Network Working Group D. Cridland
Request for Comments: 5267 C. King
Category: Standards Track Isode Limited
July 2008 Contexts for IMAP4
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.
The IMAP4rev1 protocol has powerful search facilities as part of the
core protocol, but lacks the ability to create live, updated results
that can be easily handled. This memo provides such an extension,
and shows how it can be used to provide a facility similar to virtual
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.
The SORT command defined in [SORT] provides useful features, but is
hard to use effectively on changing mailboxes over low-bandwidth
This memo borrows concepts from [ACAP], such as providing a windowed
view onto search or sort results, and making updates that are
bandwidth and round-trip efficient. These are provided by two
extensions: "ESORT" and "CONTEXT".
2. Conventions Used in This Document
In examples, "C:" and "S:" indicate lines sent by the client
messaging user agent and IMAP4rev1 ([IMAP]) server, respectively.
"//" indicates inline comments not part of the protocol exchange.
Line breaks are liberally inserted for clarity. Examples are
intended to be read in order, such that the state remains from one
example to the next.
Although the examples show a server that supports [ESEARCH], this is
not a strict 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 capitalized words are typically names of IMAP extensions or
commands -- these are uppercased for clarity only, and are case-
3. Extended Sort Syntax
Servers implementing the extended SORT provide a suite of extensions
to the SORT and UID SORT commands defined in [SORT]. This allows for
return options, as used with SEARCH and specified in [IMAP-ABNF], to
be used with SORT in a similar manner.
The SORT and UID SORT commands are extended by the addition of an
optional list of return options that follow a RETURN atom immediately
after the command. If this is missing, the server will return
results as specified in [SORT].
The extended SORT command always returns results in the requested
sort order, but is otherwise identical in its behaviour to the
extended SEARCH command defined in [IMAP-ABNF], as extended by
[ESEARCH]. In particular, the extended SORT command returns results
in an ESEARCH response.
3.1. ESORT Extension
Servers advertising the capability "ESORT" support the return options
specified in [ESEARCH] in the SORT command. These return options are
adapted as follows:
Return the message number/UID of the lowest sorted message
satisfying the search criteria.
Return the message number/UID of the highest sorted message
satisfying the search criteria.
Return all message numbers/UIDs which match the search criteria,
in the requested sort order, using a sequence-set. Note the use
of ranges described below in Section 3.2.
As in [ESEARCH].
3.2. Ranges in Extended Sort Results
Any ranges given by the server, including those given as part of the
sequence-set, in an ESEARCH response resulting from an extended SORT
or UID SORT command, MUST be ordered in increasing numerical order
after expansion, as per usual [IMAP] rules.
In particular this means that 10:12 is equivalent to 12:10, and
10,11,12. To avoid confusion, servers SHOULD present ranges only
when the first seq-number is lower than the second; that is, either
of the forms 10:12 or 10,11,12 is acceptable, but 12:10 SHOULD be
3.3. Extended SORT Example
If the list of return options is present but empty, then the server
provides the ALL return data item in an ESEARCH response. This is
functionally equivalent to an unextended UID SORT command, but can
use a smaller representation:
C: E01 UID SORT RETURN () (REVERSE DATE) UTF-8 UNDELETED
S: * ESEARCH (TAG "E01") UID ALL 23765,23764,23763,23761,[...]
S: E01 OK Sort completed
Note that the initial three results are not represented as the range
23765:23763 as mandated in Section 3.2.
The Contexts extension is present in any IMAP4rev1 server that
includes the string "CONTEXT=SEARCH", and/or "CONTEXT=SORT", within
its advertised capabilities.
In the case of CONTEXT=SEARCH, the server supports the extended
SEARCH command syntax described in [IMAP-ABNF], and accepts three
additional return options.
Servers advertising CONTEXT=SORT also advertise the SORT capability,
as described in [SORT], support the extended SORT command syntax
described in Section 3, and accept three additional return options
for this extended SORT.
