Network Working Group D. Cridland Internet-Draft C. King Expires: November 19, 2007 Isode Limited May 18, 2007 Contexts for IMAP4 draft-cridland-imap-context-02 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on November 19, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract The IMAP4rev1 protocol has powerful search facilities as part of the core protocol, but lacks the ability to create live, updated results which can be easily handled. This memo provides such an extension, and shows how it can be used to provide a facility similar to virtual mailboxes. Cridland & King Expires November 19, 2007 [Page 1] Internet-Draft IMAP CONTEXT May 2007 Table of Contents 1. Conventions used in this document . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Protocol Changes . . . . . . . . . . . . . . . . . . . . . . . 3 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.2. Context Hint . . . . . . . . . . . . . . . . . . . . . . . 4 3.3. Notifications of changes . . . . . . . . . . . . . . . . . 4 3.3.1. Refusing to update contexts . . . . . . . . . . . . . 5 3.3.2. Common Features of ADDTO and REMOVEFROM . . . . . . . 6 3.3.3. ADDTO Return Data Item . . . . . . . . . . . . . . . . 6 3.3.4. REMOVEFROM Return Data Item . . . . . . . . . . . . . 7 3.3.5. The FREECONTEXT command . . . . . . . . . . . . . . . 7 3.4. Partial results . . . . . . . . . . . . . . . . . . . . . 8 3.5. Caching results . . . . . . . . . . . . . . . . . . . . . 9 4. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 9 5. Security Considerations . . . . . . . . . . . . . . . . . . . 11 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8.1. Normative References . . . . . . . . . . . . . . . . . . . 11 8.2. Informative References . . . . . . . . . . . . . . . . . . 12 Appendix A. Cookbook . . . . . . . . . . . . . . . . . . . . . . 12 A.1. Virtual Mailboxes . . . . . . . . . . . . . . . . . . . . 12 A.2. Trash Mailboxes . . . . . . . . . . . . . . . . . . . . . 12 A.3. Immediate EXPUNGE notifications . . . . . . . . . . . . . 13 A.4. Other uses . . . . . . . . . . . . . . . . . . . . . . . . 13 A.5. Resynchronizing Contexts . . . . . . . . . . . . . . . . . 13 Appendix B. Server Implementation Notes . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14 Intellectual Property and Copyright Statements . . . . . . . . . . 15 Cridland & King Expires November 19, 2007 [Page 2] Internet-Draft IMAP CONTEXT May 2007 1. Conventions used in this document In examples, "C:" and "S:" indicate lines sent by the client messaging user agent and IMAP4rev1 ([IMAP]) server respectively. Although the examples show a server which supports [ESEARCH], this is not a requirement of this specification. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [KEYWORDS]. Other capitalised words are typically names of IMAP extensions or commands - these are uppercased for clarity only, and are case- insensitive. [[ Editorial comments are like this. XML2RFC working source is held at http://svn.dave.cridland.net/svn/ietf-drafts/ draft-cridland-imap-contexts.xml ]] 2. Introduction Although the basic SEARCH command defined in [IMAP], and as enhanced by [ESEARCH], is relatively compact in its representation, this reduction saves only a certain amount of data, and huge mailboxes might overwhelm the storage available for results on even relatively high-end desktop machines. This memo borrows concepts from [ACAP], providing a windowed view onto search results, as well as bandwidth and round-trip efficient updates. It is intended that the protocol may be easily adapted onto the SORT command specified in [SORT]. 3. Protocol Changes 3.1. Overview This extension is present in any IMAP4rev1 server which includes the string "CONTEXT", or any string beginning "CONTEXT=", within its advertised capabilities. Such servers also accept three additional return options, and provide three new result data items, and no new responses. The first search return option is CONTEXT, an optional hint that the criteria will be used repeatedly, and is defined in Section 3.2. Cridland & King Expires November 19, 2007 [Page 3] Internet-Draft IMAP CONTEXT May 2007 The second is UPDATE, which causes the server to provide efficient notifications of changes to the results. This is defined in Section 3.3. Finally, the PARTIAL return specifier causes the server to return a subset of the results in set-syntax. This allows for "virtual scrollbars" and other UI conveniences to be achieved without having to preload the entire result set, and is described in Section 3.4. All of the return specifiers have no interaction with either each other or any return specifiers defined in [ESEARCH]. 3.2. Context Hint The return option CONTEXT SHOULD be used by a client to indicate that subsequent use of the criteria are likely. Servers MAY ignore this return option, or use it as a hint to maintain a full result set, or index. A client might choose to obtain a count of matching messages prior to obtaining actual results. Here, the client signifies its intention to fetch the results themselves: C: A01 SEARCH RETURN (CONTEXT COUNT) UNDELETED UNKEYWORD $Junk S: * ESEARCH (TAG "A01") COUNT 23765 S: A01 OK Search completed. 