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Gondwana, Ed. 3 Internet-Draft FastMail 4 Updates: 3501 (if approved) July 17, 2018 5 Intended status: Standards Track 6 Expires: January 18, 2019 8 IMAP Extension for object identifiers 9 draft-ietf-extra-imap-objectid-04 11 Abstract 13 This document updates RFC3501 (IMAP4rev1) with persistent identifiers 14 on mailboxes and messages to allow clients to more efficiently re-use 15 cached data when resources have changed location on the server. 17 Status of This Memo 19 This Internet-Draft is submitted in full conformance with the 20 provisions of BCP 78 and BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF). Note that other groups may also distribute 24 working documents as Internet-Drafts. The list of current Internet- 25 Drafts is at https://datatracker.ietf.org/drafts/current/. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 This Internet-Draft will expire on January 18, 2019. 34 Copyright Notice 36 Copyright (c) 2018 IETF Trust and the persons identified as the 37 document authors. All rights reserved. 39 This document is subject to BCP 78 and the IETF Trust's Legal 40 Provisions Relating to IETF Documents 41 (https://trustee.ietf.org/license-info) in effect on the date of 42 publication of this document. Please review these documents 43 carefully, as they describe your rights and restrictions with respect 44 to this document. Code Components extracted from this document must 45 include Simplified BSD License text as described in Section 4.e of 46 the Trust Legal Provisions and are provided without warranty as 47 described in the Simplified BSD License. 49 Table of Contents 51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 52 2. Conventions Used In This Document . . . . . . . . . . . . . . 3 53 3. CAPABILITY Identification . . . . . . . . . . . . . . . . . . 3 54 4. MAILBOXID object identifier . . . . . . . . . . . . . . . . . 3 55 4.1. New response code for CREATE . . . . . . . . . . . . . . 4 56 4.2. New OK Untagged Response for SELECT and EXAMINE . . . . . 4 57 4.3. New attribute for STATUS . . . . . . . . . . . . . . . . 5 58 5. EMAILID object identifier and THREADID correlator . . . . . . 6 59 5.1. EMAILID identifier for identical messages . . . . . . . . 6 60 5.2. THREADID identifer for related messages . . . . . . . . . 6 61 5.3. New Message Data Items in FETCH and UID FETCH Commands . 7 62 6. New Filters on SEARCH command . . . . . . . . . . . . . . . . 9 63 7. Formal syntax . . . . . . . . . . . . . . . . . . . . . . . . 9 64 8. Implementation considerations . . . . . . . . . . . . . . . . 10 65 8.1. Assigning object identifiers . . . . . . . . . . . . . . 10 66 8.2. Interaction with special cases . . . . . . . . . . . . . 11 67 8.3. Client usage . . . . . . . . . . . . . . . . . . . . . . 11 68 9. Future considerations . . . . . . . . . . . . . . . . . . . . 11 69 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 70 11. Security Considerations . . . . . . . . . . . . . . . . . . . 12 71 12. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 72 12.1. draft-ietf-extra-imap-objectid-04 . . . . . . . . . . . 12 73 12.2. draft-ietf-extra-imap-objectid-03 . . . . . . . . . . . 13 74 12.3. draft-ietf-extra-imap-objectid-02 . . . . . . . . . . . 13 75 12.4. draft-ietf-extra-imap-objectid-01 . . . . . . . . . . . 13 76 12.5. draft-ietf-extra-imap-objectid-00 . . . . . . . . . . . 14 77 12.6. draft-ietf-extra-imap-uniqueid-00 . . . . . . . . . . . 14 78 12.7. draft-gondwana-imap-uniqueid-01 . . . . . . . . . . . . 14 79 12.8. draft-gondwana-imap-uniqueid-00 . . . . . . . . . . . . 14 80 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14 81 13.1. Appendix 1: ideas for implementing object identifiers . 15 82 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 83 14.1. Normative References . . . . . . . . . . . . . . . . . . 15 84 14.2. Informative References . . . . . . . . . . . . . . . . . 16 85 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16 87 1. Introduction 89 IMAP stores are often used by many clients. Each client may cache 90 data from the server so that they don't need to re-download 91 information. [RFC3501] defines that a mailbox can be uniquely 92 referenced by its name and UIDVALIDITY, and a message within that 93 mailbox can be uniquely referenced by its mailbox (name + 94 UIDVALIDITY) and UID. The triple of mailbox name, UIDVALIDITY and 95 UID is guaranteed to be immutable. 