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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group A. Melnikov 3 Internet-Draft Isode Ltd. 4 Obsoletes: 4551 (if approved) August 21, 2013 5 Updates: 3501, 2683 (if approved) 6 Intended status: Standards Track 7 Expires: February 22, 2014 9 IMAP Extension for Conditional STORE Operation or Quick Flag Changes 10 Resynchronization 11 draft-ietf-qresync-rfc4551bis-03.txt 13 Abstract 15 Often, multiple IMAP (RFC 3501) clients need to coordinate changes to 16 a common IMAP mailbox. Examples include different clients working on 17 behalf of the same user, and multiple users accessing shared 18 mailboxes. These clients need a mechanism to synchronize state 19 changes for messages within the mailbox. They must be able to 20 guarantee that only one client can change message state (e.g., 21 message flags) at any time. An example of such an application is use 22 of an IMAP mailbox as a message queue with multiple dequeueing 23 clients. 25 The Conditional Store facility provides a protected update mechanism 26 for message state information that can detect and resolve conflicts 27 between multiple writing mail clients. 29 The Conditional Store facility also allows a client to quickly 30 resynchronize mailbox flag changes. 32 This document defines an extension to IMAP (RFC 3501). 34 Status of This Memo 36 This Internet-Draft is submitted in full conformance with the 37 provisions of BCP 78 and BCP 79. 39 Internet-Drafts are working documents of the Internet Engineering 40 Task Force (IETF). Note that other groups may also distribute 41 working documents as Internet-Drafts. The list of current Internet- 42 Drafts is at http://datatracker.ietf.org/drafts/current/. 44 Internet-Drafts are draft documents valid for a maximum of six months 45 and may be updated, replaced, or obsoleted by other documents at any 46 time. It is inappropriate to use Internet-Drafts as reference 47 material or to cite them other than as "work in progress." 48 This Internet-Draft will expire on February 22, 2014. 50 Copyright Notice 52 Copyright (c) 2013 IETF Trust and the persons identified as the 53 document authors. All rights reserved. 55 This document is subject to BCP 78 and the IETF Trust's Legal 56 Provisions Relating to IETF Documents 57 (http://trustee.ietf.org/license-info) in effect on the date of 58 publication of this document. Please review these documents 59 carefully, as they describe your rights and restrictions with respect 60 to this document. Code Components extracted from this document must 61 include Simplified BSD License text as described in Section 4.e of 62 the Trust Legal Provisions and are provided without warranty as 63 described in the Simplified BSD License. 65 Table of Contents 67 1. Introduction and Overview . . . . . . . . . . . . . . . . . . 3 68 2. Conventions Used in This Document . . . . . . . . . . . . . . 5 69 3. IMAP Protocol Changes . . . . . . . . . . . . . . . . . . . . 6 70 3.1. New OK Untagged Responses for SELECT and EXAMINE . . . . 6 71 3.1.1. HIGHESTMODSEQ Response Code . . . . . . . . . . . . . 6 72 3.1.2. NOMODSEQ Response Code . . . . . . . . . . . . . . . 7 73 3.2. STORE and UID STORE Commands . . . . . . . . . . . . . . 8 74 3.3. FETCH and UID FETCH Commands . . . . . . . . . . . . . . 14 75 3.3.1. CHANGEDSINCE FETCH Modifier . . . . . . . . . . . . . 14 76 3.3.2. MODSEQ Message Data Item in FETCH Command . . . . . . 14 77 3.4. MODSEQ Search Criterion in SEARCH . . . . . . . . . . . . 16 78 3.5. Modified SEARCH Untagged Response . . . . . . . . . . . . 18 79 3.6. HIGHESTMODSEQ Status Data Items . . . . . . . . . . . . . 18 80 3.7. CONDSTORE Parameter to SELECT and EXAMINE . . . . . . . . 18 81 3.8. Interaction with IMAP SORT and THREAD extensions . . . . 19 82 3.9. Interaction with IMAP ESORT and ESEARCH extensions . . . 19 83 3.10. Additional Quality-of-Implementation Issues . . . . . . . 20 84 4. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 20 85 5. Server Implementation Considerations . . . . . . . . . . . . 23 86 6. Long Command Lines . . . . . . . . . . . . . . . . . . . . . 24 87 7. Security Considerations . . . . . . . . . . . . . . . . . . . 24 88 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 89 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25 90 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 91 10.1. Normative References . . . . . . . . . . . . . . . . . . 25 92 10.2. Informative References . . . . . . . . . . . . . . . . . 25 93 Appendix A. Changes since RFC 4551 . . . . . . . . . . . . . . . 26 94 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 26 96 1. Introduction and Overview 98 The Conditional STORE extension is present in any IMAP4 99 implementation that returns "CONDSTORE" as one of the supported 100 capabilities in the CAPABILITY command response. 102 An IMAP server that supports this extension MUST associate a positive 103 unsigned 64-bit value called a modification sequence (mod-sequence) 104 with every IMAP message. This is an opaque value updated by the 105 server whenever a metadata item is modified. The server MUST 106 guarantee that each STORE command performed on the same mailbox 107 (including simultaneous stores to different metadata items from 108 different connections) will get a different mod-sequence value. 