3.3. Notifications of changes The search return option UPDATE, if used by a client, causes the server to issue unsolicited notifications containing updates to the SEARCH results which would be returned by an unmodified SEARCH. These results are carried in ADDTO and REMOVEFROM data items in ESEARCH/ESORT responses. Both ADDTO and REMOVEFROM data items SHOULD be delivered to clients in a timely manner, as and when results change, whether by new messages arriving in the mailbox, metadata such as flags being changed, or messages being expunged. Typically, this would occur at the same time as the FETCH, EXISTS or EXPUNGE responses carrying the source of the change. Updates will cease only when the mailbox is no longer selected, or when the FREECONTEXT command is issued by the client, whichever is sooner. Cridland & King Expires November 19, 2007 [Page 4] Internet-Draft IMAP CONTEXT May 2007 Unlike [ACAP], there is no requirement that a context need be created with CONTEXT to use UPDATE, and in addition, the lack of UPDATE with a CONTEXT does not affect the results caused by later SEARCH commands - there is no snapshot facility. There is no interaction between UPDATE and any other return options; therefore use of RETURN (UPDATE MIN), for example, does not notify about the minimum UID or sequence number, but notifies instead about all changes to the set of matching messages. In particular, this means that a client using UPDATE and PARTIAL on the same search program MAY receive notifications about messages which do not currently interest it. This time, the client will require notifications of updates, and chooses to obtain a count: C: B01 UID SEARCH RETURN (UPDATE COUNT) DELETED KEYWORD $Junk S: * ESEARCH (TAG "B01") COUNT 74 S: B01 OK Search completed, will notify. 3.3.1. Refusing to update contexts In some cases, the server MAY refuse to provide updates, such as if an internal limit on the number of update contexts is reached. In this case, an untagged NO is generated during processing of the command with a response-code of NOUPDATE. The response-code contains, as argument, the tag of the search command for which the server is refusing to honour the UPDATE request. Other return options specified will still be honoured. Servers MUST provide at least one updating context per client, and SHOULD provide more - see Appendix B for strategies on reducing the impact of additional updating contexts. This time, the client will require notifications of updates, and chooses to obtain a count: C: B02 UID SEARCH RETURN (UPDATE COUNT) $Junk KEYWORD $Junk S: * ESEARCH (TAG "B01") COUNT 74 S: * NO [NOUPDATE "B01"] Too many contexts S: B02 OK Search completed, will not notify. Cridland & King Expires November 19, 2007 [Page 5] Internet-Draft IMAP CONTEXT May 2007 3.3.2. Common Features of ADDTO and REMOVEFROM The result update set included in the return data item is specified as UIDs or message numbers, depending on how the UPDATE was specified. If the UPDATE was present in a SEARCH command, the results will be message numbers; in a UID SEARCH command, they will be UIDs. The client MUST process (and the server MUST generate) ADDTO and REMOVEFROM return data items in order, including those within a single ESEARCH response. As with any response aside from EXPUNGE, ESEARCH responses carrying ADDTO and/or REMOVEFROM return data items MAY be sent at any time. In particular, servers MAY send such responses when no command is in progress, during the processing of any command, or when the client is using the IDLE facility described in [IDLE]. Implementors are recommended to read [NOTIFY] as a mechanism for clients to signal servers that they are willing to process responses at any time, and are also recommended to pay close attention to Section 5.3 of [IMAP]. Client implementors SHOULD NOT reuse tags with any command; in this case the tag is used to identify the SEARCH command which created the context. 3.3.3. ADDTO Return Data Item The ADDTO return data item contains, as payload, a list containing pairs of a position and a set of result updates to be inserted at the position. For ESEARCH responses, the position MAY be zero, and MAY be ignored by clients. If the position is non-zero, the new results in the update are to be inserted at the given position; meaning that the new results will occupy the indicated start position and all existing results starting from that position are shifted to the position after the insertion. The client MUST update the position numbers of the shifted results. The ADDTO return data item MAY include several new results to be inserted - therefore it is imporant to note that the positions included in a single ADDTO return data item contain positions before the shifting due to other new results take place. Cridland & King Expires November 19, 2007 [Page 6] Internet-Draft IMAP CONTEXT May 2007 [...] S: * 23762 FETCH (FLAGS (\Deleted \Seen)) S: * 23763 FETCH (FLAGS ($Junk \Seen)) S: * ESEARCH (TAG "B01") UID ADDTO (0 32768:32769) Note that this example assumes messages 23762 and 23763 with UIDs 32768 and 32769 respectively previously had neither \Deleted nor $Junk set. Also note that only the ADDTO is included, and not the (now changed) COUNT. 