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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-05 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-05 . . . . . . . . . . . 12 73 12.2. draft-ietf-extra-imap-objectid-04 . . . . . . . . . . . 13 74 12.3. draft-ietf-extra-imap-objectid-03 . . . . . . . . . . . 13 75 12.4. draft-ietf-extra-imap-objectid-02 . . . . . . . . . . . 13 76 12.5. draft-ietf-extra-imap-objectid-01 . . . . . . . . . . . 13 77 12.6. draft-ietf-extra-imap-objectid-00 . . . . . . . . . . . 14 78 12.7. draft-ietf-extra-imap-uniqueid-00 . . . . . . . . . . . 14 79 12.8. draft-gondwana-imap-uniqueid-01 . . . . . . . . . . . . 14 80 12.9. draft-gondwana-imap-uniqueid-00 . . . . . . . . . . . . 15 81 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15 82 13.1. Appendix 1: ideas for implementing object identifiers . 15 83 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 16 84 14.1. Normative References . . . . . . . . . . . . . . . . . . 16 85 14.2. Informative References . . . . . . . . . . . . . . . . . 17 86 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17 88 1. Introduction 90 IMAP stores are often used by many clients. Each client may cache 91 data from the server so that they don't need to re-download 92 information. [RFC3501] defines that a mailbox can be uniquely 93 referenced by its name and UIDVALIDITY, and a message within that 94 mailbox can be uniquely referenced by its mailbox (name + 95 UIDVALIDITY) and UID. The triple of mailbox name, UIDVALIDITY and 96 UID is guaranteed to be immutable. 98 [RFC4315] defines a COPYUID response which allows a client which 99 copies messages to know the mapping between the UIDs in the source 100 and destination mailboxes, and hence update its local cache. 102 If a mailbox is successfully renamed by a client, that client will 103 know that the same messages exist in the destination mailbox name as 104 previously existed in the source mailbox name. 106 The result is that the client which copies (or [RFC6851] moves) 107 messages or renames a mailbox can update its local cache, but any 108 other client connected to the same store can not know with certainty 109 that the messages are identical, and so will re-download everything. 111 This extension adds new properties to a message (EMAILID) and mailbox 112 (MAILBOXID) which allow a client to quickly identify messages or 113 mailboxes which have been renamed by another client. 115 This extension also adds an optional thread identifier (THREADID) to 116 messages, which can be used by the server to indicate messages which 117 it has identified to be related. A server that does not implement 118 threading will return NIL to all requests for THREADID. 120 2. Conventions Used In This Document 122 In examples, "C:" indicates lines sent by a client that is connected 123 to a server. "S:" indicates lines sent by the server to the client. 125 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 126 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 127 document are to be interpreted as described in [RFC2119] when they 128 appear in ALL CAPS. These words may also appear in this document in 129 lower case as plain English words, absent their normative meanings. 131 3. CAPABILITY Identification 133 IMAP servers that support this extension MUST include "OBJECTID" in 134 the response list to the CAPABILITY command. 136 4. MAILBOXID object identifier 138 The MAILBOXID is a server-allocated unique identifer for each 139 mailbox. 141 The server MUST return the same MAILBOXID for a mailbox with the same 142 name and UIDVALIDITY. 144 The server MUST NOT report the same MAILBOXID for two mailboxes at 145 the same time. 147 The server MUST NOT reuse the same MAILBOXID for a mailbox which does 148 not obey all the invarients that [RFC3501] defines for a mailbox 149 which does not change name or UIDVALIDITY. 