109 Also, for any two successful STORE operations performed in the same 110 session on the same mailbox, the mod-sequence of the second completed 111 operation MUST be greater than the mod-sequence of the first 112 completed. Note that the latter rule disallows the use of the system 113 clock as a mod-sequence, because if system time changes (e.g., an NTP 114 [RFC1305] client adjusting the time), the next generated value might 115 be less than the previous one. 117 Mod-sequences allow a client that supports the CONDSTORE extension to 118 determine if a message metadata has changed since some known moment. 119 Whenever the state of a flag changes (i.e., the flag is added where 120 previously it wasn't set, or the flag is removed and before it was 121 set) the value of the modification sequence for the message MUST be 122 updated. Adding the flag when it is already present or removing when 123 it is not present SHOULD NOT change the mod-sequence. 125 When a message is appended to a mailbox (via the IMAP APPEND command, 126 COPY to the mailbox, or using an external mechanism) the server 127 generates a new modification sequence that is higher than the highest 128 modification sequence of all messages in the mailbox and assigns it 129 to the appended message. 131 The server MAY store separate (per-message) modification sequence 132 values for different metadata items. If the server does so, per- 133 message mod-sequence is the highest mod-sequence of all metadata 134 items for the specified message. 136 The server that supports this extension is not required to be able to 137 store mod-sequences for every available mailbox. Section 3.1.2 138 describes how the server may act if a particular mailbox doesn't 139 support the persistent storage of mod-sequences. 141 This extension makes the following changes to the IMAP4 protocol: 143 a. adds UNCHANGEDSINCE STORE modifier. 145 b. adds the MODIFIED response code which should be used with an OK 146 response to the STORE command. (It can also be used in a NO 147 response.) 149 c. adds a new MODSEQ message data item for use with the FETCH 150 command. 152 d. adds CHANGEDSINCE FETCH modifier. 154 e. adds a new MODSEQ search criterion. 156 f. extends the syntax of untagged SEARCH responses to include mod- 157 sequence. 159 g. adds new OK untagged responses for the SELECT and EXAMINE 160 commands. 162 h. defines an additional parameter to SELECT/EXAMINE commands. 164 i. adds the HIGHESTMODSEQ status data item to the STATUS command. 166 A client supporting CONDSTORE extension indicates its willingness to 167 receive mod-sequence updates in all untagged FETCH responses by 168 issuing: 170 o a SELECT or EXAMINE command with the CONDSTORE parameter, 172 o a STATUS (HIGHESTMODSEQ) command, 174 o a FETCH or SEARCH command that includes the MODSEQ message data 175 item, 177 o a FETCH command with the CHANGEDSINCE modifier, or 179 o a STORE command with the UNCHANGEDSINCE modifier. 181 o a ENABLE command containing "CONDSTORE" as one of the parameters. 182 (This requirement only applies to servers that also implement the 183 ENABLE extension [RFC5161].) 185 The server MUST include mod-sequence data in all subsequent untagged 186 FETCH responses (until the connection is closed), whether they were 187 caused by a regular STORE, a STORE with UNCHANGEDSINCE modifier, or 188 an external agent. 190 This document uses the term "CONDSTORE-aware client" to refer to a 191 client that announces its willingness to receive mod-sequence updates 192 as described above. The term "CONDSTORE enabling command" will refer 193 any of the commands listed above. A future extension to this 194 document may extend the list of CONDSTORE enabling commands. A first 195 CONDSTORE enabling command executed in the session with a mailbox 196 selected MUST cause the server to return HIGHESTMODSEQ 197 (Section 3.1.1) for the mailbox, unless the server has sent NOMODSEQ 198 (Section 3.1.2) response code when the currently selected mailbox was 199 selected. 201 The rest of this document describes the protocol changes more 202 rigorously. 204 2. Conventions Used in This Document 206 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 207 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 208 document are to be interpreted as described in RFC 2119 [RFC2119]. 210 In examples, lines beginning with "S:" are sent by the IMAP server, 211 and lines beginning with "C:" are sent by the client. Line breaks 212 may appear in example commands solely for editorial clarity; when 213 present in the actual message, they are represented by "CRLF". 215 Formal syntax is defined using ABNF [RFC5234]. 217 The term "metadata" or "metadata item" is used throughout this 218 document. It refers to any system or user-defined keyword. Future 219 documents may extend "metadata" to include other dynamic message 220 data. 222 Some IMAP mailboxes are private, accessible only to the owning user. 223 Other mailboxes are not, either because the owner has set an Access 224 Control List [RFC4314] that permits access by other users, or because 225 it is a shared mailbox. Let's call a metadata item "shared" for the 226 mailbox if any changes to the metadata items are persistent and 227 visible to all other users accessing the mailbox. Otherwise, the 228 metadata item is called "private". Note that private metadata items 229 are still visible to all sessions accessing the mailbox as the same 230 user. Also note that different mailboxes may have different metadata 231 items as shared. 