These additional return options allow for notifications of changes to
the results of SEARCH or SORT commands, and also allow for access to
A server advertising the CONTEXT=SEARCH extension will order all
SEARCH results, whether from a UID SEARCH or SEARCH command, in
mailbox order -- that is, by message number and UID. Therefore, the
UID SEARCH, SEARCH, UID SORT, or SORT command used -- collectively
known as the searching command -- will always have an order, the
requested order, which will be the mailbox order for UID SEARCH and
All of the return specifiers have no interaction with either each
other or any return specifiers defined in [ESEARCH] or Section 3.1;
however, it is believed that implementations supporting CONTEXT will
also support ESEARCH and ESORT.
4.2. Context Hint
The return option CONTEXT SHOULD be used by a client to indicate that
subsequent use of the search criteria are likely. Servers MAY ignore
this return option or use it as a hint to maintain a full result
cache, or index.
A client might choose to obtain a count of matching messages prior to
obtaining actual results. Here, the client signals its intention to
fetch the results themselves:
C: A01 SEARCH RETURN (CONTEXT COUNT) UNDELETED
S: * ESEARCH (TAG "A01") COUNT 23765
S: A01 OK Search completed.
4.3. Notifications of Changes
The search return option UPDATE, if used by a client, causes the
server to issue unsolicited notifications containing updates to the
results that would be returned by an unmodified searching command.
These update sets are carried in ADDTO and REMOVEFROM data items in
These ESEARCH responses carry a search correlator of the searching
command, hence clients MUST NOT reuse tags, as already specified in
Section 2.2.1 of [IMAP]. An attempt to use UPDATE where a tag is
already in use with a previous searching command that itself used
UPDATE SHALL result in the server rejecting the searching command
with a BAD response.
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 when the mailbox is no longer selected, or when
the CANCELUPDATE command, defined in Section 4.3.5, is issued by the
client, whichever is sooner.
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 searching
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 could receive notifications about messages that do not
currently interest it.
Finally, as specified in the errata to [IMAP], any message sequence
numbers used in the search program are evaluated at the time the
command is received; therefore, if the messages referred to by such
message sequence numbers change, no notifications will be emitted.
This time, the client will require notifications of updates and
chooses to obtain a count:
C: B01 UID SEARCH RETURN (UPDATE COUNT) DELETED
S: * ESEARCH (TAG "B01") COUNT 74
S: B01 OK Search completed, will notify.
// Note that the following is rejected, and has no effect:
C: B01 SORT RETURN (UPDATE) FLAGGED
S: B01 BAD Tag reuse
4.3.1. 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
such a 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 SHALL 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. Since sorted contexts
require a higher implementation cost than unsorted contexts, refusal
to provide updates for a SORT command does not imply that SEARCH
contexts will also be refused.
This time, the client will require notifications of updates, and
chooses to obtain a count:
C: B02 UID SORT RETURN (UPDATE COUNT) UTF-8
S: * ESEARCH (TAG "B02") COUNT 74
S: * NO [NOUPDATE "B02"] Too many contexts
S: B02 OK Search completed, will not notify.
Client handling might be to retry with a UID SEARCH command, or else
cancel an existing context; see Section 4.3.5.
4.3.2. 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 or SORT command,
the results will be message numbers; in a UID SEARCH or UID SORT
command, they will be UIDs.
The client MUST process ADDTO and REMOVEFROM return data items in the
order they appear, including those within a single ESEARCH response.
Correspondingly, servers MUST generate ADDTO and REMOVEFROM responses
such that the results are maintained in the requested order.
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].
It is anticipated that typical server implementations will emit ADDTO
when they normally emit the causal FETCH or EXISTS, and similarly
emit REMOVEFROM when they normally emit the causal FETCH or EXPUNGE.
4.3.3. ADDTO Return Data Item
The ADDTO return data item contains, as payload, a list containing
pairs of a context position and a set of result updates in the
requested order to be inserted at the context position. Where the
searching command is a SEARCH or UID SEARCH command, the context
position MAY be zero. Each pair is processed in the order that it
Note that an ADDTO containing message sequence numbers added as a
result of those messages being delivered or appended MUST be sent
after the EXISTS notification itself, in order that those sequence
numbers are valid.
If the context position is non-zero, the result update is inserted at
the given context position, meaning that the first result in the set
will occupy the new context position after insertion, and any prior
existing result at that context position will be shifted to a later
Where the context position is zero, the client MAY insert the message
numbers or UIDs in the result list such that the result list is
maintained in mailbox order. In this case, servers are RECOMMENDED
to order the result update into mailbox order to produce the shortest
representation in set-syntax.