151 The server SHOULD NOT change MAILBOXID when renaming a folder, as 152 this loses the main benefit of having a unique identifier. 154 4.1. New response code for CREATE 156 This document extends the CREATE command to have the response code 157 MAILBOXID on successful mailbox creation. 159 A server advertising the OBJECTID capability MUST include the 160 MAILBOXID response code in the tagged OK response to all successful 161 CREATE commands. 163 Syntax: "MAILBOXID" SP "(" ")" 165 Response code in tagged OK for successful CREATE command. 167 Example: 169 C: 3 create foo 170 S: 3 OK [MAILBOXID (F2212ea87-6097-4256-9d51-71338625)] Completed 171 C: 4 create bar 172 S: 4 OK [MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3)] Completed 173 C: 5 create foo 174 S: 5 NO Mailbox already exists 176 4.2. New OK Untagged Response for SELECT and EXAMINE 178 This document adds a new untagged response code to the SELECT and 179 EXAMINE commands. 181 A server advertising the OBJECTID capability MUST return an untagged 182 OK response with the MAILBOXID response code on all successful SELECT 183 and EXAMINE commands. 185 Syntax: "OK" SP "[" "MAILBOXID" SP "(" ")" "]" text 187 Untagged OK response to SELECT or EXAMINE. 189 Example: 191 C: 27 select "foo" 192 [...] 193 S: * OK [MAILBOXID (F2212ea87-6097-4256-9d51-71338625)] Ok 194 [...] 195 S: 27 OK [READ-WRITE] Completed 197 4.3. New attribute for STATUS 199 This document adds the MAILBOXID attribute to the STATUS command 200 using the extended syntax defined in [RFC4466]. 202 A server that advertises the OBJECTID capability MUST support the 203 MAILBOXID status attribute. 205 Syntax: "MAILBOXID" 207 The attribute in the STATUS command. 209 Syntax: "MAILBOXID" SP "(" ")" 211 The response item in the STATUS response contains the objectid 212 assigned by the server for this mailbox. 214 Example: 216 C: 6 status foo (mailboxid) 217 S: * STATUS foo (MAILBOXID (F2212ea87-6097-4256-9d51-71338625)) 218 S: 6 OK Completed 219 C: 7 status bar (mailboxid) 220 S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3)) 221 S: 7 OK Completed 222 C: 8 rename foo renamed 223 S: * OK rename foo renamed 224 S: 8 OK Completed 225 C: 9 status renamed (mailboxid) 226 S: * STATUS renamed (MAILBOXID (F2212ea87-6097-4256-9d51-71338625)) 227 S: 9 OK Completed 228 C: 10 status bar (mailboxid) 229 S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3)) 230 S: 10 OK Completed 232 When the LIST-STATUS IMAP capability defined in [RFC5819] is also 233 available, the STATUS command can be combined with the LIST command. 235 Example: 237 C: 11 list "" "*" return (status (mailboxid)) 238 S: * LIST (\HasNoChildren) "." INBOX 239 S: * STATUS INBOX (MAILBOXID (Ff8e3ead4-9389-4aff-adb1-d8d89efd8cbf)) 240 S: * LIST (\HasNoChildren) "." bar 241 S: * STATUS bar (MAILBOXID (F6352ae03-b7f5-463c-896f-d8b48ee3)) 242 S: * LIST (\HasNoChildren) "." renamed 243 S: * STATUS renamed (MAILBOXID (F2212ea87-6097-4256-9d51-71338625)) 244 S: 11 OK Completed (0.001 secs 3 calls) 246 5. EMAILID object identifier and THREADID correlator 248 5.1. EMAILID identifier for identical messages 250 The EMAILID data item is an objectid which uniquely identifies the 251 content of a single message. Anything which must remain immutable on 252 a {name, uidvalidity, uid} triple must also be the same between 253 messages with the same EMAILID. 255 The server MUST return the same EMAILID for the same triple, hence 256 EMAILID is immutable. 258 The server MUST return the same EMAILID as the source message for the 259 matching destination message in the COPYUID pairing after a COPY or 260 [RFC6851] MOVE command. 262 The server MAY assign the same EMAILID as an existing message upon 263 APPEND (e.g. if it detects that the new message has exactly identical 264 content to that of an existing message) 266 NOTE: EMAILID only identifies the immutable content of the message. 