draft-ietf-imapext-sort-20.txt   rfc5256.txt 
IMAP Extensions Working Group M. Crispin
Internet-Draft K. Murchison
Intended status: Proposed Standard March 10, 2008
Expires: September 10, 2008
Document: internet-drafts/draft-ietf-imapext-sort-20.txt
INTERNET MESSAGE ACCESS PROTOCOL - SORT AND THREAD EXTENSIONS
Status of this Memo
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Internet-Drafts are draft documents valid for a maximum of six months Network Working Group M. Crispin
and may be updated, replaced, or obsoleted by other documents at any Request for Comments: 5256 Panda Programming
time. It is inappropriate to use Internet-Drafts as reference Category: Standards Track K. Murchison
material or to cite them other than as "work in progress." Carnegie Mellon University
June 2008
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Distribution of this memo is unlimited. This document specifies an Internet standards track protocol for the
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Abstract Abstract
This document describes the base-level server-based sorting and This document describes the base-level server-based sorting and
threading extensions to the [IMAP] protocol. These extensions threading extensions to the IMAP protocol. These extensions provide
provide substantial performance improvements for IMAP clients which substantial performance improvements for IMAP clients that offer
offer sorted and threaded views. sorted and threaded views.
1. Introduction 1. Introduction
The SORT and THREAD extensions to the [IMAP] protocol provide a means The SORT and THREAD extensions to the [IMAP] protocol provide a means
of server-based sorting and threading of messages, without requiring of server-based sorting and threading of messages, without requiring
that the client download the necessary data to do so itself. This is that the client download the necessary data to do so itself. This is
particularly useful for online clients as described in [IMAP-MODELS]. particularly useful for online clients as described in [IMAP-MODELS].
A server which supports the base-level SORT extension indicates this A server that supports the base-level SORT extension indicates this
with a capability name which starts with "SORT". Future, with a capability name which starts with "SORT". Future, upwards-
upwards-compatible extensions to the SORT extension will all start compatible extensions to the SORT extension will all start with
with "SORT", indicating support for this base level. "SORT", indicating support for this base level.
A server which supports the THREAD extension indicates this with one A server that supports the THREAD extension indicates this with one
or more capability names consisting of "THREAD=" followed by a or more capability names consisting of "THREAD=" followed by a
supported threading algorithm name as described in this document. supported threading algorithm name as described in this document.
This provides for future upwards-compatible extensions. This provides for future upwards-compatible extensions.
A server which implements the SORT and/or THREAD extensions MUST A server that implements the SORT and/or THREAD extensions MUST
collate strings in accordance with the requirements of I18NLEVEL=1, collate strings in accordance with the requirements of I18NLEVEL=1,
as described in [IMAP-I18N], and SHOULD implement and advertise the as described in [IMAP-I18N], and SHOULD implement and advertise the
I18NLEVEL=1 extension. Alternatively, a server MAY implement I18NLEVEL=1 extension. Alternatively, a server MAY implement
I18NLEVEL=2 (or higher) and comply with the rules of that level. I18NLEVEL=2 (or higher) and comply with the rules of that level.
Discussion: the SORT and THREAD extensions predate [IMAP-I18N] by Discussion: The SORT and THREAD extensions predate [IMAP-I18N] by
several years. At the time of this writing, all known server several years. At the time of this writing, all known server
implementations of SORT and THREAD comply with the rules of implementations of SORT and THREAD comply with the rules of
I18NLEVEL=1, but do not necessarily advertise it. As discussed I18NLEVEL=1, but do not necessarily advertise it. As discussed in
in [IMAP-I18N] section 4.5, all server implementations should [IMAP-I18N] section 4.5, all server implementations should
eventually be updated to comply with the I18NLEVEL=2 extension. eventually be updated to comply with the I18NLEVEL=2 extension.
Historical note: the REFERENCES threading algorithm is based on the Historical note: The REFERENCES threading algorithm is based on the
[THREADING] algorithm written used in "Netscape Mail and News" [THREADING] algorithm written and used in "Netscape Mail and News"
versions 2.0 through 3.0. versions 2.0 through 3.0.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS]. document are to be interpreted as described in [KEYWORDS].
The word "can" (not "may") is used to refer to a possible The word "can" (not "may") is used to refer to a possible
circumstance or situation, as opposed to an optional facility of the circumstance or situation, as opposed to an optional facility of the
protocol. protocol.
"User" is used to refer to a human user, whereas "client" refers to "User" is used to refer to a human user, whereas "client" refers to
the software being run by the user. the software being run by the user.
In examples, "C:" and "S:" indicate lines sent by the client and In examples, "C:" and "S:" indicate lines sent by the client and
server respectively. server, respectively.
2.1 Base Subject 2.1. Base Subject
Subject sorting and threading use the "base subject," which has Subject sorting and threading use the "base subject", which has
specific subject artifacts removed. Due to the complexity of these specific subject artifacts removed. Due to the complexity of these
artifacts, the formal syntax for the subject extraction rules is artifacts, the formal syntax for the subject extraction rules is
ambiguous. The following procedure is followed to determine the ambiguous. The following procedure is followed to determine the
"base subject", using the [ABNF] formal syntax rules described in "base subject", using the [ABNF] formal syntax rules described in
section 5: section 5:
(1) Convert any RFC 2047 encoded-words in the subject to (1) Convert any RFC 2047 encoded-words in the subject to [UTF-8]
UTF-8 as described in "internationalization as described in "Internationalization Considerations".
considerations." Convert all tabs and continuations to Convert all tabs and continuations to space. Convert all
space. Convert all multiple spaces to a single space. multiple spaces to a single space.
(2) Remove all trailing text of the subject that matches (2) Remove all trailing text of the subject that matches the
the subj-trailer ABNF, repeat until no more matches are subj-trailer ABNF; repeat until no more matches are possible.
possible.
