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diff --git a/doc/rfc/rfc4551.txt b/doc/rfc/rfc4551.txt new file mode 100644 index 0000000..894b510 --- /dev/null +++ b/doc/rfc/rfc4551.txt @@ -0,0 +1,1403 @@ + + + + + + +Network Working Group A. Melnikov +Request for Comments: 4551 Isode Ltd. +Updates: 3501 S. Hole +Category: Standards Track ACI WorldWide/MessagingDirect + June 2006 + + + IMAP Extension for Conditional STORE Operation + or Quick Flag Changes Resynchronization + +Status of This Memo + + This document specifies an Internet standards track protocol for the + Internet community, and requests discussion and suggestions for + improvements. Please refer to the current edition of the "Internet + Official Protocol Standards" (STD 1) for the standardization state + and status of this protocol. Distribution of this memo is unlimited. + +Copyright Notice + + Copyright (C) The Internet Society (2006). + +Abstract + + Often, multiple IMAP (RFC 3501) clients need to coordinate changes to + a common IMAP mailbox. Examples include different clients working on + behalf of the same user, and multiple users accessing shared + mailboxes. These clients need a mechanism to synchronize state + changes for messages within the mailbox. They must be able to + guarantee that only one client can change message state (e.g., + message flags) at any 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 writing mail clients. + + The Conditional Store facility also allows a client to quickly + resynchronize mailbox flag changes. + + This document defines an extension to IMAP (RFC 3501). + + + + + + + + + +Melnikov & Hole Standards Track [Page 1] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + +Table of Contents + + 1. Introduction and Overview ................................. 3 + 2. Conventions Used in This Document ......................... 5 + 3. IMAP Protocol Changes ..................................... 6 + 3.1. New OK untagged responses for SELECT and EXAMINE ......... 6 + 3.1.1. HIGHESTMODSEQ response code ............................ 6 + 3.1.2. NOMODSEQ response code ................................. 7 + 3.2. STORE and UID STORE Commands ............................. 7 + 3.3 FETCH and UID FETCH Commands ..............................13 + 3.3.1. CHANGEDSINCE FETCH modifier ............................13 + 3.3.2. MODSEQ message data item in FETCH Command ..............14 + 3.4. MODSEQ search criterion in SEARCH ........................16 + 3.5. Modified SEARCH untagged response ........................17 + 3.6. HIGHESTMODSEQ status data items ..........................17 + 3.7. CONDSTORE parameter to SELECT and EXAMINE ................18 + 3.8. Additional quality of implementation issues ..............18 + 4. Formal Syntax .............................................19 + 5. Server implementation considerations ......................21 + 6. Security Considerations ...................................22 + 7. IANA Considerations .......................................22 + 8. References ................................................23 + 8.1. Normative References .....................................23 + 8.2. Informative References ...................................23 + 9. Acknowledgements ..........................................23 + + + + + + + + + + + + + + + + + + + + + + + + + + +Melnikov & Hole Standards Track [Page 2] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + +1. Introduction and Overview + + The Conditional STORE extension is present in any IMAP4 + implementation that returns "CONDSTORE" as one of the supported + capabilities in the CAPABILITY command response. + + An IMAP server that supports this extension MUST associate a positive + unsigned 64-bit value called a modification sequence (mod-sequence) + with every IMAP message. This is an opaque value updated by the + server whenever a metadata item is modified. The server MUST + guarantee that each STORE command performed on the same mailbox + (including simultaneous stores to different metadata items from + different connections) will get a different mod-sequence value. + Also, for any two successful STORE operations performed in the same + session on the same mailbox, the mod-sequence of the second completed + operation MUST be greater than the mod-sequence of the first + completed. Note that the latter rule disallows the use of the system + clock as a mod-sequence, because if system time changes (e.g., an NTP + [NTP] client adjusting the time), the next generated value might be + less than the previous one. + + Mod-sequences allow a client that supports the CONDSTORE extension to + determine if a message metadata has changed since some known moment. + Whenever the state of a flag changes (i.e., the flag is added where + previously it wasn't set, or the flag is removed and before it was + set) the value of the modification sequence for the message MUST be + updated. Adding the flag when it is already present or removing when + it is not present SHOULD NOT change the mod-sequence. + + When a message is appended to a mailbox (via the IMAP APPEND command, + COPY to the mailbox, or using an external mechanism) the server + generates a new modification sequence that is higher than the highest + modification sequence of all messages in the mailbox and assigns it + to the appended message. + + The server MAY store separate (per-message) modification sequence + values for