Internet-Draft IMAP PARTIAL January 2022
Melnikov, et al. Expires 16 July 2022 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-melnikov-imap-partial-02
Updates:
5267, 4731 (if approved)
Published:
Intended Status:
Standards Track
Expires:
Authors:
A. Melnikov
Isode
A. P. Achuthan
Yahoo!
V. Nagulakonda
Yahoo!
L. Alves
Yahoo!

IMAP Paged SEARCH & FETCH Extension

Abstract

The PARTIAL extension of the Internet Message Access Protocol (RFC 3501/RFC 9051) allows clients to limit the number of search results returned, as well as to perform incremental (paged) searches. This also helps servers to optimize resource usage when performing searches.

This document extends PARTIAL SEARCH return option originally specified in RFC 5267. It also clarifies some interactions between RFC 5267 and RFC 4731/RFC 9051.

This document also describes the MESSAGELIMIT extension for announcing a limit on the number of messages that can be processed in a single FETCH/SEARCH/STORE/COPY/MOVE command.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents 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 16 July 2022.

Table of Contents

1. Introduction and Overview

This document defines an extension to the Internet Message Access Protocol [RFC3501] for performing incremental searches and fetches. This extension is compatible with both IMAP4rev1 [RFC3501] and IMAP4rev2 [RFC9051].

The PARTIAL extension of the Internet Message Access Protocol (RFC 3501/RFC 9051) allows clients to limit the number of search results returned, as well as to perform incremental (paged) searches. This also helps servers to optimize resource usage when performing searches.

This document extends PARTIAL SEARCH return option originally specified in RFC 5267. It also clarifies some interactions between RFC 5267 and RFC 4731/RFC 9051.

This document also describes the MESSAGELIMIT extension for announcing a limit on the number of messages that can be processed in a single FETCH/SEARCH/STORE/COPY/MOVE command.

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 keywords [RFC3501][RFC9051] or keywords from this document.

3. The PARTIAL extension

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

Clients that implement support for PARTIAL extension MUST also support the MESSAGELIMIT response code (see Section 4).

3.1. Incremental SEARCH and partial results

The PARTIAL search return option causes the server to provide in an ESEARCH response a subset of the results denoted by the sequence range given as the mandatory argument. The first result (message with the lowest matching UID) is 1; thus, the first 500 results would be obtained by a return option of "PARTIAL 1:500", and the second 500 by "PARTIAL 501:1000". This intentionally mirrors message sequence numbers.

It is also possible to direct the server to start SEARCH from the latest matching (with the highest UID) message. This can be done by prepeding "-" to the index. For example -1 is the last message, -2 is next to the last and so on. Using this syntax helps server implementations to optimize their SEARCHes.

A single command MUST NOT contain more than one PARTIAL or ALL search return option -- that is, either one PARTIAL, one ALL, or neither PARTIAL nor ALL is allowed.

For SEARCH results, the entire result list MUST be ordered in mailbox order, that is, in UID or message sequence number order.

Where a PARTIAL search return option references results that do not exist, by using a range which starts or ends higher (or lower) than the current number of results, then the server returns the results that are in the set. This yields a PARTIAL return data item that has, as payload, the original range and a potentially missing set of results that may be shorter than the extent of the range. If the whole range references results that do not exist, a special value "NIL" is returned by the server instead of the sequence set.

Clients need not request PARTIAL results in any particular order. Because mailboxes may change, clients might wish to use PARTIAL in combination with UPDATE (see [RFC5267] if the server also advertises CONTEXT=SEARCH capability, especially if the intent is to walk a large set of results; however, these return options do not interact -- the UPDATE will provide notifications for all matching results.

  // Let's assume that the A01 SEARCH without PARTIAL would return
  // 23764 results.
  C: A01 UID SEARCH RETURN (PARTIAL -1:-100) UNDELETED
    UNKEYWORD $Junk
  S: * ESEARCH (TAG "A01") UID PARTIAL (-1:-100 ...)
  // 100 most recent results in set syntax elided.
  S: A01 OK Completed.
  // Let's assume that the A02 SEARCH without PARTIAL would return
  // 23764 results.
  C: A02 UID SEARCH RETURN (PARTIAL 23500:24000) UNDELETED
    UNKEYWORD $Junk
  C: A03 UID SEARCH RETURN (PARTIAL 1:500) UNDELETED
    UNKEYWORD $Junk
  C: A04 UID SEARCH RETURN (PARTIAL 24000:24500) UNDELETED
    UNKEYWORD $Junk
  S: * ESEARCH (TAG "A02") UID PARTIAL (23500:24000 ...)
  // 264 results in set syntax elided,
  // this spans the end of the results.
  S: A02 OK Completed.
  S: * ESEARCH (TAG "A03") UID PARTIAL (1:500 ...)
  // 500 results in set syntax elided.
  S: A03 OK Completed.
  S: * ESEARCH (TAG "A04") UID PARTIAL (24000:24500 NIL)
  // No results are present, this is beyond the end of the results.
  S: A04 OK Completed.