(3) Remove all prefix text of the subject that matches the (3) Remove all prefix text of the subject that matches the subj-
subj-leader ABNF. leader ABNF.
(4) If there is prefix text of the subject that matches the (4) If there is prefix text of the subject that matches the subj-
subj-blob ABNF, and removing that prefix leaves a non-empty blob ABNF, and removing that prefix leaves a non-empty subj-
subj-base, then remove the prefix text. base, then remove the prefix text.
(5) Repeat (3) and (4) until no matches remain. (5) Repeat (3) and (4) until no matches remain.
Note: it is possible to defer step (2) until step (6), but this Note: It is possible to defer step (2) until step (6), but this
requires checking for subj-trailer in step (4). requires checking for subj-trailer in step (4).
(6) If the resulting text begins with the subj-fwd-hdr ABNF (6) If the resulting text begins with the subj-fwd-hdr ABNF and
and ends with the subj-fwd-trl ABNF, remove the ends with the subj-fwd-trl ABNF, remove the subj-fwd-hdr and
subj-fwd-hdr and subj-fwd-trl and repeat from step (2). subj-fwd-trl and repeat from step (2).
(7) The resulting text is the "base subject" used in the (7) The resulting text is the "base subject" used in the SORT.
SORT.
All servers and disconnected (as described in [IMAP-MODELS]) clients All servers and disconnected (as described in [IMAP-MODELS]) clients
MUST use exactly this algorithm to determine the "base subject". MUST use exactly this algorithm to determine the "base subject".
Otherwise there is potential for a user to get inconsistent results Otherwise, there is potential for a user to get inconsistent results
based on whether they are running in connected or disconnected mode. based on whether they are running in connected or disconnected mode.
2.2 Sent Date 2.2. Sent Date
As used in this document, the term "sent date" refers to the date and 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 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. 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". 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 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 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 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. SHOULD be treated as 00:00:00 on the earliest possible date.
This differs from the date-related criteria in the SEARCH command 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 (described in [IMAP] section 6.4.4), which use just the date and not
the time, and are not adjusted by time zone. the time, and are not adjusted by time zone.
If the sent date can not be determined (a Date: header is missing or If the sent date cannot be determined (a Date: header is missing or
can not be parsed), the INTERNALDATE for that message is used as the cannot be parsed), the INTERNALDATE for that message is used as the
sent date. sent date.
When comparing two sent dates that match exactly, the order in which When comparing two sent dates that match exactly, the order in which
the two messages appear in the mailbox (that is, by sequence number) the two messages appear in the mailbox (that is, by sequence number)
is used as a tie-breaker to determine the order. is used as a tie-breaker to determine the order.
3. Additional Commands 3. Additional Commands
These commands are extension to the [IMAP] base protocol. These commands are extensions to the [IMAP] base protocol.
The section headings are intended to correspond with where they would The section headings are intended to correspond with where they would
be located in the main document if they were part of the base be located in the main document if they were part of the base
specification. specification.
BASE.6.4.SORT. SORT Command BASE.6.4.SORT. SORT Command
Arguments: sort program Arguments: sort program
charset specification charset specification
searching criteria (one or more) searching criteria (one or more)
Data: untagged responses: SORT Data: untagged responses: SORT
Result: OK - sort completed Result: OK - sort completed
NO - sort error: can't sort that charset or NO - sort error: can't sort that charset or
criteria criteria
BAD - command unknown or arguments invalid BAD - command unknown or arguments invalid
The SORT command is a variant of SEARCH with sorting semantics for The SORT command is a variant of SEARCH with sorting semantics for
the results. Sort has two arguments before the searching criteria the results. There are two arguments before the searching
argument; a parenthesized list of sort criteria, and the searching criteria argument: a parenthesized list of sort criteria, and the
charset. searching charset.
The charset argument is mandatory (unlike SEARCH) and indicates The charset argument is mandatory (unlike SEARCH) and indicates
the [CHARSET] of the strings that appear in the searching the [CHARSET] of the strings that appear in the searching
criteria. The US-ASCII and UTF-8 charsets MUST be implemented. criteria. The US-ASCII and [UTF-8] charsets MUST be implemented.
All other charsets are optional. All other charsets are optional.
There is also a UID SORT command which returns unique identifiers There is also a UID SORT command that returns unique identifiers
instead of message sequence numbers. Note that there are separate instead of message sequence numbers. Note that there are separate
searching criteria for message sequence numbers and UIDs; thus the searching criteria for message sequence numbers and UIDs; thus,
arguments to UID SORT are interpreted the same as in SORT. This the arguments to UID SORT are interpreted the same as in SORT.
is analogous to the behavior of UID SEARCH, as opposed to UID This is analogous to the behavior of UID SEARCH, as opposed to UID
COPY, UID FETCH, or UID STORE. COPY, UID FETCH, or UID STORE.
The SORT command first searches the mailbox for messages that The SORT command first searches the mailbox for messages that
match the given searching criteria using the charset argument for match the given searching criteria using the charset argument for
the interpretation of strings in the searching criteria. It then the interpretation of strings in the searching criteria. It then
returns the matching messages in an untagged SORT response, sorted returns the matching messages in an untagged SORT response, sorted
according to one or more sort criteria. according to one or more sort criteria.
Sorting is in ascending order. Earlier dates sort before later Sorting is in ascending order. Earlier dates sort before later
dates; smaller sizes sort before larger sizes; and strings are dates; smaller sizes sort before larger sizes; and strings are
sorted according to ascending values established by their sorted according to ascending values established by their
collation algorithm (see under "Internationalization collation algorithm (see "Internationalization Considerations").
Considerations").