different metadata items. If the server does so, per- + message mod-sequence is the highest mod-sequence of all metadata + items for the specified message. + + The server that supports this extension is not required to be able to + store mod-sequences for every available mailbox. Section 3.1.2 + describes how the server may act if a particular mailbox doesn't + support the persistent storage of mod-sequences. + + + + + + + +Melnikov & Hole Standards Track [Page 3] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + This extension makes the following changes to the IMAP4 protocol: + + a) adds UNCHANGEDSINCE STORE modifier. + + b) adds the MODIFIED response code which should be used with an OK + response to the STORE command. (It can also be used in a NO + response.) + + c) adds a new MODSEQ message data item for use with the FETCH + command. + + d) adds CHANGEDSINCE FETCH modifier. + + e) adds a new MODSEQ search criterion. + + f) extends the syntax of untagged SEARCH responses to include + mod-sequence. + + g) adds new OK untagged responses for the SELECT and EXAMINE + commands. + + h) defines an additional parameter to SELECT/EXAMINE commands. + + i) adds the HIGHESTMODSEQ status data item to the STATUS command. + + A client supporting CONDSTORE extension indicates its willingness to + receive mod-sequence updates in all untagged FETCH responses by + issuing: + + - a SELECT or EXAMINE command with the CONDSTORE parameter, + - a STATUS (HIGHESTMODSEQ) command, + - a FETCH or SEARCH command that includes the MODSEQ message data + item, + - a FETCH command with the CHANGEDSINCE modifier, or + - a STORE command with the UNCHANGEDSINCE modifier. + + The server MUST include mod-sequence data in all subsequent untagged + FETCH responses (until the connection is closed), whether they were + caused by a regular STORE, a STORE with UNCHANGEDSINCE modifier, or + an external agent. + + This document uses the term "CONDSTORE-aware client" to refer to a + client that announces its willingness to receive mod-sequence updates + as described above. The term "CONDSTORE enabling command" will refer + any of the commands listed above. A future extension to this + document may extend the list of CONDSTORE enabling commands. A first + CONDSTORE enabling command executed in the session MUST cause the + + + + +Melnikov & Hole Standards Track [Page 4] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + server to return HIGHESTMODSEQ (Section 3.1.1) unless the server has + sent NOMODSEQ (Section 3.1.2) response code when the currently + selected mailbox was selected. + + The rest of this document describes the protocol changes more + rigorously. + +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]. + + In examples, lines beginning with "S:" are sent by the IMAP server, + and lines beginning with "C:" are sent by the client. Line breaks + may appear in example commands solely for editorial clarity; when + present in the actual message, they are represented by "CRLF". + + Formal syntax is defined using ABNF [ABNF]. + + The term "metadata" or "metadata item" is used throughout this + document. It refers to any system or user-defined keyword. Future + documents may extend "metadata" to include other dynamic message + data. + + Some IMAP mailboxes are private, accessible only to the owning user. + Other mailboxes are not, either because the owner has set an Access + Control List [ACL] that permits access by other users, or because it + is a shared mailbox. Let's call a metadata item "shared" for the + mailbox if any changes to the metadata items are persistent and + visible to all other users accessing the mailbox. Otherwise, the + metadata item is called "private". Note that private metadata items + are still visible to all sessions accessing the mailbox as the same + user. Also note that different mailboxes may have different metadata + items as shared. + + See Section 1 for the definition of a "CONDSTORE-aware client" and a + "CONDSTORE enabling command". + + + + + + + + + + + + + +Melnikov & Hole Standards Track [Page 5] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + +3. IMAP Protocol Changes + +3.1. New OK Untagged Responses for SELECT and EXAMINE + + This document adds two new response codes, HIGHESTMODSEQ and + NOMODSEQ. One of those response codes MUST be returned in the OK + untagged response for a successful SELECT/EXAMINE command. + + When opening a mailbox, the server must check if the mailbox supports + the persistent storage of mod-sequences. If the mailbox supports the + persistent storage of mod-sequences and the mailbox open operation + succeeds, the server MUST send the OK untagged response including + HIGHESTMODSEQ response code. If the persistent storage for the + mailbox is not supported, the server MUST send the OK untagged + response including NOMODSEQ response code instead. + +3.1.1. HIGHESTMODSEQ Response Code + + This document adds a new response code that is returned in the OK + untagged response for the SELECT and EXAMINE commands. A server + supporting the persistent storage of mod-sequences for the mailbox + MUST send the OK untagged response including HIGHESTMODSEQ response + code with every successful SELECT or EXAMINE command: + + OK [HIGHESTMODSEQ <mod-sequence-value>] + + where <mod-sequence-value> is the highest mod-sequence value of + all messages in the mailbox. When the server changes UIDVALIDITY + for a mailbox, it