97 [RFC4315] defines a COPYUID response which allows a client which 98 copies messages to know the mapping between the UIDs in the source 99 and destination mailboxes, and hence update its local cache. 101 If a mailbox is successfully renamed by a client, that client will 102 know that the same messages exist in the destination mailbox name as 103 previously existed in the source mailbox name. 105 The result is that the client which copies (or [RFC6851] moves) 106 messages or renames a mailbox can update its local cache, but any 107 other client connected to the same store can not know with certainty 108 that the messages are identical, and so will re-download everything. 110 This extension adds new properties to a message (EMAILID) and mailbox 111 (MAILBOXID) which allow a client to quickly identify messages or 112 mailboxes which have been renamed by another client. 114 This extension also adds an optional thread identifier (THREADID) to 115 messages, which can be used by the server to indicate messages which 116 it has identified to be related. A server that does not implement 117 threading will return NIL to all requests for THREADID. 119 2. Conventions Used In This Document 121 In examples, "C:" indicates lines sent by a client that is connected 122 to a server. "S:" indicates lines sent by the server to the client. 124 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 125 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 126 document are to be interpreted as described in [RFC2119] when they 127 appear in ALL CAPS. These words may also appear in this document in 128 lower case as plain English words, absent their normative meanings. 130 3. CAPABILITY Identification 132 IMAP servers that support this extension MUST include "OBJECTID" in 133 the response list to the CAPABILITY command. 135 4. MAILBOXID object identifier 137 The MAILBOXID is a server-allocated unique identifer for each 138 mailbox. 140 The server MUST return the same MAILBOXID for a mailbox with the same 141 name and UIDVALIDITY. 143 The server MUST NOT report the same MAILBOXID for two mailboxes at 144 the same time. 146 The server MUST NOT reuse the same MAILBOXID for a mailbox which does 147 not obey all the invarients that [RFC3501] defines for a mailbox 148 which does not change name or UIDVALIDITY. 150 The server SHOULD NOT change MAILBOXID when renaming a folder, as 151 this loses the main benefit of having a unique identifier. 153 4.1. New response code for CREATE 155 This document extends the CREATE command to have the response code 156 MAILBOXID on successful mailbox creation. 158 A server advertising the OBJECTID capability MUST include the 159 MAILBOXID response code in the tagged OK response to all successful 160 CREATE commands. 162 Syntax: "MAILBOXID" SP "(" ")" 164 Response code in tagged OK for successful CREATE command. 166 Example: 168 C: 3 create foo 169 S: 3 OK [MAILBOXID (F2212ea87-6097-4256-9d51-71338625)] Completed 170 C: 4 create bar 171 S: 4 OK [MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3)] Completed 172 C: 5 create foo 173 S: 5 NO Mailbox already exists 175 4.2. New OK Untagged Response for SELECT and EXAMINE 177 This document adds a new untagged response code to the SELECT and 178 EXAMINE commands. 180 A server advertising the OBJECTID capability MUST return an untagged 181 OK response with the MAILBOXID response code on all successful SELECT 182 and EXAMINE commands. 184 Syntax: "OK" SP "[" "MAILBOXID" SP "(" ")" "]" text 186 Untagged OK response to SELECT or EXAMINE. 188 Example: 190 C: 27 select "foo" 191 [...] 192 S: * OK [MAILBOXID (F2212ea87-6097-4256-9d51-71338625)] Ok 193 [...] 194 S: 27 OK [READ-WRITE] Completed 196 4.3. New attribute for STATUS 198 This document adds the MAILBOXID attribute to the STATUS command 199 using the extended syntax defined in [RFC4466]. 201 A server that advertises the OBJECTID capability MUST support the 202 MAILBOXID status attribute. 204 Syntax: "MAILBOXID" 206 The attribute in the STATUS command. 208 Syntax: "MAILBOXID" SP "(" ")" 210 The response item in the STATUS response contains the objectid 211 assigned by the server for this mailbox. 