If two or more messages exactly match according to the sorting If two or more messages exactly match according to the sorting
criteria, these messages are sorted according to the order in criteria, these messages are sorted according to the order in
which they appear in the mailbox. In other words, there is an which they appear in the mailbox. In other words, there is an
implicit sort criterion of "sequence number". implicit sort criterion of "sequence number".
When multiple sort criteria are specified, the result is sorted in When multiple sort criteria are specified, the result is sorted in
the priority order that the criteria appear. For example, the priority order that the criteria appear. For example,
(SUBJECT DATE) will sort messages in order by their base subject (SUBJECT DATE) will sort messages in order by their base subject
text; and for messages with the same base subject text will sort text; and for messages with the same base subject text, it will
by their sent date. sort by their sent date.
Untagged EXPUNGE responses are not permitted while the server is Untagged EXPUNGE responses are not permitted while the server is
responding to a SORT command, but are permitted during a UID SORT responding to a SORT command, but are permitted during a UID SORT
command. command.
The defined sort criteria are as follows. Refer to the Formal The defined sort criteria are as follows. Refer to the Formal
Syntax section for the precise syntactic definitions of the Syntax section for the precise syntactic definitions of the
arguments. If the associated RFC-822 header for a particular arguments. If the associated RFC-822 header for a particular
criterion is absent, it is treated as the empty string. The empty criterion is absent, it is treated as the empty string. The empty
string always collates before non-empty strings. string always collates before non-empty strings.
skipping to change at line 252 skipping to change at page 5, line 44
Sent date and time, as described in section 2.2. Sent date and time, as described in section 2.2.
FROM FROM
[IMAP] addr-mailbox of the first "From" address. [IMAP] addr-mailbox of the first "From" address.
REVERSE REVERSE
Followed by another sort criterion, has the effect of that Followed by another sort criterion, has the effect of that
criterion but in reverse (descending) order. criterion but in reverse (descending) order.
Note: REVERSE only reverses a single criterion, and does not Note: REVERSE only reverses a single criterion, and does not
affect the implicit "sequence number" sort criterion if all affect the implicit "sequence number" sort criterion if all
other criteria are identicial. Consequently, a sort of other criteria are identical. Consequently, a sort of
REVERSE SUBJECT is not the same as a reverse ordering of a REVERSE SUBJECT is not the same as a reverse ordering of a
SUBJECT sort. This can be avoided by use of additional SUBJECT sort. This can be avoided by use of additional
criteria, e.g. SUBJECT DATE vs. REVERSE SUBJECT REVERSE criteria, e.g., SUBJECT DATE vs. REVERSE SUBJECT REVERSE
DATE. In general, however, it's better (and faster, if the DATE. In general, however, it's better (and faster, if the
client has a "reverse current ordering" command) to reverse client has a "reverse current ordering" command) to reverse
the results in the client instead of issuing a new SORT. the results in the client instead of issuing a new SORT.
SIZE SIZE
Size of the message in octets. Size of the message in octets.
SUBJECT SUBJECT
Base subject text. Base subject text.
skipping to change at line 294 skipping to change at page 6, line 39
Data: untagged responses: THREAD Data: untagged responses: THREAD
Result: OK - thread completed Result: OK - thread completed
NO - thread error: can't thread that charset or NO - thread error: can't thread that charset or
criteria criteria
BAD - command unknown or arguments invalid BAD - command unknown or arguments invalid
The THREAD command is a variant of SEARCH with threading semantics The THREAD command is a variant of SEARCH with threading semantics
for the results. Thread has two arguments before the searching for the results. Thread has two arguments before the searching
criteria argument; a threading algorithm, and the searching criteria argument: a threading algorithm and the searching
charset. charset.
The charset argument is mandatory (unlike SEARCH) and indicates The charset argument is mandatory (unlike SEARCH) and indicates
the [CHARSET] of the strings that appear in the searching the [CHARSET] of the strings that appear in the searching
criteria. The US-ASCII and UTF-8 charsets MUST be implemented. criteria. The US-ASCII and [UTF-8] charsets MUST be implemented.
All other charsets are optional. All other charsets are optional.
There is also a UID THREAD command which returns unique There is also a UID THREAD command that returns unique identifiers
identifiers instead of message sequence numbers. Note that there instead of message sequence numbers. Note that there are separate
are separate searching criteria for message sequence numbers and searching criteria for message sequence numbers and UIDs; thus the
UIDs; thus the arguments to UID THREAD are interpreted the same as arguments to UID THREAD are interpreted the same as in THREAD.
in THREAD. This is analogous to the behavior of UID SEARCH, as This is analogous to the behavior of UID SEARCH, as opposed to UID
opposed to UID COPY, UID FETCH, or UID STORE. COPY, UID FETCH, or UID STORE.
The THREAD command first searches the mailbox for messages that The THREAD command first searches the mailbox for messages that
match the given searching criteria using the charset argument for match the given searching criteria using the charset argument for
the interpretation of strings in the searching criteria. It then the interpretation of strings in the searching criteria. It then
returns the matching messages in an untagged THREAD response, returns the matching messages in an untagged THREAD response,
threaded according to the specified threading algorithm. threaded according to the specified threading algorithm.
All collation is in ascending order. Earlier dates collate before All collation is in ascending order. Earlier dates collate before
later dates and strings are collated according to ascending values later dates and strings are collated according to ascending values
established by their collation algorithm (see under established by their collation algorithm (see
"Internationalization Considerations"). "Internationalization Considerations").
Untagged EXPUNGE responses are not permitted while the server is Untagged EXPUNGE responses are not permitted while the server is
responding to a THREAD command, but are permitted during a UID responding to a THREAD command, but are permitted during a UID
THREAD command. THREAD command.
