MailMaint D. Eggert, Ed. Internet-Draft Apple Inc Intended status: Standards Track 30 December 2024 Expires: 3 July 2025 IMAP UIDBATCHES Extension draft-ietf-mailmaint-imap-uidbatches-01 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 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 3 July 2025. Copyright Notice Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved. Eggert Expires 3 July 2025 [Page 1] Internet-Draft IMAP UIDBATCHES Extension December 2024 This document is subject to BCP 78 and the IETF Trust's Legal 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 and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. 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. Eggert Expires 3 July 2025 [Page 2] Internet-Draft IMAP UIDBATCHES Extension December 2024 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 messages, requesting batches of size returns UID ranges corresponding to the sequence numbers Eggert Expires 3 July 2025 [Page 3] Internet-Draft IMAP UIDBATCHES Extension December 2024 : : : ... 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 Eggert Expires 3 July 2025 [Page 4] Internet-Draft IMAP UIDBATCHES Extension December 2024 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. Eggert Expires 3 July 2025 [Page 5] Internet-Draft IMAP UIDBATCHES Extension December 2024 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. Eggert Expires 3 July 2025 [Page 6] Internet-Draft IMAP UIDBATCHES Extension December 2024 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, Eggert Expires 3 July 2025 [Page 7] Internet-Draft IMAP UIDBATCHES Extension December 2024 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: Eggert Expires 3 July 2025 [Page 8] Internet-Draft IMAP UIDBATCHES Extension December 2024 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 () ,,,... 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. Eggert Expires 3 July 2025 [Page 9] Internet-Draft IMAP UIDBATCHES Extension December 2024 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. Eggert Expires 3 July 2025 [Page 10] Internet-Draft IMAP UIDBATCHES Extension December 2024 capability =/ "UIDBATCHES" ;; 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, . Eggert Expires 3 July 2025 [Page 11] Internet-Draft IMAP UIDBATCHES Extension December 2024 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, . [RFC5182] Melnikov, A., "IMAP Extension for Referencing the Last SEARCH Result", RFC 5182, DOI 10.17487/RFC5182, March 2008, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9051] Melnikov, A., Ed. and B. Leiba, Ed., "Internet Message Access Protocol (IMAP) - Version 4rev2", RFC 9051, DOI 10.17487/RFC9051, August 2021, . [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, . Author's Address Daniel Eggert (editor) Apple Inc One Apple Park Way Cupertino, CA 95014 United States of America Email: deggert@apple.com Eggert Expires 3 July 2025 [Page 12]