213 Example: 215 C: 6 status foo (mailboxid) 216 S: * STATUS foo (MAILBOXID (F2212ea87-6097-4256-9d51-71338625)) 217 S: 6 OK Completed 218 C: 7 status bar (mailboxid) 219 S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3)) 220 S: 7 OK Completed 221 C: 8 rename foo renamed 222 S: * OK rename foo renamed 223 S: 8 OK Completed 224 C: 9 status renamed (mailboxid) 225 S: * STATUS renamed (MAILBOXID (F2212ea87-6097-4256-9d51-71338625)) 226 S: 9 OK Completed 227 C: 10 status bar (mailboxid) 228 S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3)) 229 S: 10 OK Completed 231 When the LIST-STATUS IMAP capability defined in [RFC5819] is also 232 available, the STATUS command can be combined with the LIST command. 234 Example: 236 C: 11 list "" "*" return (status (mailboxid)) 237 S: * LIST (\HasNoChildren) "." INBOX 238 S: * STATUS INBOX (MAILBOXID (Ff8e3ead4-9389-4aff-adb1-d8d89efd8cbf)) 239 S: * LIST (\HasNoChildren) "." bar 240 S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3)) 241 S: * LIST (\HasNoChildren) "." renamed 242 S: * STATUS renamed (MAILBOXID (F2212ea87-6097-4256-9d51-71338625)) 243 S: 11 OK Completed (0.001 secs 3 calls) 245 5. EMAILID object identifier and THREADID correlator 247 5.1. EMAILID identifier for identical messages 249 The EMAILID data item is an objectid which uniquely identifies the 250 content of a single message. Anything which must remain immutable on 251 a {name, uidvalidity, uid} triple must also be the same between 252 messages with the same EMAILID. 254 The server MUST return the same EMAILID for the same triple, hence 255 EMAILID is immutable. 257 The server MUST return the same EMAILID as the source message for the 258 matching destination message in the COPYUID pairing after a COPY or 259 [RFC6851] MOVE command. 261 The server MAY assign the same EMAILID as an existing message upon 262 APPEND (e.g. if it detects that the new message has exactly identical 263 content to that of an existing message) 265 NOTE: EMAILID only identifies the immutable content of the message. 266 In particular, it is possible for different messages with the same 267 EMAILID to have different keywords. This document does not specify a 268 way to STORE by EMAILID. 270 5.2. THREADID identifer for related messages 272 The THREADID data item is an objectid which uniquely identifies a set 273 of messages which the server believes should be grouped together when 274 presented. 276 THREADID calculation is generally based on some combination of 277 References, In-Reply-To and Subject, but the exact logic is left up 278 to the server implementation. [RFC5256] describes some algorithms 279 that could be used, however this specfication does not mandate any 280 particular strategy. 282 The server MUST return the same THREADID for all messages with the 283 same EMAILID. 285 The server SHOULD return the same THREADID for related messages even 286 if they are in different mailboxes. 288 THREADID is optional, if the server doesn't support THREADID or is 289 unable to calculate relationships between messages, it MUST return 290 NIL to all FETCH responses for the THREADID data item, and a SEARCH 291 for THREADID MUST NOT match any messages. 293 The server MUST NOT use the same objectid value for both EMAILIDs and 294 THREADIDs. If they are stored with the same value internally, the 295 server can generate prefixed values (as shown in the examples below 296 with M and T prefixes) to avoid clashes. 298 5.3. New Message Data Items in FETCH and UID FETCH Commands 300 This document defines two FETCH items: 302 Syntax: EMAILID 304 The EMAILID message data item causes the server to return EMAILID 305 FETCH response data items. 307 Syntax: THREADID 309 The THREADID message data item causes the server to return THREADID 310 FETCH response data items. 312 And the following responses: 314 Syntax: EMAILID ( ) 316 The EMAILID response data item contains the server-assigned objectid 317 for each message. 319 Syntax: THREADID ( ) 321 The THREADID response data item contains the server-assigned objectid 322 for the set of related messages to which this message belongs. 324 Syntax: THREADID NIL 326 The NIL value to the THREADID response data item is returned when 327 the server mailbox does not support THREADID calculation. 329 Example: 331 C: 5 append inbox "20-Mar-2018 03:07:37 +1100" {733} 332 [...] 333 Subject: Message A 334 Message-ID: 335 [...] 336 S: 5 OK [APPENDUID 1521475658 1] Completed 338 C: 11 append inbox "20-Mar-2018 03:07:37 +1100" {793} 339 [...] 