3.2. Interaction between PARTIAL, MIN, MAX and SAVE SEARCH return options

This section only applies if the server advertises PARTIAL IMAP capability or CONTEXT=SEARCH [RFC5267], together with ESEARCH [RFC4731] and/or IMAP4rev2"[RFC9051].

The SAVE result option doesn't change whether the server would return items corresponding to PARTIAL SEARCH result options.

As specified in Section 3.1, it is an error to specify both PARTIAL and ALL result options in the same SEARCH command.

When the SAVE result option is combined with the PARTIAL result option, and none of MIN/MAX/COUNT result options is present, the corresponding PARTIAL is returned, and the "$" marker would contain all messages returned by the PARTIAL result option.

When the SAVE + PARTIAL result options are combined with the MIN or the MAX result option, and the COUNT result option is absent, the corresponding PARTIAL result and MIN/MAX is returned (if the search result is not empty), and the "$" marker would contain all messages returned by the PARTIAL result option + the corresponding MIN/MAX message.

If the SAVE + PARTIAL result options are combined with both MIN and MAX result options, and the COUNT result options is absent, the PARTIAL, MIN and MAX are returned (if the search result is not empty), and the "$" marker would contain all messages returned by the PARTIAL result option plus MIN and MAX messages.

If the SAVE + PARTIAL result options are combined with the COUNT result option, the PARTIAL and COUNT are returned, and the "$" marker would always contain all messages found by the SEARCH or UID SEARCH command.

The following table summarizes the additional requirement on ESEARCH server implementations described in this section.

Table 1
Combination of Result option "$" marker value
SAVE PARTIAL PARTIAL
SAVE PARTIAL MIN PARTIAL & MIN
SAVE PARTIAL MAX PARTIAL & MAX
SAVE PARTIAL MIN MAX PARTIAL & MIN & MAX
SAVE PARTIAL COUNT [m] all found messages

where '[m]' means optional "MIN" and/or "MAX"

3.3. Extension to UID FETCH

The PARTIAL extension also extends the UID FETCH command with a PARTIAL FETCH modifier. The PARTIAL FETCH modifier has the same syntax as the PARTIAL SEARCH result option. Presence of the PARTIAL FETCH modifier instructs the server to only return FETCH results for messages in the specified range. It is useful when the sequence-set (first) parameter to the UID FETCH command includes unknown number of messages.

  // Returning information for the last 3 messages in the UID range
  C: 10 UID FETCH 25900:26600 (UID FLAGS) (PARTIAL -1:-3)
  S: * 12888 FETCH (FLAGS (\Seen) UID 25996)
  S: * 12889 FETCH (FLAGS (\Flagged \Answered) UID 25997)
  S: * 12890 FETCH (FLAGS () UID 26600)
  S: 10 OK FETCH completed
  // Returning information for the first 5 messages in the UID range
  C: 11 UID FETCH 25900:26600 (UID FLAGS) (PARTIAL 1:5)
  S: * 12591 FETCH (FLAGS (\Seen) UID 25900)
  S: * 12592 FETCH (FLAGS (\Flagged) UID 25902)
  S: * 12593 FETCH (FLAGS (\Answered) UID 26310)
  S: * 12594 FETCH (FLAGS () UID 26311)
  S: * 12595 FETCH (FLAGS (\Answered) UID 26498)
  S: 11 OK FETCH completed

3.4. Use of PARTIAL and CONDSTORE IMAP extensions together

This section is informative.

The PARTIAL FETCH modifier can be combined with the CHANGEDSINCE FETCH modifier.

  // Returning information for the last 30 messages in the UID range
  // that have any flag/keyword modified since modseq 98305
  C: 101 UID FETCH 25900:26600 (UID FLAGS) (PARTIAL -1:-30 CHANGEDSINCE 98305)
  S: * 12888 FETCH (FLAGS (\Flagged \Answered) MODSEQ (98306) UID 25997)
  S: * 12890 FETCH (FLAGS () MODSEQ (98312) UID 26600)
  S: 101 OK FETCH completed

The above example causes the server to first select the last 30 messages and then only return flag changes for subset of these messages which have MODSEQ higher than 98305.

Note that the order of PARTIAL and CHANGEDSINCE FETCH modifiers in the UID FETCH command is not important, i.e. the above example can also use "UID FETCH 25900:26600 (UID FLAGS) (CHANGEDSINCE 98305 PARTIAL -1:-30)" command and it would result in the same responses.

4. 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 SEARCH/FETCH/STORE/COPY/MOVE command.