267 In particular, it is possible for different messages with the same 268 EMAILID to have different keywords. This document does not specify a 269 way to STORE by EMAILID. 271 5.2. THREADID identifer for related messages 273 The THREADID data item is an objectid which uniquely identifies a set 274 of messages which the server believes should be grouped together when 275 presented. 277 THREADID calculation is generally based on some combination of 278 References, In-Reply-To and Subject, but the exact logic is left up 279 to the server implementation. [RFC5256] describes some algorithms 280 that could be used, however this specfication does not mandate any 281 particular strategy. 283 The server MUST return the same THREADID for all messages with the 284 same EMAILID. 286 The server SHOULD return the same THREADID for related messages even 287 if they are in different mailboxes. 289 The server MUST NOT change the THREADID of a message once reported. 291 THREADID is optional, if the server doesn't support THREADID or is 292 unable to calculate relationships between messages, it MUST return 293 NIL to all FETCH responses for the THREADID data item, and a SEARCH 294 for THREADID MUST NOT match any messages. 296 The server MUST NOT use the same objectid value for both EMAILIDs and 297 THREADIDs. If they are stored with the same value internally, the 298 server can generate prefixed values (as shown in the examples below 299 with M and T prefixes) to avoid clashes. 301 5.3. New Message Data Items in FETCH and UID FETCH Commands 303 This document defines two FETCH items: 305 Syntax: EMAILID 307 The EMAILID message data item causes the server to return EMAILID 308 FETCH response data items. 310 Syntax: THREADID 312 The THREADID message data item causes the server to return THREADID 313 FETCH response data items. 315 And the following responses: 317 Syntax: EMAILID ( ) 319 The EMAILID response data item contains the server-assigned objectid 320 for each message. 322 Syntax: THREADID ( ) 324 The THREADID response data item contains the server-assigned objectid 325 for the set of related messages to which this message belongs. 327 Syntax: THREADID NIL 329 The NIL value to the THREADID response data item is returned when 330 the server mailbox does not support THREADID calculation. 332 Example: 334 C: 5 append inbox "20-Mar-2018 03:07:37 +1100" {733} 335 [...] 336 Subject: Message A 337 Message-ID: 338 [...] 339 S: 5 OK [APPENDUID 1521475658 1] Completed 341 C: 11 append inbox "20-Mar-2018 03:07:37 +1100" {793} 342 [...] 343 Subject: Re: Message A 344 Message-ID: 345 References: 346 [...] 347 S: 11 OK [APPENDUID 1521475658 2] Completed 349 C: 17 append inbox "20-Mar-2018 03:07:37 +1100" {736} 350 [...] 351 Subject: Message C 352 Message-ID: 353 [...] 354 S: 17 OK [APPENDUID 1521475658 3] Completed 356 C: 22 fetch 1:* (emailid threadid) 357 S: * 1 FETCH (EMAILID (M6d99ac3275bb4e) THREADID (T64b478a75b7ea9)) 358 S: * 2 FETCH (EMAILID (M288836c4c7a762) THREADID (T64b478a75b7ea9)) 359 S: * 3 FETCH (EMAILID (M5fdc09b49ea703) THREADID (T11863d02dd95b5)) 360 S: 22 OK Completed (0.000 sec) 362 C: 23 move 2 foo 363 S: * OK [COPYUID 1521475659 2 1] Completed 364 S: * 2 EXPUNGE 365 S: 23 OK Completed 367 C: 24 fetch 1:* (emailid threadid) 368 S: * 1 FETCH (EMAILID (M6d99ac3275bb4e) THREADID (T64b478a75b7ea9)) 369 S: * 2 FETCH (EMAILID (M5fdc09b49ea703) THREADID (T11863d02dd95b5)) 370 S: 24 OK Completed (0.000 sec) 371 C: 25 select "foo" 373 C: 25 select "foo" 374 [...] 375 S: 25 OK [READ-WRITE] Completed 376 C: 26 fetch 1:* (emailid threadid) 377 S: * 1 FETCH (EMAILID (M288836c4c7a762) THREADID (T64b478a75b7ea9)) 378 S: 26 OK Completed (0.000 sec) 380 Example: (no THREADID support) 381 C: 26 fetch 1:* (emailid threadid) 382 S: * 1 FETCH (EMAILID (M00000001) THREADID NIL) 383 S: * 2 FETCH (EMAILID (M00000002) THREADID NIL) 384 S: 26 OK Completed (0.000 sec) 386 6. New Filters on SEARCH command 388 This document defines filters EMAILID and THREADID on the SEARCH 389 command. 