340 Subject: Re: Message A 341 Message-ID: 342 References: 343 [...] 344 S: 11 OK [APPENDUID 1521475658 2] Completed 346 C: 17 append inbox "20-Mar-2018 03:07:37 +1100" {736} 347 [...] 348 Subject: Message C 349 Message-ID: 350 [...] 351 S: 17 OK [APPENDUID 1521475658 3] Completed 353 C: 22 fetch 1:* (emailid threadid) 354 S: * 1 FETCH (EMAILID (M6d99ac3275bb4e) THREADID (T64b478a75b7ea9)) 355 S: * 2 FETCH (EMAILID (M288836c4c7a762) THREADID (T64b478a75b7ea9)) 356 S: * 3 FETCH (EMAILID (M5fdc09b49ea703) THREADID (T11863d02dd95b5)) 357 S: 22 OK Completed (0.000 sec) 359 C: 23 move 2 foo 360 S: * OK [COPYUID 1521475659 2 1] Completed 361 S: * 2 EXPUNGE 362 S: 23 OK Completed 364 C: 24 fetch 1:* (emailid threadid) 365 S: * 1 FETCH (EMAILID (M6d99ac3275bb4e) THREADID (T64b478a75b7ea9)) 366 S: * 2 FETCH (EMAILID (M5fdc09b49ea703) THREADID (T11863d02dd95b5)) 367 S: 24 OK Completed (0.000 sec) 368 C: 25 select "foo" 370 C: 25 select "foo" 371 [...] 372 S: 25 OK [READ-WRITE] Completed 373 C: 26 fetch 1:* (emailid threadid) 374 S: * 1 FETCH (EMAILID (M288836c4c7a762) THREADID (T64b478a75b7ea9)) 375 S: 26 OK Completed (0.000 sec) 377 Example: (no THREADID support) 378 C: 26 fetch 1:* (emailid threadid) 379 S: * 1 FETCH (EMAILID (M00000001) THREADID NIL) 380 S: * 2 FETCH (EMAILID (M00000002) THREADID NIL) 381 S: 26 OK Completed (0.000 sec) 383 6. New Filters on SEARCH command 385 This document defines filters EMAILID and THREADID on the SEARCH 386 command. 388 EMAILID 390 Messages whose EMAILID is exactly the specified objectid. 392 THREADID 394 Messages whose THREADID is exactly the specified objectid. 396 Example: (as if run before the MOVE above when the mailbox had 3 397 messages) 399 C: 27 search emailid M6d99ac3275bb4e 400 S: * SEARCH 1 401 S: 27 OK Completed (1 msgs in 0.000 secs) 402 C: 28 search threadid T64b478a75b7ea9 403 S: * SEARCH 1 2 404 S: 28 OK Completed (2 msgs in 0.000 secs) 406 7. Formal syntax 408 The following syntax specification uses the Augmented Backus-Naur 409 Form (ABNF) [RFC5234] notation. Elements not defined here can be 410 found in the formal syntax of the ABNF [RFC5234], IMAP [RFC3501], and 411 IMAP ABNF extensions [RFC4466] specifications. 413 Except as noted otherwise, all alphabetic characters are case- 414 insensitive. The use of upper- or lowercase characters to define 415 token strings is for editorial clarity only. Implementations MUST 416 accept these strings in a case-insensitive fashion. 418 capability =/ "OBJECTID" 420 fetch-att =/ "EMAILID" / "THREADID" 422 fetch-emailid-resp = "EMAILID" SP "(" objectid ")" ; follows tagged- 423 ext production from [RFC4466] 424 fetch-threadid-resp = "THREADID" SP "(" objectid ")" / "THREADID" NIL 425 ; follows tagged-ext production from [RFC4466] 427 objectid = 1*255(ALPHA / DIGIT / "_" / "-") ; characters in object 428 identifiers are case ; significant 430 resp-text-code =/ "MAILBOXID" SP "(" objectid ")" ; incorporated 431 before the expansion rule of ; atom [SP 1*] 432 ; that appears in [RFC3501] 434 search-key =/ "EMAILID" SP objectid / "THREADID" SP objectid 436 status-att =/ "MAILBOXID" 438 status-att-value =/ "MAILBOXID" SP "(" objectid ")" ; follows tagged- 439 ext production from [RFC4466] 441 8. Implementation considerations 443 8.1. Assigning object identifiers 445 All objectid values are allocated by the server. 447 In the interests of reducing the possibilities of encoding mistakes, 448 objectids are restricted to a safe subset of possible byte values, 449 and in order to allow clients to allocate storage, they are 450 restricted in length. 452 An objectid is a string of 1 to 255 characters from the following set 453 of 64 codepoints. a-z, A-Z, 0-9, '_', '-'. These characters are safe 454 to use in almost any context (e.g. filesystems, URIs, IMAP atoms). 456 For maximum safety, servers SHOULD also follow defensive allocation 457 strategies to avoid creating risks where glob completion or data type 458 detection may be present (e.g. on filesystems or in spreadsheets). 459 In particular it is wise to avoid: 461 o ids starting with - 463 o ids starting with digits 465 o ids which contain only digits 467 o ids which differ only by ASCII case (A vs a) 469 o the specific sequence of 3 characters NIL 470 A good solution to these issues is to prefix every ID with a single 471 alphabetical character. 