IMAP Extensions Working Group M. Crispin
INTERNET-DRAFT: IMAP SORT K. Murchison
Document: internet-drafts/draft-ietf-imapext-sort-15.txt April 2004
INTERNET MESSAGE ACCESS PROTOCOL - SORT AND THREAD EXTENSIONS
Status of this Memo
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Distribution of this memo is unlimited.
Abstract
This document describes the base-level server-based sorting and
threading extensions to the [IMAP] protocol. These extensions
provide substantial performance improvements for IMAP clients which
offer sorted and threaded views.
1. Introduction
The SORT and THREAD extensions to the [IMAP] protocol provide a means
of server-based sorting and threading of messages, without requiring
that the client download the necessary data to do so itself. This is
particularly useful for online clients as described in [IMAP-MODELS].
A server which supports the base-level SORT extension indicates this
with a capability name which starts with "SORT". Future,
upwards-compatible extensions to the SORT extension will all start
with "SORT", indicating support for this base level.
A server which supports the THREAD extension indicates this with one
or more capability names consisting of "THREAD=" followed by a
supported threading algorithm name as described in this document.
This provides for future upwards-compatible extensions.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "MAY", and "OPTIONAL" in this document are to
be interpreted as described in [KEYWORDS].
The word "can" (not "may") is used to refer to a possible
circumstance or situation, as opposed to an optional facility of the
protocol.
"User" is used to refer to a human user, whereas "client" refers to
the software being run by the user.
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
2.1 Base Subject
Subject sorting and threading use the "base subject," which has
specific subject artifacts removed. Due to the complexity of these
artifacts, the formal syntax for the subject extraction rules is
ambiguous. The following procedure is followed to determine the
actual "base subject" which is used to sort by subject, using the
[ABNF] formal syntax rules described in section 5:
(1) Convert any RFC 2047 encoded-words in the subject to
UTF-8 as described in "internationalization
considerations." Convert all tabs and continuations to
space. Convert all multiple spaces to a single space.
(2) Remove all trailing text of the subject that matches
the subj-trailer ABNF, repeat until no more matches are
possible.
(3) Remove all prefix text of the subject that matches the
subj-leader ABNF.
(4) If there is prefix text of the subject that matches the
subj-blob ABNF, and removing that prefix leaves a non-empty
subj-base, then remove the prefix text.
(5) Repeat (3) and (4) until no matches remain.
Note: it is possible to defer step (2) until step (6), but this
requires checking for subj-trailer in step (4).
(6) If the resulting text begins with the subj-fwd-hdr ABNF
and ends with the subj-fwd-trl ABNF, remove the
subj-fwd-hdr and subj-fwd-trl and repeat from step (2).
(7) The resulting text is the "base subject" used in the
SORT.
All servers and disconnected (as described in [IMAP-MODEL]) clients
MUST use exactly this algorithm when sorting by subject. Otherwise
there is potential for a user to get inconsistent results based on
whether they are running in connected or disconnected mode.
2.2 Sent Date
As used in this document, the term "sent date" refers to the date and
time from the Date: header, adjusted by time zone to normalize to UTC.
For example, "31 Dec 2000 16:01:33 -0800" is equivalent to the UTC
date and time of "1 Jan 2001 00:01:33 +0000".
If the time zone is invalid, the date and time SHOULD be treated as UTC.
If the time is also invalid, the time SHOULD be treated as 00:00:00. If
there is no valid date or time, the date and time SHOULD be treated as
00:00:00 on the earliest possible date.
This differs from the date-related criteria in the SEARCH command
(described in [IMAP] section 6.4.4), which use just the date and not
the time, and are not adjusted by time zone.
3. Additional Commands
These commands are extension to the [IMAP] base protocol.
The section headings are intended to correspond with where they would
be located in the main document if they were part of the base
specification.
BASE.6.4.SORT. SORT Command
Arguments: sort program
charset specification
searching criteria (one or more)
Data: untagged responses: SORT
Result: OK - sort completed
NO - sort error: can't sort that charset or
criteria
BAD - command unknown or arguments invalid
The SORT command is a variant of SEARCH with sorting semantics for
the results. Sort has two arguments before the searching criteria
argument; a parenthesized list of sort criteria, and the searching
charset.
