Internet Draft: IMAP Extension for Conditional STORE A. Melnikov Document: draft-melnikov-imap-condstore-00.txt S. Hole Expires: August 2001 ACI WorldWide/MessagingDirect February 2001 IMAP Extension for Conditional STORE operation Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. Copyright Notice Copyright (C) The Internet Society 2001. All Rights Reserved. 0.1. Open issues 1). modtime: timestamp or sequence counter? 2). How specify different UNCHANGESINCE for different flags in the same STORE? Do we want such granularity anyway? 3). The document assumes that each flag has a corresponding ANNOTATE attribute. How to specify both entry name and attribute in attr-name for annotations? This has to be synchronized with ANNOTATE draft. 4). What MODTIME has a user-defined flag that was never set? What about system flags? 5). Untagged Modtime response used with FETCH/STORE to specify that client has all data till modtime? MODTIME Message Data Item in STATUS? 6). Add support for SORT extension? Table of Contents <<To be completed later>> 1. Abstract There is a class of applications that wish to have multiple IMAP clients coordinate activities with a single shared mailbox. These applications need some mechanism to synchronize state changes for messages within the mailbox. They must be able to guaranty that only one client can change message state (e.g. message flags or annotations) at any point in time. An example of such an application is use of an IMAP mailbox as a message queue with multiple dequeueing clients. The Conditional Store facility provides a protected update mechanism for message state information that can detect and resolve conflicts between multiple writers. 2. Conventions Used in This Document 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 RFC 2119 [KEYWORDS]. Formal syntax is defined using ABNF [ABNF] as modified by [IMAP4]. In examples, "C:" and "S:" indicate lines sent by the client and server respectively. The term metadata or metadata item will be used throughout this document. It references any system or user defined flag or an annotation [ANNOTATION]. 3. Introduction and Overview The Conditional STORE extension is present in any IMAP4 implementation which returns "CONDSTORE" as one of the supported capabilities in the CAPABILITY command response. Every read-write metadata of an IMAP message has an associated value called modification timestamp (modtime). This is an opaque value updated by the server whenever metadata item is modified. The value is intended to be used only for comparisons within a server, not as an accurate timestamp. However the server MUST guarantie that each STORE command (including simultaneous stores from different connections on different attributes) will use different modtime values. Modtime is described more fully in section 3.1.1 of [ACAP]. Modtime allows the client that supports CONDSTORE extension to track whether the value of particular flag was changed since some moment in time. Whenever the state of a flag change (i.e. the flag is added and before it wasn't set or the flags is removed and before it was set) the value of modification timestamp for that flag MUST be updated. Adding the flag when it is already present of removing when it is not SHOULD NOT change modtime. Each flag SHOULD have separate modtime, for example change to \Draft flag SHOULD NOT affect modtime for \Deleted flag. When message is appended to mailbox (via APPEND command or using external mechanism) the server assigns the current server timestamp to every flag or annotation specified in the APPEND command. When an annotation is removed modtime SHOULD be preserved. This extension makes the following changes to the IMAP4 protocol: a) extends syntax of the STORE command for allowing to specify STORE modifiers b) adds MODIFIED response code that should be used with NO response to STORE c) adds a new MODTIME message data item for use in the FETCH command d) adds a new MODTIME message data item for use in the SEARCH command The rest of this document describes protocol changes more rigorously. 4. IMAP Protocol Changes 4.1. STORE and UID STORE Commands Arguments: message set OPTIONAL store modifiers message data item name value for message data item Responses: untagged responses: FETCH Result: OK - store completed NO - store error: can't store that data BAD - command unknown or arguments invalid This document extends syntax of the STORE (and UID STORE respectively) command (see section 6.4.6 of [IMAP]) to include optional STORE modifiers. The document defines the following modifier: UNCHANGEDSINCE If the "modtime" of any metadata item specified in STORE operation for any message in the message set is greater than the unchangedsince value, then the store fails with a MODIFIED response code that includes message set of all messages that failed UNCHANGESINCE test. Example: C: a101 STORE 7,5,9 (UNCHANGEDSINCE "20000320162338") +FLAGS.SILENT (\Deleted) S: a101 NO [MODIFIED 7,9] Conditional STORE failed Use of UNCHANGEDSINCE with a time of "00000101000000" will always fail if the metadata item exists. Example: C: a102 STORE 12 (UNCHANGEDSINCE "00000101000000") +FLAGS.SILENT ($MDNSent) S: a102 NO [MODIFIED 12] Conditional STORE failed If operation is successful the server MUST update the "modtime" attribute for every metadata item that was changed. Untagged FETCH response MUST be sent even if .SILENT is specified and it