The defined threading algorithms are as follows: The defined threading algorithms are as follows:
ORDEREDSUBJECT ORDEREDSUBJECT
The ORDEREDSUBJECT threading algorithm is also referred to as The ORDEREDSUBJECT threading algorithm is also referred to as
"poor man's threading." The searched messages are sorted by "poor man's threading". The searched messages are sorted by
base subject and then by the sent date. The messages are then base subject and then by the sent date. The messages are then
split into separate threads, with each thread containing split into separate threads, with each thread containing
messages with the same base subject text. Finally, the threads messages with the same base subject text. Finally, the threads
are sorted by the sent date of the first message in the thread. 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 top level or "root" in ORDEREDSUBJECT threading contains
(the "root"). The second message of a thread is the child of the first message of every thread. All messages in the root
the first message, and subsequent messages of the thread are are siblings of each other. The second message of a thread is
siblings of the second message and hence children of the the child of the first message, and subsequent messages of the
message at the root. Hence, there are no grandchildren in thread are siblings of the second message and hence children of
the message at the root. Hence, there are no grandchildren in
ORDEREDSUBJECT threading. ORDEREDSUBJECT threading.
Children in ORDEREDSUBJECT threading do not have descendents. Children in ORDEREDSUBJECT threading do not have descendents.
Client implementations SHOULD treat descendents of a child in Client implementations SHOULD treat descendents of a child in a
a server response as being siblings of that child. server response as being siblings of that child.
REFERENCES REFERENCES
The REFERENCES threading algorithm threads the searched The REFERENCES threading algorithm threads the searched
messages by grouping them together in parent/child messages by grouping them together in parent/child
relationships based on which messages are replies to others. relationships based on which messages are replies to others.
The parent/child relationships are built using two methods: The parent/child relationships are built using two methods:
reconstructing a message's ancestry using the references reconstructing a message's ancestry using the references
contained within it; and checking the original (not base) contained within it; and checking the original (not base)
subject of a message to see if it is a reply to (or forward of) subject of a message to see if it is a reply to (or forward of)
another message. another message.
Note: "Message ID" in the following description refers to a Note: "Message ID" in the following description refers to a
normalized form of the msg-id in [RFC-2822]. The actual normalized form of the msg-id in [RFC2822]. The actual text
text in an RFC 2822 may use quoting, resulting in multiple in RFC 2822 may use quoting, resulting in multiple ways of
ways of expressing the same Message ID. Implementations of expressing the same Message ID. Implementations of the
the REFERENCES threading algorithm MUST normalize any msg-id REFERENCES threading algorithm MUST normalize any msg-id in
in order to avoid false non-matches due to differences in order to avoid false non-matches due to differences in
quoting. quoting.
For example, the msg-id For example, the msg-id
<"01KF8JCEOCBS0045PS"@xxx.yyy.com> <"01KF8JCEOCBS0045PS"@xxx.yyy.com>
and the msg-id and the msg-id
<01KF8JCEOCBS0045PS@xxx.yyy.com> <01KF8JCEOCBS0045PS@xxx.yyy.com>
MUST be interpreted as being the same Message ID. MUST be interpreted as being the same Message ID.
The references used for reconstructing a message's ancestry are The references used for reconstructing a message's ancestry are
found using the following rules: found using the following rules:
If a message contains a References header line, then use the If a message contains a References header line, then use the
Message IDs in the References header line as the references. Message IDs in the References header line as the references.
If a message does not contain a References header line, or If a message does not contain a References header line, or
the References header line does not contain any valid the References header line does not contain any valid
Message IDs, then use the first (if any) valid Message ID Message IDs, then use the first (if any) valid Message ID
found in the In-Reply-To header line as the only reference found in the In-Reply-To header line as the only reference
(parent) for this message. (parent) for this message.
Note: Although [RFC-2822] permits multiple Message IDs in Note: Although [RFC2822] permits multiple Message IDs in
the In-Reply-To header, in actual practice this the In-Reply-To header, in actual practice this
discipline has not been followed. For example, discipline has not been followed. For example,
In-Reply-To headers have been observed with message In-Reply-To headers have been observed with message
addresses after the Message ID, and there are no good addresses after the Message ID, and there are no good
heuristics for software to determine the difference. heuristics for software to determine the difference.
This is not a problem with the References header however. This is not a problem with the References header,
however.
If a message does not contain an In-Reply-To header line, or 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 the In-Reply-To header line does not contain a valid Message
ID, then the message does not have any references (NIL). ID, then the message does not have any references (NIL).
A message is considered to be a reply or forward if the base A message is considered to be a reply or forward if the base
subject extraction rules, applied to the original subject, subject extraction rules, applied to the original subject,
remove any of the following: a subj-refwd, a "(fwd)" remove any of the following: a subj-refwd, a "(fwd)" subj-
subj-trailer, or a subj-fwd-hdr and subj-fwd-trl. trailer, or a subj-fwd-hdr and subj-fwd-trl.
The REFERENCES algorithm is significantly more complex than The REFERENCES algorithm is significantly more complex than
ORDEREDSUBJECT and consists of six main steps. These steps are ORDEREDSUBJECT and consists of six main steps. These steps are
outlined in detail below. outlined in detail below.
(1) For each searched message: (1) For each searched message:
(A) Using the Message IDs in the message's references, link (A) Using the Message IDs in the message's references, link
the corresponding messages (those whose Message-ID header the corresponding messages (those whose Message-ID
line contains the given reference Message ID) together as header line contains the given reference Message ID)
parent/child. Make the first reference the parent of the together as parent/child. Make the first reference the
second (and the second a child of the first), the second the parent of the second (and the second a child of the
parent of the third (and the third a child of the second), first), the second the parent of the third (and the
etc. The following rules govern the creation of these third a child of the second), etc. The following rules
links: govern the creation of these links:
If a message does not contain a Message-ID header line, If a message does not contain a Message-ID header
or the Message-ID header line does not contain a valid line, or the Message-ID header line does not
Message ID, then assign a unique Message ID to this contain a valid Message ID, then assign a unique
message. Message ID to this message.
