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

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Abstract

   This document describes the base-level server-based sorting and
   threading extensions to the [IMAP] protocol.  These extensions
   provide substantial performance improvements for IMAP clients which
   offer sorted and threaded views.

1. Introduction

   The SORT and THREAD extensions to the [IMAP] protocol provide a means
   of server-based sorting and threading of messages, without requiring
   that the client download the necessary data to do so itself.  This is
   particularly useful for online clients as described in [IMAP-MODELS].

   A server which supports the base-level SORT extension indicates this
   with a capability name which starts with "SORT".  Future,
   upwards-compatible extensions to the SORT extension will all start
   with "SORT", indicating support for this base level.

   A server which supports the THREAD extension indicates this with one
   or more capability names consisting of "THREAD=" followed by a
   supported threading algorithm name as described in this document.
   This provides for future upwards-compatible extensions.

   A server which 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.

      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 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.

        (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 can not be determined (a Date: header is missing or
   can not 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.

3. Additional Commands

   These commands are extension to the [IMAP] base protocol.

   The section headings are intended to correspond with where they would
   be located in the main document if they were part of the base
   specification.

BASE.6.4.SORT. SORT Command

   Arguments:  sort program
               charset specification
               searching criteria (one or more)

   Data:       untagged responses: SORT

   Result:     OK - sort completed
               NO - sort error: can't sort that charset or
                    criteria
               BAD - command unknown or arguments invalid

      The SORT command is a variant of SEARCH with sorting semantics for
      the results.  Sort has two arguments before the searching criteria
      argument; a parenthesized list of sort criteria, and the searching
      charset.

      The charset argument is mandatory (unlike SEARCH) and indicates
      the [CHARSET] of the strings that appear in the searching
      criteria.  The US-ASCII and UTF-8 charsets MUST be implemented.
      All other charsets are optional.

      There is also a UID SORT command which returns unique identifiers
      instead of message sequence numbers.  Note that there are separate
      searching criteria for message sequence numbers and UIDs; thus the
      arguments to UID SORT are interpreted the same as in SORT.  This
      is analogous to the behavior of UID SEARCH, as opposed to UID
      COPY, UID FETCH, or UID STORE.

      The SORT command first searches the mailbox for messages that
      match the given searching criteria using the charset argument for
      the interpretation of strings in the searching criteria.  It then
      returns the matching messages in an untagged SORT response, sorted
      according to one or more sort criteria.

      Sorting is in ascending order.  Earlier dates sort before later
      dates; smaller sizes sort before larger sizes; and strings are
      sorted according to ascending values established by their
      collation algorithm (see under "Internationalization
      Considerations").

      If two or more messages exactly match according to the sorting
      criteria, these messages are sorted according to the order in
      which they appear in the mailbox.  In other words, there is an
      implicit sort criterion of "sequence number".

      When multiple sort criteria are specified, the result is sorted in
      the priority order that the criteria appear.  For example,
      (SUBJECT DATE) will sort messages in order by their base subject
      text; and for messages with the same base subject text will sort
      by their sent date.

      Untagged EXPUNGE responses are not permitted while the server is
      responding to a SORT command, but are permitted during a UID SORT
      command.

      The defined sort criteria are as follows.  Refer to the Formal
      Syntax section for the precise syntactic definitions of the
      arguments.  If the associated RFC-822 header for a particular
      criterion is absent, it is treated as the empty string.  The empty
      string always collates before non-empty strings.

      ARRIVAL
         Internal date and time of the message.  This differs from the
         ON criteria in SEARCH, which uses just the internal date.

      CC
         [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 identicial.  Consequently, a sort of
            REVERSE SUBJECT is not the same as a reverse ordering of a
            SUBJECT sort.  This can be avoided by use of additional
            criteria, e.g. SUBJECT DATE vs. REVERSE SUBJECT REVERSE
            DATE.  In general, however, it's better (and faster, if the
            client has a "reverse current ordering" command) to reverse
            the results in the client instead of issuing a new SORT.

      SIZE
         Size of the message in octets.

      SUBJECT
         Base subject text.

      TO
         [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 which returns unique
      identifiers instead of message sequence numbers.  Note that there
      are separate searching criteria for message sequence numbers and
      UIDs; thus the arguments to UID THREAD are interpreted the same as
      in THREAD.  This is analogous to the behavior of UID SEARCH, as
      opposed to UID COPY, UID FETCH, or UID STORE.