The charset argument is mandatory (unlike SEARCH) and indicates
the [CHARSET] of the strings that appear in the searching criteria.
The US-ASCII and UTF-8 charsets MUST be implemented. All other
charsets are optional.
There is also a UID SORT command which returns unique identifiers
instead of message sequence numbers. Note that there are separate
searching criteria for message sequence numbers and UIDs; thus the
arguments to UID SORT are interpreted the same as in SORT. This is
analogous to the behavior of UID SEARCH, as opposed to UID COPY, UID
FETCH, or UID STORE.
The SORT command first searches the mailbox for messages that
match the given searching criteria using the charset argument for
the interpretation of strings in the searching criteria. It then
returns the matching messages in an untagged SORT response, sorted
according to one or more sort criteria.
Sorting is in ascending order. Earlier dates sort before later
dates; smaller sizes sort before larger sizes; and strings are
sorted according to ascending values established by their
collation algorithm (see under "Internationalization
Considerations").
If two or more messages exactly match according to the sorting
criteria, these messages are sorted according to the order in
which they appear in the mailbox. In other words, there is an
implicit sort criterion of "sequence number".
When multiple sort criteria are specified, the result is sorted in
the priority order that the criteria appear. For example,
(SUBJECT DATE) will sort messages in order by their base subject
text; and for messages with the same base subject text will sort
by their sent date.
Untagged EXPUNGE responses are not permitted while the server is
responding to a SORT command, but are permitted during a UID SORT
command.
The defined sort criteria are as follows. Refer to the Formal
Syntax section for the precise syntactic definitions of the
arguments. If the associated RFC-822 header for a particular
criterion is absent, it is treated as the empty string. The empty
string always collates before non-empty strings.
ARRIVAL
Internal date and time of the message. This differs from the
ON criteria in SEARCH, which uses just the internal date.
CC
RFC-822 local-part of the first "cc" address.
DATE
Sent date and time from the Date: header, adjusted by time
zone. This differs from the SENTON criteria in SEARCH, which
uses just the date and not the time, nor adjusts by time zone.
FROM
RFC-822 local-part of the first "From" address.
REVERSE
Followed by another sort criterion, has the effect of that
criterion but in reverse (descending) order.
Note: REVERSE only reverses a single criterion, and does not
affect the implicit "sequence number" sort criterion if all
other criteria are identicial. Consequently, a sort of
REVERSE SUBJECT is not the same as a reverse ordering of a
SUBJECT sort. This can be avoided by use of additional
criteria, e.g. SUBJECT DATE vs. REVERSE SUBJECT REVERSE
DATE. In general, however, it's better (and faster, if the
client has a "reverse current ordering" command) to reverse
the results in the client instead of issuing a new SORT.
SIZE
Size of the message in octets.
SUBJECT
Base subject text.
TO
RFC-822 local-part of the first "To" address.
Example: C: A282 SORT (SUBJECT) UTF-8 SINCE 1-Feb-1994
S: * SORT 2 84 882
S: A282 OK SORT completed
C: A283 SORT (SUBJECT REVERSE DATE) UTF-8 ALL
S: * SORT 5 3 4 1 2
S: A283 OK SORT completed
C: A284 SORT (SUBJECT) US-ASCII TEXT "not in mailbox"
S: * SORT
S: A284 OK SORT completed
BASE.6.4.THREAD. THREAD Command
Arguments: threading algorithm
charset specification
searching criteria (one or more)
Data: untagged responses: THREAD
Result: OK - thread completed
NO - thread error: can't thread that charset or
criteria
BAD - command unknown or arguments invalid
The THREAD command is a variant of SEARCH with threading semantics
for the results. Thread has two arguments before the searching
criteria argument; a threading algorithm, and the searching
charset.
The charset argument is mandatory (unlike SEARCH) and indicates
the [CHARSET] of the strings that appear in the searching criteria.
The US-ASCII and UTF-8 charsets MUST be implemented. All other
charsets are optional.
There is also a UID THREAD command which returns unique identifiers
instead of message sequence numbers. Note that there are separate
searching criteria for message sequence numbers and UIDs; thus the
arguments to UID THREAD are interpreted the same as in THREAD. This is
analogous to the behavior of UID SEARCH, as opposed to UID COPY, UID
FETCH, or UID STORE.
