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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (January 2002) is 8109 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Missing reference section? 'ABNF' on line 528 looks like a reference -- Missing reference section? 'NEWS' on line 531 looks like a reference Summary: 7 errors (**), 0 flaws (~~), 2 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IMAP Extensions Working Group M. Crispin 3 Internet Draft: IMAP THREAD K. Murchison 4 Document: internet-drafts/draft-ietf-imapext-thread-08.txt January 2002 6 INTERNET MESSAGE ACCESS PROTOCOL - THREAD EXTENSION 8 Status of this Memo 10 This document is an Internet-Draft and is in full conformance with 11 all provisions of Section 10 of RFC 2026. 13 Internet-Drafts are working documents of the Internet Engineering 14 Task Force (IETF), its areas, and its working groups. Note that 15 other groups may also distribute working documents as Internet- 16 Drafts. 18 Internet-Drafts are draft documents valid for a maximum of six months 19 and may be updated, replaced, or obsoleted by other documents at any 20 time. It is inappropriate to use Internet-Drafts as reference 21 material or to cite them other than as "work in progress." 23 The list of current Internet-Drafts can be accessed at 24 http://www.ietf.org/ietf/1id-abstracts.txt 26 To view the list Internet-Draft Shadow Directories, see 27 http://www.ietf.org/shadow.html. 29 A revised version of this draft document will be submitted to the RFC 30 editor as a Proposed Standard for the Internet Community. Discussion 31 and suggestions for improvement are requested, and should be sent to 32 ietf-imapext@IMC.ORG. This document will expire before 4 July 2002. 33 Distribution of this memo is unlimited. 35 Abstract 37 This document describes the server-based threading extension to the 38 IMAP4rev1 protocol. This extension provides substantial performance 39 improvements for IMAP clients which offer threaded views. 41 A server which supports this extension indicates this with one or 42 more capability names consisting of "THREAD=" followed by a supported 43 threading algorithm name as described in this document. This 44 provides for future upwards-compatible extensions. 46 Extracted Subject Text 48 Threading uses a version of the subject which has specific subject 49 artifacts of deployed Internet mail software removed. Due to the 50 complexity of these artifacts, the formal syntax for the subject 51 extraction rules is ambiguous. The following procedure is followed 52 to determine the actual "base subject" which is used to thread: 54 (1) Convert any RFC 2047 encoded-words in the subject to 55 UTF-8. Convert all tabs and continuations to space. 56 Convert all multiple spaces to a single space. 58 (2) Remove all trailing text of the subject that matches 59 the subj-trailer ABNF, repeat until no more matches are 60 possible. 62 (3) Remove all prefix text of the subject that matches the 63 subj-leader ABNF. 65 (4) If there is prefix text of the subject that matches the 66 subj-blob ABNF, and removing that prefix leaves a non-empty 67 subj-base, then remove the prefix text. 69 (5) Repeat (3) and (4) until no matches remain. 71 Note: it is possible to defer step (2) until step (6), 72 but this requires checking for subj-trailer in step (4). 74 (6) If the resulting text begins with the subj-fwd-hdr ABNF 75 and ends with the subj-fwd-trl ABNF, remove the 76 subj-fwd-hdr and subj-fwd-trl and repeat from step (2). 78 (7) The resulting text is the "base subject" used in 79 threading. 81 All servers and disconnected clients MUST use exactly this algorithm 82 when threading. Otherwise there is potential for a user to get 83 inconsistent results based on whether they are running in connected 84 or disconnected IMAP mode. 86 Sent Date 88 As used in this document, the term "sent date" refers to the date and 89 time from the Date: header, adjusted by time zone. This differs from 90 date-related criteria in SEARCH, which use just the date and not the 91 time, nor adjusts by time zone. 93 Additional Commands 95 This command is an extension to the IMAP4rev1 base protocol. 97 The section header is intended to correspond with where it would be 98 located in the main document if it was part of the base 99 specification. 