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draft-melnikov-imap-condstore-02.txt --> draft-melnikov-imap-condstore-03.txt

Document: draft-melnikov-imap-condstore-02.txt draft-melnikov-imap-condstore-03.txt S. Hole
Expires: January 2001 March 2002 ACI WorldWide/MessagingDirect
July
September 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.

0.1. Open issues

1). Should conditional STORE be atomic accross message set (i.e. either
all messages in message set weren't changed since and condtiontal
STORE succeeds or operation fails for all messages)?
This can be very expensive for some servers.

2). How specify different UNCHANGESINCE for different flags in the same
STORE? Do we want such granularity anyway?

2).

3). The document assumes that each flag has a corresponding ANNOTATE
entry. This has to be synchronized with ANNOTATE draft.

3). "If the "modtime" of any metadata item specified in STORE
operation

4). Add support for any message in the message set is greater
than the unchangedsince value, then the store fails with a
MODIFIED SORT extension?

0.2. Change History

Changes from -02 to -03:
1. Changed MODTIME untagged response to MODTIME response code.
2. Added MODTIME response code that includes message set of all
messages that failed UNCHANGESINCE test."

This basically means that server must continue conditional store
operation to find all messages for which test fails. Is it
reasonable, or should the text be changed from "all messages" to
"at least one message".

4). MODTIME Untagged Response specifies tagged OK response for SEARCH.
Updated examples accordingly.
3. Changed rule for sending untagged FETCH response as a message set modtime applies to.
Can anybody think result of a reason why it
STORE when .SILENT prefix is a bad idea? Message set used. If .SILENT prefix is not really required, because MODTIME untagged response must
follow corresponding used,
server doesn't have to send untagged FETCH responses.
Should we use UID set instead?

5). Should SEARCH MODTIME require response, because
MODTIME untagged response?

SEARCH doesn't cause updates, unless they are the result of some
other operation. However SEARCH response code already contains modtime.
4. Renamed MODTIME forces server to read
modtime, so server can report this information anyway.

6). Add support for SORT extension?

0.2. Change History MODSEQ to make sure there is no confusion
between mod-sequence and ACAP modtime.
5. Minor ABNF changes.
6. Minor language corrections.

Changes from -01 to -02:
1. Added MODTIME data item to STATUS command.
2. Added OK untagged response to SELECT/EXAMINE.
3. Clarified that MODIFIED response code contains list of UIDs for
conditional UID STORE and message set for STORE.
4. Added per-message modtime.
5. Added PERFLAGMODTIME capability.
6. Fixed several bugs in examples.
7. Added more comments to ABNF.

Changes from -00 to -01:
1. Refreshed the list of Open Issues.
2. Changed "attr-name" to "entry-name", because modtime applies to
entry, not attribute.
3. Added MODTIME untagged response.
4. Cleaned up ABNF.
5. Added "Acknowledgments" section.
6. Fixed some spelling mistakes.

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 unsigned
64-bit value called modification timestamp (modtime). sequence (mod-sequence). 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. server.
However the server MUST guaranty that each STORE command (including
simultaneous stores from different connections on different attributes)
will use different modtime mod-sequence values. Also, for any two successfull
consequent conditional store operations performed in the same session, modtime
mod-sequence of the second MUST be greater than modtime mod-sequence of the
first. Modtime is described more fully in section 3.1.1 Note, that the latter rule disallows the use of [ACAP].

Modtime system clock as
mod-sequence, because if system time changes (e.g. NTP client adjusting
time), next generated value can be less than previous.

Mod-sequence allows the client that supports CONDSTORE extension to
track whether the value of particular flag was has changed since some moment in
time. known
moment. 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 sequence for that flag MUST be updated. Adding the
flag when it is already present of removing when it is not SHOULD NOT
change modtime. mod-sequence. Change to any flag MUST also update per message
mod-sequence. Each flag SHOULD have separate modtime, mod-sequence, for example
change to \Draft flag SHOULD NOT affect modtime mod-sequence for \Deleted flag.
Server that supports per flag modtimes (i.e. per message mod-sequences (i.e. satisfies
the latter SHOULD) MUST also report "PERFLAGMODTIME" "PERFLAGMODSEQ" in CAPABILITY
command response.

When message is appended to mailbox (via APPEND command or using
external mechanism) the server assigns the current server
timestamp modification
sequence to every flag or annotation specified in the APPEND command.