doesn't have to keep the same HIGHESTMODSEQ for + the mailbox. + + A disconnected client can use the value of HIGHESTMODSEQ to check if + it has to refetch metadata from the server. If the UIDVALIDITY value + has changed for the selected mailbox, the client MUST delete the + cached value of HIGHESTMODSEQ. If UIDVALIDITY for the mailbox is the + same, and if the HIGHESTMODSEQ value stored in the client's cache is + less than the value returned by the server, then some metadata items + on the server have changed since the last synchronization, and the + client needs to update its cache. The client MAY use SEARCH MODSEQ + (Section 3.4) to find out exactly which metadata items have changed. + Alternatively, the client MAY issue FETCH with the CHANGEDSINCE + modifier (Section 3.3.1) in order to fetch data for all messages that + have metadata items changed since some known modification sequence. + + Example 1: + + C: A142 SELECT INBOX + S: * 172 EXISTS + + + +Melnikov & Hole Standards Track [Page 6] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + S: * 1 RECENT + S: * OK [UNSEEN 12] Message 12 is first unseen + S: * OK [UIDVALIDITY 3857529045] UIDs valid + S: * OK [UIDNEXT 4392] Predicted next UID + S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft) + S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited + S: * OK [HIGHESTMODSEQ 715194045007] + S: A142 OK [READ-WRITE] SELECT completed + +3.1.2. NOMODSEQ Response Code + + A server that doesn't support the persistent storage of mod-sequences + for the mailbox MUST send the OK untagged response including NOMODSEQ + response code with every successful SELECT or EXAMINE command. A + server that returned NOMODSEQ response code for a mailbox, which + subsequently receives one of the following commands while the mailbox + is selected: + + - a FETCH command with the CHANGEDSINCE modifier, + - a FETCH or SEARCH command that includes the MODSEQ message data + item, or + - a STORE command with the UNCHANGEDSINCE modifier + + MUST reject any such command with the tagged BAD response. + + Example 2: + + C: A142 SELECT INBOX + S: * 172 EXISTS + S: * 1 RECENT + S: * OK [UNSEEN 12] Message 12 is first unseen + S: * OK [UIDVALIDITY 3857529045] UIDs valid + S: * OK [UIDNEXT 4392] Predicted next UID + S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft) + S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited + S: * OK [NOMODSEQ] Sorry, this mailbox format doesn't support + modsequences + S: A142 OK [READ-WRITE] SELECT completed + +3.2. STORE and UID STORE Commands + + This document defines the following STORE modifier (see Section 2.5 + of [IMAPABNF]): + + UNCHANGEDSINCE <mod-sequence> + + For each message specified in the message set, the server performs + the following. If the mod-sequence of any metadata item of the + + + +Melnikov & Hole Standards Track [Page 7] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + message is equal or less than the specified UNCHANGEDSINCE value, + then the requested operation (as described by the message data + item) is performed. If the operation is successful, the server + MUST update the mod-sequence attribute of the message. An + untagged FETCH response MUST be sent, even if the .SILENT suffix + is specified, and the response MUST include the MODSEQ message + data item. This is required to update the client's cache with the + correct mod-sequence values. See Section 3.3.2 for more details. + + However, if the mod-sequence of any metadata item of the message + is greater than the specified UNCHANGEDSINCE value, then the + requested operation MUST NOT be performed. In this case, the + mod-sequence attribute of the message is not updated, and the + message number (or unique identifier in the case of the UID STORE + command) is added to the list of messages that failed the + UNCHANGESINCE test. + + When the server finished performing the operation on all the + messages in the message set, it checks for a non-empty list of + messages that failed the UNCHANGESINCE test. If this list is + non-empty, the server MUST return in the tagged response a + MODIFIED response code. The MODIFIED response code includes the + message set (for STORE) or set of UIDs (for UID STORE) of all + messages that failed the UNCHANGESINCE test. + + Example 3: + + All messages pass the UNCHANGESINCE test. + + C: a103 UID STORE 6,4,8 (UNCHANGEDSINCE 12121230045) + +FLAGS.SILENT (\Deleted) + S: * 1 FETCH (UID 4 MODSEQ (12121231000)) + S: * 2 FETCH (UID 6 MODSEQ (12121230852)) + S: * 4 FETCH (UID 8 MODSEQ (12121130956)) + S: a103 OK Conditional Store completed + + Example 4: + + C: a104 STORE * (UNCHANGEDSINCE 12121230045) +FLAGS.SILENT + (\Deleted $Processed) + S: * 50 FETCH (MODSEQ (12111230047)) + S: a104 OK Store (conditional) completed + + Example 5: + + C: c101 STORE 1 (UNCHANGEDSINCE 12121230045) -FLAGS.SILENT + (\Deleted) + S: * OK [HIGHESTMODSEQ 12111230047] + + + +Melnikov & Hole Standards Track [Page 8] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + S: * 50 FETCH (MODSEQ (12111230048)) + S: c101 OK Store (conditional) completed + + HIGHESTMODSEQ response code was sent by the server presumably + because this was the first CONDSTORE enabling command. + + Example 6: + + In spite of the failure of the conditional STORE operation for + message 7, the server continues to process the conditional STORE + in order to find all messages that fail the test. + + C: d105 STORE 7,5,9 (UNCHANGEDSINCE 320162338) + +FLAGS.SILENT (\Deleted) + S: * 5 FETCH (MODSEQ (320162350)) + S: d105 OK [MODIFIED 7,9] Conditional STORE failed + + Example 7: + + Same as above, but the server follows the SHOULD recommendation in + Section 6.4.6 of [IMAP4]. + + C: d105 STORE 7,5,9 (UNCHANGEDSINCE 320162338) + +FLAGS.SILENT (\Deleted) + S: * 7 FETCH (MODSEQ (320162342) FLAGS (\Seen \Deleted)) + S: * 5 FETCH (MODSEQ (320162350)) + S: * 9 FETCH (MODSEQ (320162349) FLAGS (\Answered)) + S: d105 OK [MODIFIED 7,9] Conditional STORE failed + + Use of UNCHANGEDSINCE with a modification sequence of 0 always + fails if the metadata item exists. A system flag MUST always be + considered existent, whether it was set or not. + + Example 8: + + C: a102 STORE 12 (UNCHANGEDSINCE 0) + +FLAGS.SILENT ($MDNSent) + S: a102 OK [MODIFIED 12] Conditional STORE failed + + The client has tested the presence of the $MDNSent user-defined + keyword. + + Note: A client trying to make an atomic change to the state of a + particular metadata item (or a set of metadata items) should be + prepared to deal with the case when the server returns the MODIFIED + response code if the state of the metadata item being watched hasn't + changed (but the state of some other metadata item has). This is + necessary, because some servers don't store separate mod-sequences + + + +Melnikov & Hole Standards Track [Page 9] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + for different metadata items. However, a server implementation + SHOULD avoid generating spurious MODIFIED responses for +FLAGS/-FLAGS + STORE operations, even when the server stores a single mod-sequence + per message. Section 5 describes how this can be achieved. + + Unless the server has included an unsolicited FETCH to update + client's knowledge about messages that have failed the UNCHANGEDSINCE + test, upon receipt of the MODIFIED response code, the client SHOULD + try to figure out if the required metadata items have indeed changed + by issuing FETCH or NOOP command. It is RECOMMENDED that the server + avoids the need for the client to do that by sending an unsolicited + FETCH response (Examples 9 and 10). + + If the required metadata items haven't changed, the client SHOULD + retry the command with the new mod-sequence. The client SHOULD allow + for a configurable but reasonable number of retries (at least 2). + + Example 9: + + In the example below, the server returns the MODIFIED response + code without sending information describing why the STORE + UNCHANGEDSINCE operation has failed. + + C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000) + +FLAGS.SILENT ($Processed) + S: * 100 FETCH (MODSEQ (303181230852)) + S: * 102 FETCH (MODSEQ (303181230852)) + ... + S: * 150 FETCH (MODSEQ (303181230852)) + S: a106 OK [MODIFIED 101] Conditional STORE failed + + The flag $Processed was set on the message 101... + + C: a107 NOOP + S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed)) + S: a107 OK + + Or the flag hasn't changed, but another has (note that this server + behaviour is discouraged. Server implementers should also see + Section 5)... + + C: b107 NOOP + S: * 101 FETCH (MODSEQ (303011130956) FLAGS (\Deleted \Answered)) + S: b107 OK + + ...and the client retries the operation for the message 101 with + the updated UNCHANGEDSINCE value + + + + +Melnikov & Hole Standards Track [Page 10] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + C: b108 STORE 101 (UNCHANGEDSINCE 303011130956) + +FLAGS.SILENT ($Processed) + S: * 101 FETCH (MODSEQ (303181230852)) + S: b108 OK Conditional Store completed + + Example 10: + + Same as above, but the server avoids the need for the client to + poll for changes. + + The flag $Processed was set on the message 101 by another + client... + + C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000) + +FLAGS.SILENT ($Processed) + S: * 100 FETCH (MODSEQ (303181230852)) + S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed)) + S: * 102 FETCH (MODSEQ (303181230852)) + ... + S: * 150 FETCH (MODSEQ (303181230852)) + S: a106 OK [MODIFIED 101] Conditional STORE failed + + Or the flag hasn't changed, but another has (note that this server + behaviour is discouraged. Server implementers should also see + Section 5)... + + C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000) + +FLAGS.SILENT ($Processed) + S: * 100 FETCH (MODSEQ (303181230852)) + S: * 101 FETCH (MODSEQ (303011130956) FLAGS (\Deleted \Answered)) + S: * 102 FETCH (MODSEQ (303181230852)) + ... + S: * 150 FETCH (MODSEQ (303181230852)) + S: a106 OK [MODIFIED 101] Conditional STORE failed + + ...and the client retries the operation for the message 101 with + the updated UNCHANGEDSINCE value + + C: b108 STORE 101 (UNCHANGEDSINCE 303011130956) + +FLAGS.SILENT ($Processed) + S: * 101 FETCH (MODSEQ (303181230852)) + S: b108 OK Conditional Store completed + + Or the flag hasn't changed, but another has (nice server + behaviour. Server implementers should also see Section 5)... + + C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000) + +FLAGS.SILENT ($Processed) + + + +Melnikov & Hole Standards Track [Page 11] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + S: * 100 FETCH (MODSEQ (303181230852)) + S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed \Deleted + \Answered)) + S: * 102 FETCH (MODSEQ (303181230852)) + ... + S: * 150 FETCH (MODSEQ (303181230852)) + S: a106 OK Conditional