If two or more messages have the same Message ID, then If two or more messages have the same Message ID,
only use that Message ID in the first (lowest sequence then only use that Message ID in the first (lowest
number) message, and assign a unique Message ID to each sequence number) message, and assign a unique
of the subsequent messages with a duplicate of that Message ID to each of the subsequent messages with
Message ID. a duplicate of that Message ID.
If no message can be found with a given Message ID, If no message can be found with a given Message ID,
create a dummy message with this ID. Use this dummy create a dummy message with this ID. Use this
message for all subsequent references to this ID. dummy message for all subsequent references to this
ID.
If a message already has a parent, don't change the If a message already has a parent, don't change the
existing link. This is done because the References existing link. This is done because the References
header line may have been truncated by a MUA. As a header line may have been truncated by a Mail User
result, there is no guarantee that the messages Agent (MUA). As a result, there is no guarantee
corresponding to adjacent Message IDs in the References that the messages corresponding to adjacent Message
header line are parent and child. IDs in the References header line are parent and
child.
Do not create a parent/child link if creating that link Do not create a parent/child link if creating that
would introduce a loop. For example, before making link would introduce a loop. For example, before
message A the parent of B, make sure that A is not a making message A the parent of B, make sure that A
descendent of B. is not a descendent of B.
Note: Message ID comparisons are case-sensitive. Note: Message ID comparisons are case-sensitive.
(B) Create a parent/child link between the last reference (B) Create a parent/child link between the last reference
(or NIL if there are no references) and the current message. (or NIL if there are no references) and the current
If the current message already has a parent, it is probably message. If the current message already has a parent,
the result of a truncated References header line, so break it is probably the result of a truncated References
the current parent/child link before creating the new header line, so break the current parent/child link
correct one. As in step 1.A, do not create the parent/child before creating the new correct one. As in step 1.A,
link if creating that link would introduce a loop. Note do not create the parent/child link if creating that
that if this message has no references, that it will now link would introduce a loop. Note that if this message
have no parent. has no references, it will now have no parent.
Note: The parent/child links created in steps 1.A and 1.B Note: The parent/child links created in steps 1.A
MUST be kept consistent with one another at ALL times. and 1.B MUST be kept consistent with one another at
ALL times.
(2) Gather together all of the messages that have no parents (2) Gather together all of the messages that have no parents
and make them all children (siblings of one another) of a dummy and make them all children (siblings of one another) of a
parent (the "root"). These messages constitute the first dummy parent (the "root"). These messages constitute the
(head) message of the threads created thus far. first (head) message of the threads created thus far.
(3) Prune dummy messages from the thread tree. Traverse each (3) Prune dummy messages from the thread tree. Traverse each
thread under the root, and for each message: 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 NO children, delete it.
If it is a dummy message with children, delete it, but If it is a dummy message with children, delete it, but
promote its children to the current level. In other words, promote its children to the current level. In other
splice them in with the dummy's siblings. words, splice them in with the dummy's siblings.
Do not promote the children if doing so would make them Do not promote the children if doing so would make them
children of the root, unless there is only one child. children of the root, unless there is only one child.
(4) Sort the messages under the root (top-level siblings only) (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 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 dummy message, sort its children by sent date and then use
first child for the top-level sort. the first child for the top-level sort.
(5) Gather together messages under the root that have the same (5) Gather together messages under the root that have the same
base subject text. base subject text.
(A) Create a table for associating base subjects with (A) Create a table for associating base subjects with
messages, called the subject table. messages, called the subject table.
(B) Populate the subject table with one message per each (B) Populate the subject table with one message per each
base subject. For each child of the root: base subject. For each child of the root:
(i) Find the subject of this thread, by using the base (i) Find the subject of this thread, by using the
subject from either the current message or its first base subject from either the current message or
child if the current message is a dummy. This is the its first child if the current message is a
thread subject. dummy. This is the thread subject.
(ii) If the thread subject is empty, skip this message. (ii) If the thread subject is empty, skip this
message.
(iii) Look up the message associated with the thread (iii) Look up the message associated with the thread
subject in the subject table. subject in the subject table.
(iv) If there is no message in the subject table with the (iv) If there is no message in the subject table with
thread subject, add the current message and the thread the thread subject, add the current message and
subject to the subject table. the thread subject to the subject table.
Otherwise, if the message in the subject table is not a Otherwise, if the message in the subject table is
dummy, AND either of the following criteria are true: not a dummy, AND either of the following criteria
are true:
The current message is a dummy, OR The current message is a dummy, OR
The message in the subject table is a reply or forward The message in the subject table is a reply
and the current message is not. or forward and the current message is not.
then replace the message in the subject table with the then replace the message in the subject table
current message. with the current message.
(C) Merge threads with the same thread subject. For each (C) Merge threads with the same thread subject. For each
child of the root: child of the root:
(i) Find the message's thread subject as in step 5.B.i (i) Find the message's thread subject as in step
above. 5.B.i above.
(ii) If the thread subject is empty, skip this message. (ii) If the thread subject is empty, skip this
message.
(iii) Lookup the message associated with this thread (iii) Lookup the message associated with this thread
subject in the subject table. subject in the subject table.
(iv) If the message in the subject table is the current (iv) If the message in the subject table is the
message, skip this message. current message, skip this message.
Otherwise, merge the current message with the one in the Otherwise, merge the current message with the one in
subject table using the following rules: the subject table using the following rules:
If both messages are dummies, append the current If both messages are dummies, append the current
message's children to the children of the message in message's children to the children of the message
the subject table (the children of both messages in the subject table (the children of both messages
become siblings), and then delete the current message. become siblings), and then delete the current
message.
