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
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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.
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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
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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.
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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.
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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.
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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.
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[...]
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.
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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.
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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
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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)
;; <command-select> from [IMAP]
addto-position = number
;; Number may be 0 for SEARCH result additions.
;; <number> from [IMAP]
modifier-context = "CONTEXT"
modifier-partial = "PARTIAL" SP seq-range
;; <seq-range> from [IMAP]
modifier-update = "UPDATE"
search-return-opt =/ modifier-context / modifier-partial /
modifier-update
;; All conform to <search-return-opt>, from [IMAP-ABNF]
resp-text-code =/ "NOUPDATE" SP quoted
;; <resp-text-code> from [IMAP]
ret-data-addto = "ADDTO"
SP "(" addto-position SP sequence-set
*(SP addto-position SP sequence-set)
")"
;; <sequence-set> from [IMAP]
ret-data-partial = "PARTIAL"
SP "(" seq-range SP partial-results ")"
;; <seq-range> is the requested range.
;; <seq-range> from [IMAP]
partial-results = sequence-set / "NIL"
;; <sequence-set> from [IMAP]
;; NIL indicates no results correspond to the requested range.
ret-data-removefrom = "REMOVEFROM" SP sequence-set
;; <sequence-set> from [IMAP]
search-return-data =/ ret-data-partial / ret-data-addto /
ret-data-removefrom
;; All conform to <search-return-data>, from [IMAP-ABNF]
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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.
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[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
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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.
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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
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