IMAP UIDBATCHES Extension
draft-ietf-mailmaint-imap-uidbatches-02

MailMaint                                                 D. Eggert, Ed.
Internet-Draft                                                 Apple Inc
Intended status: Standards Track                          2 January 2025
Expires: 6 July 2025

                       IMAP UIDBATCHES Extension
                draft-ietf-mailmaint-imap-uidbatches-02

Abstract

   The UIDBATCHES extension of the Internet Message Access Protocol
   (IMAP) allows clients to retrieve UIDs from the server such that
   these UIDs split the messages of a mailbox into equally sized
   batches.  It enables the client to perform operations such as
   FETCH/SEARCH/STORE on these specific batches.  This limits the number
   of messages that each command operates on, enabling better control
   over resource usage and response sizes.

Status of This Memo

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Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Document Conventions  . . . . . . . . . . . . . . . . . . . .   3
   3.  The UIDBATCHES extension  . . . . . . . . . . . . . . . . . .   3
     3.1.  UIDBATCHES Command  . . . . . . . . . . . . . . . . . . .   3
       3.1.1.  Usage and Example . . . . . . . . . . . . . . . . . .   4
       3.1.2.  Response Format . . . . . . . . . . . . . . . . . . .   5
       3.1.3.  Batch Sizes . . . . . . . . . . . . . . . . . . . . .   5
       3.1.4.  UIDs  . . . . . . . . . . . . . . . . . . . . . . . .   6
       3.1.5.  Batch Ranges  . . . . . . . . . . . . . . . . . . . .   7
       3.1.6.  Empty Responses . . . . . . . . . . . . . . . . . . .   8
       3.1.7.  Large Mailboxes . . . . . . . . . . . . . . . . . . .   8
     3.2.  Similarity to UID SEARCH Command  . . . . . . . . . . . .   9
     3.3.  Similarity to PARTIAL Extension . . . . . . . . . . . . .   9
     3.4.  Interaction with MESSAGELIMIT Extension . . . . . . . . .  10
     3.5.  Interaction with UIDONLY Extension  . . . . . . . . . . .  10
     3.6.  Interaction with SEARCHRES Extension  . . . . . . . . . .  10
   4.  Formal syntax . . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Changes/additions to the IMAP4 capabilities registry  . .  11
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   This document defines an extension to the Internet Message Access
   Protocol [RFC3501] to retrieve UIDs that partition a mailbox's
   messages into evenly sized batches.  This extension is compatible
   with both IMAP4rev1 [RFC3501] and IMAP4rev2 [RFC9051].

   The purpose of this extension is to allow clients to (pre-)determine
   UID ranges that limit the number of messages that each command
   operates on.  This is especially beneficial with [RFC9586] UIDONLY
   mode, where sequence numbers are unavailable to the client.

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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] or keywords from
   this document.

3.  The UIDBATCHES extension

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

3.1.  UIDBATCHES Command

   Arguments:
      Message count per batch.
      Optional batch range.

   Responses:
      UIDBATCHES response

   Result:
      OK
      BAD command unknown or arguments invalid

   When the client sends a UIDBATCHES command to the server, the server
   will return the UID ranges that partition the messages in the
   currently selected mailbox into equally sized batches.

   Batches are arranged by descending UID order, with the first batch
   containing the highest UIDs.

   For a mailbox with <M> messages, requesting batches of size <N>
   returns UID ranges corresponding to the sequence numbers

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   <M>:<M-N+1>
   <M-N>:<M-2*N+1>
   <M-2*N>:<M-3*N+1>
   ...