MUST include MODTIME message data item as described in 4.2. Example: C: a103 UID STORE 6,4,8 (UNCHANGEDSINCE "200012121230045") +FLAGS.SILENT (\Deleted) S: * 1 FETCH (UID 4 MODTIME ("/message/flags/system/\\Deleted" "200012111230045")) S: * 2 FETCH (UID 6 MODTIME ("/message/flags/system/\\Deleted" "200012101230045")) S: * 4 FETCH (UID 8 MODTIME ("/message/flags/system/\\Deleted" "200012121130045")) S: a103 OK Store completed Example: C: a104 STORE * (UNCHANGEDSINCE "200012121230045") +FLAGS.SILENT (\Deleted $Processed) S: * 50 FETCH (MODTIME ("/message/flags/system/\\Deleted" "200012111230045" "/message/flags/system/$Processed" "200012111230045")) S: a104 OK Store completed In the latter example UNCHANGEDSINCE value is checked against modtimes for both flags. Note: If the message is specified multiple times in the message set and the server doesn't internally eliminate duplicates from the message set it MUST NOT fail conditional STORE operation for the second occurence of the message in the message set if operation completed succesfully for the first occurence. For example, if the client specifies: a100 STORE 7,3:9 (UNCHANGEDSINCE "200012121230045") +FLAGS.SILENT (\Deleted) the server must not fail operation for the message 7 as a part of processing "3:9" if it succeeded when the message 7 was processed the first time. 4.2. MODTIME message data item in FETCH Command This extension adds an MODTIME message data item to the FETCH command. This allows clients to retrieve modtime for various metadata items for a range of messages in the currently selected mailbox. MODTIME <attr-names> The MODTIME message data item, when used by the client in the FETCH command, takes a list of metadata items. For a flag <flagname> the corresponding attr-name has a form "/message/flags/system/<flagname>" Example: C: a FETCH 1 (MODTIME ("/message/comment" "/message/flags/system/$MDNSent")) S: * 1 FETCH (MODTIME ("/message/comment" 112 "/message/flags/system/$MDNSent" 64)) S: a OK Fetch complete 4.3 MODTIME criterion in SEARCH The MODTIME criterion for the SEARCH command allows a client to search for the specified modtime of a metadata item in a message. MODTIME <attr-name> <modtime-value> Messages that have modification counter for metadata item <attr-name> with value equal or greater than <modtime-value>. This allows a client, for example, to find out which messages contain metadata items that have changed since the last time it updated its disconnected cache. Examples: C: a SEARCH MODTIME "/message/flags/system/\\draft" "20010320162338" ANNOTATION "/message/comment" "value" "IMAP4" S: * SEARCH 2 3 5 7 11 13 17 19 23 S: a OK Search complete In the above example, the message numbers of any messages containing the string "IMAP4" in the "value" attribute of the "/message/comment" entry and having modtime "20010320162338" for flag \Draft are returned in the search results. 5. 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]. 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. store = "STORE" SP set SP store-modifiers store-att-flags store-modifiers = [ "(" 1*store-modifier ")" ] store-modifier = "UNCHANGEDSINCE" time fetch-att =/ fetch-modtime ; modifies original IMAP4 fetch-att fetch-modtime = "MODTIME" SP "(" 1*attr-name ")" fetch-mod-resp = "MODTIME" SP "(" 1*(attr-name SP counter) ")" search-key =/ search-modtime ; modifies original IMAP4 search-key search-modtime = "MODTIME" SP attr-name SP counter resp-text-code =/ "MODIFIED" SP set attr-name = "/message/flags/system/" attr-flag ;; each system or user defined flag <flag> is mapped to ;; "/message/flags/system/<flag>" <<Borrowed from IMAP4rev1 and modified accordingly:>> attr-flag = "\\Answered" / "\\Flagged" / "\\Deleted" / "\\Seen" / "\\Draft" / attr-flag-keyword / attr-flag-extension ; Does not include "\Recent" attr-flag-extension = "\\" atom ; Future expansion. Client implementations ; MUST accept flag-extension flags. Server ; implementations MUST NOT generate ; flag-extension flags except as defined by ; future standard or standards-track ; revisions of this specification. attr-flag-keyword = atom <<Borrowed from ACAP:>> time = <"> time-year time-month time-day time-hour time-minute time-second time-subsecond <"> ;; Timestamp in UTC time-day = 2DIGIT ;; 01-31 time-hour = 2DIGIT ;; 00-23 time-minute = 2DIGIT ;; 00-59 time-month = 2DIGIT ;; 01-12 time-second = 2DIGIT ;; 00-60 time-subsecond = *DIGIT time-year = 4DIGIT 6. Security Considerations There are no known security issues with this extension. 7. References [ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd, November 1997. [IMAP4] Crispin, M., "Internet Message Access Protocol - Version 4rev1", RFC 2060, University of Washington, December 1996. [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, Harvard University, March 1997. [ACAP] Newman, Myers, "ACAP -- Application Configuration Access Protocol", RFC 2244, Innosoft, Netscape, November 1997. <ftp://ftp.isi.edu/in-notes/rfc2244.txt> [ANNOTATION] Gellens, R., Daboo, C., "IMAP ANNOTATE Extension", work in progress. <http://www.ietf.org/internet-drafts/draft-ietf-imapext-annotate-xx.txt> [SORT-EXT] Crispin, M., "Internet Message Access Protocol -- SORT Extension", work in progress. <http://www.ietf.org/internet-drafts/draft-crispin-imapext-sort-xx.txt> 8. Full Copyright Statement Copyright (C) The Internet Society 2001. All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.