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

Do we need a way to specify SEARCH criterion for "all UIDs after" or "all UIDs before" a specific UID?

If a server implementation doesn't allow more than <N> messages to be operated on by a single SEARCH/FETCH/STORE/COPY/MOVE 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 command. The lowest processed UID is <LastUID>. The client needs to repeat the operation for remaining messages, if required.
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 matching messages exceeds the server's published 1000 messages limit.
  C: 04 UID SEARCH UID 22000:25000 UNDELETED
  S: * SEARCH 25000 24998 (... 997 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.
  C: 05 UID COPY 18000:21000 "Trash"
  S: * NO [MESSAGELIMIT 1000 20001] Too many messages to copy
  S: 05 OK [COPYUID 1397597919 20001:21000 21363:22362] COPY completed for the last 1000 messages
Open Issue: Note that the above example shows a UID COPY that partially fails. This is assumed to be better for clients that don't understand the MESSAGELIMIT response code. However this might cause naive clients to believe that the COPY fully completed and that all messages were copied. (An alternative would be to return MESSAGELIMIT in the tagged NO response, meaning that no messages could be copied. However this wouldn't work well with clients that don't support MESSAGELIMIT response code.)
The following example shows MOVE of messages with UIDs between 18000:21000. The total message count exceeds the server's published 1000 messages limit. The client that wants to move all messages in the range and observes a MESSAGELIMIT response code, can repeat the command by updating the UID set parameter specified in the command. The client needs to keep doing this until 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: * 11335 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 message count 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 command by updating the UID set parameter specified in the command. The client needs to keep doing this until 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 use of MESSAGELIMIT response code together with the PARTIAL extension. The total message count exceeds the server's published 1000 messages limit.
  C: 08 UID FETCH 22000:25000 (UID FLAGS MODSEQ) (PARTIAL -1:-1500)
  S: 08 NO [MESSAGELIMIT 1000] 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: 031 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: 031 OK [EXPUNGEISSUED] Some messages were also expunged

5. 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.


SP                  = <Defined in RFC 5234>
MINUS               = "-"

capability          =/ "PARTIAL"
                       ;; <capability> from [RFC3501]

modifier-partial    = "PARTIAL" SP partial-range

partial-range-first = nz-number ":" nz-number
    ;; Request to search from oldest (lowest UIDs) to
    ;; more recent messages.
    ;; A range 500:400 is the same as 400:500.
    ;; This is similar to <seq-range> from [RFC3501],
    ;; but cannot contain "*".

partial-range-last  = MINUS nz-number ":" MINUS nz-number
    ;; Request to search from newest (highest UIDs) to
    ;; oldest messages.
    ;; A range -500:-400 is the same as -400:-500.

partial-range       = partial-range-first / partial-range-last

search-return-opt   =/ modifier-partial
    ;; All conform to <search-return-opt>, from [IMAP-ABNF]/[RFC9051]

search-return-data  =/ ret-data-partial

ret-data-partial    = "PARTIAL"
                      SP "(" partial-range SP partial-results ")"
    ;; <partial-range> is the requested range.

partial-results     = sequence-set / "NIL"
    ;; <sequence-set> from [RFC3501].
    ;; NIL indicates no results correspond to the requested range.

tagged-ext-simple   =/ partial-range-last

fetch-modifier      =/ modifier-partial



capability          =/ "MESSAGELIMIT=" 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.

6. Security Considerations

TBD.

7. IANA Considerations

7.1. 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 definition of the PARTIAL extension to point to this document.

8. Acknowledgments

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

Editor of this document would like to thank the following people who provided useful comments or participated in discussions of this document: Timo Sirainen.

This document uses lots of text from RFC 5267. Thus work of the RFC 5267 authors Dave Cridland and Curtis King is appreciated.

9. References

9.1. Normative References

[ABNF]
Crocker, D., Ed. and P. Overell, Ed., "Augmented BNF for Syntax Specifications: ABNF", RFC 5234, , <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, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC3501]
Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, , <https://www.rfc-editor.org/info/rfc3501>.
[RFC4731]
Melnikov, A. and D. Cridland, "IMAP4 Extension to SEARCH Command for Controlling What Kind of Information Is Returned", RFC 4731, DOI 10.17487/RFC4731, , <https://www.rfc-editor.org/info/rfc4731>.
[RFC5267]
Cridland, D. and C. King, "Contexts for IMAP4", RFC 5267, DOI 10.17487/RFC5267, , <https://www.rfc-editor.org/info/rfc5267>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <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, , <https://www.rfc-editor.org/info/rfc9051>.

9.2. Informative References

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

Index

M

Authors' Addresses

Alexey Melnikov
Isode Limited
Arun Prakash Achuthan
Yahoo!
Vikram Nagulakonda
Yahoo!
Luis Alves
Yahoo!