391 EMAILID 393 Messages whose EMAILID is exactly the specified objectid. 395 THREADID 397 Messages whose THREADID is exactly the specified objectid. 399 Example: (as if run before the MOVE above when the mailbox had 3 400 messages) 402 C: 27 search emailid M6d99ac3275bb4e 403 S: * SEARCH 1 404 S: 27 OK Completed (1 msgs in 0.000 secs) 405 C: 28 search threadid T64b478a75b7ea9 406 S: * SEARCH 1 2 407 S: 28 OK Completed (2 msgs in 0.000 secs) 409 7. Formal syntax 411 The following syntax specification uses the Augmented Backus-Naur 412 Form (ABNF) [RFC5234] notation. Elements not defined here can be 413 found in the formal syntax of the ABNF [RFC5234], IMAP [RFC3501], and 414 IMAP ABNF extensions [RFC4466] specifications. 416 Except as noted otherwise, all alphabetic characters are case- 417 insensitive. The use of upper- or lowercase characters to define 418 token strings is for editorial clarity only. Implementations MUST 419 accept these strings in a case-insensitive fashion. 421 capability =/ "OBJECTID" 423 fetch-att =/ "EMAILID" / "THREADID" 425 fetch-emailid-resp = "EMAILID" SP "(" objectid ")" ; follows tagged- 426 ext production from [RFC4466] 427 fetch-threadid-resp = "THREADID" SP "(" objectid ")" / "THREADID" NIL 428 ; follows tagged-ext production from [RFC4466] 430 objectid = 1*255(ALPHA / DIGIT / "_" / "-") ; characters in object 431 identifiers are case ; significant 433 resp-text-code =/ "MAILBOXID" SP "(" objectid ")" ; incorporated 434 before the expansion rule of ; atom [SP 1*] 435 ; that appears in [RFC3501] 437 search-key =/ "EMAILID" SP objectid / "THREADID" SP objectid 439 status-att =/ "MAILBOXID" 441 status-att-value =/ "MAILBOXID" SP "(" objectid ")" ; follows tagged- 442 ext production from [RFC4466] 444 8. Implementation considerations 446 8.1. Assigning object identifiers 448 All objectid values are allocated by the server. 450 In the interests of reducing the possibilities of encoding mistakes, 451 objectids are restricted to a safe subset of possible byte values, 452 and in order to allow clients to allocate storage, they are 453 restricted in length. 455 An objectid is a string of 1 to 255 characters from the following set 456 of 64 codepoints. a-z, A-Z, 0-9, '_', '-'. These characters are safe 457 to use in almost any context (e.g. filesystems, URIs, IMAP atoms). 459 For maximum safety, servers should also follow defensive allocation 460 strategies to avoid creating risks where glob completion or data type 461 detection may be present (e.g. on filesystems or in spreadsheets). 462 In particular it is wise to avoid: 464 o ids starting with - 466 o ids starting with digits 468 o ids which contain only digits 470 o ids which differ only by ASCII case (A vs a) 472 o the specific sequence of 3 characters NIL 473 A good solution to these issues is to prefix every ID with a single 474 alphabetical character. 476 8.2. Interaction with special cases 478 The case of RENAME INBOX may need special handling, as it has special 479 behaviour as defined in [RFC3501] section 6.3.5. 481 It is advisable (though not required) to have MAILBOXID be globally 482 unique, but it is only required to be unique within messages offered 483 to a single client login to a single server hostname. For example, a 484 proxy which aggregates multiple independent servers MUST NOT 485 advertise the OBJECTID capability unless it can guarantee that 486 different objects will never use the same identifiers, even if 487 backend object collide. 489 8.3. Client usage 491 Servers that implement both RFC 6154 and this specification should 492 optimize their execution of command like UID SEARCH OR EMAILID 1234 493 EMAILID 4321. 495 Clients can assume that searching the all-mail mailbox using OR/ 496 EMAILID or OR/THREADID is a fast way to find messages again if some 497 other client has moved them out of the mailbox where they were 498 previously seen. 500 Clients that cache data offline should fetch the EMAILID of all new 501 messages to avoid re-downloading already cached message details. 