473 8.2. Interaction with special cases 475 The case of RENAME INBOX may need special handling, as it has special 476 behaviour as defined in [RFC3501] section 6.3.5. 478 It is advisable (though not required) to have MAILBOXID be globally 479 unique, but it is only required to be unique within messages offered 480 to a single client login to a single server hostname. For example, a 481 proxy which aggregates multiple independent servers MUST NOT 482 advertise the OBJECTID capability unless it can guarantee that 483 different objects will never use the same identifiers, even if 484 backend object collide. 486 8.3. Client usage 488 Servers that implement both RFC 6154 and this specification SHOULD 489 optimize their execution of command like UID SEARCH OR EMAILID 1234 490 EMAILID 4321. 492 Clients can assume that searching the all-mail mailbox using OR/ 493 EMAILID or OR/THREADID is a fast way to find messages again if some 494 other client has moved them out of the mailbox where they were 495 previously seen. 497 Clients that cache data offline SHOULD fetch the EMAILID of all new 498 messages to avoid re-downloading already cached message details. 500 Clients SHOULD fetch the MAILBOXID for any new mailboxes before 501 discarding cache data for any mailbox which is no longer present on 502 the server, so that they can detect renames and avoid re-downloading 503 data. 505 9. Future considerations 507 This extension is intentionally defined to be compatible with the 508 data model in [I-D.ietf-jmap-mail]. 510 A future extension could be proposed to give a way to SELECT a 511 mailbox by MAILBOXID rather than name. 513 A future extension to [RFC5228] could allow fileinto by MAILBOXID 514 rather than name. 516 An extension to allow fetching message content directly via EMAILID 517 and message listings by THREADID could be proposed. 519 10. IANA Considerations 521 IANA is requested to add "OBJECTID" to the "IMAP Capabilities" 522 registry located at . 525 IANA is requested to add "MAILBOXID" to the "IMAP Response Codes" 526 registry located at with a Reference of [[THIS RFC]]. 529 11. Security Considerations 531 It is strongly advised that servers generate OBJECTIDs which are safe 532 to use as filesystem names, and unlikely to be auto-detected as 533 numbers. See implementation considerations. 535 If a digest is used for ID generation, it must have a collision 536 resistent property, so server implementations are advised to monitor 537 current security research and choose secure digests. As the IDs are 538 generated by the server, it will be possible to migrate to a new hash 539 by just creating new IDs with the new algorithm. This is 540 particularly true if a prefix is used on each ID, which can be 541 changed when the algorithm changes. 543 The use of a digest for ID generation may be used as proof that a 544 particular sequence of bytes was seen by the server, however this is 545 only a risk if IDs are leaked to clients who don't have permission to 546 fetch the data directly. Servers that are expected to handle highly 547 sensitive data should consider using a ID generation mechanism which 548 doesn't derive from a digest. 550 See also the security considerations in [RFC3501] section 11. 552 12. Changes 554 To be removed by the editor before publication 556 12.1. draft-ietf-extra-imap-objectid-04 558 o described NIL THREADID in more detail (ad review) 560 o made RFC5256 a normative reference (ad review) 562 o fixed ABNF missing quote (ad review) 564 o documented hash upgrade process (ad review) 566 o referenced RFC3501 for INBOX rename (ad review) 567 o referenced RFC3501 security considerations (secdir review) 569 o turned mealy-mouthed "SHOULDs" in to "MUSTs" on immutability 570 (genart review) 572 o remove suggested algorithms which are no longer legitimate (genart 573 review) 575 o updated proxy advice to suggest rewriting ids (genart review) 577 o fixed minor gramatical issues (genart review) 579 o required that EMAILID and THREADID are not identical (own 580 decision) 582 12.2. draft-ietf-extra-imap-objectid-03 584 o added RFC3501 to Abstract 586 o updated [[THIS RFC]] to not fail idnits 588 o changed jmap-mail to be informative rather than normative 590 o shortened IDs to stop wrapping and outdents in IMAP examples 592 12.3. draft-ietf-extra-imap-objectid-02 