3.1.1.  Usage and Example

   When the client selects a mailbox, it can use the UIDBATCHES command
   to find the UIDs that split the mailbox’s messages into batches.  For
   example:

   C: A142 SELECT INBOX
   S: * 6823 EXISTS
   S: * 1 RECENT
   S: * OK [UNSEEN 12] Message 12 is first unseen
   S: * OK [UIDVALIDITY 3857529045] UIDs valid
   S: * OK [UIDNEXT 215296] Predicted next UID
   S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
   S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
   S: A142 OK [READ-WRITE] SELECT completed
   C: A143 UIDBATCHES 2000
   S: * UIDBATCHES (TAG "A143") 215295:99695,99696:20350,20351:7829,7830:1
   S: A143 OK UIDBATCHES Completed

   The server’s response provides four UID ranges:

   1.  215295:99695

   2.  99696:20350

   3.  20351:7829

   4.  7830:1

   Each range contains up to 2,000 messages, except the last range,
   which contains the remaining 823 messages.

   As new messages cannot appear within these UID ranges, the number of
   messages in each range will not increase.  It may decrease, though,
   as messages are deleted.

   The client MUST NOT re-run UIDBATCHES unless at least one of the
   following conditions is met:

   5.  A different mailbox has been selected

   6.  More than N/2 messages have been expunged from the mailbox

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   7.  More than N/2 new messages have been received into the mailbox

   These limits are in place to protect servers, since it may be
   expensive for the server to run this command.  The client MUST NOT
   excessively re-run UIDBATCHES while the mailbox remains selected.

   The client can keep track of the number of EXPUNGE or VANISH messages
   and re-run UIDBATCHES if many messages are deleted.

   As new messages arrive into the mailbox, the client should add these
   to a new message batch (starting at UID 215296 in the above example).
   Once N/2 or more new messages have been added to the mailbox, the
   client can ask for updated batches by re-running the UIDBATCHES
   command.

   The server MAY reject UIDBATCHES commands with a BAD response if the
   client exceeds these limits.

3.1.2.  Response Format

   The server MUST reply with a UIDBATCHES response, even if no ranges
   are returned (see Section 3.1.6).  The UIDBATCHES response MUST
   include the tag of the command it relates to (similar to an ESEARCH
   response), and it MUST include the UID indicator.

   The UID ranges in the response MUST be ordered in descending
   sequence, from the highest to the lowest UIDs.

3.1.3.  Batch Sizes

   This extension enforces a hard limit on the minimum batch size that a
   client can request, and it gives the server some flexibility in the
   actual size being returned.  This ensures the server has
   implementation flexibility, making the operation less resource-
   intensive.  And to prevent clients from misusing this extension to
   infer message sequence numbers.

   The intent of this extension is to work well in combination with
   [RFC9586] UIDONLY mode without creating a de-facto loophole that re-
   introduces sequence numbers.

   Section 3.2 also outlines some reasoning for these limitation.

   The server MUST support batch sizes of 500 messages or larger.

   The server MUST respond with BAD and a response code TOOSMALL if the
   client uses a batch size that is smaller than the minimum allowed by
   the server, e.g.

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   S: A302 BAD [TOOSMALL] Minimum batch size is 500

   The server MUST NOT return ranges that contain more than the number
   of messages per batch requested by the client (2,000 in the above
   example).  But the server MAY return fewer messages per range,
   notably if that makes the implementation simpler and/or more
   efficient.

   Servers SHOULD NOT return batches that are substantially smaller, and
   SHOULD aim to be within 90% of the requested size.  The client is
   likely to pick a batch count based on what it wants to display to the
   user.  A client may e.g. request 2 batches of size 1,000 if it wants
   to be able to display the last 2,000 messages to the user.

   A server MAY return batches that are substantially smaller if there
   are changes in mailbox state during the execution of the UIDBATCHES
   command, namely messages being expunged, such that the overall size
   of the mailbox changes.  The client would be able to infer this from
   the `EXPUNGE`, `VANISH`, or `EXISTS` messages it receives.

   If the total number of messages is not evenly divisible by the
   requested batch size, the last batch will contain the remainder.
   Thus, the last batch in the mailbox (i.e. the batch with the lowest
   UIDs) will usually have fewer messages than the requested number of
   messages.

   If the requested batch size is equal to or larger than the number of
   messages in the mailbox, the server MUST return a response with a
   single UID range that spans all messages.