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.
If the searching command "C01" initially generated a result list of
2734:2735, then the following three responses are equivalent, and
yield a result list of 2731:2735:
S: * ESEARCH (TAG "C01") UID ADDTO (1 2733 1 2732 1 2731)
S: * ESEARCH (TAG "C01") UID ADDTO (1 2733) ADDTO (1 2731:2732)
S: * ESEARCH (TAG "C01") UID ADDTO (1 2731:2733)
The last is the preferred representation.
4.3.4. REMOVEFROM Return Data Item
The REMOVEFROM return data item contains, as payload, a list
containing pairs of a context position and a set of result updates in
the requested order to be removed starting from the context position.
Where the searching command is a SEARCH or UID SEARCH command, the
context position MAY be zero. Each pair is processed in the order
that it appears.
If the context position is non-zero, the results are removed at the
given context position, meaning that the first result in the set will
occupy the given context position before removal, and any prior
existing result at that context position will be shifted to an
earlier context position.
Where the context position is zero, the client removes the message
numbers or UIDs in the result list wherever they occur, and servers
are RECOMMENDED to order the result list in mailbox order to obtain
the best benefit from the set-syntax.
Note that a REMOVEFROM containing message sequence numbers removed as
a result of those messages being expunged MUST be sent prior to the
expunge notification itself, in order that those sequence numbers
Here, a message in the result list is expunged. The REMOVEFROM is
shown to happen without any command in progress; see Section 4.3.2.
Note that EXPUNGE responses do not have this property.
S: * ESEARCH (TAG "B01") UID REMOVEFROM (0 32768)
C: B03 NOOP
S: * 23762 EXPUNGE
S: B03 OK Nothing done.
4.3.5. The CANCELUPDATE Command
When a client no longer wishes to receive updates, it may issue the
CANCELUPDATE 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
The server MAY free any resource associated with a context so
disabled -- however, the client is free to issue further searching
commands with the same criteria and requested order, including
C: B04 CANCELUPDATE "B01"
S: B04 OK No further updates.
4.4. Partial Results
The PARTIAL search return option causes the server to provide in an
ESEARCH response a subset of 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.
A single command MUST NOT contain more than one PARTIAL or ALL search
return option -- that is, either one PARTIAL, one ALL, or neither
PARTIAL nor ALL is allowed.
For SEARCH results, the entire result list MUST be ordered in mailbox
order, that is, in UID or message sequence number order.
Where a PARTIAL search return option references results that do not
exist, by using a range which starts or ends higher than the current
number of results, then the server returns the results that are in
the set. This yields a PARTIAL return data item that has, as
payload, the original range and a potentially missing set of results
that may be shorter than the extent of the range.
Clients need not request PARTIAL results in any particular order.
Because mailboxes may change, clients will often wish to use PARTIAL
in combination with UPDATE, especially if the intent is to walk a
large set of results; however, these return options do not interact
-- the UPDATE will provide notifications for all matching results.
// Recall from A01 that there are 23764 results.
C: A02 UID SEARCH RETURN (PARTIAL 23500:24000) UNDELETED
C: A03 UID SEARCH RETURN (PARTIAL 1:500) UNDELETED
C: A04 UID SEARCH RETURN (PARTIAL 24000:24500) UNDELETED
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.
4.5. 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.
5. Formal Syntax
The collected formal syntax. This uses ABNF as defined in [ABNF].
It includes definitions from [IMAP], [IMAP-ABNF], and [SORT].
capability =/ "CONTEXT=SEARCH" / "CONTEXT=SORT" / "ESORT"
;; <capability> from [IMAP]
command-select =/ "CANCELUPDATE" 1*(SP quoted)
;; <command-select> from [IMAP]
context-position = number
;; Context position may be 0 for SEARCH result additions.
;; <number> from [IMAP]
modifier-context = "CONTEXT"
modifier-partial = "PARTIAL" SP partial-range
partial-range = nz-number ":" nz-number
;; A range 500:400 is the same as 400:500.