The THREAD command first searches the mailbox for messages that
match the given searching criteria using the charset argument for
the interpretation of strings in the searching criteria. It then
returns the matching messages in an untagged THREAD response,
threaded according to the specified threading algorithm.
All collation is in ascending order. Earlier dates collate before
later dates and strings are collated according to ascending values
established by their collation algorithm (see under
"Internationalization Considerations").
The defined threading algorithms are as follows:
ORDEREDSUBJECT
The ORDEREDSUBJECT threading algorithm is also referred to as
"poor man's threading." The searched messages are sorted by
base subject and then by the sent date. The messages are then
split into separate threads, with each thread containing
messages with the same base subject text. Finally, the threads
are sorted by the sent date of the first message in the thread.
The first message of each thread are siblings of each other
(the "root"). The second message of a thread is the child of
the first message, and subsequent messages of the thread are
siblings of the second message and hence children of the
message at the root. Hence, there are no grandchildren in
ORDEREDSUBJECT threading.
Note: early drafts of this specification specified
that each message in an ORDEREDSUBJECT thread is a child
(as opposed to a sibling) of the previous message. This
is now deprecated. For compatibility with servers which
may still use the old definition, client implementations
SHOULD treat descendents of a child as being siblings of
that child.
This is because the old definition mistakenly indicated
that there was a parent/child relationship between
successive messages in a thread; when in fact there was
only a chronological relationship. In clients which
indicate parent/child relationships in a thread tree,
this would indicate levels of descent which did not
exist.
REFERENCES
The REFERENCES threading algorithm is based on the [THREADING]
algorithm written used in "Netscape Mail and News" versions 2.0
through 3.0. This algorithm threads the searched messages by
grouping them together in parent/child relationships based on
which messages are replies to others. The parent/child
relationships are built using two methods: reconstructing a
message's ancestry using the references contained within it;
and checking the original (not base) subject of a message to
see if it is a reply to (or forward of) another message.
Note: "Message ID" in the following description refers to a
normalized form of the msg-id in [RFC 2822]. The actual
text in an RFC 2822 may use quoting, resulting in multiple
ways of expressing the same Message ID. Implementations of
the REFERENCES threading algorithm MUST normalize any msg-id
in order to avoid false non-matches due to differences in
quoting.
For example, the msg-id
<"01KF8JCEOCBS0045PS"@xxx.yyy.com>
and the msg-id
<01KF8JCEOCBS0045PS@xxx.yyy.com>
MUST be interpreted as being the same Message ID.
The references used for reconstructing a message's ancestry are
found using the following rules:
If a message contains a References header line, then use the
Message IDs in the References header line as the references.
If a message does not contain a References header line, or
the References header line does not contain any valid
Message IDs, then use the first (if any) valid Message ID
found in the In-Reply-To header line as the only reference
(parent) for this message.
Note: Although [RFC 2822] permits multiple Message IDs in
the In-Reply-To header, in actual practice this
discipline has not been followed. For example,
In-Reply-To headers have been observed with message
addresses after the Message ID, and there are no good
heuristics for software to determine the difference.
This is not a problem with the References header however.
If a message does not contain an In-Reply-To header line, or
the In-Reply-To header line does not contain a valid Message
ID, then the message does not have any references (NIL).
A message is considered to be a reply or forward if the base
subject extraction rules, applied to the original subject,
remove any of the following: a subj-refwd, a "(fwd)"
subj-trailer, or a subj-fwd-hdr and subj-fwd-trl.
The REFERENCES algorithm is significantly more complex than
ORDEREDSUBJECT and consists of six main steps. These steps are
outlined in detail below.
(1) For each searched message:
(A) Using the Message IDs in the message's references, link
the corresponding messages (those whose Message-ID header
line contains the given reference Message ID) together as
parent/child. Make the first reference the parent of the
second (and the second a child of the first), the second the
parent of the third (and the third a child of the second),
etc. The following rules govern the creation of these
links:
If a message does not contain a Message-ID header line,
or the Message-ID header line does not contain a valid
Message ID, then assign a unique Message ID to this
message.