101 6.3.THREAD. THREAD Command 103 Arguments: threading algorithm 104 charset specification 105 searching criteria (one or more) 107 Data: untagged responses: THREAD 109 Result: OK - thread completed 110 NO - thread error: can't thread that charset or 111 criteria 112 BAD - command unknown or arguments invalid 114 The THREAD command is a variant of SEARCH with threading semantics 115 for the results. Thread has two arguments before the searching 116 criteria argument; a threading algorithm, and the searching 117 charset. Note that unlike SEARCH, the searching charset argument 118 is mandatory. 120 There is also a UID THREAD command which corresponds to THREAD the 121 way that UID SEARCH corresponds to SEARCH. 123 The THREAD command first searches the mailbox for messages that 124 match the given searching criteria using the charset argument for 125 the interpretation of strings in the searching criteria. It then 126 returns the matching messages in an untagged THREAD response, 127 threaded according to the specified threading algorithm. 129 The defined threading algorithms are as follows: 131 ORDEREDSUBJECT 132 The ORDEREDSUBJECT threading algorithm is also referred to as 133 "poor man's threading." The searched messages are sorted by 134 subject and then by the sent date. The messages are then split 135 into separate threads, with each thread containing messages 136 with the same extracted subject text. Finally, the threads are 137 sorted by the sent date of the first message in the thread. 139 Note that each message in a thread is a child (as opposed to a 140 sibling) of the previous message. 142 REFERENCES 143 The REFERENCES threading algorithm is based on the algorithm 144 written by Jamie Zawinski which was used in "Netscape Mail and 145 News" versions 2.0 through 3.0. For details, see 146 http://www.jwz.org/doc/threading.html. 148 This algorithm threads the searched messages by grouping them 149 together in parent/child relationships based on which messages 150 are replies to others. The parent/child relationships are 151 built using two methods: reconstructing a message's ancestry 152 using the references contained within it; and checking the 153 subject of a message to see if it is a reply to (or forward of) 154 another. 156 The references used for reconstructing a message's ancestry are 157 found using the following rules: 159 If a message contains a [NEWS]-style References header line, 160 then use the Message IDs in the References header line as 161 the references. 163 If a message does not contain a References header line, or 164 the References header line does not contain any valid 165 Message IDs, then use the first (if any) valid Message ID 166 found in the In-Reply-To header line as the only reference 167 (parent) for this message. 169 Note: Although RFC 822 permits multiple Message IDs in 170 the In-Reply-To header, in actual practice this 171 discipline has not been followed. For example, 172 In-Reply-To headers have been observed with email 173 addresses after the Message ID, and there are no good 174 heuristics for software to determine the difference. 175 This is not a problem with the References header however. 177 If a message does not contain an In-Reply-To header line, or 178 the In-Reply-To header line does not contain a valid Message 179 ID, then the message does not have any references (NIL). 181 The REFERENCES algorithm is significantly more complex than 182 ORDEREDSUBJECT and consists of six main steps. These steps are 183 outlined in detail below. 185 (1) For each searched message: 187 (A) Using the Message IDs in the message's references, link 188 the corresponding messages (those whose Message-ID header 189 line contains the given reference Message ID) together as 190 parent/child. Make the first reference the parent of the 191 second (and the second a child of the first), the second the 192 parent of the third (and the third a child of the second), 193 etc. The following rules govern the creation of these 194 links: 196 If a message does not contain a Message-ID header line, 197 or the Message-ID header line does not contain a valid 198 Message ID, then assign a unique Message ID to this 199 message. 201 If two or more messages have the same Message ID, assign 202 a unique Message ID to each of the duplicates. 