;; This is similar to <seq-range> from [IMAP],
;; but cannot contain "*".
modifier-update = "UPDATE"
search-return-opt =/ modifier-context / modifier-partial /
;; 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 "(" context-position SP sequence-set
*(SP context-position SP sequence-set)
;; <sequence-set> from [IMAP]
ret-data-partial = "PARTIAL"
SP "(" partial-range SP partial-results ")"
;; <partial-range> is the requested range.
partial-results = sequence-set / "NIL"
;; <sequence-set> from [IMAP]
;; NIL indicates no results correspond to the requested range.
ret-data-removefrom = "REMOVEFROM"
SP "(" context-position SP sequence-set
*(SP context-position SP sequence-set)
;; <sequence-set> from [IMAP]
search-return-data =/ ret-data-partial / ret-data-addto /
;; All conform to <search-return-data>, from [IMAP-ABNF]
sort =/ extended-sort
;; <sort> from [SORT]
extended-sort = ["UID" SP] "SORT" search-return-opts
SP sort-criteria SP search-criteria
;; <search-return-opts> from [IMAP-ABNF]
;; <sort-criteria> and <search-criteria> from [SORT]
6. Security Considerations
This document defines additional IMAP4 capabilities. As such, it
does not change the underlying security considerations of [IMAP].
The authors and reviewers believe that no new security issues are
introduced with these additional IMAP4 capabilities.
Creation of a large number of contexts may provide an avenue for
denial-of-service attacks by authorized users. Implementors may
reduce this by limiting the number of contexts possible to create,
via the protocol features described in Section 4.3.1; by reducing the
impact of contexts by the implementation strategies described in
Appendix B; and by logging context creation and usage so that
administrative remedies may be applied.
7. IANA Considerations
IMAP4 capabilities are registered by publishing a Standards Track or
IESG-approved Experimental RFC.
This document defines the ESORT, CONTEXT=SEARCH, and CONTEXT=SORT
IMAP capabilities. IANA has added them to the registry accordingly.
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,
Peter Coates, Philip Van Hoof, Randall Gellens, Timo Sirainen, 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.
9.1. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[ESEARCH] Melnikov, A. and D. Cridland, "IMAP4 Extension to SEARCH
Command for Controlling What Kind of Information Is
Returned", RFC 4731, November 2006.
[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[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.
[SORT] Crispin, M. and K. Murchison, "Internet Message Access
Protocol - SORT and THREAD Extensions", RFC 5256,
9.2. Informative References
[ACAP] Newman, C. and J. Myers, "ACAP -- Application
Configuration Access Protocol", RFC 2244, November 1997.
[IDLE] Leiba, B., "IMAP4 IDLE command", RFC 2177, June 1997.
[NOTIFY] Melnikov, A., Gulbrandsen, A., and C. King, "The IMAP
NOTIFY Extension", Work in Progress, March 2008.
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 RETURN (COUNT
UPDATE CONTEXT), and using RETURN (PARTIAL x:y) as the user scrolls,
feeding the results into a FETCH as required to populate summary
A typically useful view is "UID SORT (DATE) RETURN (...) UTF-8
UNSEEN UNDELETED", which can be used to show the mailbox sorted into
INTERNALDATE order, filtered to only show messages which are unread
and not yet deleted.
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
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 that 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
behavior without losing the benefit of sequence numbering.
A.4. Monitoring Counts
A client need not maintain any result cache at all, but instead it
can maintain a simple count of messages matching the search criteria.
Typically, this would use the SEARCH command, as opposed to UID
SEARCH, due to its smaller representation. Such usage might prove
useful in monitoring the number of flagged, but unanswered, messages,
for example, with "SEARCH RETURN (UPDATE COUNT) FLAGGED UNANSWERED".
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
Appendix B. Server Implementation Notes
Although a server may cache the results, this is neither mandated nor
required, especially when the client uses SEARCH or UID SEARCH
commands. 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
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.
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.
Note that in the absence of a server-side results cache, it may be
impossible to know if an expunged message previously matched unless
the original message is still available. Therefore, some
implementations may be forced into using a results cache in many
UPDATE contexts created with SORT or UID SORT will almost certainly
require some form of results caching, however.
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