STORE completed + + Example 11: + + The following example is based on the example from the Section + 4.2.3 of [RFC-2180] and demonstrates that the MODIFIED response + code may be also returned in the tagged NO response. + + Client tries to conditionally STORE flags on a mixture of expunged + and non-expunged messages; one message fails the UNCHANGEDSINCE + test. + + C: B001 STORE 1:7 (UNCHANGEDSINCE 320172338) +FLAGS (\SEEN) + S: * 1 FETCH (MODSEQ (320172342) FLAGS (\SEEN)) + S: * 3 FETCH (MODSEQ (320172342) FLAGS (\SEEN)) + S: B001 NO [MODIFIED 2] Some of the messages no longer exist. + + C: B002 NOOP + S: * 4 EXPUNGE + S: * 4 EXPUNGE + S: * 4 EXPUNGE + S: * 4 EXPUNGE + S: * 2 FETCH (MODSEQ (320172340) FLAGS (\Deleted \Answered)) + S: B002 OK NOOP Completed. + + By receiving FETCH responses for messages 1 and 3, and EXPUNGE + responses that indicate that messages 4 through 7 have been + expunged, the client retries the operation only for the message 2. + The updated UNCHANGEDSINCE value is used. + + C: b003 STORE 2 (UNCHANGEDSINCE 320172340) +FLAGS (\Seen) + S: * 2 FETCH (MODSEQ (320180050)) + S: b003 OK Conditional Store completed + + Note: If a 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 the conditional STORE operation for the + second (or subsequent) occurrence of the message if the operation + completed successfully for the first occurrence. For example, if the + client specifies: + + + + + +Melnikov & Hole Standards Track [Page 12] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + e105 STORE 7,3:9 (UNCHANGEDSINCE 12121230045) + +FLAGS.SILENT (\Deleted) + + the server must not fail the operation for message 7 as part of + processing "3:9" if it succeeded when message 7 was processed the + first time. + + Once the client specified the UNCHANGEDSINCE modifier in a STORE + command, the server MUST include the MODSEQ fetch response data items + in all subsequent unsolicited FETCH responses. + + This document also changes the behaviour of the server when it has + performed a STORE or UID STORE command and the UNCHANGEDSINCE + modifier is not specified. If the operation is successful for a + message, the server MUST update the mod-sequence attribute of the + message. The server is REQUIRED to include the mod-sequence value + whenever it decides to send the unsolicited FETCH response to all + CONDSTORE-aware clients that have opened the mailbox containing the + message. + + Server implementers should also see Section 3.8 for additional + quality of implementation issues related to the STORE command. + +3.3. FETCH and UID FETCH Commands + +3.3.1. CHANGEDSINCE FETCH Modifier + + This document defines the following FETCH modifier (see Section 2.4 + of [IMAPABNF]): + + CHANGEDSINCE <mod-sequence> + + CHANGEDSINCE FETCH modifier allows to create a further subset of + the list of messages described by sequence set. The information + described by message data items is only returned for messages that + have mod-sequence bigger than <mod-sequence>. + + When CHANGEDSINCE FETCH modifier is specified, it implicitly adds + MODSEQ FETCH message data item (Section 3.3.2). + + Example 12: + + C: s100 UID FETCH 1:* (FLAGS) (CHANGEDSINCE 12345) + S: * 1 FETCH (UID 4 MODSEQ (65402) FLAGS (\Seen)) + S: * 2 FETCH (UID 6 MODSEQ (75403) FLAGS (\Deleted)) + S: * 4 FETCH (UID 8 MODSEQ (29738) FLAGS ($NoJunk $AutoJunk + $MDNSent)) + S: s100 OK FETCH completed + + + +Melnikov & Hole Standards Track [Page 13] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + +3.3.2. MODSEQ Message Data Item in FETCH Command + + This extension adds a MODSEQ message data item to the FETCH command. + The MODSEQ message data item allows clients to retrieve mod-sequence + values for a range of messages in the currently selected mailbox. + + Once the client specified the MODSEQ message data item in a FETCH + request, the server MUST include the MODSEQ fetch response data items + in all subsequent unsolicited FETCH responses. + + Syntax: MODSEQ + + The MODSEQ message data item causes the server to return MODSEQ + fetch response data items. + + Syntax: MODSEQ ( <permsg-modsequence> ) + + MODSEQ response data items contain per-message mod-sequences. + + The MODSEQ response data item is returned if the client issued + FETCH with MODSEQ message data item. It also allows the server to + notify the client about mod-sequence changes caused by conditional + STOREs (Section 3.2) and/or changes caused by external sources. + + Example 13: + + C: a FETCH 1:3 (MODSEQ) + S: * 1 FETCH (MODSEQ (624140003)) + S: * 2 FETCH (MODSEQ (624140007)) + S: * 3 FETCH (MODSEQ (624140005)) + S: a OK Fetch complete + + In this example, the client requests per-message mod-sequences for + a set of messages. + + When a flag for a message is modified in a different session, the + server sends an unsolicited FETCH response containing the mod- + sequence for the message. + + Example 14: + + (Session 1, authenticated as a user "alex"). The user adds a + shared flag \Deleted: + + C: A142 SELECT INBOX + ... + S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft) + S: * OK [PERMANENTFLAGS (\Answered \Deleted \Seen \*)] Limited + + + +Melnikov & Hole Standards Track [Page 14] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + ... + + C: A160 STORE 7 +FLAGS.SILENT (\Deleted) + S: * 7 FETCH (MODSEQ (2121231000)) + S: A160 OK Store completed + + (Session 2, also authenticated as the user "alex"). Any changes + to flags are always reported to all sessions authenticated as the + same user as in the session 1. + + C: C180 NOOP + S: * 7 FETCH (FLAGS (\Deleted \Answered) MODSEQ (12121231000)) + S: C180 OK Noop completed + + (Session 3, authenticated as a user "andrew"). As \Deleted is a + shared flag, changes in session 1 are also reported in session 3: + + C: D210 NOOP + S: * 7 FETCH (FLAGS (\Deleted \Answered) MODSEQ (12121231000)) + S: D210 OK Noop completed + + The user modifies a private flag \Seen in session 1... + + C: A240 STORE 7 +FLAGS.SILENT (\Seen) + S: * 7 FETCH (MODSEQ (12121231777)) + S: A240 OK Store completed + + ...which is only reported in session 2... + + C: C270 NOOP + S: * 7 FETCH (FLAGS (\Deleted \Answered \Seen) MODSEQ + (12121231777)) + S: C270 OK Noop completed + + ...but not in session 3. + + C: D300 NOOP + S: D300 OK Noop completed + + And finally, the user removes flags \Answered (shared) and \Seen + (private) in session 1. + + C: A330 STORE 7 -FLAGS.SILENT (\Answered \Seen) + S: * 7 FETCH (MODSEQ (12121245160)) + S: A330 OK Store completed + + + + + + +Melnikov & Hole Standards Track [Page 15] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + Both changes are reported in the session 2... + + C: C360 NOOP + S: * 7 FETCH (FLAGS (\Deleted) MODSEQ (12121245160)) + S: C360 OK Noop completed + + ...and only changes to shared flags are reported in session 3. + + C: D390 NOOP + S: * 7 FETCH (FLAGS (\Deleted) MODSEQ (12121245160)) + S: D390 OK Noop completed + + Server implementers should also see Section 3.8 for additional + quality of implementation issues related to the FETCH command. + +3.4. MODSEQ Search Criterion in SEARCH + + The MODSEQ criterion for the SEARCH command allows a client to search + for the metadata items that were modified since a specified moment. + + Syntax: MODSEQ [<entry-name> <entry-type-req>] <mod-sequence-valzer> + + Messages that have modification values that are equal to or + greater than <mod-sequence-valzer>. 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. The client may also specify <entry-name> (name of metadata + item) and <entry-type-req> (type of metadata item) before + <mod-sequence-valzer>. <entry-type-req> can be one of "shared", + "priv" (private), or "all". The latter means that the server + should use the biggest value among "priv" and "shared" mod- + sequences for the metadata item. If the server doesn't store + internally separate mod-sequences for different metadata items, it + MUST ignore <entry-name> and <entry-type-req>. Otherwise, the + server should use them to narrow down the search. + + For a flag <flagname>, the corresponding <entry-name> has a form + "/flags/<flagname>" as defined in [IMAPABNF]. Note that the + leading "\" character that denotes a system flag has to be escaped + as per Section 4.3 of [IMAP4], as the <entry-name> uses syntax for + quoted strings. + + If client specifies a MODSEQ criterion in a SEARCH command and the + server returns a non-empty SEARCH result, the server MUST also append + (to the end of the untagged SEARCH response) the highest mod-sequence + for all messages being returned. See also Section 3.5. + + + + + +Melnikov & Hole Standards Track [Page 16] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + Example 15: + + C: a SEARCH MODSEQ "/flags/\\draft" all 620162338 + S: * SEARCH 2 5 6 7 11 12 18 19 20 23 (MODSEQ 917162500) + 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 + "/comment" entry and having a mod-sequence equal to or greater + than 620162338 for the "\Draft" flag are returned in the search + results. + + Example 16: + + C: t SEARCH OR NOT MODSEQ 720162338 LARGER 50000 + S: * SEARCH + S: t OK Search complete, nothing found + +3.5. Modified SEARCH Untagged Response + + Data: zero or more numbers + mod-sequence value (omitted if no match) + + This document extends syntax of the untagged SEARCH response to + include the highest mod-sequence for all messages being returned. + + If a client specifies a MODSEQ criterion in a SEARCH (or UID SEARCH) + command and the server returns a non-empty SEARCH result, the server + MUST also append (to the end of the untagged SEARCH response) the + highest mod-sequence for all messages being returned. See Section + 3.4 for examples. + +3.6. HIGHESTMODSEQ Status Data Items + + This document defines a new status data item: + + HIGHESTMODSEQ + + The highest mod-sequence value of all messages in the mailbox. + This is the same value that is returned by the server in the + HIGHESTMODSEQ response code in an OK untagged response (see + Section 3.1.1). If the server doesn't support the persistent + storage of mod-sequences for the mailbox (see Section 3.1.2), the + server MUST return 0 as the value of HIGHESTMODSEQ status data + item. + + + + + + +Melnikov & Hole Standards Track [Page 17] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + Example 17: + + C: A042 STATUS blurdybloop (UIDNEXT MESSAGES HIGHESTMODSEQ) + S: * STATUS blurdybloop (MESSAGES 231 UIDNEXT 44292 + HIGHESTMODSEQ 7011231777) + S: A042 OK STATUS completed + +3.7. CONDSTORE Parameter to SELECT and EXAMINE + + The CONDSTORE extension defines a single optional select parameter, + "CONDSTORE", which tells the server that it MUST include the MODSEQ + fetch response data items in all subsequent unsolicited FETCH + responses. + + The CONDSTORE parameter to SELECT/EXAMINE helps avoid a race + condition that might arise