503 Clients should fetch the MAILBOXID for any new mailboxes before 504 discarding cache data for any mailbox which is no longer present on 505 the server, so that they can detect renames and avoid re-downloading 506 data. 508 9. Future considerations 510 This extension is intentionally defined to be compatible with the 511 data model in [I-D.ietf-jmap-mail]. 513 A future extension could be proposed to give a way to SELECT a 514 mailbox by MAILBOXID rather than name. 516 A future extension to [RFC5228] could allow fileinto by MAILBOXID 517 rather than name. 519 An extension to allow fetching message content directly via EMAILID 520 and message listings by THREADID could be proposed. 522 10. IANA Considerations 524 IANA is requested to add "OBJECTID" to the "IMAP Capabilities" 525 registry located at . 528 IANA is requested to add "MAILBOXID" to the "IMAP Response Codes" 529 registry located at with a Reference of [[THIS RFC]]. 532 11. Security Considerations 534 It is strongly advised that servers generate OBJECTIDs which are safe 535 to use as filesystem names, and unlikely to be auto-detected as 536 numbers. See implementation considerations. 538 If a digest is used for ID generation, it must have a collision 539 resistent property, so server implementations are advised to monitor 540 current security research and choose secure digests. As the IDs are 541 generated by the server, it will be possible to migrate to a new hash 542 by just creating new IDs with the new algorithm. This is 543 particularly true if a prefix is used on each ID, which can be 544 changed when the algorithm changes. 546 The use of a digest for ID generation may be used as proof that a 547 particular sequence of bytes was seen by the server, however this is 548 only a risk if IDs are leaked to clients who don't have permission to 549 fetch the data directly. Servers that are expected to handle highly 550 sensitive data should consider using a ID generation mechanism which 551 doesn't derive from a digest. 553 See also the security considerations in [RFC3501] section 11. 555 12. Changes 557 To be removed by the editor before publication 559 12.1. draft-ietf-extra-imap-objectid-05 561 o changed some SHOULD to lower case in advice sections (genart 562 review) 564 o clarified that THREADID MUST NOT change 566 12.2. draft-ietf-extra-imap-objectid-04 568 o described NIL THREADID in more detail (ad review) 570 o made RFC5256 a normative reference (ad review) 572 o fixed ABNF missing quote (ad review) 574 o documented hash upgrade process (ad review) 576 o referenced RFC3501 for INBOX rename (ad review) 578 o referenced RFC3501 security considerations (secdir review) 580 o turned mealy-mouthed "SHOULDs" in to "MUSTs" on immutability 581 (genart review) 583 o remove suggested algorithms which are no longer legitimate (genart 584 review) 586 o updated proxy advice to suggest rewriting ids (genart review) 588 o fixed minor gramatical issues (genart review) 590 o required that EMAILID and THREADID are not identical (own 591 decision) 593 12.3. draft-ietf-extra-imap-objectid-03 595 o added RFC3501 to Abstract 597 o updated [[THIS RFC]] to not fail idnits 599 o changed jmap-mail to be informative rather than normative 601 o shortened IDs to stop wrapping and outdents in IMAP examples 603 12.4. draft-ietf-extra-imap-objectid-02 605 o added "Client usage" section 607 12.5. draft-ietf-extra-imap-objectid-01 609 o added "updates" for RFC3501 611 o fixed domains in thread example 613 o described threading in more detail 614 o added IANA request for Response Code 616 o clarified RFC2119 references 618 o simplified some waffle in wording 620 o added security consideration to choose good digest 622 o added MAILBOXID-UID suggestion for EMAILID generation 