If two or more messages have the same Message ID, then
only use that Message ID in the first (lowest sequence
number) message, and assign a unique Message ID to each
of the subsequent messages with a duplicate of that
Message ID.
If no message can be found with a given Message ID,
create a dummy message with this ID. Use this dummy
message for all subsequent references to this ID.
If a message already has a parent, don't change the
existing link. This is done because the References
header line may have been truncated by a MUA. As a
result, there is no guarantee that the messages
corresponding to adjacent Message IDs in the References
header line are parent and child.
Do not create a parent/child link if creating that link
would introduce a loop. For example, before making
message A the parent of B, make sure that A is not a
descendent of B.
Note: Message ID comparisons are case-sensitive.
(B) Create a parent/child link between the last reference
(or NIL if there are no references) and the current message.
If the current message already has a parent, it is probably
the result of a truncated References header line, so break
the current parent/child link before creating the new
correct one. As in step 1.A, do not create the parent/child
link if creating that link would introduce a loop. Note
that if this message has no references, that it will now
have no parent.
Note: The parent/child links created in steps 1.A and 1.B
MUST be kept consistent with one another at ALL times.
(2) Gather together all of the messages that have no parents
and make them all children (siblings of one another) of a dummy
parent (the "root"). These messages constitute the first
(head) message of the threads created thus far.
(3) Prune dummy messages from the thread tree. Traverse each
thread under the root, and for each message:
If it is a dummy message with NO children, delete it.
If it is a dummy message with children, delete it, but
promote its children to the current level. In other words,
splice them in with the dummy's siblings.
Do not promote the children if doing so would make them
children of the root, unless there is only one child.
(4) Sort the messages under the root (top-level siblings only)
by sent date. In the case of an exact match on sent date or if
either of the Date: headers used in a comparison can not be
parsed, use the order in which the messages appear in the
mailbox (that is, by sequence number) to determine the order.
In the case of a dummy message, sort its children by sent date
and then use the first child for the top-level sort.
(5) Gather together messages under the root that have the same
base subject text.
(A) Create a table for associating base subjects with
messages, called the subject table.
(B) Populate the subject table with one message per each
base subject. For each child of the root:
(i) Find the subject of this thread, by using the base
subject from either the current message or its first
child if the current message is a dummy. This is the
thread subject.
(ii) If the thread subject is empty, skip this message.
(iii) Look up the message associated with the thread
subject in the subject table.
(iv) If there is no message in the subject table with the
thread subject, add the current message and the thread
subject to the subject table.
Otherwise, if the message in the subject table is not a
dummy, AND either of the following criteria are true:
The current message is a dummy, OR
The message in the subject table is a reply or forward
and the current message is not.
then replace the message in the subject table with the
current message.
(C) Merge threads with the same thread subject. For each
child of the root:
(i) Find the message's thread subject as in step 5.B.i
above.
(ii) If the thread subject is empty, skip this message.
(iii) Lookup the message associated with this thread
subject in the subject table.
(iv) If the message in the subject table is the current
message, skip this message.
Otherwise, merge the current message with the one in the
subject table using the following rules:
If both messages are dummies, append the current
message's children to the children of the message in
the subject table (the children of both messages
become siblings), and then delete the current message.
If the message in the subject table is a dummy and the
current message is not, make the current message a
child of the message in the subject table (a sibling
of its children).
If the current message is a reply or forward and the
message in the subject table is not, make the current
message a child of the message in the subject table (a
sibling of its children).
Otherwise, create a new dummy message and make both
the current message and the message in the subject
table children of the dummy. Then replace the message
in the subject table with the dummy message.
Note: Subject comparisons are case-insensitive, as
described under "Internationalization
Considerations."
(6) Traverse the messages under the root and sort each set of
siblings by sent date. Traverse the messages in such a way
that the "youngest" set of siblings are sorted first, and the
"oldest" set of siblings are sorted last (grandchildren are
sorted before children, etc). In the case of an exact match on
sent date or if either of the Date: headers used in a
comparison can not be parsed, use the order in which the
messages appear in the mailbox (that is, by sequence number) to
determine the order. In the case of a dummy message (which can
only occur with top-level siblings), use its first child for
sorting.