3.3.4. REMOVEFROM Return Data Item The REMOVEFROM return data item contains a set of results to be removed. The results to be removed are referenced by message number or UID, as appropriate, and need not be in the same order as the results, therefore servers are RECOMMENDED to sort the results into increasing UID (and sequence number) order, to take full advantage of the set-syntax representation. Note that although no position information is included, the positions of any results after those removed will change. Here, a message in the result set is expunged. The REMOVEFROM here is shown to happen without any command in progress, see Section 3.3.2. Note that EXPUNGE responses do not have this property. [...] S: * ESEARCH (TAG "B01") UID REMOVEFROM 32768 C: B03 NOOP S: * EXPUNGE 23762 S: B03 OK Nothing done. 3.3.5. The FREECONTEXT command When a client no longer wishes to receive updates, it may issue the FREECONTEXT command, which will prevent all updates to the contexts named in the arguments from being transmitted by the server. The command takes, as arguments, one or more tags of the commands used to request updates. The server MAY free any resource associated with a context so disabled. C: B04 FREECONTEXT "B01" S: B04 OK No further updates. Cridland & King Expires November 19, 2007 [Page 7] Internet-Draft IMAP CONTEXT May 2007 3.4. Partial results The PARTIAL search return option causes the server to provide in an ESEARCH response the range from the results denoted by the sequence range given as the mandatory argument. The first result 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. For SEARCH results, the entire result set MUST be ordered in "mailbox order", that is, in UID or message sequence number order. Where a PARTIAL search return option references results which do not exist, by using a range which starts or ends higher than the COUNT of results, then the server returns those results which are in the set. This yields a PARTIAL return data item which has, as payload, the original range and a potentially missing set of results which may be shorter than the extent of the range. The subset of results are returned in sequence-set syntax, and servers SHOULD order results from a SEARCH for maximum efficiency in this representation. Clients need not request PARTIAL results in any particular order. // Recall from A01 that there are 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. Cridland & King Expires November 19, 2007 [Page 8] Internet-Draft IMAP CONTEXT May 2007 3.5. Caching results Server implementations MAY cache results from a search or sort, whether or not hinted to by CONTEXT, in order to make subsequent searches more efficient, perhaps by recommencing a subsequent PARTIAL search where a previous search left off. However servers MUST behave identically whether or not internal caching is taking place, therefore any such cache is required to be updated as changes to the mailbox occur. An alternate strategy would be to discard results when any change occurs to the mailbox. 4. Formal Syntax Cridland & King Expires November 19, 2007 [Page 9] Internet-Draft IMAP CONTEXT May 2007 The collected formal syntax. This includes definitions from [IMAP] and [IMAP-ABNF], and uses ABNF as defined in [ABNF]. capability =/ "CONTEXT" / "CONTEXT=" atom command-select =/ "FREECONTEXT" 1*(SP quoted) ;; from [IMAP] addto-position = number ;; Number may be 0 for SEARCH result additions. ;; from [IMAP] modifier-context = "CONTEXT" modifier-partial = "PARTIAL" SP seq-range ;; from [IMAP] modifier-update = "UPDATE" search-return-opt =/ modifier-context / modifier-partial / modifier-update ;; All conform to , from [IMAP-ABNF] resp-text-code =/ "NOUPDATE" SP quoted ;; from [IMAP] ret-data-addto = "ADDTO" SP "(" addto-position SP sequence-set *(SP addto-position SP sequence-set) ")" ;; from [IMAP] ret-data-partial = "PARTIAL" SP "(" seq-range SP partial-results ")" ;; is the requested range. ;; from [IMAP] partial-results = sequence-set / "NIL" ;; from [IMAP] ;; NIL indicates no results correspond to the requested range. ret-data-removefrom = "REMOVEFROM" SP sequence-set ;; from [IMAP] search-return-data =/ ret-data-partial / ret-data-addto / ret-data-removefrom ;; All conform to , from [IMAP-ABNF] Cridland & King Expires November 19, 2007 [Page 10] Internet-Draft IMAP CONTEXT May 2007 5. Security Considerations It is believed that this specification introduces no serious new security considerations. However, implementors are advised to refer to [IMAP]. Creation of contexts, both for UPDATE and PARTIAL, can benefit from storing potentially large result sets on the server. Implementors are advised to take care not to provide a method for denial of service (DoS) attacks based on this; the notes in Appendix B may aid in implementation decisions. Note that a server avoiding storing the results will have much increased I/O, which may also be an avenue for DoS attacks. 6. IANA Considerations IMAP4 capabilities are registered by publishing a standards track or IESG approved experimental RFC. The registry is currently located at: http://www.iana.org/assignments/imap4-capabilities This document defines the CONTEXT IMAP capability. IANA is requested to add it to the registry accordingly. 