when one or more metadata items are + modified in another session after the server has sent the + HIGHESTMODSEQ response code and before the client was able to issue a + CONDSTORE enabling command. + + Example 18: + + C: A142 SELECT INBOX (CONDSTORE) + S: * 172 EXISTS + S: * 1 RECENT + S: * OK [UNSEEN 12] Message 12 is first unseen + S: * OK [UIDVALIDITY 3857529045] UIDs valid + S: * OK [UIDNEXT 4392] Predicted next UID + S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft) + S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited + S: * OK [HIGHESTMODSEQ 715194045007] + S: A142 OK [READ-WRITE] SELECT completed, CONDSTORE is now enabled + +3.8. Additional Quality-of-Implementation Issues + + Server implementations should follow the following rule, which + applies to any successfully completed STORE/UID STORE (with and + without UNCHANGEDSINCE modifier), as well as to a FETCH command that + implicitly sets \Seen flag: + + Adding the flag when it is already present or removing when it is + not present SHOULD NOT change the mod-sequence. + + This will prevent spurious client synchronization requests. + + + + + + + +Melnikov & Hole Standards Track [Page 18] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + However, note that client implementers MUST NOT rely on this server + behavior. A client can't distinguish between the case when a server + has violated the SHOULD mentioned above, and that when one or more + clients set and unset (or unset and set) the flag in another session. + +4. Formal Syntax + + The following syntax specification uses the Augmented Backus-Naur + Form (ABNF) [ABNF] notation. Elements not defined here can be found + in the formal syntax of the ABNF [ABNF], IMAP [IMAP4], and IMAP ABNF + extensions [IMAPABNF] specifications. + + Except as noted otherwise, all alphabetic characters are case- + insensitive. The use of upper- or lowercase characters to define + token strings is for editorial clarity only. Implementations MUST + accept these strings in a case-insensitive fashion. + + capability =/ "CONDSTORE" + + status-att =/ "HIGHESTMODSEQ" + ;; extends non-terminal defined in RFC 3501. + + status-att-val =/ "HIGHESTMODSEQ" SP mod-sequence-valzer + ;; extends non-terminal defined in [IMAPABNF]. + ;; Value 0 denotes that the mailbox doesn't + ;; support persistent mod-sequences + ;; as described in Section 3.1.2 + + store-modifier =/ "UNCHANGEDSINCE" SP mod-sequence-valzer + ;; Only a single "UNCHANGEDSINCE" may be + ;; specified in a STORE operation + + fetch-modifier =/ chgsince-fetch-mod + ;; conforms to the generic "fetch-modifier" + ;; syntax defined in [IMAPABNF]. + + chgsince-fetch-mod = "CHANGEDSINCE" SP mod-sequence-value + ;; CHANGEDSINCE FETCH modifier conforms to + ;; the fetch-modifier syntax + + fetch-att =/ fetch-mod-sequence + ;; modifies original IMAP4 fetch-att + + fetch-mod-sequence = "MODSEQ" + + fetch-mod-resp = "MODSEQ" SP "(" permsg-modsequence ")" + + msg-att-dynamic =/ fetch-mod-resp + + + +Melnikov & Hole Standards Track [Page 19] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + search-key =/ search-modsequence + ;; modifies original IMAP4 search-key + ;; + ;; This change applies to all commands + ;; referencing this non-terminal, in + ;; particular SEARCH. + + search-modsequence = "MODSEQ" [search-modseq-ext] SP + mod-sequence-valzer + + search-modseq-ext = SP entry-name SP entry-type-req + + resp-text-code =/ "HIGHESTMODSEQ" SP mod-sequence-value / + "NOMODSEQ" / + "MODIFIED" SP set + + entry-name = entry-flag-name + + entry-flag-name = DQUOTE "/flags/" attr-flag DQUOTE + ;; each system or user defined flag <flag> + ;; is mapped to "/flags/<flag>". + ;; + ;; <entry-flag-name> follows the escape rules + ;; used by "quoted" string as described in + ;; Section 4.3 of [IMAP4], e.g., for the flag + ;; \Seen the corresponding <entry-name> is + ;; "/flags/\\seen", and for the flag + ;; $MDNSent, the corresponding <entry-name> + ;; is "/flags/$mdnsent". + + entry-type-resp = "priv" / "shared" + ;; metadata item type + + entry-type-req = entry-type-resp / "all" + ;; perform SEARCH operation on private + ;; metadata item, shared metadata item or both + + permsg-modsequence = mod-sequence-value + ;; per message mod-sequence + + mod-sequence-value = 1*DIGIT + ;; Positive unsigned 64-bit integer + ;; (mod-sequence) + ;; (1 <= n < 18,446,744,073,709,551,615) + + mod-sequence-valzer = "0" / mod-sequence-value + + search-sort-mod-seq = "(" "MODSEQ" SP mod-sequence-value ")" + + + +Melnikov & Hole Standards Track [Page 20] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + select-param =/ condstore-param + ;; conforms to the generic "select-param" + ;; non-terminal syntax defined in [IMAPABNF]. + + condstore-param = "CONDSTORE" + + mailbox-data =/ "SEARCH" [1*(SP nz-number) SP + search-sort-mod-seq] + + 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 [IMAP4]. + + attr-flag-keyword = atom + +5. Server Implementation Considerations + + This section describes how a server implementation that doesn't store + separate per-metadata mod-sequences for different metadata items can + avoid sending the MODIFIED response to any of the following + conditional STORE operations: + + +FLAGS + -FLAGS + +FLAGS.SILENT + -FLAGS.SILENT + + Note that the optimization described in this section can't be + performed in case of a conditional STORE FLAGS operation. + + Let's use the following example. The client