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.
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.
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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.
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.
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.
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(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.
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.
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").
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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.
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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.
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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.
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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.
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(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,
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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.
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(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).
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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.
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.
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-- 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].
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.
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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.
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.
[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.
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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.
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