Example: C: A283 THREAD ORDEREDSUBJECT UTF-8 SINCE 5-MAR-2000
S: * THREAD (166)(167)(168)(169)(172)(170)(171)
(173)(174 (175)(176)(178)(181)(180))(179)(177
(183)(182)(188)(184)(185)(186)(187)(189))(190)
(191)(192)(193)(194 195)(196 (197)(198))(199)
(200 202)(201)(203)(204)(205)(206 207)(208)
S: A283 OK THREAD completed
C: A284 THREAD ORDEREDSUBJECT US-ASCII TEXT "gewp"
S: * THREAD
S: A284 OK THREAD completed
C: A285 THREAD REFERENCES UTF-8 SINCE 5-MAR-2000
S: * THREAD (166)(167)(168)(169)(172)((170)(179))
(171)(173)((174)(175)(176)(178)(181)(180))
((177)(183)(182)(188 (184)(189))(185 186)(187))
(190)(191)(192)(193)((194)(195 196))(197 198)
(199)(200 202)(201)(203)(204)(205 206 207)(208)
S: A285 OK THREAD completed
Note: The line breaks in the first and third client
responses are for editorial clarity and do not appear in
real THREAD responses.
4. Additional Responses
These responses are extensions to the [IMAP] base protocol.
The section headings of these responses are intended to correspond
with where they would be located in the main document.
BASE.7.2.SORT. SORT Response
Data: zero or more numbers
The SORT response occurs as a result of a SORT or UID SORT
command. The number(s) refer to those messages that match the
search criteria. For SORT, these are message sequence numbers;
for UID SORT, these are unique identifiers. Each number is
delimited by a space.
Example: S: * SORT 2 3 6
BASE.7.2.THREAD. THREAD Response
Data: zero or more threads
The THREAD response occurs as a result of a THREAD or UID THREAD
command. It contains zero or more threads. A thread consists of
a parenthesized list of thread members.
Thread members consist of zero or more message numbers, delimited
by spaces, indicating successive parent and child. This continues
until the thread splits into multiple sub-threads, at which point
the thread nests into multiple sub-threads with the first member
of each subthread being siblings at this level. There is no limit
to the nesting of threads.
The messages numbers refer to those messages that match the search
criteria. For THREAD, these are message sequence numbers; for UID
THREAD, these are unique identifiers.
Example: S: * THREAD (2)(3 6 (4 23)(44 7 96))
The first thread consists only of message 2. The second thread
consists of the messages 3 (parent) and 6 (child), after which it
splits into two subthreads; the first of which contains messages 4
(child of 6, sibling of 44) and 23 (child of 4), and the second of
which contains messages 44 (child of 6, sibling of 4), 7 (child of
44), and 96 (child of 7). Since some later messages are parents
of earlier messages, the messages were probably moved from some
other mailbox at different times.
-- 2
-- 3
\-- 6
|-- 4
| \-- 23
|
\-- 44
\-- 7
\-- 96
Example: S: * THREAD ((3)(5))
In this example, 3 and 5 are siblings of a parent which does not
match the search criteria (and/or does not exist in the mailbox);
however they are members of the same thread.
5. Formal Syntax of SORT and THREAD Commands and Responses
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [ABNF]. It also uses [ABNF]
rules defined in [IMAP].