When an annotation is removed modtime mod-sequence 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 MODSEQ message data item for use in the FETCH command

d) adds a new MODTIME MODSEQ message data item for use in the SEARCH command

e) adds a new MODTIME untagged Response MODSEQ response code

f) adds a new OK untagged responses response for SELECT and EXAMINE

g) adds MODTIME HIGHESTMODSEQ status data item

The rest of this document describes protocol changes more rigorously.

4. IMAP Protocol Changes

4.1. OK untagged responses response for SELECT and EXAMINE

This document add adds a new OK untagged responses response for SELECT and EXAMINE
commands. A server supporting CONDSTORE extension MUST send the following
OK untagged response with any successful SELECT or EXAMINE command.

OK [HIGHESTMODTIME <modtime-value>] [HIGHESTMODSEQ <mod-sequence-value>]
The highest modtime mod-sequence value for any metadata item of
any message in the mailbox.

Disconnected client can use the value of HIGHESTMODTIME HIGHESTMODSEQ to check if it has
to refetch flags from the server. If the value stored in clients cache is
less than the value returned by the server, this means that some metadata
items on the server changed since last synchronization and client has to
update its cache. Client MAY use SEARCH MODTIME MODSEQ as described in 4.4 to
find out exactly what metadata items has changed.

Example: 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: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited
S: * OK [HIGHESTMODTIME "20010715194045007"] [HIGHESTMODSEQ 20010715194045007]
S: A142 OK [READ-WRITE] SELECT completed

4.2. STORE and UID STORE Commands

Arguments: message set
OPTIONAL store modifiers
message data item name
value for message data item

Responses: untagged responses: FETCH, MODTIME FETCH
OK untagged responses: MODSEQ (See section 4.5)

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" mod-sequence 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 (for STORE) or set of
UIDs (for UID STORE) of all messages that failed UNCHANGESINCE
test.

Example:

C: a101 STORE 7,5,9 (UNCHANGEDSINCE "20000320162338") 20000320162338)
+FLAGS.SILENT (\Deleted)
S: a101 NO [MODIFIED 7,9] Conditional STORE failed

In spite of failure of conditional STORE operation for message 7
server continues to perform conditional STORE to find all
messages for which operation will fail.

Use of UNCHANGEDSINCE with a time modification sequence of "00000101000000" 0 will
always fail if the metadata item exists.

Example:

C: a102 STORE 12 (UNCHANGEDSINCE "00000101000000") 0)
+FLAGS.SILENT ($MDNSent)
S: a102 NO [MODIFIED 12] Conditional STORE failed

If operation is successful the server MUST update the "modtime"
mod-sequence attribute for every metadata item that was changed.
Untagged FETCH response MUST be sent even if (unless .SILENT is specified
specified) and it MUST include MODTIME MODSEQ message data item as
described in 4.3.

Also server MUST send untagged MODTIME MODSEQ response code to indicate that
client has received all updates to metadata items which have
modtime
mod-sequence values less than or equal to the indicated modtime
mod-sequence value.

Example:

C: a103 UID STORE 6,4,8 (UNCHANGEDSINCE "200012121230045") 200012121230045)
+FLAGS.SILENT (\Deleted)
S: * 1 FETCH (UID 4 MODTIME ("200012121231000" MODSEQ (200012121231000
"/message/flags/system/\\Deleted" "200012121231000")) 200012121231000))
S: * 2 FETCH (UID 6 MODTIME ("200012101230852" MODSEQ (200012101230852
"/message/flags/system/\\Deleted" "200012101230852")) 200012101230852))
S: * 4 FETCH (UID 8 MODTIME ("200012121130956" MODSEQ (200012121130956
"/message/flags/system/\\Deleted" "200012121130956"))
S: * MODTIME 1:4 "200012121231000" 200012121130956))
S: a103 OK [MODSEQ 4,6,8 200012121231000] Store completed

Example:

C: a104 STORE * (UNCHANGEDSINCE "200012121230045") 200012121230045) +FLAGS.SILENT
(\Deleted $Processed)
S: * 50 FETCH (MODTIME ("200012111230045" (MODSEQ (200012111230045 "/message/flags/system/\\Deleted" "200012111230045"
200012111230045 "/message/flags/system/$Processed" "200012111230045"))
S: * MODTIME 50 "200012111230045" 200012111230045))
S: a104 OK [MODSEQ 50 200012111230045] Store completed

In the latter example UNCHANGEDSINCE value is checked against modtimes
mod-sequences 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 occurrence of the message in the message set if
operation completed successfully for the first occurrence. For
example, if the client specifies:

a100 STORE 7,3:9 (UNCHANGEDSINCE "200012121230045") 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.