If the message in the subject table is a dummy and the If the message in the subject table is a dummy and
current message is not, make the current message a the current message is not, make the current
child of the message in the subject table (a sibling message a child of the message in the subject table
of its children). (a sibling of its children).
If the current message is a reply or forward and the If the current message is a reply or forward and
message in the subject table is not, make the current the message in the subject table is not, make the
message a child of the message in the subject table (a current message a child of the message in the
sibling of its children). subject table (a sibling of its children).
Otherwise, create a new dummy message and make both Otherwise, create a new dummy message and make both
the current message and the message in the subject the current message and the message in the subject
table children of the dummy. Then replace the message table children of the dummy. Then replace the
in the subject table with the dummy message. message in the subject table with the dummy
message.
Note: Subject comparisons are case-insensitive, as Note: Subject comparisons are case-insensitive,
described under "Internationalization as described under "Internationalization
Considerations." Considerations".
(6) Traverse the messages under the root and sort each set of (6) Traverse the messages under the root and sort each set of
siblings by sent date as described in section 2.2. Traverse siblings by sent date as described in section 2.2.
the messages in such a way that the "youngest" set of siblings Traverse the messages in such a way that the "youngest" set
are sorted first, and the "oldest" set of siblings are sorted of siblings are sorted first, and the "oldest" set of
last (grandchildren are sorted before children, etc). In the siblings are sorted last (grandchildren are sorted before
case of a dummy message (which can only occur with top-level children, etc). In the case of a dummy message (which can
siblings), use its first child for sorting. only occur with top-level siblings), use its first child
for sorting.
Example: C: A283 THREAD ORDEREDSUBJECT UTF-8 SINCE 5-MAR-2000 Example: C: A283 THREAD ORDEREDSUBJECT UTF-8 SINCE 5-MAR-2000
S: * THREAD (166)(167)(168)(169)(172)(170)(171) S: * THREAD (166)(167)(168)(169)(172)(170)(171)
(173)(174 (175)(176)(178)(181)(180))(179)(177 (173)(174 (175)(176)(178)(181)(180))(179)(177
(183)(182)(188)(184)(185)(186)(187)(189))(190) (183)(182)(188)(184)(185)(186)(187)(189))(190)
(191)(192)(193)(194 195)(196 (197)(198))(199) (191)(192)(193)(194 195)(196 (197)(198))(199)
(200 202)(201)(203)(204)(205)(206 207)(208) (200 202)(201)(203)(204)(205)(206 207)(208)
S: A283 OK THREAD completed S: A283 OK THREAD completed
C: A284 THREAD ORDEREDSUBJECT US-ASCII TEXT "gewp" C: A284 THREAD ORDEREDSUBJECT US-ASCII TEXT "gewp"
S: * THREAD S: * THREAD
S: A284 OK THREAD completed S: A284 OK THREAD completed
C: A285 THREAD REFERENCES UTF-8 SINCE 5-MAR-2000 C: A285 THREAD REFERENCES UTF-8 SINCE 5-MAR-2000
S: * THREAD (166)(167)(168)(169)(172)((170)(179)) S: * THREAD (166)(167)(168)(169)(172)((170)(179))
(171)(173)((174)(175)(176)(178)(181)(180)) (171)(173)((174)(175)(176)(178)(181)(180))
((177)(183)(182)(188 (184)(189))(185 186)(187)) ((177)(183)(182)(188 (184)(189))(185 186)(187))
(190)(191)(192)(193)((194)(195 196))(197 198) (190)(191)(192)(193)((194)(195 196))(197 198)
(199)(200 202)(201)(203)(204)(205 206 207)(208) (199)(200 202)(201)(203)(204)(205 206 207)(208)
S: A285 OK THREAD completed S: A285 OK THREAD completed
Note: The line breaks in the first and third server Note: The line breaks in the first and third server
responses are for editorial clarity and do not appear in responses are for editorial clarity and do not appear in
real THREAD responses. real THREAD responses.
4. Additional Responses 4. Additional Responses
These responses are extensions to the [IMAP] base protocol. These responses are extensions to the [IMAP] base protocol.
The section headings of these responses are intended to correspond The section headings of these responses are intended to correspond
with where they would be located in the main document. with where they would be located in the main document.
BASE.7.2.SORT. SORT Response BASE.7.2.SORT. SORT Response
Data: zero or more numbers Data: zero or more numbers
skipping to change at line 613 skipping to change at page 13, line 34
BASE.7.2.THREAD. THREAD Response BASE.7.2.THREAD. THREAD Response
Data: zero or more threads Data: zero or more threads
The THREAD response occurs as a result of a THREAD or UID THREAD The THREAD response occurs as a result of a THREAD or UID THREAD
command. It contains zero or more threads. A thread consists of command. It contains zero or more threads. A thread consists of
a parenthesized list of thread members. a parenthesized list of thread members.
Thread members consist of zero or more message numbers, delimited Thread members consist of zero or more message numbers, delimited
by spaces, indicating successive parent and child. This continues by spaces, indicating successive parent and child. This continues
until the thread splits into multiple sub-threads, at which point until the thread splits into multiple sub-threads, at which point,
the thread nests into multiple sub-threads with the first member the thread nests into multiple sub-threads with the first member
of each subthread being siblings at this level. There is no limit of each sub-thread being siblings at this level. There is no
to the nesting of threads. limit to the nesting of threads.
The messages numbers refer to those messages that match the search The messages numbers refer to those messages that match the search
criteria. For THREAD, these are message sequence numbers; for UID criteria. For THREAD, these are message sequence numbers; for UID
THREAD, these are unique identifiers. THREAD, these are unique identifiers.