      The THREAD command first searches the mailbox for messages that
      match the given searching criteria using the charset argument for
      the interpretation of strings in the searching criteria.  It then
      returns the matching messages in an untagged THREAD response,
      threaded according to the specified threading algorithm.

      All collation is in ascending order.  Earlier dates collate before
      later dates and strings are collated according to ascending values
      established by their collation algorithm (see under
      "Internationalization Considerations").

      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 first message of each thread are siblings of each other
         (the "root").  The second message of a thread is the child of
         the first message, and subsequent messages of the thread are
         siblings of the second message and hence children of the
         message at the root.  Hence, there are no grandchildren in
         ORDEREDSUBJECT threading.

         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.

            Note: "Message ID" in the following description refers to a
            normalized form of the msg-id in [RFC-2822].  The actual
            text in an RFC 2822 may use quoting, resulting in multiple
            ways of expressing the same Message ID.  Implementations of
            the REFERENCES threading algorithm MUST normalize any msg-id
            in order to avoid false non-matches due to differences in
            quoting.

            For example, the msg-id
               <"01KF8JCEOCBS0045PS"@xxx.yyy.com>
            and the msg-id
               <01KF8JCEOCBS0045PS@xxx.yyy.com>
            MUST be interpreted as being the same Message ID.

         The references used for reconstructing a message's ancestry are
         found using the following rules:

            If a message contains a References header line, then use the
            Message IDs in the References header line as the references.

            If a message does not contain a References header line, or
            the References header line does not contain any valid
            Message IDs, then use the first (if any) valid Message ID
            found in the In-Reply-To header line as the only reference
            (parent) for this message.

               Note: Although [RFC-2822] permits multiple Message IDs in
               the In-Reply-To header, in actual practice this
               discipline has not been followed.  For example,
               In-Reply-To headers have been observed with message
               addresses after the Message ID, and there are no good
               heuristics for software to determine the difference.
               This is not a problem with the References header however.

            If a message does not contain an In-Reply-To header line, or
            the In-Reply-To header line does not contain a valid Message
            ID, then the message does not have any references (NIL).

         A message is considered to be a reply or forward if the base
         subject extraction rules, applied to the original subject,
         remove any of the following: a subj-refwd, a "(fwd)"
         subj-trailer, or a subj-fwd-hdr and subj-fwd-trl.

         The REFERENCES algorithm is significantly more complex than
         ORDEREDSUBJECT and consists of six main steps.  These steps are
         outlined in detail below.

         (1) For each searched message:

            (A) Using the Message IDs in the message's references, link
            the corresponding messages (those whose Message-ID header
            line contains the given reference Message ID) together as
            parent/child.  Make the first reference the parent of the
            second (and the second a child of the first), the second the
            parent of the third (and the third a child of the second),
            etc.  The following rules govern the creation of these
            links:

               If a message does not contain a Message-ID header line,
               or the Message-ID header line does not contain a valid
               Message ID, then assign a unique Message ID to this
               message.

               If two or more messages have the same Message ID, then
               only use that Message ID in the first (lowest sequence
               number) message, and assign a unique Message ID to each
               of the subsequent messages with a duplicate of that
               Message ID.

               If no message can be found with a given Message ID,
               create a dummy message with this ID.  Use this dummy
               message for all subsequent references to this ID.

               If a message already has a parent, don't change the
               existing link.  This is done because the References
               header line may have been truncated by a MUA.  As a
               result, there is no guarantee that the messages
               corresponding to adjacent Message IDs in the References
               header line are parent and child.

               Do not create a parent/child link if creating that link
               would introduce a loop.  For example, before making
               message A the parent of B, make sure that A is not a
               descendent of B.

                  Note: Message ID comparisons are case-sensitive.

            (B) Create a parent/child link between the last reference
            (or NIL if there are no references) and the current message.
            If the current message already has a parent, it is probably
            the result of a truncated References header line, so break
            the current parent/child link before creating the new
            correct one.  As in step 1.A, do not create the parent/child
            link if creating that link would introduce a loop.  Note
            that if this message has no references, that it will now
            have no parent.

               Note: The parent/child links created in steps 1.A and 1.B
               MUST be kept consistent with one another at ALL times.

         (2) Gather together all of the messages that have no parents
         and make them all children (siblings of one another) of a dummy
         parent (the "root").  These messages constitute the first
         (head) message of the threads created thus far.

         (3) Prune dummy messages from the thread tree.  Traverse each
         thread under the root, and for each message:

            If it is a dummy message with NO children, delete it.