3.1.4.  UIDs

   The server MAY return UID ranges with UIDs that do not exist on the
   server.  The client as a result MUST NOT make assumptions about the
   existence of messages.  If the server returns the response

   S: * UIDBATCHES (TAG "A302") 163886:99703,99696:20358,20351:7841,7830:1
   S: A302 OK UIDBATCHES Completed

   there may not be any messages on the server with the UIDs such as
   163886, 99703, 99696, etc.

   The range 163886:99703 will span approximately the requested number
   of messages (may be less, see Section 3.1.3), but its start and end
   UIDs may not correspond to messages on the server.

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   This gives the server implementation some flexibility as to which UID
   ranges to return.  They might, e.g., return 163886:99697 and
   99696:20358 instead of 163886:99703 and 99696:20358 -- assuming that
   there are no messages in the range 99704:99697.

   If there are fewer message in the mailbox than the requested batch
   size, the server would return a single batch that contains all
   messages in the mailbox.

   Servers SHOULD end the last UID batch in the mailbox with UID 1 even
   if this UID does not exist on the server.  This makes it unambiguous
   to the client that this range is in fact the last range.

3.1.5.  Batch Ranges

   A client can optionally provide a batch range.  The server limits its
   response to UID ranges corresponding to the specified batch indices.
   For example, if the client sends

   C: A302 UIDBATCHES 2000 10:20

   for a mailbox with more than 40,000 messages, the server would return
   the 10th to 20th batches, corresponding to the 20,000th and 40,000th
   message respectively.

   Note that batches start at the highest UIDs: batch 1 is the batch
   with the highest UIDs.

   The UID ranges that the server returns would still split the
   mailbox’s messages into batches of the requested size (2,000 in the
   example).

   If the client requests more batches than exist on the server, the
   server would return those that do exist.  For example if the client
   sends

   C: A302 UIDBATCHES 2000 1:4

   and the selected mailbox has 7,000 messages, the server would then
   return a UIDBATCHES response with only 4 UID ranges.

   If the client requests a range of batches that do not exist on the
   server, the server MUST still return an empty response.  See section
   Section 3.1.6.

   Note that the number of messages per batch returned by the server may
   be approximate as detailed in Section 3.1.3.  As a result, if the
   client needs to request consecutive batch ranges such as 1:100,

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   101:200, 201:300, and so on, the client may want to make these batch
   ranges overlap by e.g. requesting 1:100, 100:200, and 200:300.  The
   client would then be able to check if the resulting UIDs do in fact
   overlap.

   Clients MUST NOT request batch ranges that span more than 100,000
   messages, i.e. the number of batches multiplied by the batch size
   MUST NOT be larger than 100,000.  The server MAY reject UIDBATCHES
   commands with a BAD response with the LIMIT response code if the
   client exceeds this limit.

   C: A302 UIDBATCHES 2000 1:100
   S: A302 BAD [LIMIT] Too many messages

3.1.6.  Empty Responses

   When the client issues any valid UIDBATCHES command and the mailbox
   is empty, the server MUST reply with a UIDBATCHES response, e.g.

   S: * UIDBATCHES (TAG "A302")
   S: A302 OK UIDBATCHES Completed

   If the client requests a range of batches that do not exist, the
   server MUST reply with an empty UIDBATCHES response.  If the mailbox
   has 7,000 messages, and the client sends

   C: A302 UIDBATCHES 2000 6:8

   the server would respond with

   S: * UIDBATCHES (TAG "A302")
   S: A302 OK UIDBATCHES Completed

3.1.7.  Large Mailboxes

   The server may not be able to return all UID ranges if the mailbox
   contains an extremely large number of messages.

   The server MUST at least support returning UID ranges spanning
   100,000 messages.  See Section 3.1.5 for details on this limit.