233 See Section 1 for the definition of a "CONDSTORE-aware client" and a 234 "CONDSTORE enabling command". 236 3. IMAP Protocol Changes 238 3.1. New OK Untagged Responses for SELECT and EXAMINE 240 This document adds two new response codes, HIGHESTMODSEQ and 241 NOMODSEQ. One of those response codes MUST be returned in the OK 242 untagged response for a successful SELECT/EXAMINE command. 244 When opening a mailbox, the server must check if the mailbox supports 245 the persistent storage of mod-sequences. If the mailbox supports the 246 persistent storage of mod-sequences and the mailbox open operation 247 succeeds, the server MUST send the OK untagged response including 248 HIGHESTMODSEQ response code. If the persistent storage for the 249 mailbox is not supported, the server MUST send the OK untagged 250 response including NOMODSEQ response code instead. 252 3.1.1. HIGHESTMODSEQ Response Code 254 This document adds a new response code that is returned in the OK 255 untagged response for the SELECT and EXAMINE commands. A server 256 supporting the persistent storage of mod-sequences for the mailbox 257 MUST send the OK untagged response including HIGHESTMODSEQ response 258 code with every successful SELECT or EXAMINE command: 260 OK [HIGHESTMODSEQ ] 262 where is the highest mod-sequence value of 263 all messages in the mailbox. When the server changes UIDVALIDITY 264 for a mailbox, it doesn't have to keep the same HIGHESTMODSEQ for 265 the mailbox. 267 Note that this requirement applies whether or not a CONDSTORE 268 enabling command was issued in the session. 270 A disconnected client can use the value of HIGHESTMODSEQ to check if 271 it has to refetch metadata from the server. If the UIDVALIDITY value 272 has changed for the selected mailbox, the client MUST delete the 273 cached value of HIGHESTMODSEQ. If UIDVALIDITY for the mailbox is the 274 same, and if the HIGHESTMODSEQ value stored in the client's cache is 275 less than the value returned by the server, then some metadata items 276 on the server have changed since the last synchronization, and the 277 client needs to update its cache. The client MAY use SEARCH MODSEQ 278 (Section 3.4) to find out exactly which metadata items have changed. 279 Alternatively, the client MAY issue FETCH with the CHANGEDSINCE 280 modifier (Section 3.3.1) in order to fetch data for all messages that 281 have metadata items changed since some known modification sequence. 283 C: A142 SELECT INBOX 284 S: * 172 EXISTS 285 S: * 1 RECENT 286 S: * OK [UNSEEN 12] Message 12 is first unseen 287 S: * OK [UIDVALIDITY 3857529045] UIDs valid 288 S: * OK [UIDNEXT 4392] Predicted next UID 289 S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft) 290 S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited 291 S: * OK [HIGHESTMODSEQ 715194045007] 292 S: A142 OK [READ-WRITE] SELECT completed 294 Example 1 296 3.1.2. NOMODSEQ Response Code 298 A server that doesn't support the persistent storage of mod-sequences 299 for the mailbox MUST send the OK untagged response including NOMODSEQ 300 response code with every successful SELECT or EXAMINE command. Note 301 that this requirement applies whether or not a CONDSTORE enabling 302 command was issued in the session. 304 A server that returned NOMODSEQ response code for a mailbox, which 305 subsequently receives one of the following commands while the mailbox 306 is selected: 308 o a FETCH command with the CHANGEDSINCE modifier, 310 o a FETCH or SEARCH command that includes the MODSEQ message data 311 item, or 313 o a STORE command with the UNCHANGEDSINCE modifier 315 MUST reject any such command with the tagged BAD response. 317 C: A142 SELECT INBOX 318 S: * 172 EXISTS 319 S: * 1 RECENT 320 S: * OK [UNSEEN 12] Message 12 is first unseen 321 S: * OK [UIDVALIDITY 3857529045] UIDs valid 322 S: * OK [UIDNEXT 4392] Predicted next UID 323 S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft) 324 S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited 325 S: * OK [NOMODSEQ] Sorry, this mailbox format doesn't support 326 modsequences 327 S: A142 OK [READ-WRITE] SELECT completed 329 Example 2 331 3.2. STORE and UID STORE Commands 333 This document defines the following STORE modifier (see Section 2.5 334 of [RFC4466]): 336 UNCHANGEDSINCE For each message specified in the 337 message set, the server performs the following. If the mod- 338 sequence of any metadata item of the message is equal or less than 339 the specified UNCHANGEDSINCE value, then the requested operation 340 (as described by the message data item) is performed. If the 341 operation is successful, the server MUST update the mod-sequence 342 attribute of the message. An untagged FETCH response MUST be 343 sent, even if the .SILENT suffix is specified, and the response 344 MUST include the MODSEQ message data item. This is required to 345 update the client's cache with the correct mod-sequence values. 