Example: S: * THREAD (2)(3 6 (4 23)(44 7 96)) Example: S: * THREAD (2)(3 6 (4 23)(44 7 96))
The first thread consists only of message 2. The second thread The first thread consists only of message 2. The second thread
consists of the messages 3 (parent) and 6 (child), after which it consists of the messages 3 (parent) and 6 (child), after which it
splits into two subthreads; the first of which contains messages 4 splits into two sub-threads; the first of which contains messages
(child of 6, sibling of 44) and 23 (child of 4), and the second of 4 (child of 6, sibling of 44) and 23 (child of 4), and the second
which contains messages 44 (child of 6, sibling of 4), 7 (child of of which contains messages 44 (child of 6, sibling of 4), 7 (child
44), and 96 (child of 7). Since some later messages are parents of 44), and 96 (child of 7). Since some later messages are
of earlier messages, the messages were probably moved from some parents of earlier messages, the messages were probably moved from
other mailbox at different times. some other mailbox at different times.
-- 2 -- 2
-- 3 -- 3
\-- 6 \-- 6
|-- 4 |-- 4
| \-- 23 | \-- 23
| |
\-- 44 \-- 44
\-- 7 \-- 7
\-- 96 \-- 96
Example: S: * THREAD ((3)(5)) Example: S: * THREAD ((3)(5))
In this example, 3 and 5 are siblings of a parent which does not 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); match the search criteria (and/or does not exist in the mailbox);
however they are members of the same thread. however they are members of the same thread.
5. Formal Syntax of SORT and THREAD Commands and Responses 5. Formal Syntax of SORT and THREAD Commands and Responses
The following syntax specification uses the Augmented Backus-Naur The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [ABNF]. It also uses [ABNF] Form (ABNF) notation as specified in [ABNF]. It also uses [ABNF]
rules defined in [IMAP]. rules defined in [IMAP].
sort = ["UID" SP] "SORT" SP sort-criteria SP search-criteria sort = ["UID" SP] "SORT" SP sort-criteria SP search-criteria
sort-criteria = "(" sort-criterion *(SP sort-criterion) ")" sort-criteria = "(" sort-criterion *(SP sort-criterion) ")"
sort-criterion = ["REVERSE" SP] sort-key sort-criterion = ["REVERSE" SP] sort-key
skipping to change at line 713 skipping to change at page 15, line 36
subj-middle = *subj-blob (subj-base / subj-fwd) subj-middle = *subj-blob (subj-base / subj-fwd)
; last subj-blob is subj-base if subj-base would ; last subj-blob is subj-base if subj-base would
; otherwise be empty ; otherwise be empty
subj-trailer = "(fwd)" / WSP subj-trailer = "(fwd)" / WSP
subj-base = NONWSP *(*WSP NONWSP) subj-base = NONWSP *(*WSP NONWSP)
; can be a subj-blob ; can be a subj-blob
BLOBCHAR = %x01-5a / %x5c / %x5e-ff BLOBCHAR = %x01-5a / %x5c / %x5e-ff
; any CHAR8 except '[' and ']' ; any CHAR8 except '[' and ']'.
; SHOULD comply with [UTF-8]
NONWSP = %x01-08 / %x0a-1f / %x21-ff NONWSP = %x01-08 / %x0a-1f / %x21-ff
; any CHAR8 other than WSP ; any CHAR8 other than WSP.
; SHOULD comply with [UTF-8]
6. Security Considerations 6. Security Considerations
The SORT and THREAD extensions do not raise any security The SORT and THREAD extensions do not raise any security
considerations that are not present in the base [IMAP] protocol, and considerations that are not present in the base [IMAP] protocol, and
these issues are discussed in [IMAP]. Nevertheless, it is important these issues are discussed in [IMAP]. Nevertheless, it is important
to remember that [IMAP] protocol transactions, including message to remember that [IMAP] protocol transactions, including message
data, are sent in the clear over the network unless protection from data, are sent in the clear over the network unless protection from
snooping is negotiated, either by the use of STARTTLS, privacy snooping is negotiated, either by the use of STARTTLS, privacy
protection is negotiated in the AUTHENTICATE command, or some other protection in AUTHENTICATE, or some other protection mechanism.
protection mechanism.
Although not a security consideration, it is important to recognize Although not a security consideration, it is important to recognize
that sorting by REFERENCES can lead to misleading threading trees. that sorting by REFERENCES can lead to misleading threading trees.
For example, a message with false References: header data will cause For example, a message with false References: header data will cause
a thread to be incorporated into another thread. a thread to be incorporated into another thread.
The process of extracting the base subject may lead to incorrect The process of extracting the base subject may lead to incorrect
collation if the extracted data was significant text as opposed to collation if the extracted data was significant text as opposed to a
a subject artifact. subject artifact.
7. Internationalization Considerations 7. Internationalization Considerations
As stated in the introduction, the rules of I18NLEVEL=1 as described As stated in the introduction, the rules of I18NLEVEL=1 as described
in [IMAP-I18N] MUST be followed; that is, the SORT and THREAD in [IMAP-I18N] MUST be followed; that is, the SORT and THREAD
extensions MUST collate strings according to the i;unicode-casemap extensions MUST collate strings according to the i;unicode-casemap
collation described in [UNICASEMAP]. Servers SHOULD also advertise collation described in [UNICASEMAP]. Servers SHOULD also advertise
the I18NLEVEL=1 extension. Alternatively, a server MAY implement the I18NLEVEL=1 extension. Alternatively, a server MAY implement
I18NLEVEL=2 (or higher) and comply with the rules of that level. I18NLEVEL=2 (or higher) and comply with the rules of that level.
As discussed in [IMAP-I18N] section 4.5, all server implementations As discussed in [IMAP-I18N] section 4.5, all server implementations
should eventually be updated to support the [IMAP-I18N] I18NLEVEL=2 should eventually be updated to support the [IMAP-I18N] I18NLEVEL=2
extension. extension.
