Network Working Group M. Crispin
Internet Draft: IMAP THREAD University of Washington
Document: internet-drafts/draft-crispin-imapext-thread-01.txt February 2000
INTERNET MESSAGE ACCESS PROTOCOL - THREAD EXTENSION
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net (US East Coast), nic.nordu.net
(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
Rim).
A revised version of this draft document will be submitted to the RFC
editor as a Proposed Standard for the Internet Community.
Discussion and suggestions for improvement are requested, and should
be sent to ietf-imapext@IMC.ORG. This document will expire before 5
July 2000. Distribution of this memo is unlimited.
Abstract
This document describes the server-based threading extension to the
IMAP4rev1 protocol. This extension provides substantial performance
improvements for IMAP clients which offer threaded views.
A server which supports this extension indicates this with more 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.
Crispin [Page 1]
INTERNET DRAFT IMAP THREAD EXPIRES 5 July 2000
Extracted Subject Text
Threading algorithms use a version of the subject which has specific
subject artifacts of deployed Internet mail software removed. Due to
the complexity of these artifacts, the above syntax is ambiguous.
The following procedure is followed to determing the actual "base
subject":
(1) Remove all trailing text of the subject that matches
the subj-trailer ABNF, repeat until no more matches are
possible.
(2) Remove all prefix text of the subject that matches
subj-leader.
(3) If there is prefix text of the subject that matches
subj-blob, and removing that prefix leaves a non-empty
subj-base, then remove the prefix text.
(4) Repeat (2) and (3) until no matches remain.
(5) Convert any RFC 2047 encoded-words in the remaining
subj-base to UTF-8.
(6) The resulting text is the "base subject" used in the
THREAD.
All servers and disconnected clients MUST use exactly this algorithm
when threading. Otherwise there is potential for a user to get
inconsistant results based on whether they are running in connected
or disconnected IMAP mode.
Crispin [Page 2]
INTERNET DRAFT IMAP THREAD EXPIRES 5 July 2000
Additional Commands
This command is an extension to the IMAP4rev1 base protocol.
The section header is intended to correspond with where it would be
located in the main document if it was part of the base
specification.
6.3.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. Note that unlike SEARCH, the searching charset argument
is mandatory.
There is also a UID THREAD command which corresponds to THREAD the
way that UID SEARCH corresponds to SEARCH.
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. Unlike
SEARCH, if no messages match the searching criteria in a THREAD
command, no untagged THREAD response is returned.
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
subject and then by sent date, equivalent to a "SORT (SUBJECT
DATE)". The messages are then split into separate threads,
with each thread containing messages with the same extracted
subject text. Finally, the threads are sorted by the sent date
Crispin [Page 3]
INTERNET DRAFT IMAP THREAD EXPIRES 5 July 2000
of the first message in the thread.
Example: C: A283 THREAD ORDEREDSUBJECT UTF-8 SINCE 5-AUG-1999
S: * THREAD (146 151)(144 145)(155)(147 148 152
167)(182)(181)(149)(154)(153)(164 170)(156)
(158 161 162)(157 160 163)(159)(183 185)(165)
(166)(168)(169)(171)(172)(173)(186)(174)(150)
(175)(176)(177 179 184)(178)(180)(187)
S: A283 OK THREAD completed
C: A284 THREAD ORDEREDSUBJECT US-ASCII TEXT "gewp"
S: A284 OK THREAD completed
Note: The line breaks in the first client response are for
editorial clarity and do not appear in a real THREAD
response.
Crispin [Page 4]
INTERNET DRAFT IMAP THREAD EXPIRES 5 July 2000
Additional Responses
This response is an extension to the IMAP4rev1 base protocol.
The section heading of this response is intended to correspond with
where it would be located in the main document.
7.2.THREAD. THREAD Response
Data: one or more threads
The THREAD response occurs as a result of a THREAD or UID THREAD
command. It contains one or more threads. A thread consists of a
parenthesized list of thread members. Thread members consist of
one or more message numbers until the thread splits into multiple
sub-threads, at which point the thread nests into multiple sub-
threads. 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))
In this example, there are two threads. The first thread
consists only of message 2. The second thread consists
of the messages 3 and 6, after which it splits into two
subthreads; the first of which contains messages 4 and
23, and the second of which contains messages 44, 7, and
96.
2
,- 4 - 23
3 - 6 -<
`- 44 - 7 - 96
Crispin [Page 5]
INTERNET DRAFT IMAP THREAD EXPIRES 5 July 2000
Formal Syntax of THREAD commands and Responses
thread-data = "THREAD" SPACE 1*thread-list
thread-list = "(" thread-members / thread-nested ")"
thread-members = nz-number *(SP nz-number) [SP thread-nested]
thread-nested = 2*thread-list
thread = ["UID" SPACE] "THREAD" SP thread-algorthm
SP search-charset 1*(SP search-key)
thread-algorithm = "ORDEREDSUBJECT" / atom
The following syntax describes subject extraction rules (1)-(5):
subject = *(subj-leader / subj-blob) subj-base *subj-trailer
subj-repeat = "[" 1*DIGIT "]"
subj-refwd = ("re" / ("fw" ["d"])) [subj-repeat] ":"
subj-leader = subj-refwd / WSP
subj-blob = "[" 1*BLOBCHAR "]" *WSP
subj-trailer = "(fwd)" / WSP
subj-base = NONWSP *([WSP] NONWSP)
BLOBCHAR = %x01-5c / %x5e-7f
; any CHAR except ']'
NONWSP = %x01-08 / %x0a-1f / %x21-7f
; any CHAR other than WSP
Security Considerations
Security issues are not discussed in this memo.
Internationalization Considerations
By default, strings are threaded according to the "minimum sorting
collation algorithm". All implementations of THREAD MUST implement
Crispin [Page 6]
INTERNET DRAFT IMAP THREAD EXPIRES 5 July 2000
the minimum sorting collation algorithm.
In the minimum sorting collation algorithm, the 26 Latin alphabetics
are sorted in a case-insensitive fashion; that is, "A" and "a" are
treated as exact equals. All other characters are sorted according
to their octet values, as expressed in UTF-8. No attempt is made to
treat composed characters specially.
Other sorting collations, and the ability to change the sorting
collation, will be defined in a separate document dealing with IMAP
internationalization.
It is anticipated that there will be a generic Unicode sorting
collation, which will provide generic case-insensitivity for
alphabetic scripts, specification of composed character handling, and
language-specific sorting collations. A server which implements
non-default sorting collations will modify its sorting behavior
according to the selected sorting collation.
Non-English translations of "Re" or "Fw"/"Fwd" are not specified for
removal in the extracted subject text process. By specifying that
only the English forms of the prefixes are used, it becomes a simple
display time task to localize the prefix language for the user. If,
on the other hand, prefixes in multiple languages are permitted, the
result is a geometrically complex, and ultimately unimplementable,
task. In order to improve the ability to support non-English display
in Internet mail clients, only the English form of these prefixes
should be transmitted in Internet mail messages.
Author's Address
Mark R. Crispin
Networks and Distributed Computing
University of Washington
4545 15th Aveneue NE
Seattle, WA 98105-4527
Phone: (206) 543-5762
EMail: MRC@CAC.Washington.EDU
Crispin [Page 7]