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RFC 5256

Internet Message Access Protocol - SORT and THREAD Extensions

Pages: 19
Proposed Standard
Updated by:  5957

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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.
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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.

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.
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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.

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.
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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").
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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.

   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.
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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.
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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]. 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]
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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.
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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.

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.
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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.

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.
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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
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