Translations of the "re" or "fw"/"fwd" tokens are not specified for Translations of the "re" or "fw"/"fwd" tokens are not specified for
removal in the base subject extraction process. An attempt to add removal in the base subject extraction process. An attempt to add
such translated tokens would result in a geometrically complex, and such translated tokens would result in a geometrically complex, and
ultimately unimplementable, task. ultimately unimplementable, task.
Instead, note that [RFC-2822] section 3.6.5 recommends that "re:" Instead, note that [RFC2822] section 3.6.5 recommends that "re:"
(from the Latin "res", in the matter of) be used to identify a reply. (from the Latin "res", meaning "in the matter of") be used to
Although it is evident that, from the multiple forms of token to identify a reply. Although it is evident that, from the multiple
identify a forwarded message, there is considerable variation found forms of token to identify a forwarded message, there is considerable
in the wild, the variations are (still) manageable. Consequently, it variation found in the wild, the variations are (still) manageable.
is suggested that "re:" and one of the variations of the tokens for Consequently, it is suggested that "re:" and one of the variations of
forward supported by the base subject extraction rules be adopted for the tokens for a forward supported by the base subject extraction
Internet mail messages, since doing so makes it a simple display time rules be adopted for Internet mail messages, since doing so makes it
task to localize the token language for the user. a simple display-time task to localize the token language for the
user.
8. IANA Considerations 8. IANA Considerations
[IMAP] capabilities are registered by publishing a standards track or [IMAP] capabilities are registered by publishing a standards track or
IESG approved experimental RFC. This document constitutes IESG-approved experimental RFC. This document constitutes
registration of the SORT and THREAD capabilities in the [IMAP] registration of the SORT and THREAD capabilities in the [IMAP]
capabilities registry. capabilities registry.
This document creates a new [IMAP] threading algorithms registry, This document creates a new [IMAP] threading algorithms registry,
which registers threading algorithms by publishing a standards track which registers threading algorithms by publishing a standards track
or IESG approved experimental RFC. This document constitutes or IESG-approved experimental RFC. This document constitutes
registration of the ORDEREDSUBJECT and REFERENCES algorithms in that registration of the ORDEREDSUBJECT and REFERENCES algorithms in that
registry. registry.
9. Normative References 9. Normative References
The following documents are normative to this document:
[ABNF] Crocker, D. and Overell, P. "Augmented BNF
for Syntax Specifications: ABNF", RFC 5234
January 2008
[CHARSET] Freed, N. and Postel, J. "IANA Character Set [ABNF] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Registration Procedures", RFC 2978, October Syntax Specifications: ABNF", STD 68, RFC 5234, January
2000. 2008.
[IMAP] Crispin, M. "Internet Message Access Protocol - [CHARSET] Freed, N. and J. Postel, "IANA Charset Registration
Version 4rev1", RFC 3501, March 2003. Procedures", BCP 19, RFC 2978, October 2000.
[IMAP-I18N] Newman, C. and Gulbrandsen, A. "Internet [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
Message Access Protocol Internationalization", VERSION 4rev1", RFC 3501, March 2003.
Work in Progress.
[KEYWORDS] Bradner, S. "Key words for use in RFCs to [IMAP-I18N] Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet
Indicate Requirement Levels", BCP 14, RFC 2119, Message Access Protocol Internationalization", RFC
March 1997. 5255, June 2008.
[RFC-2822] Resnick, P. "Internet Message Format", RFC [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
2822, April 2001. Requirement Levels", BCP 14, RFC 2119, March 1997.
[UNICASEMAP] Crispin, M. "i;unicode-casemap - Simple Unicode [RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822,
Collation Algorithm", RFC 5051. April 2001.
10. Informative References [UNICASEMAP] Crispin, M., "i;unicode-casemap - Simple Unicode
Collation Algorithm", RFC 5051, October 2007.
The following documents are informative to this document: [UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[IMAP-MODELS] Crispin, M. "Distributed Electronic Mail Models 10. Informative References
in IMAP4", RFC 1733, December 1994.
[THREADING] Zawinski, J. "Message Threading", [IMAP-MODELS] Crispin, M., "Distributed Electronic Mail Models in
http://www.jwz.org/doc/threading.html, IMAP4", RFC 1733, December 1994.
1997-2002.
Appendices [THREADING] Zawinski, J. "Message Threading",
http://www.jwz.org/doc/threading.html, 1997-2002.
Author's Address Authors' Addresses
Mark R. Crispin Mark R. Crispin
Networks and Distributed Computing Panda Programming
University of Washington 6158 NE Lariat Loop
4545 15th Avenue NE Bainbridge Island, WA 98110-2098
Seattle, WA 98105-4527
Phone: +1 (206) 543-5762
EMail: MRC@CAC.Washington.EDU Phone: +1 (206) 842-2385
EMail: IMAP+SORT+THREAD@Lingling.Panda.COM
Kenneth Murchison Kenneth Murchison
Carnegie Mellon University Carnegie Mellon University
5000 Forbes Avenue 5000 Forbes Avenue
Cyert Hall 285 Cyert Hall 285
Pittsburgh, PA 15213 Pittsburgh, PA 15213
Phone: +1 (412) 268-2638 Phone: +1 (412) 268-2638
Email: murch@andrew.cmu.edu EMail: murch@andrew.cmu.edu
Full Copyright Statement Full Copyright Statement
Copyright (C) The IETF Trust (2008). Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors contained in BCP 78, and except as set forth therein, the authors
retain all their rights. retain all their rights.
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
skipping to change at line 879 skipping to change at page 19, line 42
Copies of IPR disclosures made to the IETF Secretariat and any Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an 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 attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr. http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf- this standard. Please address the information to the IETF at
ipr@ietf.org. ietf-ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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