594 o added "Client usage" section 596 12.4. draft-ietf-extra-imap-objectid-01 598 o added "updates" for RFC3501 600 o fixed domains in thread example 602 o described threading in more detail 604 o added IANA request for Response Code 606 o clarified RFC2119 references 608 o simplified some waffle in wording 610 o added security consideration to choose good digest 612 o added MAILBOXID-UID suggestion for EMAILID generation 614 o updated ABNF normative reference to RFC5234 616 12.5. draft-ietf-extra-imap-objectid-00 618 o renamed draft to be objectid rather than uniqueid 620 o renamed UNIQUEID (capability) to OBJECTID 622 o restricted objectid to 64 safe characters 624 o added security considerations and advice about choosing objectid 626 o wrapped all responses in () for RFC4466 compatibility 628 o signifiant rewrite of all sections 630 12.6. draft-ietf-extra-imap-uniqueid-00 632 o renamed draft to be an EXTRA document 634 o added example for LIST RETURN STATUS 636 o started work on ABNF 638 o attempted to add response codes for EMAILID and THREADID 640 12.7. draft-gondwana-imap-uniqueid-01 642 o renamed UNIQUEID (status item) to MAILBOXID 644 o renamed MSGID to EMAILID 646 o renamed THRID to THREADID 648 o added TODO section 650 12.8. draft-gondwana-imap-uniqueid-00 652 o initial upload with names UNIQUEID/MSGID/THRID 654 13. Acknowledgments 656 The EXTRA working group at IETF. In particular feedback from Arnt 657 Gulbrandsen, Brandon Long, Chris Newman and Josef Sipek. 659 The Gmail X-GM-THRID and X-GM-MSGID implementation as currently 660 defined at . 663 Dovecot X-GUID implementation. 665 13.1. Appendix 1: ideas for implementing object identifiers 667 Ideas for calculating MAILBOXID: 669 o [RFC4122] UUID 671 o Server assigned sequence number (guaranteed not to be reused) 673 Ideas for implementing EMAILID: 675 o Digest of message content (RFC822 bytes) - expensive unless cached 677 o [RFC4122] UUID 679 o Server assigned sequence number (guaranteed not to be reused) 681 Ideas for implementing THREADID: 683 o Derive from EMAILID of first seen message in the thread. 685 o [RFC4122] UUID 687 o Server assigned sequence number (guaranteed not to be reused) 689 There is a need to index and look up reference/in-reply-to data at 690 message creation to efficiently find matching messages for threading. 691 Threading may be either across folders, or within each folder only. 692 The server has significant leeway here. 694 14. References 696 14.1. Normative References 698 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 699 Requirement Levels", BCP 14, RFC 2119, 700 DOI 10.17487/RFC2119, March 1997, 701 . 703 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 704 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 705 . 707 [RFC4315] Crispin, M., "Internet Message Access Protocol (IMAP) - 708 UIDPLUS extension", RFC 4315, DOI 10.17487/RFC4315, 709 December 2005, . 711 [RFC4466] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4 712 ABNF", RFC 4466, DOI 10.17487/RFC4466, April 2006, 713 . 715 [RFC5228] Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An Email 716 Filtering Language", RFC 5228, DOI 10.17487/RFC5228, 717 January 2008, . 719 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 720 Specifications: ABNF", STD 68, RFC 5234, 721 DOI 10.17487/RFC5234, January 2008, 722 . 724 [RFC5256] Crispin, M. and K. Murchison, "Internet Message Access 725 Protocol - SORT and THREAD Extensions", RFC 5256, 726 DOI 10.17487/RFC5256, June 2008, 727 . 729 [RFC5819] Melnikov, A. and T. Sirainen, "IMAP4 Extension for 730 Returning STATUS Information in Extended LIST", RFC 5819, 731 DOI 10.17487/RFC5819, March 2010, 732 . 734 [RFC6851] Gulbrandsen, A. and N. Freed, Ed., "Internet Message 735 Access Protocol (IMAP) - MOVE Extension", RFC 6851, 736 DOI 10.17487/RFC6851, January 2013, 737 . 739 14.2. Informative References 741 [I-D.ietf-jmap-mail] 742 Jenkins, N., "JMAP for Mail", draft-ietf-jmap-mail-06 743 (work in progress), July 2018. 745 [RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally 746 Unique IDentifier (UUID) URN Namespace", RFC 4122, 747 DOI 10.17487/RFC4122, July 2005, 748 . 750 Author's Address 751 Bron Gondwana (editor) 752 FastMail 753 Level 2, 114 William St 754 Melbourne VIC 3000 755 Australia 757 Email: brong@fastmailteam.com 758 URI: https://www.fastmail.com