IMAP PARTIAL Extension for Paged SEARCH and FETCH
RFC 9394



Internet Engineering Task Force (IETF)                       A. Melnikov
Request for Comments: 9394                                         Isode
Updates: 4731, 5267                                       A. P. Achuthan
Category: Standards Track                                 V. Nagulakonda
ISSN: 2070-1721                                                   Yahoo!
                                                                L. Alves
                                                               June 2023

           IMAP PARTIAL Extension for Paged SEARCH and FETCH

Abstract

   The PARTIAL extension of the Internet Message Access Protocol (see
   RFCs 3501 and 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 the PARTIAL SEARCH return option originally
   specified in RFC 5267.  It also clarifies some interactions between
   RFC 5267 and RFCs 4731 and 9051.

   This document updates RFCs 4731 and 5267.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc9394.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   than English.

Table of Contents

   1.  Introduction and Overview
   2.  Document Conventions
   3.  The PARTIAL Extension
     3.1.  Incremental SEARCH and Partial Results
     3.2.  Interaction between PARTIAL, MIN, MAX, and SAVE SEARCH
           Return Options
     3.3.  Extension to UID FETCH
     3.4.  Use of "PARTIAL" and "CONDSTORE" IMAP Extensions Together
   4.  Formal Syntax
   5.  Security Considerations
   6.  IANA Considerations
     6.1.  Changes/Additions to the IMAP Capabilities Registry
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction and Overview

   This document defines an extension to the Internet Message Access
   Protocol [RFC3501] [RFC9051] for performing incremental searches and
   fetches.  This extension is compatible with both IMAP4rev1 [RFC3501]
   and IMAP4rev2 [RFC9051].  This extension uses IMAP extensibility
   rules defined in [RFC4466].

   The PARTIAL extension of the Internet Message Access Protocol 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 the PARTIAL SEARCH return option originally
   specified in RFC 5267.  It also clarifies some interactions between
   RFC 5267 and RFCs 4731 and 9051.

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

3.  The PARTIAL Extension

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

3.1.  Incremental SEARCH and Partial Results

   The PARTIAL SEARCH return option causes the server to provide in an
   ESEARCH response [RFC4731] [RFC9051] a subset of the results denoted
   by the sequence range given as the mandatory argument.  The first
   result (message with the lowest matching Unique Identifier (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 the SEARCH from the
   latest matching (with the highest UID) message.  This can be done by
   prepending "-" 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 list of results MUST be ordered in
   mailbox order -- that is, in UID or message sequence number order.

   In cases where a PARTIAL SEARCH return option references results that
   do not exist by using a range that starts or ends higher (or lower)
   than the current number of results, 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
   the "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 the "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 the
   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 the MIN/MAX/COUNT result options are present, the
   corresponding PARTIAL is returned, and the "$" marker would contain
   references to all messages returned by the PARTIAL result option.

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

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

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

   Table 1 summarizes additional requirements for ESEARCH server
   implementations described in this section.

   Note regarding Table 1: "[m]" means optional "MIN" and/or "MAX".

          +===============================+=====================+
          | Combination of Result Options |   "$" 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 |
          +-------------------------------+---------------------+

                                  Table 1

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.  The 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 in the UID FETCH command includes an
   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 [RFC7162].

     // Returning information for the last 30 messages in the UID range
     // that have any flags/keywords 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 a subset of those
   messages that 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 the "UID FETCH 25900:26600 (UID FLAGS) (CHANGEDSINCE 98305
   PARTIAL -1:-30)" command and it would result in the same responses.

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
   IMAP4rev1 [RFC3501] or IMAP4rev2 [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>
   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
       ;; [RFC4466] and [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 that no results correspond to
       ;; the requested range.

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

   fetch-modifier      =/ modifier-partial
                          ;; <fetch-modifier> from [RFC4466].

5.  Security Considerations

   This document defines an additional IMAP4 capability.  As such, it
   does not change the underlying security considerations of IMAP4rev1
   [RFC3501] and IMAP4rev2 [RFC9051].  The authors and reviewers believe
   that no new security issues are introduced with these additional
   IMAP4 capabilities.

   This document defines an optimization that can reduce both the amount
   of work performed by the server and 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 new implementation bugs.

6.  IANA Considerations

6.1.  Changes/Additions to the IMAP 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/imap-
   capabilities>.

   IANA has added the PARTIAL extension to the "IMAP Capabilities"
   registry with RFC 9394 as the reference.

7.  References

7.1.  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>.

   [RFC4466]  Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
              ABNF", RFC 4466, DOI 10.17487/RFC4466, April 2006,
              <https://www.rfc-editor.org/info/rfc4466>.

   [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, November 2006,
              <https://www.rfc-editor.org/info/rfc4731>.

   [RFC5267]  Cridland, D. and C. King, "Contexts for IMAP4", RFC 5267,
              DOI 10.17487/RFC5267, July 2008,
              <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,
              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>.

7.2.  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, May 2014,
              <https://www.rfc-editor.org/info/rfc7162>.

Acknowledgments

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

   The authors of this document would like to thank the following
   people, who provided useful comments or participated in discussions
   of this document: Timo Sirainen and Barry Leiba.

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

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