346 See Section 3.3.2 for more details. 348 However, if the mod-sequence of any metadata item of the message is 349 greater than the specified UNCHANGEDSINCE value, then the requested 350 operation MUST NOT be performed. In this case, the mod-sequence 351 attribute of the message is not updated, and the message number (or 352 unique identifier in the case of the UID STORE command) is added to 353 the list of messages that failed the UNCHANGEDSINCE test. 355 When the server finished performing the operation on all the messages 356 in the message set, it checks for a non-empty list of messages that 357 failed the UNCHANGEDSINCE test. If this list is non-empty, the 358 server MUST return in the tagged response a MODIFIED response code. 359 The MODIFIED response code includes the message set (for STORE) or 360 set of UIDs (for UID STORE) of all messages that failed the 361 UNCHANGEDSINCE test. 363 All messages pass the UNCHANGEDSINCE test. 365 C: a103 UID STORE 6,4,8 (UNCHANGEDSINCE 12121230045) 366 +FLAGS.SILENT (\Deleted) 367 S: * 1 FETCH (UID 4 MODSEQ (12121231000)) 368 S: * 2 FETCH (UID 6 MODSEQ (12121230852)) 369 S: * 4 FETCH (UID 8 MODSEQ (12121130956)) 370 S: a103 OK Conditional Store completed 372 Example 3 374 C: a104 STORE * (UNCHANGEDSINCE 12121230045) +FLAGS.SILENT 375 (\Deleted $Processed) 376 S: * 50 FETCH (MODSEQ (12111230047)) 377 S: a104 OK Store (conditional) completed 379 Example 4 381 C: c101 STORE 50 (UNCHANGEDSINCE 12121230045) -FLAGS.SILENT 382 (\Deleted) 383 S: * OK [HIGHESTMODSEQ 12111230047] 384 S: * 50 FETCH (MODSEQ (12111230048)) 385 S: c101 OK Store (conditional) completed 387 HIGHESTMODSEQ response code was sent by the server presumably because 388 this was the first CONDSTORE enabling command. 390 Example 5 392 In spite of the failure of the conditional STORE operation for 393 message 7, the server continues to process the conditional STORE in 394 order to find all messages that fail the test. 396 C: d105 STORE 7,5,9 (UNCHANGEDSINCE 320162338) 397 +FLAGS.SILENT (\Deleted) 398 S: * 5 FETCH (MODSEQ (320162350)) 399 S: d105 OK [MODIFIED 7,9] Conditional STORE failed 401 Example 6 403 Same as above, but the server follows the SHOULD recommendation in 404 Section 6.4.6 of [RFC3501]. 406 C: d105 STORE 7,5,9 (UNCHANGEDSINCE 320162338) 407 +FLAGS.SILENT (\Deleted) 408 S: * 7 FETCH (MODSEQ (320162342) FLAGS (\Seen \Deleted)) 409 S: * 5 FETCH (MODSEQ (320162350)) 410 S: * 9 FETCH (MODSEQ (320162349) FLAGS (\Answered)) 411 S: d105 OK [MODIFIED 7,9] Conditional STORE failed 413 Use of UNCHANGEDSINCE with a modification sequence of 0 always fails 414 if the metadata item exists. A system flag MUST always be considered 415 existent, whether it was set or not. 417 Example 7 419 C: a102 STORE 12 (UNCHANGEDSINCE 0) 420 +FLAGS.SILENT ($MDNSent) 421 S: a102 OK [MODIFIED 12] Conditional STORE failed 422 The client has tested the presence of the $MDNSent user-defined 423 keyword. 425 Example 8 427 Note: A client trying to make an atomic change to the state of a 428 particular metadata item (or a set of metadata items) should be 429 prepared to deal with the case when the server returns the MODIFIED 430 response code if the state of the metadata item being watched hasn't 431 changed (but the state of some other metadata item has). This is 432 necessary, because some servers don't store separate mod-sequences 433 for different metadata items. However, a server implementation 434 SHOULD avoid generating spurious MODIFIED responses for +FLAGS/-FLAGS 435 STORE operations, even when the server stores a single mod-sequence 436 per message. Section 5 describes how this can be achieved. 438 Unless the server has included an unsolicited FETCH to update 439 client's knowledge about messages that have failed the UNCHANGEDSINCE 440 test, upon receipt of the MODIFIED response code, the client SHOULD 441 try to figure out if the required metadata items have indeed changed 442 by issuing FETCH or NOOP command. It is RECOMMENDED that the server 443 avoids the need for the client to do that by sending an unsolicited 444 FETCH response (Examples 9 and 10). 446 If the required metadata items haven't changed, the client SHOULD 447 retry the command with the new mod-sequence. The client SHOULD allow 448 for a configurable but reasonable number of retries (at least 2). 450 In the example below, the server returns the MODIFIED response code 451 without sending information describing why the STORE UNCHANGEDSINCE 452 operation has failed. 454 C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000) 455 +FLAGS.SILENT ($Processed) 456 S: * 100 FETCH (MODSEQ (303181230852)) 457 S: * 102 FETCH (MODSEQ (303181230852)) 458 ... 459 S: * 150 FETCH (MODSEQ (303181230852)) 460 S: a106 OK [MODIFIED 101] Conditional STORE failed 462 The flag $Processed was set on the message 101... 464 C: a107 NOOP 465 S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed)) 466 S: a107 OK 468 Example 9 470 Or the flag hasn't changed, but another has (note that this server 471 behaviour is discouraged. Server implementers should also see 472 Section 5)... 