            If it is a dummy message with children, delete it, but
            promote its children to the current level.  In other words,
            splice them in with the dummy's siblings.

            Do not promote the children if doing so would make them
            children of the root, unless there is only one child.

         (4) Sort the messages under the root (top-level siblings only)
         by sent date 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.

               (iii) Look up the message associated with the thread
               subject in the subject table.

               (iv) If there is no message in the subject table with the
               thread subject, add the current message and the thread
               subject to the subject table.

               Otherwise, if the message in the subject table is not a
               dummy, AND either of the following criteria are true:

                  The current message is a dummy, OR

                  The message in the subject table is a reply or forward
                  and the current message is not.

            then replace the message in the subject table with the
            current message.

            (C) Merge threads with the same thread subject.  For each
            child of the root:

               (i) Find the message's thread subject as in step 5.B.i
               above.

               (ii) If the thread subject is empty, skip this message.

               (iii) Lookup the message associated with this thread
               subject in the subject table.

               (iv) If the message in the subject table is the current
               message, skip this message.

               Otherwise, merge the current message with the one in the
               subject table using the following rules:

                  If both messages are dummies, append the current
                  message's children to the children of the message in
                  the subject table (the children of both messages
                  become siblings), and then delete the current message.

                  If the message in the subject table is a dummy and the
                  current message is not, make the current message a
                  child of the message in the subject table (a sibling
                  of its children).

                  If the current message is a reply or forward and the
                  message in the subject table is not, make the current
                  message a child of the message in the subject table (a
                  sibling of its children).

                  Otherwise, create a new dummy message and make both
                  the current message and the message in the subject
                  table children of the dummy.  Then replace the message
                  in the subject table with the dummy message.

                     Note: Subject comparisons are case-insensitive, as
                     described under "Internationalization
                     Considerations."

         (6) Traverse the messages under the root and sort each set of
         siblings by sent date 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.

4. Additional Responses

   These responses are extensions to the [IMAP] base protocol.

   The section headings of these responses are intended to correspond
   with where they would be located in the main document.

BASE.7.2.SORT. SORT Response

   Data:       zero or more numbers

      The SORT response occurs as a result of a SORT or UID SORT
      command.  The number(s) refer to those messages that match the
      search criteria.  For SORT, these are message sequence numbers;
      for UID SORT, these are unique identifiers.  Each number is
      delimited by a space.

   Example:    S: * SORT 2 3 6

BASE.7.2.THREAD. THREAD Response

   Data:       zero or more threads

      The THREAD response occurs as a result of a THREAD or UID THREAD
      command.  It contains zero or more threads.  A thread consists of
      a parenthesized list of thread members.

      Thread members consist of zero or more message numbers, delimited
      by spaces, indicating successive parent and child.  This continues
      until the thread splits into multiple sub-threads, at which point
      the thread nests into multiple sub-threads with the first member
      of each subthread being siblings at this level.  There is no limit
      to the nesting of threads.

      The messages numbers refer to those messages that match the search
      criteria.  For THREAD, these are message sequence numbers; for UID
      THREAD, these are unique identifiers.

   Example:    S: * THREAD (2)(3 6 (4 23)(44 7 96))

      The first thread consists only of message 2.  The second thread
      consists of the messages 3 (parent) and 6 (child), after which it
      splits into two subthreads; the first of which contains messages 4
      (child of 6, sibling of 44) and 23 (child of 4), and the second of
      which contains messages 44 (child of 6, sibling of 4), 7 (child of
      44), and 96 (child of 7).  Since some later messages are parents
      of earlier messages, the messages were probably moved from some
      other mailbox at different times.

      -- 2

      -- 3
         \-- 6
             |-- 4
             |   \-- 23
             |
             \-- 44
                  \-- 7
                      \-- 96

   Example:    S: * THREAD ((3)(5))

      In this example, 3 and 5 are siblings of a parent which does not
      match the search criteria (and/or does not exist in the mailbox);
      however they are members of the same thread.