624 o updated ABNF normative reference to RFC5234 626 12.6. draft-ietf-extra-imap-objectid-00 628 o renamed draft to be objectid rather than uniqueid 630 o renamed UNIQUEID (capability) to OBJECTID 632 o restricted objectid to 64 safe characters 634 o added security considerations and advice about choosing objectid 636 o wrapped all responses in () for RFC4466 compatibility 638 o signifiant rewrite of all sections 640 12.7. draft-ietf-extra-imap-uniqueid-00 642 o renamed draft to be an EXTRA document 644 o added example for LIST RETURN STATUS 646 o started work on ABNF 648 o attempted to add response codes for EMAILID and THREADID 650 12.8. draft-gondwana-imap-uniqueid-01 652 o renamed UNIQUEID (status item) to MAILBOXID 654 o renamed MSGID to EMAILID 656 o renamed THRID to THREADID 658 o added TODO section 660 12.9. draft-gondwana-imap-uniqueid-00 662 o initial upload with names UNIQUEID/MSGID/THRID 664 13. Acknowledgments 666 The EXTRA working group at IETF. In particular feedback from Arnt 667 Gulbrandsen, Brandon Long, Chris Newman and Josef Sipek. 669 The Gmail X-GM-THRID and X-GM-MSGID implementation as currently 670 defined at . 673 Dovecot X-GUID implementation. 675 13.1. Appendix 1: ideas for implementing object identifiers 677 Ideas for calculating MAILBOXID: 679 o [RFC4122] UUID 681 o Server assigned sequence number (guaranteed not to be reused) 683 Ideas for implementing EMAILID: 685 o Digest of message content (RFC822 bytes) - expensive unless cached 687 o [RFC4122] UUID 689 o Server assigned sequence number (guaranteed not to be reused) 691 Ideas for implementing THREADID: 693 o Derive from EMAILID of first seen message in the thread. 695 o [RFC4122] UUID 697 o Server assigned sequence number (guaranteed not to be reused) 699 There is a need to index and look up reference/in-reply-to data at 700 message creation to efficiently find matching messages for threading. 701 Threading may be either across folders, or within each folder only. 702 The server has significant leeway here. 704 14. References 706 14.1. Normative References 708 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 709 Requirement Levels", BCP 14, RFC 2119, 710 DOI 10.17487/RFC2119, March 1997, 711 . 713 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 714 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 715 . 717 [RFC4315] Crispin, M., "Internet Message Access Protocol (IMAP) - 718 UIDPLUS extension", RFC 4315, DOI 10.17487/RFC4315, 719 December 2005, . 721 [RFC4466] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4 722 ABNF", RFC 4466, DOI 10.17487/RFC4466, April 2006, 723 . 725 [RFC5228] Guenther, P., Ed. and T. Showalter, Ed., "Sieve: An Email 726 Filtering Language", RFC 5228, DOI 10.17487/RFC5228, 727 January 2008, . 729 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 730 Specifications: ABNF", STD 68, RFC 5234, 731 DOI 10.17487/RFC5234, January 2008, 732 . 734 [RFC5256] Crispin, M. and K. Murchison, "Internet Message Access 735 Protocol - SORT and THREAD Extensions", RFC 5256, 736 DOI 10.17487/RFC5256, June 2008, 737 . 739 [RFC5819] Melnikov, A. and T. Sirainen, "IMAP4 Extension for 740 Returning STATUS Information in Extended LIST", RFC 5819, 741 DOI 10.17487/RFC5819, March 2010, 742 . 744 [RFC6851] Gulbrandsen, A. and N. Freed, Ed., "Internet Message 745 Access Protocol (IMAP) - MOVE Extension", RFC 6851, 746 DOI 10.17487/RFC6851, January 2013, 747 . 749 14.2. Informative References 751 [I-D.ietf-jmap-mail] 752 Jenkins, N., "JMAP for Mail", draft-ietf-jmap-mail-06 753 (work in progress), July 2018. 755 [RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally 756 Unique IDentifier (UUID) URN Namespace", RFC 4122, 757 DOI 10.17487/RFC4122, July 2005, 758 . 760 Author's Address 762 Bron Gondwana (editor) 763 FastMail 764 Level 2, 114 William St 765 Melbourne VIC 3000 766 Australia 768 Email: brong@fastmailteam.com 769 URI: https://www.fastmail.com