474 C: b107 NOOP 475 S: * 101 FETCH (MODSEQ (303011130956) FLAGS (\Deleted \Answered)) 476 S: b107 OK 478 ...and the client retries the operation for the message 101 with 479 the updated UNCHANGEDSINCE value 481 C: b108 STORE 101 (UNCHANGEDSINCE 303011130956) 482 +FLAGS.SILENT ($Processed) 483 S: * 101 FETCH (MODSEQ (303181230852)) 484 S: b108 OK Conditional Store completed 486 Same as above, but the server avoids the need for the client to poll 487 for changes. 489 The flag $Processed was set on the message 101 by another 490 client... 492 C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000) 493 +FLAGS.SILENT ($Processed) 494 S: * 100 FETCH (MODSEQ (303181230852)) 495 S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed)) 496 S: * 102 FETCH (MODSEQ (303181230852)) 497 ... 498 S: * 150 FETCH (MODSEQ (303181230852)) 499 S: a106 OK [MODIFIED 101] Conditional STORE failed 501 Example 10 503 Or the flag hasn't changed, but another has (note that this server 504 behaviour is discouraged. Server implementers should also see 505 Section 5)... 507 C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000) 508 +FLAGS.SILENT ($Processed) 509 S: * 100 FETCH (MODSEQ (303181230852)) 510 S: * 101 FETCH (MODSEQ (303011130956) FLAGS (\Deleted \Answered)) 511 S: * 102 FETCH (MODSEQ (303181230852)) 512 ... 513 S: * 150 FETCH (MODSEQ (303181230852)) 514 S: a106 OK [MODIFIED 101] Conditional STORE failed 515 ...and the client retries the operation for the message 101 with 516 the updated UNCHANGEDSINCE value 518 C: b108 STORE 101 (UNCHANGEDSINCE 303011130956) 519 +FLAGS.SILENT ($Processed) 520 S: * 101 FETCH (MODSEQ (303181230852)) 521 S: b108 OK Conditional Store completed 523 Or the flag hasn't changed, but another has (nice server behaviour. 524 Server implementers should also see Section 5)... 526 C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000) 527 +FLAGS.SILENT ($Processed) 528 S: * 100 FETCH (MODSEQ (303181230852)) 529 S: * 101 FETCH (MODSEQ (303011130956) FLAGS ($Processed \Deleted 530 \Answered)) 531 S: * 102 FETCH (MODSEQ (303181230852)) 532 ... 533 S: * 150 FETCH (MODSEQ (303181230852)) 534 S: a106 OK Conditional STORE completed 536 The following example is based on the example from the Section 4.2.3 537 of [RFC2180] and demonstrates that the MODIFIED response code may be 538 also returned in the tagged NO response. 540 Client tries to conditionally STORE flags on a mixture of expunged 541 and non-expunged messages; one message fails the UNCHANGEDSINCE 542 test. 544 C: B001 STORE 1:7 (UNCHANGEDSINCE 320172338) +FLAGS (\SEEN) 545 S: * 1 FETCH (MODSEQ (320172342) FLAGS (\SEEN)) 546 S: * 3 FETCH (MODSEQ (320172342) FLAGS (\SEEN)) 547 S: B001 NO [MODIFIED 2] Some of the messages no longer exist. 549 C: B002 NOOP 550 S: * 4 EXPUNGE 551 S: * 4 EXPUNGE 552 S: * 4 EXPUNGE 553 S: * 4 EXPUNGE 554 S: * 2 FETCH (MODSEQ (320172340) FLAGS (\Deleted \Answered)) 555 S: B002 OK NOOP Completed. 557 By receiving FETCH responses for messages 1 and 3, and EXPUNGE 558 responses that indicate that messages 4 through 7 have been 559 expunged, the client retries the operation only for the message 2. 560 The updated UNCHANGEDSINCE value is used. 562 C: b003 STORE 2 (UNCHANGEDSINCE 320172340) +FLAGS (\Seen) 563 S: * 2 FETCH (MODSEQ (320180050)) 564 S: b003 OK Conditional Store completed 566 Example 11 568 Note: If a message is specified multiple times in the message set, 569 and the server doesn't internally eliminate duplicates from the 570 message set, it MUST NOT fail the conditional STORE operation for the 571 second (or subsequent) occurrence of the message if the operation 572 completed successfully for the first occurrence. For example, if the 573 client specifies: 575 e105 STORE 7,3:9 (UNCHANGEDSINCE 12121230045) +FLAGS.SILENT 576 (\Deleted) 578 the server must not fail the operation for message 7 as part of 579 processing "3:9" if it succeeded when message 7 was processed the 580 first time. 582 Once the client specified the UNCHANGEDSINCE modifier in a STORE 583 command, the server MUST include the MODSEQ fetch response data items 584 in all subsequent unsolicited FETCH responses. 586 This document also changes the behaviour of the server when it has 587 performed a STORE or UID STORE command and the UNCHANGEDSINCE 588 modifier is not specified. If the operation is successful for a 589 message, the server MUST update the mod-sequence attribute of the 590 message. The server is REQUIRED to include the mod-sequence value 591 whenever it decides to send the unsolicited FETCH response to all 592 CONDSTORE-aware clients that have opened the mailbox containing the 593 message. 595 Server implementers should also see Section 3.10 for additional 596 quality of implementation issues related to the STORE command. 598 3.3. FETCH and UID FETCH Commands 600 3.3.1. CHANGEDSINCE FETCH Modifier 602 This document defines the following FETCH modifier (see Section 2.4 603 of [RFC4466]): 605 CHANGEDSINCE CHANGEDSINCE FETCH modifier allows to 606 create a further subset of the list of messages described by 607 sequence set. The information described by message data items is 608 only returned for messages that have mod-sequence bigger than 609 . 611 When CHANGEDSINCE FETCH modifier is specified, it implicitly adds 612 MODSEQ FETCH message data item (Section 3.3.2). 