Internet-Draft	IMAP MESSAGELIMIT	February 2024
Melnikov, et al.	Expires 4 August 2024	[Page]

Workgroup:: Network Working Group
Internet-Draft:: draft-ietf-extra-imap-messagelimit-08
Published:: 1 February 2024
Intended Status:: Experimental
Expires:: 4 August 2024
Authors:: A. Melnikov

Isode

A. P. Achuthan

Yahoo!

V. Nagulakonda

Yahoo!

L. Alves

IMAP MESSAGELIMIT Extension

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/APPEND/EXPUNGE command. This helps servers to control resource usage when performing various IMAP operations. This helps clients to know the message limit enforced by corresponding IMAP server and avoid issuing commands that would exceed such limit.¶

Status of This Memo

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This Internet-Draft will expire on 4 August 2024.¶

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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.¶

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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/MOVE/APPEND/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 words are IMAP key words [RFC3501][RFC9051] or key words from this document.¶

3. The MESSAGELIMIT extension

An IMAP server advertises support for the MESSAGELIMIT extension by including "MESSAGELIMIT=<limit>" capability in the CAPABILITY response/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/APPEND or UID EXPUNGE command.¶

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

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

3.1. Returning limits on the number of messages processed in a single SEARCH/FETCH/STORE/COPY/MOVE/APPEND/EXPUNGE command

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

Note that when the MESSAGELIMIT response code is similar to the LIMIT ([RFC9051]) response code, 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 the <N> value is 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 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 copy of messages with UIDs between 18000:21000. The total message count exceeds the server's published 1000 messages limit. As COPY/UID COPY needs to 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 of messages with UIDs between 18000:21000. The total message count exceeds the server's published 1000 messages limit. (Unlike COPY/UID COPY, MOVE/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, can repeat the UID MOVE command with the same parameter. (For the MOVE command, the message set parameter need 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 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 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 limit. The client that wants to change flags for all messages in the range and observes a MESSAGELIMIT response 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 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 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 removal of messages (using UID EXPUNGE) that have \Deleted flag set with UIDs between 11000:13000. The total message count of messages with \Deleted flag set exceeds the server's published 1000 messages limit. The client that wants to remove all messages marked as \Deleted in the range and observes a MESSAGELIMIT response 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 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 \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 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 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 message limit

Without the PARTIAL 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 limit

Note that when the server needs to return both EXPUNGEISSUED ([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 IMAP MULTIAPPEND [RFC3502] extension is also supported by the server, the message limit also applies to the APPEND command.¶

3.2. UIDAFTER and UIDBEFORE SEARCH criteria

The MESSAGELIMIT extension also defines 2 extra SEARCH 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 SEARCH keys are particularly useful when the SEARCHRES [RFC5182] extension 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

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

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

3.4. Interaction with SEARCHRES extension and IMAP4rev2

Servers that support both MESSAGELIMIT and SEARCHRES [RFC5182] extensions 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

3.5. Effects of MESSAGELIMIT/SAVELIMIT extensions on non compliant 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 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 and MOVE will be effectively restricted to the last N messages of the mailbox. In particular unextended SEARCHes intended for counting of messages with or without a particular set of flags would return incorrect counts.¶

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 by IMAP4 [RFC3501].¶

Except as noted otherwise, all alphabetic characters are case-insensitive. The use of upper or lower case 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, when not known.

5. Security Considerations

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

6. IANA Considerations

6.1. Changes/additions to the IMAP4 capabilities registry

IMAP4 capabilities are registered by publishing a standards track or IESG approved Informational or Experimental RFC. The registry is currently located at:¶

   https://www.iana.org/assignments/imap4-capabilities

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

[ABNF]: Crocker, D., Ed. and P. Overell, Ed., "Augmented BNF for Syntax Specifications: ABNF", RFC 5234, 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>.

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

M ¶

M ¶
- MESSAGELIMIT (response code)
  
  Section 3.1, Paragraph 2.2.1 ¶

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