in the
$ variable:

C: D0004 UID SEARCH RETURN (SAVE) SINCE 1-Jan-2004 NOT FROM "Smith"
UID 22000:25000 UNDELETED
S: D0004 OK [MESSAGELIMIT 1000 1179] SEARCH completed with 1000
partial results saved

4. Interoperability Considerations

4.1. Effects of MESSAGELIMIT/SAVELIMIT extensions MESSAGELIMIT and SAVELIMIT Extensions on non compliant
clients Noncompliant
Clients

A server that advertises the MESSAGELIMIT=N capability would have the
following effect on clients that don't support this capability:

* Operations on a mailbox that has <= N messages are not affected.

* In a mailbox with more than N messages:

- An attempt to COPY/UID COPY or UID COPY more than N messages will always
fail.

- EXPUNGE and CLOSE will always operate on the full mailbox, so
they are not affected.

- Other commands like FETCH, SEARCH SEARCH, and MOVE will be effectively
restricted to the last N messages of the mailbox. In
particular
particular, unextended SEARCHes intended (intended for counting of
messages with or without a particular set of flags flags) would
return incorrect counts.

4.2. Maintaining Compatibility

It is important to understand that the above effects essentially
abandon existing clients with respect to operation on large
mailboxes. Suppose, for example, that a user is accessing a large
and active mailing list via IMAP - IMAP, and the mailing list gets on the
order of 1500 posts per week. When the user returns from a week-long
vacation and catches up on the mailing list, the user’s user's client will
be fetching information about 1500 messages. If the server has a
MESSAGELIMIT of 1000, the client will only be able to download 1000
of the most recent messages; the client will not understand why, will
not be prepared to recover from the situation, and will act as if it
is interacting with a broken server.

In order to give clients time to implement this extension, servers
should not be strict about applying the MESSAGELIMIT at first. One
possible approach is to advertise a MESSAGELIMIT but not enforce it
at all for a while. Clients that understand this extension will
comply, reducing load on the server, but clients that do not
understand the limit will continue to work in all situations.

Another approach, which perhaps could be phased in later, is to
advertise one limit but to treat it as a soft limit and to begin
enforcement at a higher, unadvertised hard limit. In the above
example, perhaps the server might advertise 1000 but actually enforce
a limit of 10,000. Again, clients that understand MESSAGELIMIT will
comply with the limit of 1000, but other clients will still
interoperate up to the higher threshold.

Attempts to go beyond the advertised limit can be logged so that
client understanding of MESSAGELIMIT can be tracked. If
implementation and deployment of this extension becomes common, it
may at some point be acceptable to strictly enforce the advertised
limit and to accept that the remaining clients will, indeed, no
longer work properly with mailboxes above that limit.

5. Formal syntax 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 by
IMAP4 [RFC3501].

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

capability =/ "MESSAGELIMIT=" message-limit /
"SAVELIMIT=" message-limit
;; <capability> from [RFC3501]

message-limit = nz-number

resp-text-code =/ "MESSAGELIMIT" SP message-limit [SP uniqueid]
;; No more than nz-number messages can be processed
;; by any command at a time. The last (lowest) processed
;; UID is uniqueid.
;; The last parameter is omitted, omitted when not known.

6. Security Considerations

This document defines an additional IMAP4 capability. As such, it
does not change the underlying security considerations of IMAP4rev1
[RFC3501]
and or IMAP4rev2 [RFC9051].

This document defines an optimization that can both reduce the amount
of work performed by the server, as well at the amount of data
returned to the client. Use of this extension is likely to cause the
server and the client to use less memory than when the extension is
not used, which can in turn help to protect from Denial-of-Service denial-of-service
attacks. However, as this is going to be new code in both the client
and the server, rigorous testing of such code is required in order to
avoid introducing of new implementation bugs.

7. IANA Considerations

7.1. Changes/additions Additions to the IMAP4 capabilities registry IMAP Capabilities Registry

IMAP4 capabilities are registered by publishing a standards track Standards Track or
IESG approved
IESG-approved Informational or Experimental RFC. The "IMAP
Capabilities" registry is currently located at:

https://www.iana.org/assignments/imap-capabilities/
<https://www.iana.org/assignments/imap-capabilities/>.

IANA is requested to add registrations of has added "MESSAGELIMIT=" and "SAVELIMIT=" capabilities to this
registry, both pointing to this
document.

8. Acknowledgments

This document was motivated by the Yahoo! team and their questions
about best client practices for dealing with large mailboxes.

Editor of this document would like to thank as the following people who
provided useful comments, contributed text or participated in
discussions of this document: Timo Sirainen, Barry Leiba, Ken
Murchison and Arnt Gulbrandsen.

9. reference.

8. References

9.1.

8.1. Normative References

[ABNF] Crocker, D., Ed. and P. Overell, Ed., "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>.

[RFC3502] Crispin, M., "Internet Message Access Protocol (IMAP) -
MULTIAPPEND Extension", RFC 3502, DOI 10.17487/RFC3502,
March 2003, <https://www.rfc-editor.org/info/rfc3502>.

[RFC5182] Melnikov, A., "IMAP Extension for Referencing the Last
SEARCH Result", RFC 5182, DOI 10.17487/RFC5182, March
2008, <https://www.rfc-editor.org/info/rfc5182>.

[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>.

[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.

8.2. Informative References

[RFC9394] Melnikov, A., Achuthan, A. P., Nagulakonda, V., and L.
Alves, "IMAP PARTIAL Extension for Paged SEARCH and
FETCH", RFC 9394, DOI 10.17487/RFC9394, June 2023,
<https://www.rfc-editor.org/info/rfc9394>.

Index

MESSAGELIMIT (response code)
Section 3.1, Paragraph 2.2.1

Acknowledgments

This document was motivated by the Yahoo! team and their questions
about best client practices for dealing with large mailboxes.

The editor of this document would like to thank the following people
who provided useful comments, contributed text, or participated in
discussions of this document: Timo Sirainen, Barry Leiba, Ken
Murchison, and Arnt Gulbrandsen.

Authors' Addresses

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

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

Vikram Nagulakonda
Yahoo!
Email: nvikram_imap@yahoo.com

Luis Alves
Email: luis.alves@lafaspot.com