614 C: s100 UID FETCH 1:* (FLAGS) (CHANGEDSINCE 12345) 615 S: * 1 FETCH (UID 4 MODSEQ (65402) FLAGS (\Seen)) 616 S: * 2 FETCH (UID 6 MODSEQ (75403) FLAGS (\Deleted)) 617 S: * 4 FETCH (UID 8 MODSEQ (29738) FLAGS ($NoJunk $AutoJunk 618 $MDNSent)) 619 S: s100 OK FETCH completed 621 Example 12 623 3.3.2. MODSEQ Message Data Item in FETCH Command 625 This extension adds a MODSEQ message data item to the FETCH command. 626 The MODSEQ message data item allows clients to retrieve mod-sequence 627 values for a range of messages in the currently selected mailbox. 629 Once the client specified the MODSEQ message data item in a FETCH 630 request, the server MUST include the MODSEQ fetch response data items 631 in all subsequent unsolicited FETCH responses. 633 Syntax: MODSEQ The MODSEQ message data item causes the server to 634 return MODSEQ fetch response data items. 636 Syntax: MODSEQ ( ) MODSEQ response data items 637 contain per-message mod-sequences. 639 The MODSEQ response data item is returned if the client issued 640 FETCH with MODSEQ message data item. It also allows the server to 641 notify the client about mod-sequence changes caused by conditional 642 STOREs (Section 3.2) and/or changes caused by external sources. 644 C: a FETCH 1:3 (MODSEQ) 645 S: * 1 FETCH (MODSEQ (624140003)) 646 S: * 2 FETCH (MODSEQ (624140007)) 647 S: * 3 FETCH (MODSEQ (624140005)) 648 S: a OK Fetch complete 650 In this example, the client requests per-message mod-sequences for a 651 set of messages. 653 Example 13 655 When a flag for a message is modified in a different session, the 656 server sends an unsolicited FETCH response containing the mod- 657 sequence for the message. 659 (Session 1, authenticated as a user "alex"). The user adds a 660 shared flag \Deleted: 662 C: A142 SELECT INBOX 663 ... 664 S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft) 665 S: * OK [PERMANENTFLAGS (\Answered \Deleted \Seen \*)] Limited 666 ... 667 C: A160 STORE 7 +FLAGS.SILENT (\Deleted) 668 S: * 7 FETCH (MODSEQ (2121231000)) 669 S: A160 OK Store completed 671 (Session 2, also authenticated as the user "alex"). Any changes 672 to flags are always reported to all sessions authenticated as the 673 same user as in the session 1. 675 C: C180 NOOP 676 S: * 7 FETCH (FLAGS (\Deleted \Answered) MODSEQ (12121231000)) 677 S: C180 OK Noop completed 679 (Session 3, authenticated as a user "andrew"). As \Deleted is a 680 shared flag, changes in session 1 are also reported in session 3: 682 C: D210 NOOP 683 S: * 7 FETCH (FLAGS (\Deleted \Answered) MODSEQ (12121231000)) 684 S: D210 OK Noop completed 686 The user modifies a private flag \Seen in session 1... 688 C: A240 STORE 7 +FLAGS.SILENT (\Seen) 689 S: * 7 FETCH (MODSEQ (12121231777)) 690 S: A240 OK Store completed 692 ...which is only reported in session 2... 694 C: C270 NOOP 695 S: * 7 FETCH (FLAGS (\Deleted \Answered \Seen) MODSEQ 696 (12121231777)) 697 S: C270 OK Noop completed 699 ...but not in session 3. 701 C: D300 NOOP 702 S: D300 OK Noop completed 704 And finally, the user removes flags \Answered (shared) and \Seen 705 (private) in session 1. 707 C: A330 STORE 7 -FLAGS.SILENT (\Answered \Seen) 708 S: * 7 FETCH (MODSEQ (12121245160)) 709 S: A330 OK Store completed 711 Both changes are reported in the session 2... 713 C: C360 NOOP 714 S: * 7 FETCH (FLAGS (\Deleted) MODSEQ (12121245160)) 715 S: C360 OK Noop completed 717 ...and only changes to shared flags are reported in session 3. 719 C: D390 NOOP 720 S: * 7 FETCH (FLAGS (\Deleted) MODSEQ (12121245160)) 721 S: D390 OK Noop completed 723 Example 14 725 Server implementers should also see Section 3.10 for additional 726 quality of implementation issues related to the FETCH command. 728 3.4. MODSEQ Search Criterion in SEARCH 729 The MODSEQ criterion for the SEARCH (or UID SEARCH) command allows a 730 client to search for the metadata items that were modified since a 731 specified moment. 733 Syntax: MODSEQ [ ] 735 Messages that have modification values that are equal to or 736 greater than . This allows a client, for 737 example, to find out which messages contain metadata items that 738 have changed since the last time it updated its disconnected 739 cache. The client may also specify (name of metadata 740 item) and (type of metadata item) before . can be one of "shared", "priv" 742 (private), or "all". The latter means that the server should use 743 the biggest value among "priv" and "shared" mod- sequences for the 744 metadata item. If the server doesn't store internally separate 745 mod-sequences for different metadata items, it MUST ignore and . Otherwise, the server should use them 747 to narrow down the search. 749 For a flag , the corresponding has a form " 750 /flags/" as defined in [RFC4466]. Note that the leading 751 "\" character that denotes a system flag has to be escaped as per 752 Section 4.3 of [RFC3501], as the uses syntax for 753 quoted strings. 755 If client specifies a MODSEQ criterion in a SEARCH (or UID SEARCH) 756 command and the server returns a non-empty SEARCH result, the server 757 MUST also append (to the end of the untagged SEARCH response) the 758 highest mod-sequence for all messages being returned. See also 759 Section 3.5. Note that other IMAP extensions such as ESEARCH 760 [RFC4731] can override this requirement (see Section 3.9 for more 761 details.) 