Network Working Group A. Melnikov Internet-Draft Isode Updates: 5267, 4731 (if approved) A. P. Achuthan Intended status: Standards Track V. Nagulakonda Expires: 13 February 2023 Yahoo! L. Alves 12 August 2022 IMAP Paged SEARCH & FETCH Extension draft-ietf-extra-imap-partial-02 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. 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 13 February 2023. Copyright Notice Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved. Melnikov, et al. Expires 13 February 2023 [Page 1] Internet-Draft IMAP PARTIAL August 2022 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. 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. Table of Contents 1. Introduction and Overview . . . . . . . . . . . . . . . . . . 2 2. Document Conventions . . . . . . . . . . . . . . . . . . . . 3 3. The PARTIAL extension . . . . . . . . . . . . . . . . . . . . 3 3.1. Incremental SEARCH and partial results . . . . . . . . . 3 3.2. Interaction between PARTIAL, MIN, MAX and SAVE SEARCH return options . . . . . . . . . . . . . . . . . . . . . 5 3.3. Extension to UID FETCH . . . . . . . . . . . . . . . . . 6 3.4. Use of PARTIAL and CONDSTORE IMAP extensions together . . 7 4. Formal syntax . . . . . . . . . . . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 6.1. Changes/additions to the IMAP4 capabilities registry . . 9 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 8.1. Normative References . . . . . . . . . . . . . . . . . . 9 8.2. Informative References . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 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]. Melnikov, et al. Expires 13 February 2023 [Page 2] Internet-Draft IMAP PARTIAL August 2022 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. 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 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 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. Melnikov, et al. Expires 13 February 2023 [Page 3] Internet-Draft IMAP PARTIAL August 2022 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. Melnikov, et al. Expires 13 February 2023 [Page 4] Internet-Draft IMAP PARTIAL August 2022 // 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 "$" marker would contain all messages returned by the PARTIAL result option + the corresponding MIN/MAX message. Melnikov, et al. Expires 13 February 2023 [Page 5] Internet-Draft IMAP PARTIAL August 2022 If the SAVE + PARTIAL result options are combined with both MIN and MAX result options, and the COUNT result options is absent, the PARTIAL, MIN and MAX are returned (if the search result is not empty), and the "$" 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 "$" marker would always contain all messages found by the SEARCH or UID SEARCH command. The following table summarizes the additional requirement on ESEARCH server implementations described in this section. +==============================+=====================+ | Combination of Result option | "$" 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 | +------------------------------+---------------------+ Table 1 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 Melnikov, et al. Expires 13 February 2023 [Page 6] Internet-Draft IMAP PARTIAL August 2022 // 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. 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. Melnikov, et al. Expires 13 February 2023 [Page 7] Internet-Draft IMAP PARTIAL August 2022 SP = MINUS = "-" capability =/ "PARTIAL" ;; 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 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 , from [IMAP-ABNF]/[RFC9051] search-return-data =/ ret-data-partial ret-data-partial = "PARTIAL" SP "(" partial-range SP partial-results ")" ;; is the requested range. partial-results = sequence-set / "NIL" ;; from [RFC3501]. ;; NIL indicates no results correspond to the requested range. tagged-ext-simple =/ partial-range-last fetch-modifier =/ modifier-partial 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]. The authors and reviewers believe that no new security issues are introduced with these additional IMAP4 capabilities. Melnikov, et al. Expires 13 February 2023 [Page 8] Internet-Draft IMAP PARTIAL August 2022 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 definition of the PARTIAL extension with RFC XXXX as the reference. 7. 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 and Barry Leiba. This document uses lots of text from RFC 5267. Thus work of the RFC 5267 authors Dave Cridland and Curtis King is appreciated. 8. References 8.1. Normative References [ABNF] Crocker, D., Ed. and P. Overell, Ed., "Augmented BNF for Syntax Specifications: ABNF", RFC 5234, January 2008, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . Melnikov, et al. Expires 13 February 2023 [Page 9] Internet-Draft IMAP PARTIAL August 2022 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, . [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, . [RFC5256] Crispin, M. and K. Murchison, "Internet Message Access Protocol - SORT and THREAD Extensions", RFC 5256, DOI 10.17487/RFC5256, June 2008, . [RFC5267] Cridland, D. and C. King, "Contexts for IMAP4", RFC 5267, DOI 10.17487/RFC5267, July 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, . 8.2. 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, . Authors' Addresses Alexey Melnikov Isode Limited Email: alexey.melnikov@isode.com URI: https://www.isode.com Arun Prakash Achuthan Yahoo! Email: arunprakash@myyahoo.com Melnikov, et al. Expires 13 February 2023 [Page 10] Internet-Draft IMAP PARTIAL August 2022 Vikram Nagulakonda Yahoo! Email: nvikram_imap@yahoo.com Luis Alves Email: luis.alves@lafaspot.com Melnikov, et al. Expires 13 February 2023 [Page 11]