4.3 MODTIME MODSEQ message data item in FETCH Command

This extension adds an MODTIME MODSEQ message data item to the FETCH command.
This allows clients to retrieve modtime mod-sequence for various metadata items
for a range of messages in the currently selected mailbox.

Syntax: MODTIME MODSEQ <entry-names>

The MODTIME MODSEQ message data item, when used by the client in the FETCH
command, takes a list of metadata items. For a flag <flagname> the
corresponding entry-name has a form
"/message/flags/system/<flagname>". Empty list requests server to
return only per-message modtime.

MODTIME mod-sequence.

MODSEQ message data item causes server to return MODTIME MODSEQ fetch response
data item.

Syntax: MODTIME MODSEQ ( <permessage-modtime> <permsg-modsequence> <entry-name> <modtime-value> <mod-sequence-value> ... )

MODTIME

MODSEQ response data item contains per-message modtime mod-sequence and
possible empty list of requested metadata items and their
corresponding
modtimes. mod-sequences.

Example:

C: a FETCH 1 (MODTIME (MODSEQ ("/message/comment" "/message/flags/system/$MDNSent"))
S: * 1 FETCH (MODTIME ("20000624140000" (MODSEQ (20000624140000 "/message/comment" "20000624140000" 20000624140000
"/message/flags/system/$MDNSent" "20000624140000")) 20000624140000))
S: a OK Fetch complete

4.4 MODTIME MODSEQ criterion in SEARCH

The MODTIME MODSEQ criterion for the SEARCH command allows a client to search
for the specified modtime mod-sequence of a metadata item in a message.

Syntax: MODTIME MODSEQ <entry-name> <modtime-value> <mod-sequence-value>

Messages that have modification counter for metadata item
<entry-name> with value equal or greater than <modtime-value>.
<mod-sequence-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:

If client specifies MODSEQ creterion in SEARCH command and server returns
non empty SEARCH result, server MUST also return MODSEQ response code in
the tagged OK response for all messages returned in untagged SEARCH. See
also 4.5.

Example:
C: a SEARCH MODTIME MODSEQ "/message/flags/system/\\draft" "20010320162338" 20010320162338
ANNOTATION "/message/comment" "value" "IMAP4"
S: * SEARCH 2 3 5 6 7 11 13 17 12 18 19 20 23
S: a OK [MODSEQ 2,5:7,11:12,18:20,23 20010917162338] 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" mod-sequence 20010320162338
for flag \Draft are returned in the search results.

Example:
C: a SEARCH OR NOT MODSEQ "/message/flags/system/$MDNSent" 20010320162338
LARGER 50000
S: * SEARCH
S: a OK Search complete, nothing found

4.5 MODTIME Untagged MODSEQ Response code for successful FETCH and FETCH, STORE and SEARCH

Data: message set
modtime
mod-sequence value

MODSEQ response code can be sent in the following three cases:

1) Successful STORE UNCHANGEDSINCE results in untagged MODTIME MODSEQ response code
being sent in tagged OK response to the client to inform of the
latest modtime mod-sequence of all metadata items specified in the STORE
command. It would also

MODSEQ response code contains message set mod-sequence applies to if
is caused by STORE command or UID set it is caused by UID STORE.

Example:
C: a103 STORE 9 (UNCHANGEDSINCE 200012121230045) +FLAGS.SILENT ($Forwarded)
S: * 9 FETCH (MODSEQ (200012121231000 "/message/flags/system/$Forwarded"
200012121130046))
S: a103 OK [MODSEQ 9 200012121231000] Store completed

2) MODSEQ response code in untagged OK response MUST be issued sent by the
server following a group of one or more unsolicited FETCH responses
to indicate that the client has received all updates to metadata
items which have modtime mod-sequence values less than or equal to the
indicated modtime mod-sequence value.

MODSEQ response code terminating a group of unsolicited FETCH
responses always contain message set mod-sequence applies to.