763 C: a SEARCH MODSEQ "/flags/\\draft" all 620162338 764 S: * SEARCH 2 5 6 7 11 12 18 19 20 23 (MODSEQ 917162500) 765 S: a OK Search complete 767 In the above example, the message numbers of any messages containing 768 the string "IMAP4" in the "value" attribute of the "/comment" entry 769 and having a mod-sequence equal to or greater than 620162338 for the 770 "\Draft" flag are returned in the search results. 772 Example 15 774 C: t SEARCH OR NOT MODSEQ 720162338 LARGER 50000 775 S: * SEARCH 776 S: t OK Search complete, nothing found 777 Example 16 779 3.5. Modified SEARCH Untagged Response 781 Data: zero or more numbers 782 mod-sequence value (omitted if no match) 784 This document extends syntax of the untagged SEARCH response to 785 include the highest mod-sequence for all messages being returned. 787 If a client specifies a MODSEQ criterion in a SEARCH (or UID SEARCH) 788 command and the server returns a non-empty SEARCH result, the server 789 MUST also append (to the end of the untagged SEARCH response) the 790 highest mod-sequence for all messages being returned. See 791 Section 3.4 for examples. 793 3.6. HIGHESTMODSEQ Status Data Items 795 This document defines a new status data item: 797 HIGHESTMODSEQ The highest mod-sequence value of all messages in the 798 mailbox. This is the same value that is returned by the server in 799 the HIGHESTMODSEQ response code in an OK untagged response (see 800 Section 3.1.1). If the server doesn't support the persistent 801 storage of mod-sequences for the mailbox (see Section 3.1.2), the 802 server MUST return 0 as the value of HIGHESTMODSEQ status data 803 item. 805 C: A042 STATUS blurdybloop (UIDNEXT MESSAGES HIGHESTMODSEQ) 806 S: * STATUS blurdybloop (MESSAGES 231 UIDNEXT 44292 807 HIGHESTMODSEQ 7011231777) 808 S: A042 OK STATUS completed 810 Example 17 812 3.7. CONDSTORE Parameter to SELECT and EXAMINE 814 The CONDSTORE extension defines a single optional select parameter, 815 "CONDSTORE", which tells the server that it MUST include the MODSEQ 816 fetch response data items in all subsequent unsolicited FETCH 817 responses. 819 The CONDSTORE parameter to SELECT/EXAMINE helps avoid a race 820 condition that might arise when one or more metadata items are 821 modified in another session after the server has sent the 822 HIGHESTMODSEQ response code and before the client was able to issue a 823 CONDSTORE enabling command. 825 C: A142 SELECT INBOX (CONDSTORE) 826 S: * 172 EXISTS 827 S: * 1 RECENT 828 S: * OK [UNSEEN 12] Message 12 is first unseen 829 S: * OK [UIDVALIDITY 3857529045] UIDs valid 830 S: * OK [UIDNEXT 4392] Predicted next UID 831 S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft) 832 S: * OK [PERMANENTFLAGS (\Deleted \Seen \*)] Limited 833 S: * OK [HIGHESTMODSEQ 715194045007] 834 S: A142 OK [READ-WRITE] SELECT completed, CONDSTORE is now enabled 836 Example 18 838 3.8. Interaction with IMAP SORT and THREAD extensions 840 MODSEQ Search Criterion (see Section 3.4) causes modifications to 841 SORT [RFC5256] responses similar to modifications to SEARCH responses 842 defined in Section 3.5: 844 SORT response Data: zero or more numbers 845 mod-sequence value (omitted if no match) 847 This document extends syntax of the untagged SORT response to include 848 the highest mod-sequence for all messages being returned. 850 If a client specifies a MODSEQ criterion in a SORT (or UID SORT) 851 command and the server returns a non-empty SORT result, the server 852 MUST also append (to the end of the untagged SORT response) the 853 highest mod-sequence for all messages being returned. Note that 854 other IMAP extensions such as ESORT [RFC5267] can override this 855 requirement (see Section 3.9 for more details.) 857 THREAD commands which include a MODSEQ Search Criterion return THREAD 858 responses as specified in [RFC5256]. 860 3.9. Interaction with IMAP ESORT and ESEARCH extensions 862 If a client specifies a MODSEQ criterion in an extended SEARCH (or 863 extended UID SEARCH) [RFC4731] command and the server returns a non- 864 empty SEARCH result, the server MUST return the ESEARCH response 865 containing the MODSEQ result option as defined in Section 3.2 of 866 [RFC4731]. 868 C: a SEARCH RETURN (ALL) MODSEQ 1234 869 S: * ESEARCH (TAG "a") ALL 1:3,5 MODSEQ 1236 870 S: a OK Extended SEARCH completed 872 Example 19 874 If a client specifies a MODSEQ criterion in an extended SORT (or 875 extended UID SORT) [RFC5267] command and the server returns a non- 876 empty SORT result, the server MUST return the ESEARCH response 877 containing the MODSEQ result option defined in Section 3.2 of 878 [RFC4731]. 880 C: a SORT RETURN (ALL) (DATE) UTF-8 MODSEQ 1234 881 S: * ESEARCH (TAG "a") ALL 5,3,2,1 MODSEQ 1236 882 S: a OK Extended SORT completed 884 Example 20 886 3.10. Additional Quality-of-Implementation Issues 888 Server implementations should follow the following rule, which 889 applies to any successfully completed STORE/UID STORE (with and 890 without UNCHANGEDSINCE modifier), as well as to a FETCH command that 891 implicitly sets \Seen flag: 893 Adding the flag when it is already present or removing when it is 894 not present SHOULD NOT change the mod-sequence. 