204 If no message can be found with a given Message ID, 205 create a dummy message with this ID. Use this dummy 206 message for all subsequent references to this ID. 208 If a message already has a parent, don't change the 209 existing link. This is done because the References 210 header line may have been truncated by a MUA. As a 211 result, there is no guarantee that the messages 212 corresponding to adjacent Message IDs in the References 213 header line are parent and child. 215 Do not create a parent/child link if creating that link 216 would introduce a loop. For example, before making 217 message A the parent of B, make sure that A is not a 218 descendent of B. 220 Note: Message ID comparisons are case-sensitive. 222 (B) Create a parent/child link between the last reference 223 (or NIL if there are no references) and the current message. 224 If the current message already has a parent, it is probably 225 the result of a truncated References header line, so break 226 the current parent/child link before creating the new 227 correct one. As in step 1.A, do not create the parent/child 228 link if creating that link would introduce a loop. Note 229 that if this message has no references, that it will now 230 have no parent. 232 Note: The parent/child links created in steps 1.A and 1.B 233 MUST be kept consistent with one another at ALL times. 235 (2) Gather together all of the messages that have no parents 236 and make them all children (siblings of one another) of a dummy 237 parent (the "root"). These messages constitute the first 238 (head) message of the threads created thus far. 240 (3) Prune dummy messages from the thread tree. Traverse each 241 thread under the root, and for each message: 243 If it is a dummy message with NO children, delete it. 245 If it is a dummy message with children, delete it, but 246 promote its children to the current level. In other words, 247 splice them in with the dummy's siblings. 249 Do not promote the children if doing so would make them 250 children of the root, unless there is only one child. 252 (4) Sort the messages under the root (top-level siblings only) 253 by sent date. In the case of an exact match on sent date or if 254 either of the Date: headers used in a comparison can not be 255 parsed, use the order in which the messages appear in the 256 mailbox (that is, by sequence number) to determine the order. 257 In the case of a dummy message, sort its children by sent date 258 and then use the first child for the top-level sort. 260 (5) Gather together messages under the root that have the same 261 extracted subject text. 263 (A) Create a table for associating extracted subjects with 264 messages. 266 (B) Populate the subject table with one message per 267 extracted subject. For each child of the root: 269 (i) Find the subject of this thread by extracting the 270 base subject from the current message, or its first child 271 if the current message is a dummy. 273 (ii) If the extracted subject is empty, skip this 274 message. 276 (iii) Lookup the message associated with this extracted 277 subject in the table. 279 (iv) If there is no message in the table with this 280 subject, add the current message and the extracted 281 subject to the subject table. 283 Otherwise, replace the message in the table with the 284 current message if the message in the table is not a 285 dummy AND either of the following criteria are true: 287 The current message is a dummy, OR 289 The message in the table is a reply or forward (its 290 original subject contains a subj-refwd part and/or a 291 "(fwd)" subj-trailer) and the current message is not. 293 (C) Merge threads with the same subject. For each child of 294 the root: 296 (i) Find the subject of this thread as in step 4.B.i 297 above. 299 (ii) If the extracted subject is empty, skip this 300 message. 302 (iii) Lookup the message associated with this extracted 303 subject in the table. 305 (iv) If the message in the table is the current message, 306 skip this message. 308 Otherwise, merge the current message with the one in the 309 table using the following rules: 311 If both messages are dummies, append the current 312 message's children to the children of the message in 313 the table (the children of both messages become 314 siblings), and then delete the current message. 316 If the message in the table is a dummy and the current 317 message is not, make the current message a child of 318 the message in the table (a sibling of it's children). 