Example:
C: a103 STORE 9 (UNCHANGEDSINCE "200012121230045") +FLAGS.SILENT ($Forwarded) UID FETCH 20138:* (FLAGS INTERNALDATE)
S: * 9 101 FETCH (MODTIME ("200012121231000" "/message/flags/system/$Forwarded"
"200012121130046")) (UID 20140 FLAGS (\Seen \Answered $Forwarded)
INTERNALDATE "28-Oct-2001 15:44:25 +0300")
S: * MODTIME 9 "200012121231000" 12 FETCH (FLAGS (\Flagged) MODSEQ (200109121231000 "/message/flags/system/\\Flagged"
200109121231000))
S: * 16 FETCH (FLAGS (\Answered \Flagged) MODSEQ (200109121231000
"/message/flags/system/\\Flagged" 200109121231000 "/message/flags/system/\\Answered"
200109121231000))
S: * OK [MODSEQ 12,16 200109121231000]
S: a103 OK Store completed

3) If client specifies MODSEQ creterion in SEARCH command and server
returns non empty SEARCH result, server MUST also return MODSEQ
response code in the tagged OK response for all messages returned in
untagged SEARCH response.

MODSEQ response code contains message set mod-sequence applies to if
is caused by SEARCH command or UID set it is caused by UID SEARCH.

4.6 MODTIME HIGHESTMODSEQ status data items

This document defines a new status data item:

HIGHESTMODTIME

HIGHESTMODSEQ
The highest modtime mod-sequence value for any metadata item of any message
in the mailbox. This is the same value that is returned by server
in
HIGHESTMODTIME HIGHESTMODSEQ response code in Ok untagged response (See 4.1).

Example: C: A042 STATUS blurdybloop (UIDNEXT MESSAGES HIGHESTMODTIME) HIGHESTMODSEQ)
S: * STATUS blurdybloop (MESSAGES 231 UIDNEXT 44292 HIGHESTMODTIME "200201011231777") HIGHESTMODSEQ
200201011231777)
S: A042 OK STATUS completed

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.

capability =/ "CONDSTORE" / "PERFLAGMODTIME"

resp_text_code =/ "HIGHESTMODTIME" SP modtime-value "PERFLAGMODSEQ"

mailbox-data =/ "STATUS" SP mailbox SP "("
[status-att SP status-value *(SP status-att SP status-value)] ")"

status-value = number / modtime-value mod-sequence-value

store = "STORE" SP set store-modifiers SP store-att-flags

store-modifiers = [ SP "(" 1*store-modifier ")" ]

store-modifier = "UNCHANGEDSINCE" SP modtime-value mod-sequence-value

fetch-att =/ fetch-modtime fetch-mod-sequence
;; modifies original IMAP4 fetch-att

fetch-modtime

fetch-mod-sequence = "MODTIME" "MODSEQ" SP entry-names

fetch-mod-resp = "MODTIME" "MODSEQ" SP "(" permessage-modtime permsg-modsequence
*(entry-name SP modtime-value) mod-sequence-value) ")"

search-key =/ search-modtime search-modsequence
;; modifies original IMAP4 search-key

search-modtime

search-modsequence = "MODTIME" "MODSEQ" SP entry-name SP modtime-value mod-sequence-value

resp-text-code =/ "HIGHESTMODSEQ" SP mod-sequence-value /
"MODIFIED" SP set /
"MODSEQ" SP set SP mod-sequence-value
;; set of message numbers for STORE STORE/FETCH or
;; set of UIDs for UID STORE STORE/UID FECTH

entry-names = "(" *entry-name ")"
;; empty list means that only per-message MODTIME MODSEQ should be returned

entry-name = '"' "/message/flags/system/" attr-flag '"'
;; each system or user defined flag <flag> is mapped to
;; "/message/flags/system/<flag>".
;; system IMAP flags must have two leading "\",
;; because "\" is an escape character.

permessage-modtime

permsg-modsequence = modtime-value mod-sequence-value
;; per message modtime, mod-sequence, if server supports per flag modtimes, mod-sequences,
;; this is the highest modtime mod-sequence between all metadata items

modtime-value

mod-sequence-value = time

mailbox-data =/ "MODTIME" SP set SP modtime-value 1*DIGIT
;; Unsigned 64-bit integer (mod-sequence)
;; (0 <= n < 18,446,744,073,709,551,615)

<<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. Acknowledgments

Many thanks to Randall Gellens for his comments on how MODTIME should
work together with ANNOTATE extension.

Some text was borrowed from "IMAP ANNOTATE Extension" by Randall Gellens
and Cyrus Daboo and "ACAP -- Application Configuration Access Protocol"
by Chris Newman and John Myers.

Many thanks to Randall Gellens for his comments on how CONDSTORE should
interact with ANNOTATE extension.

Authors also acknowledge the feedback provided by Cyrus Daboo and Larry
Greenfield.

8. Full Copyright Statement

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.
----