   If the server can not return all of the requested UID ranges, it MUST
   respond with a BAD response with the LIMIT response code.  Notably,
   when the client requests all UID ranges and the mailbox has more than
   100,000 messages, the server MAY reply with a BAD response.  For
   example:

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   C: A302 UIDBATCHES 2000
   S: A302 BAD [LIMIT] Too many messages in mailbox

   The client should know what the message count in the mailbox is, and
   if the message count exceeds 100,000 it may choose to always request
   batch ranges as discussed in Section 3.1.5 instead of requesting all
   batches.

3.2.  Similarity to UID SEARCH Command

   The UIDBATCHES is in effect nothing more than shorthand for a UID
   SEARCH command of the form

   C: A145 UID SEARCH RETURN () <N-M>,<N-2*M>,<N-3*M>,...

   where N is the number of messages in the mailbox and M is the
   requested batch count.

   The special purpose UIDBATCHES command, though, tries to address two
   problems:

   (a)  for many servers, UID SEARCH commands specifying sequence
        numbers are costly, especially for mailboxes with many messages.

   (b)  the UIDONLY extension disallows the use of sequence numbers and
        thus makes it difficult for the client to split its commands
        into batches of a size that works well for the client and
        server.

   By providing a special purpose command, servers can implement a
   different, optimized code path for determining message batches.  And
   servers using the UIDONLY extension can provide a facility to let the
   client determine message batches without using sequence numbers in a
   UID SEARCH command.

   Section 3.1.3 describes some implementation restrictions to ensure
   this.

3.3.  Similarity to PARTIAL Extension

   The PARTIAL extension provides a different way for the client to
   split its commands into batches by using pages SEARCH and FETCH.

   The intention of the UIDBATCHES command is to let the client pre-
   determine message batches of a desired size.

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   This makes it easier for the client to share implementation between
   servers regardless of their support of PARTIAL.  And additionally,
   because the client can issue a corresponding UID SEARCH command to
   servers that do not implement UIDBATCHES, the client can use similar
   batching implementations for servers that support UIDBATCHES and
   those that do not.

3.4.  Interaction with MESSAGELIMIT Extension

   When the server supports both the MESSAGELIMIT and UIDBATCHES
   extension, the client SHOULD request batches no larger than the
   specified maximum number of messages that can be processed in a
   single command.  The client MAY choose to use a smaller batch size.

   Additionally, since servers MAY limit the number of UIDs returned in
   response to UIDBATCHES, it is reasonable to assume that they would at
   most return N UIDs where N is the limit the server announced as its
   MESSAGELIMIT.

3.5.  Interaction with UIDONLY Extension

   As noted above, the UIDBATCHES extension allows for clients to create
   UID ranges for message batches even when the connection operates in
   UIDONLY mode which otherwise doesn't allow for using message sequence
   numbers.

3.6.  Interaction with SEARCHRES Extension

   Servers that support SEARCHRES [RFC5182] MUST NOT store the result of
   UIDBATCHES in the $ variable.

4.  Formal syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [RFC5234].

   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.

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

   command-select      =/ message-batches

   message-batches     = "UIDBATCHES" SP nz-number
                         [SP nz-number ":" nz-number]

   uidbatches-response = "UIDBATCHES" search-correlator
                         [SP uid-range *("," uid-range) ]

   mailbox-data        =/ uidbatches-response

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 as 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 the introduction 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 the "UIDBATCHES" capability
   to this registry, pointing to this document.

7.  References

7.1.  Normative References

   [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>.

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   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <https://www.rfc-editor.org/info/rfc3501>.

   [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>.

   [RFC5234]  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>.

   [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>.

   [RFC9586]  Melnikov, A., Achuthan, A. P., Nagulakonda, V., Singh, A.,
              and L. Alves, "IMAP Extension for Using and Returning
              Unique Identifiers (UIDs) Only", RFC 9586,
              DOI 10.17487/RFC9586, May 2024,
              <https://www.rfc-editor.org/info/rfc9586>.

Author's Address

   Daniel Eggert (editor)
   Apple Inc
   One Apple Park Way
   Cupertino, CA 95014
   United States of America
   Email: deggert@apple.com

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