320 If the current message is a reply or forward and the 321 message in the table is not, make the current message 322 a child of the message in the table (a sibling of it's 323 children). 325 Otherwise, create a new dummy message and make both 326 the current message and the message in the table 327 children of the dummy. Then replace the message in 328 the table with the dummy message. 330 Note: Subject comparisons are case-insensitive, as 331 described under "Internationalization 332 Considerations." 334 (6) Traverse the messages under the root and sort each set of 335 siblings by sent date. Traverse the messages in such a way 336 that the "youngest" set of siblings are sorted first, and the 337 "oldest" set of siblings are sorted last (grandchildren are 338 sorted before children, etc). In the case of an exact match on 339 sent date or if either of the Date: headers used in a 340 comparison can not be parsed, use the order in which the 341 messages appear in the mailbox (that is, by sequence number) to 342 determine the order. In the case of a dummy message (which can 343 only occur with top-level siblings), use its first child for 344 sorting. 346 Example: C: A283 THREAD ORDEREDSUBJECT UTF-8 SINCE 5-MAR-2000 347 S: * THREAD (166)(167)(168)(169)(172)(170)(171) 348 (173)(174 175 176 178 181 180)(179)(177 183 349 182 188 184 185 186 187 189)(190)(191)(192) 350 (193)(194 195)(196 197 198)(199)(200 202)(201) 351 (203)(204)(205)(206 207)(208) 352 S: A283 OK THREAD completed 353 C: A284 THREAD ORDEREDSUBJECT US-ASCII TEXT "gewp" 354 S: * THREAD 355 S: A284 OK THREAD completed 356 C: A285 THREAD REFERENCES UTF-8 SINCE 5-MAR-2000 357 S: * THREAD (166)(167)(168)(169)(172)((170)(179)) 358 (171)(173)((174)(175)(176)(178)(181)(180)) 359 ((177)(183)(182)(188 (184)(189))(185 186)(187)) 360 (190)(191)(192)(193)((194)(195 196))(197 198) 361 (199)(200 202)(201)(203)(204)(205 206 207)(208) 362 S: A285 OK THREAD completed 364 Note: The line breaks in the first and third client 365 responses are for editorial clarity and do not appear in 366 real THREAD responses. 368 Additional Responses 370 This response is an extension to the IMAP4rev1 base protocol. 372 The section heading of this response is intended to correspond with 373 where it would be located in the main document. 375 7.2.THREAD. THREAD Response 377 Data: zero or more threads 379 The THREAD response occurs as a result of a THREAD or UID THREAD 380 command. It contains zero or more threads. A thread consists of 381 a parenthesized list of thread members. 383 Thread members consist of zero or more message numbers, delimited 384 by spaces, indicating successive parent and child. This continues 385 until the thread splits into multiple sub-threads, at which point 386 the thread nests into multiple sub-threads with the first member 387 of each subthread being siblings at this level. There is no limit 388 to the nesting of threads. 390 The messages numbers refer to those messages that match the search 391 criteria. For THREAD, these are message sequence numbers; for UID 392 THREAD, these are unique identifiers. 394 Example: S: * THREAD (2)(3 6 (4 23)(44 7 96)) 396 The first thread consists only of message 2. The second thread 397 consists of the messages 3 (parent) and 6 (child), after which it 398 splits into two subthreads; the first of which contains messages 4 399 (child of 6, sibling of 44) and 23 (child of 4), and the second of 400 which contains messages 44 (child of 6, sibling of 4), 7 (child of 401 44), and 96 (child of 7). Since some later messages are parents 402 of earlier messages, the messages were probably moved from some 403 other mailbox at different times. 405 -- 2 407 -- 3 408 \-- 6 409 |-- 4 410 | \-- 23 411 | 412 \-- 44 413 \-- 7 414 \-- 96 416 Example: S: * THREAD ((3)(5)) 418 In this example, 3 and 5 are siblings of a parent which does not 419 match the search criteria (and/or does not exist in the mailbox); 420 however they are members of the same thread. 