7. Acknowledgements Much of the design of this extension can be found in ACAP. Valuable comments, both in agreement and in dissent, were received from Alexey Melnikov, Arnt Gulbrandsen, Cyrus Daboo, Filip Navara, Mark Crispin, Randall Gellens, Zoltan Ordogh and others, and many of these comments have had significant influence on the design or the text. The authors are grateful to all those involved, including those not mentioned here. 8. References 8.1. Normative References [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003. Cridland & King Expires November 19, 2007 [Page 11] Internet-Draft IMAP CONTEXT May 2007 [IMAP-ABNF] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4 ABNF", RFC 4466, April 2006. [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 8.2. Informative References [ACAP] Newman, C. and J. Myers, "ACAP -- Application Configuration Access Protocol", RFC 2244, November 1997. [ESEARCH] Melnikov, A. and D. Cridland, "IMAP4 Extension to SEARCH Command for Controlling What Kind of Information Is Returned", RFC 4731, November 2006. [IDLE] Leiba, B., "IMAP4 IDLE command", RFC 2177, June 1997. [NOTIFY] King, C., "The IMAP NOTIFY Extension", draft-gulbrandsen-imap-notify-03 (work in progress), March 2007. [SORT] Crispin, M. and K. Murchison, "INTERNET MESSAGE ACCESS PROTOCOL - SORT AND THREAD EXTENSIONS", draft-ietf-imapext-sort-18 (work in progress), November 2006. Appendix A. Cookbook A.1. Virtual Mailboxes It is possible to use the facilities described within this memo to create a facility largely similar to a virtual mailbox, but handled on the client side. Initially, the client SELECTs the real "backing" mailbox. Next, it can switch to a filtered view at any time by issuing a SEARCH RETURN (COUNT UPDATE CONTEXT), and using SEARCH RETURN (PARTIAL x:y) as the user scrolls, feeding the results into a FETCH as required to populate summary views. A.2. Trash Mailboxes Certain contexts are particularly useful for client developers wishing to present something similar to the common trash mailbox metaphor in limited bandwidth. The simple criteria of UNDELETED only Cridland & King Expires November 19, 2007 [Page 12] Internet-Draft IMAP CONTEXT May 2007 matches undeleted messages, and the corresponding DELETED search key can be used to display a per-mailbox trash-like virtual mailbox. A.3. Immediate EXPUNGE notifications The command "SEARCH RETURN (UPDATE) ALL" can be used to create a context which notifies immediately about expunged messages, yet will not affect message sequence numbers until the normal EXPUNGE message can be sent. This may be useful for clients wishing to show this behaviour without losing the benefit of sequence numbering. A.4. Other uses It is entirely possible to simultaneously have two or more UPDATE contexts in operation. This can be used to build a grouped message display in some cases, and also allows for monitoring counts of messages matching certain complex criteria. A.5. Resynchronizing Contexts The creation of a context, and immediate access to it, can all be accomplished in a single round-trip. Therefore, whilst it is possible to elide resynchronization if no changes have occurred, it is simpler in most cases to resynchronize by simply recreating the context. Appendix B. Server Implementation Notes Although a server may cache the results, this is not mandated nor required. UPDATE processing, for example, can be achieved efficiently by comparison of the old flag state (if any) and the new, and PARTIAL can be achieved by re-running the search until the suitable window is required. This is a result of there being no snapshot facility. For example, on a new message, the server can simply test for matches against all current UPDATE context search programs, and for any that match, send the ADDTO return data. Similarly, for a flag change on an existing message, the server can check whether the message matched with its old flags, whether it matches with new flags, and provide ADDTO or REMOVEFROM return data accordingly if these results differ. For PARTIAL requests, the server can perform a full search, discarding results until the lower bound is hit, and stopping the search when sufficient results have been obtained. Cridland & King Expires November 19, 2007 [Page 13] Internet-Draft IMAP CONTEXT May 2007 With some additional state, it is possible to restart PARTIAL searches, thus avoiding performing the initial discard phase. For the best performance, however, caching the full search results is needed, which can allow for faster responses at the expense of memory. One reasonable strategy would be to balance this trade-off at run-time, discarding search results after a suitable timeout, and regenerating them as required. This yields state requirements of storing the search program for any UPDATE contexts, and optionally storing both search program and (updated) results for further contexts as required. Authors' Addresses Dave Cridland Isode Limited 5 Castle Business Village 36, Station Road Hampton, Middlesex TW12 2BX GB Email: dave.cridland@isode.com Curtis King Isode Limited 5 Castle Business Village 36, Station Road Hampton, Middlesex TW12 2BX GB Email: cking@mumbo.ca Cridland & King Expires November 19, 2007 [Page 14] Internet-Draft IMAP CONTEXT May 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Cridland & King Expires November 19, 2007 [Page 15]