has issued + + C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000) + +FLAGS.SILENT ($Processed) + + When the server receives the command and parses it successfully, it + iterates through the message set and tries to execute the conditional + STORE command for each message. + + + + +Melnikov & Hole Standards Track [Page 21] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + + Each server internally works as a client, i.e., it has to cache the + current state of all IMAP flags as it is known to the client. In + order to report flag changes to the client, the server compares the + cached values with the values in its database for IMAP flags. + + Imagine that another client has changed the state of a flag \Deleted + on the message 101 and that the change updated the mod-sequence for + the message. The server knows that the mod-sequence for the mailbox + has changed; however, it also knows that: + + a) the client is not interested in \Deleted flag, as it hasn't + included it in +FLAGS.SILENT operation; and + + b) the state of the flag $Processed hasn't changed (the server can + determine this by comparing cached flag state with the state of + the flag in the database). + + Therefore, the server doesn't have to report MODIFIED to the client. + Instead, the server may set $Processed flag, update the mod-sequence + for the message 101 once again and send an untagged FETCH response + with new mod-sequence and flags: + + S: * 101 FETCH (MODSEQ (303011130956) + FLAGS ($Processed \Deleted \Answered)) + + See also Section 3.8 for additional quality-of-implementation issues. + +6. Security Considerations + + It is believed that the Conditional STORE extension doesn't raise any + new security concerns that are not already discussed in [IMAP4]. + However, the availability of this extension may make it possible for + IMAP4 to be used in critical applications it could not be used for + previously, making correct IMAP server implementation and operation + even more important. + +7. 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 CONDSTORE IMAP capability. IANA has added + it to the registry accordingly. + + + + + +Melnikov & Hole Standards Track [Page 22] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + +8. References + +8.1. Normative References + + [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, March 1997. + + [ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax + Specifications: ABNF", RFC 4234, October 2005. + + [IMAP4] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION + 4rev1", RFC 3501, March 2003. + + [IMAPABNF] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4 + ABNF", RFC 4466, April 2006. + +8.2. Informative References + + [ACAP] Newman, C. and J. Myers, "ACAP -- Application + Configuration Access Protocol", RFC 2244, November 1997. + + [ACL] Melnikov, A., "IMAP4 Access Control List (ACL) Extension", + RFC 4314, December 2005. + + [ANN] Daboo, C. and R. Gellens, "IMAP ANNOTATE Extension", Work + in Progress, March 2006. + + [NTP] Mills, D., "Network Time Protocol (Version 3) + Specification, Implementation and Analysis", RFC 1305, + March 1992. + + [RFC-2180] Gahrns, M., "IMAP4 Multi-Accessed Mailbox Practice", RFC + 2180, July 1997. + +9. Acknowledgements + + Some text was borrowed from "IMAP ANNOTATE Extension" [ANN] by + Randall Gellens and Cyrus Daboo and from "ACAP -- Application + Configuration Access Protocol" [ACAP] by Chris Newman and John Myers. + + Many thanks to Randall Gellens for his thorough review of the + document. + + The authors also acknowledge the feedback provided by Cyrus Daboo, + Larry Greenfield, Chris Newman, Harrie Hazewinkel, Arnt Gulbrandsen, + Timo Sirainen, Mark Crispin, Ned Freed, Ken Murchison, and Dave + Cridland. + + + + +Melnikov & Hole Standards Track [Page 23] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + +Authors' Addresses + + Alexey Melnikov + Isode Limited + 5 Castle Business Village + 36 Station Road + Hampton, Middlesex + TW12 2BX, + United Kingdom + + EMail: Alexey.Melnikov@isode.com + + + Steve Hole + ACI WorldWide/MessagingDirect + #1807, 10088 102 Ave + Edmonton, AB + T5J 2Z1 + Canada + + EMail: Steve.Hole@messagingdirect.com + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Melnikov & Hole Standards Track [Page 24] + +RFC 4551 IMAP Extension for Conditional STORE June 2006 + + +Full Copyright Statement + + Copyright (C) The Internet Society (2006). + + 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. + + This document and the information contained herein are provided on an + "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS + OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET + ENGINEERING TASK FORCE DISCLAIM 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. + +Intellectual Property + + The IETF takes no position regarding the validity or scope of any + Intellectual Property Rights or other rights that might be claimed to + pertain to the implementation or use of the technology described in + this document or the extent to which any license under such rights + might or might not be available; nor does it represent that it has + made any independent effort to identify any such rights. 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. + +Acknowledgement + + Funding for the RFC Editor function is provided by the IETF + Administrative Support Activity (IASA). + + + + + + + +Melnikov & Hole Standards Track [Page 25] + |