422 Formal Syntax of THREAD commands and Responses 424 thread-data = "THREAD" [SP 1*thread-list] 426 thread-list = "(" thread-members / thread-nested ")" 428 thread-members = nz-number *(SP nz-number) [SP thread-nested] 430 thread-nested = 2*thread-list 432 thread = ["UID" SP] "THREAD" SP thread-algorithm 433 SP search-charset 1*(SP search-key) 435 thread-algorithm = "ORDEREDSUBJECT" / "REFERENCES" / atom 437 The following syntax describes subject extraction rules (2)-(6): 439 subject = *subj-leader [subj-middle] *subj-trailer 441 subj-refwd = ("re" / ("fw" ["d"])) *WSP [subj-blob] ":" 443 subj-blob = "[" *BLOBCHAR "]" *WSP 445 subj-fwd = subj-fwd-hdr subject subj-fwd-trl 447 subj-fwd-hdr = "[fwd:" 449 subj-fwd-trl = "]" 451 subj-leader = (*subj-blob subj-refwd) / WSP 453 subj-middle = *subj-blob (subj-base / subj-fwd) 454 ; last subj-blob is subj-base if subj-base would 455 ; otherwise be empty 457 subj-trailer = "(fwd)" / WSP 459 subj-base = NONWSP *([*WSP] NONWSP) 460 ; can be a subj-blob 462 BLOBCHAR = %x01-5a / %x5c / %x5e-7f 463 ; any CHAR except '[' and ']' 465 NONWSP = %x01-08 / %x0a-1f / %x21-7f 466 ; any CHAR other than WSP 468 Security Considerations 470 Security issues are not discussed in this memo. 472 Internationalization Considerations 474 By default, strings are threaded according to the "minimum sorting 475 collation algorithm". All implementations of THREAD MUST implement 476 the minimum sorting collation algorithm. 478 In the minimum sorting collation algorithm, the Basic Latin 479 alphabetics (U+0041 to U+005A uppercase, U+0061 to U+007A lowercase) 480 are sorted in a case-insensitive fashion; that is, "A" (U+0041) and 481 "a" (U+0061) are treated as exact equals. The characters U+005B to 482 U+0060 are sorted after the Basic Latin alphabetics; for example, 483 U+005E is sorted after U+005A and U+007A. All other characters are 484 sorted according to their octet values, as expressed in UTF-8. No 485 attempt is made to treat composed characters specially, or to do 486 case-insensitive comparisons of composed characters. 488 Note: this means, among other things, that the composed 489 characters in the Latin-1 Supplement are not compared in 490 what would be considered an ISO 8859-1 "case-insensitive" 491 fashion. Case comparison rules for characters with 492 diacriticals differ between languages; the minimum sorting 493 collation does not attempt to deal with this at all. This 494 is reserved for other sorting collations, which may be 495 language-specific. 497 ;;; *** ITEM FOR DISCUSSION *** 498 ;;; THERE IS SOME CONCERN THAT THIS MINIMUM COLLATION IS TOO MINIMAL, 499 ;;; AND THAT THE "GENERIC UNICODE SORTING COLLATION" DISCUSSED BELOW 500 ;;; NEEDS TO BE THE MINIMUM. ONE SUGGESTION IS UNICODE TECHNICAL 501 ;;; STANDARD 10 (TR-10). IF THIS IS THE MINIMUM, THAT REQUIRES THAT 502 ;;; ALL IMPLEMENTATIONS OF SORT AND THREAD BE UNICODE-SAVVY AT LEAST 503 ;;; TO THE POINT OF IMPLEMENTATION TR-10. IS THIS REALISTIC? DOES 504 ;;; THIS RAISE EXCESSIVE IMPLEMENTATION BARRIERS? 505 Other sorting collations, and the ability to change the sorting 506 collation, will be defined in a separate document dealing with IMAP 507 internationalization. 509 It is anticipated that there will be a generic Unicode sorting 510 collation, which will provide generic case-insensitivity for 511 alphabetic scripts, specification of composed character handling, and 512 language-specific sorting collations. A server which implements 513 non-default sorting collations will modify its sorting behavior 514 according to the selected sorting collation. 516 Non-English translations of "Re" or "Fw"/"Fwd" are not specified for 517 removal in the extracted subject text process. By specifying that 518 only the English forms of the prefixes are used, it becomes a simple 519 display time task to localize the prefix language for the user. If, 520 on the other hand, prefixes in multiple languages are permitted, the 521 result is a geometrically complex, and ultimately unimplementable, 522 task. In order to improve the ability to support non-English display 523 in Internet mail clients, only the English form of these prefixes 524 should be transmitted in Internet mail messages. 526 A. References 528 [ABNF] Crocker, D., and Overell, P. "Augmented BNF for Syntax 529 Specifications: ABNF", RFC 2234, November 1997. 531 [NEWS] Horton, M., and Adams, R., "Standard for interchange of USENET 532 messages", RFC-1036, AT&T Bell Laboratories and Center for Seismic 533 Studies, December, 1987. 535 Author's Address 537 Mark R. Crispin 538 Networks and Distributed Computing 539 University of Washington 540 4545 15th Avenue NE 541 Seattle, WA 98105-4527 543 Phone: (206) 543-5762 545 EMail: MRC@CAC.Washington.EDU 547 Kenneth Murchison 548 Oceana Matrix Ltd. 549 21 Princeton Place 550 Orchard Park, NY 14127 552 Phone: (716) 662-8973 x26 554 EMail: ken@oceana.com