Network Working Group M. Crispin
Request for Comments: 5256 Panda Programming
Category: Standards Track K. Murchison
Carnegie Mellon University
June 2008
Internet Message Access Protocol - SORT and THREAD Extensions
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.
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 that 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 that 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 that 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.
A server that implements the SORT and/or THREAD extensions MUST
collate strings in accordance with the requirements of I18NLEVEL=1,
as described in [IMAP-I18N], and SHOULD implement and advertise the
I18NLEVEL=1 extension. Alternatively, a server MAY implement
I18NLEVEL=2 (or higher) and comply with the rules of that level.
Crispin & Murchison Standards Track [Page 1]
RFC 5256 IMAP Sort June 2008
Discussion: The SORT and THREAD extensions predate [IMAP-I18N] by
several years. At the time of this writing, all known server
implementations of SORT and THREAD comply with the rules of
I18NLEVEL=1, but do not necessarily advertise it. As discussed in
[IMAP-I18N] section 4.5, all server implementations should
eventually be updated to comply with the I18NLEVEL=2 extension.
Historical note: The REFERENCES threading algorithm is based on the
[THREADING] algorithm written and used in "Netscape Mail and News"
versions 2.0 through 3.0.
2. Terminology
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].
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
"base 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.
Crispin & Murchison Standards Track [Page 2]
RFC 5256 IMAP Sort June 2008
(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-MODELS]) clients
MUST use exactly this algorithm to determine the "base 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.
If the sent date cannot be determined (a Date: header is missing or
cannot be parsed), the INTERNALDATE for that message is used as the
sent date.
When comparing two sent dates that match exactly, the order in which
the two messages appear in the mailbox (that is, by sequence number)
is used as a tie-breaker to determine the order.
Crispin & Murchison Standards Track [Page 3]
RFC 5256 IMAP Sort June 2008
3. Additional Commands
These commands are extensions 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. There are 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 that 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 "Internationalization Considerations").
Crispin & Murchison Standards Track [Page 4]
RFC 5256 IMAP Sort June 2008
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, it 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
[IMAP] addr-mailbox of the first "cc" address.
DATE
Sent date and time, as described in section 2.2.
FROM
[IMAP] addr-mailbox 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 identical. 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.
Crispin & Murchison Standards Track [Page 5]
RFC 5256 IMAP Sort June 2008
SIZE
Size of the message in octets.
SUBJECT
Base subject text.
TO
[IMAP] addr-mailbox 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 that 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.
Crispin & Murchison Standards Track [Page 6]
RFC 5256 IMAP Sort June 2008
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
"Internationalization Considerations").
Untagged EXPUNGE responses are not permitted while the server is
responding to a THREAD command, but are permitted during a UID
THREAD command.
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 top level or "root" in ORDEREDSUBJECT threading contains
the first message of every thread. All messages in the root
are siblings of each other. 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.
Children in ORDEREDSUBJECT threading do not have descendents.
Client implementations SHOULD treat descendents of a child in a
server response as being siblings of that child.
REFERENCES
The REFERENCES threading 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.
Crispin & Murchison Standards Track [Page 7]
RFC 5256 IMAP Sort June 2008
Note: "Message ID" in the following description refers to a
normalized form of the msg-id in [RFC2822]. The actual text
in 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 [RFC2822] 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.
Crispin & Murchison Standards Track [Page 8]
RFC 5256 IMAP Sort June 2008
(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 Mail User
Agent (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,
Crispin & Murchison Standards Track [Page 9]
RFC 5256 IMAP Sort June 2008
do not create the parent/child link if creating that
link would introduce a loop. Note that if this message
has no references, 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 as described in section 2.2. 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.
Crispin & Murchison Standards Track [Page 10]
RFC 5256 IMAP Sort June 2008
(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).
Crispin & Murchison Standards Track [Page 11]
RFC 5256 IMAP Sort June 2008
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 as described in section 2.2.
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 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 server
responses are for editorial clarity and do not appear in
real THREAD responses.
Crispin & Murchison Standards Track [Page 12]
RFC 5256 IMAP Sort June 2008
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 sub-thread 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 sub-threads; 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.
Crispin & Murchison Standards Track [Page 13]
RFC 5256 IMAP Sort June 2008
-- 2
-- 3
\-- 6
|-- 4
| \-- 23
|
\-- 44
\-- 7
\-- 96
Example: S: * THREAD ((3)(5))
In this example, 3 and 5 are siblings of a parent that 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 = atom / quoted
; CHARSET values MUST be registered with IANA
sort-data = "SORT" *(SP nz-number)
thread-data = "THREAD" [SP 1*thread-list]
Crispin & Murchison Standards Track [Page 14]
RFC 5256 IMAP Sort June 2008
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-ff
; any CHAR8 except '[' and ']'.
; SHOULD comply with [UTF-8]
NONWSP = %x01-08 / %x0a-1f / %x21-ff
; any CHAR8 other than WSP.
; SHOULD comply with [UTF-8]
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 in AUTHENTICATE, or some other protection mechanism.
Crispin & Murchison Standards Track [Page 15]
RFC 5256 IMAP Sort June 2008
Although not a security consideration, it is important to recognize
that sorting by REFERENCES can lead to misleading threading trees.
For example, a message with false References: header data will cause
a thread to be incorporated into another thread.
The process of extracting the base subject may lead to incorrect
collation if the extracted data was significant text as opposed to a
subject artifact.
7. Internationalization Considerations
As stated in the introduction, the rules of I18NLEVEL=1 as described
in [IMAP-I18N] MUST be followed; that is, the SORT and THREAD
extensions MUST collate strings according to the i;unicode-casemap
collation described in [UNICASEMAP]. Servers SHOULD also advertise
the I18NLEVEL=1 extension. Alternatively, a server MAY implement
I18NLEVEL=2 (or higher) and comply with the rules of that level.
As discussed in [IMAP-I18N] section 4.5, all server implementations
should eventually be updated to support the [IMAP-I18N] I18NLEVEL=2
extension.
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 [RFC2822] section 3.6.5 recommends that "re:"
(from the Latin "res", meaning "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 a 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.
Crispin & Murchison Standards Track [Page 16]
RFC 5256 IMAP Sort June 2008
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.
9. Normative References
[ABNF] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234, January
2008.
[CHARSET] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.
[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.
[IMAP-I18N] Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet
Message Access Protocol Internationalization", RFC
5255, June 2008.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822,
April 2001.
[UNICASEMAP] Crispin, M., "i;unicode-casemap - Simple Unicode
Collation Algorithm", RFC 5051, October 2007.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
10. Informative References
[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.
Crispin & Murchison Standards Track [Page 17]
RFC 5256 IMAP Sort June 2008
Authors' Addresses
Mark R. Crispin
Panda Programming
6158 NE Lariat Loop
Bainbridge Island, WA 98110-2098
Phone: +1 (206) 842-2385
EMail: IMAP+SORT+THREAD@Lingling.Panda.COM
Kenneth Murchison
Carnegie Mellon University
5000 Forbes Avenue
Cyert Hall 285
Pittsburgh, PA 15213
Phone: +1 (412) 268-2638
EMail: murch@andrew.cmu.edu
Crispin & Murchison Standards Track [Page 18]
RFC 5256 IMAP Sort June 2008
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Crispin & Murchison Standards Track [Page 19]
RFC, FYI, BCP