5. Formal Syntax of SORT and THREAD Commands and Responses

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) notation as specified in [ABNF].  It also uses [ABNF]
   rules defined in [IMAP].

sort            = ["UID" SP] "SORT" SP sort-criteria SP search-criteria

sort-criteria   = "(" sort-criterion *(SP sort-criterion) ")"

sort-criterion  = ["REVERSE" SP] sort-key

sort-key        = "ARRIVAL" / "CC" / "DATE" / "FROM" / "SIZE" /
                  "SUBJECT" / "TO"

thread          = ["UID" SP] "THREAD" SP thread-alg SP search-criteria

thread-alg      = "ORDEREDSUBJECT" / "REFERENCES" / thread-alg-ext

thread-alg-ext  = atom
                    ; New algorithms MUST be registered with IANA

search-criteria = charset 1*(SP search-key)

charset         = atom / quoted
                    ; CHARSET values MUST be registered with IANA

sort-data       = "SORT" *(SP nz-number)

thread-data     = "THREAD" [SP 1*thread-list]

thread-list     = "(" (thread-members / thread-nested) ")"

thread-members  = nz-number *(SP nz-number) [SP thread-nested]

thread-nested   = 2*thread-list

   The following syntax describes base subject extraction rules (2)-(6):

subject         = *subj-leader [subj-middle] *subj-trailer

subj-refwd      = ("re" / ("fw" ["d"])) *WSP [subj-blob] ":"

subj-blob       = "[" *BLOBCHAR "]" *WSP

subj-fwd        = subj-fwd-hdr subject subj-fwd-trl

subj-fwd-hdr    = "[fwd:"

subj-fwd-trl    = "]"

subj-leader     = (*subj-blob subj-refwd) / WSP

subj-middle     = *subj-blob (subj-base / subj-fwd)
                    ; last subj-blob is subj-base if subj-base would
                    ; otherwise be empty

subj-trailer    = "(fwd)" / WSP

subj-base       = NONWSP *(*WSP NONWSP)
                    ; can be a subj-blob

BLOBCHAR        = %x01-5a / %x5c / %x5e-ff
                    ; any CHAR8 except '[' and ']'

NONWSP          = %x01-08 / %x0a-1f / %x21-ff
                    ; any CHAR8 other than WSP

6. Security Considerations

   The SORT and THREAD extensions do not raise any security
   considerations that are not present in the base [IMAP] protocol, and
   these issues are discussed in [IMAP].  Nevertheless, it is important
   to remember that [IMAP] protocol transactions, including message
   data, are sent in the clear over the network unless protection from
   snooping is negotiated, either by the use of STARTTLS, privacy
   protection is negotiated in the AUTHENTICATE command, or some other
   protection mechanism.

   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 [RFC-2822] section 3.6.5 recommends that "re:"
   (from the Latin "res", in the matter of) be used to identify a reply.
   Although it is evident that, from the multiple forms of token to
   identify a forwarded message, there is considerable variation found
   in the wild, the variations are (still) manageable.  Consequently, it
   is suggested that "re:" and one of the variations of the tokens for
   forward supported by the base subject extraction rules be adopted for
   Internet mail messages, since doing so makes it a simple display time
   task to localize the token language for the user.

8. IANA Considerations

   [IMAP] capabilities are registered by publishing a standards track or
   IESG approved experimental RFC.  This document constitutes
   registration of the SORT and THREAD capabilities in the [IMAP]
   capabilities registry.

   This document creates a new [IMAP] threading algorithms registry,
   which registers threading algorithms by publishing a standards track
   or IESG approved experimental RFC.  This document constitutes
   registration of the ORDEREDSUBJECT and REFERENCES algorithms in that
   registry.

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
                         Registration Procedures", RFC 2978, October
                         2000.

   [IMAP]                Crispin, M. "Internet Message Access Protocol -
                         Version 4rev1", RFC 3501, March 2003.

   [IMAP-I18N]           Newman, C. and Gulbrandsen, A. "Internet
                         Message Access Protocol Internationalization",
                         Work in Progress.

   [KEYWORDS]            Bradner, S. "Key words for use in RFCs to
                         Indicate Requirement Levels", BCP 14, RFC 2119,
                         March 1997.

   [RFC-2822]            Resnick, P. "Internet Message Format", RFC
                         2822, April 2001.

   [UNICASEMAP]          Crispin, M. "i;unicode-casemap - Simple Unicode
                         Collation Algorithm", RFC 5051.

10. Informative References

   The following documents are informative to this document:

   [IMAP-MODELS]         Crispin, M. "Distributed Electronic Mail Models
                         in IMAP4", RFC 1733, December 1994.

   [THREADING]           Zawinski, J. "Message Threading",
                         http://www.jwz.org/doc/threading.html,
                         1997-2002.

Appendices

Author's Address

   Mark R. Crispin
   Networks and Distributed Computing
   University of Washington
   4545 15th Avenue NE
   Seattle, WA  98105-4527

   Phone: +1 (206) 543-5762

   EMail: MRC@CAC.Washington.EDU

   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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