sort = ["UID" SP] "SORT" SP sort-criteria SP search-criteria
sort-criteria = "(" sort-criterion *(SP sort-criterion) ")"
sort-criterion = ["REVERSE" SP] sort-key
sort-key = "ARRIVAL" / "CC" / "DATE" / "FROM" / "SIZE" /
"SUBJECT" / "TO"
thread = ["UID" SP] "THREAD" SP thread-alg SP search-criteria
thread-alg = "ORDEREDSUBJECT" / "REFERENCES" / thread-alg-ext
thread-alg-ext = atom
; New algorithms MUST be registered with IANA
search-criteria = charset 1*(SP search-key)
charset = astring
; CHARSET values MUST be registered with IANA
sort-data = "SORT" *(SP nz-number)
thread-data = "THREAD" [SP 1*thread-list]
thread-list = "(" (thread-members / thread-nested) ")"
thread-members = nz-number *(SP nz-number) [SP thread-nested]
thread-nested = 2*thread-list
The following syntax describes base subject extraction rules (2)-(6):
subject = *subj-leader [subj-middle] *subj-trailer
subj-refwd = ("re" / ("fw" ["d"])) *WSP [subj-blob] ":"
subj-blob = "[" *BLOBCHAR "]" *WSP
subj-fwd = subj-fwd-hdr subject subj-fwd-trl
subj-fwd-hdr = "[fwd:"
subj-fwd-trl = "]"
subj-leader = (*subj-blob subj-refwd) / WSP
subj-middle = *subj-blob (subj-base / subj-fwd)
; last subj-blob is subj-base if subj-base would
; otherwise be empty
subj-trailer = "(fwd)" / WSP
subj-base = NONWSP *(*WSP NONWSP)
; can be a subj-blob
BLOBCHAR = %x01-5a / %x5c / %x5e-7f
; any CHAR except '[' and ']'
NONWSP = %x01-08 / %x0a-1f / %x21-7f
; any CHAR other than WSP
6. Security Considerations
The SORT and THREAD extensions do not raise any security
considerations that are not present in the base [IMAP] protocol, and
these issues are discussed in [IMAP]. Nevertheless, it is important
to remember that [IMAP] protocol transactions, including message
data, are sent in the clear over the network unless protection from
snooping is negotiated, either by the use of STARTTLS, privacy
protection is negotiated in the AUTHENTICATE command, or some other
protection mechanism is in effect.
7. Internationalization Considerations
Strings in charsets other than US-ASCII and UTF-8 must be converted
to UTF-8 prior to any comparisons. String comparisons used in SORT
and THREAD collations are performed as described in [IMAP-I18N].
Translations of the "re" or "fw"/"fwd" tokens are not specified for
removal in the base subject extraction process. An attempt to add such
translated tokens would result in a geometrically complex, and
ultimately unimplementable, task.
Instead, note that [RFC-2822] section 3.6.5 recommends that "re:" (from
the Latin "res", in the matter of) be used to identify a reply.
Although it is evident that, from the multiple forms of token to
identify a forwarded message, there is considerable variation found in
the wild, the variations are (still) manageable. Consequently, it is
suggested that "re:" and one of the variations of the tokens for forward
supported by the base subject extraction rules be adopted for Internet
mail messages, since doing so makes it a simple display time task to
localize the token language for the user.
8. IANA Considerations
[IMAP] capabilities are registered by publishing a standards track or
IESG approved experimental RFC. This document constitutes registration
of the SORT and THREAD capabilities in the [IMAP] capabilities registry.
This document creates a new [IMAP] threading algorithms registry, which
registers threading algorithms by publishing a standards track or IESG
approved experimental RFC. This document constitutes registration of
the ORDEREDSUBJECT and REFERENCES algorithms in that registry.
Appendices
A. Normative References
The following documents are normative to this document:
[ABNF] Crocker, D., and Overell, P. "Augmented BNF
for Syntax Specifications: ABNF", RFC 2234,
November 1997.
[CHARSET] Freed, N. and J. Postel, "IANA Character Set
Registration Procedures", RFC 2978, October
2000.
[IMAP] Crispin, M., "Internet Message Access Protocol -
Version 4rev1", RFC 3501, March 2003.
[IMAP-I18N] Newman, C. "Internet Message Access Protocol
Internationalization", Work in Progress.
[KEYWORDS] Bradner, S. "Key words for use in RFCs to
Indicate Requirement Levels", RFC 2119, Harvard
University, March 1997.
[RFC-2822] Resnick, P. "Internet Message Format", RFC 2822,
April 2001.
B. Informative References
The following documents are informative to this document:
[IMAP-MODELS] Crispin, M., "Distributed Electronic Mail Models
in IMAP4", RFC 1733, December 1994.
[THREADING] Zawinski, J. "Message Threading",
http://www.jwz.org/doc/threading.html, 1997-2002.
Author's Address
Mark R. Crispin
Networks and Distributed Computing
University of Washington
4545 15th Avenue NE
Seattle, WA 98105-4527
Phone: (206) 543-5762
EMail: MRC@CAC.Washington.EDU
Kenneth Murchison
Oceana Matrix Ltd.
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