896 This will prevent spurious client synchronization requests. 898 However, note that client implementers MUST NOT rely on this server 899 behavior. A client can't distinguish between the case when a server 900 has violated the SHOULD mentioned above, and that when one or more 901 clients set and unset (or unset and set) the flag in another session. 903 4. Formal Syntax 905 The following syntax specification uses the Augmented Backus-Naur 906 Form (ABNF) [RFC5234] notation. Elements not defined here can be 907 found in the formal syntax of the ABNF [RFC5234], IMAP [RFC3501], and 908 IMAP ABNF extensions [RFC4466] specifications. 910 Except as noted otherwise, all alphabetic characters are case- 911 insensitive. The use of upper- or lowercase characters to define 912 token strings is for editorial clarity only. Implementations MUST 913 accept these strings in a case-insensitive fashion. 915 capability =/ "CONDSTORE" 916 status-att =/ "HIGHESTMODSEQ" 917 ;; extends non-terminal defined in RFC 3501. 919 status-att-val =/ "HIGHESTMODSEQ" SP mod-sequence-valzer 920 ;; extends non-terminal defined in [RFC4466]. 921 ;; Value 0 denotes that the mailbox doesn't 922 ;; support persistent mod-sequences 923 ;; as described in Section 3.1.2 [[Check the ref]] 925 store-modifier =/ "UNCHANGEDSINCE" SP mod-sequence-valzer 926 ;; Only a single "UNCHANGEDSINCE" may be 927 ;; specified in a STORE operation 929 fetch-modifier =/ chgsince-fetch-mod 930 ;; conforms to the generic "fetch-modifier" 931 ;; syntax defined in [RFC4466]. 933 chgsince-fetch-mod = "CHANGEDSINCE" SP mod-sequence-value 934 ;; CHANGEDSINCE FETCH modifier conforms to 935 ;; the fetch-modifier syntax 937 fetch-att =/ fetch-mod-sequence 938 ;; modifies original IMAP4 fetch-att 940 fetch-mod-sequence = "MODSEQ" 942 fetch-mod-resp = "MODSEQ" SP "(" permsg-modsequence ")" 944 msg-att-dynamic =/ fetch-mod-resp 946 search-key =/ search-modsequence 947 ;; modifies original IMAP4 search-key 948 ;; 949 ;; This change applies to all commands 950 ;; referencing this non-terminal, in 951 ;; particular SEARCH, SORT and THREAD. 953 search-modsequence = "MODSEQ" [search-modseq-ext] SP 954 mod-sequence-valzer 956 search-modseq-ext = SP entry-name SP entry-type-req 958 resp-text-code =/ "HIGHESTMODSEQ" SP mod-sequence-value / 959 "NOMODSEQ" / 960 "MODIFIED" SP sequence-set 962 entry-name = entry-flag-name 963 entry-flag-name = DQUOTE "/flags/" attr-flag DQUOTE 964 ;; each system or user defined flag 965 ;; is mapped to "/flags/". 966 ;; 967 ;; follows the escape rules 968 ;; used by "quoted" string as described in 969 ;; Section 4.3 of [RFC3501], e.g., for the flag 970 ;; \Seen the corresponding is 971 ;; "/flags/\\seen", and for the flag 972 ;; $MDNSent, the corresponding 973 ;; is "/flags/$mdnsent". 975 entry-type-resp = "priv" / "shared" 976 ;; metadata item type 978 entry-type-req = entry-type-resp / "all" 979 ;; perform SEARCH operation on private 980 ;; metadata item, shared metadata item or both 982 permsg-modsequence = mod-sequence-value 983 ;; per message mod-sequence 985 mod-sequence-value = 1*DIGIT 986 ;; Positive unsigned 64-bit integer 987 ;; (mod-sequence) 988 ;; (1 <= n < 18,446,744,073,709,551,615) 990 mod-sequence-valzer = "0" / mod-sequence-value 992 search-sort-mod-seq = "(" "MODSEQ" SP mod-sequence-value ")" 994 select-param =/ condstore-param 995 ;; conforms to the generic "select-param" 996 ;; non-terminal syntax defined in [RFC4466]. 998 condstore-param = "CONDSTORE" 1000 mailbox-data =/ "SEARCH" [1*(SP nz-number) SP 1001 search-sort-mod-seq] 1003 sort-data = "SORT" [1*(SP nz-number) SP 1004 search-sort-mod-seq] 1005 ; Updates SORT response from RFC 5256 1007 attr-flag = "\\Answered" / "\\Flagged" / "\\Deleted" / 1008 "\\Seen" / "\\Draft" / attr-flag-keyword / 1009 attr-flag-extension 1010 ;; Does not include "\\Recent" 1012 attr-flag-extension = "\\" atom 1013 ;; Future expansion. Client implementations 1014 ;; MUST accept flag-extension flags. Server 1015 ;; implementations MUST NOT generate 1016 ;; flag-extension flags except as defined by 1017 ;; future standard or standards-track 1018 ;; revisions of [RFC3501]. 1020 attr-flag-keyword = atom 1022 5. Server Implementation Considerations 1024 This section describes how a server implementation that doesn't store 1025 separate per-metadata mod-sequences for different metadata items can 1026 avoid sending the MODIFIED response to any of the following 1027 conditional STORE operations: 1029 +FLAGS 1031 -FLAGS 1033 +FLAGS.SILENT 1035 -FLAGS.SILENT 1037 Note that the optimization described in this section can't be 1038 performed in case of a conditional STORE FLAGS operation. 1040 Let's use the following example. The client has issued 1042 C: a106 STORE 100:150 (UNCHANGEDSINCE 212030000000) 1043 +FLAGS.SILENT ($Processed) 1045 When the server receives the command and parses it successfully, it 1046 iterates through the message set and tries to execute the conditional 1047 STORE command for each message. 1049 Each server internally works as a client, i.e., it has to cache the 1050 current state of all IMAP flags as it is known to the client. In 1051 order to report flag changes to the client, the server compares the 1052 cached values with the values in its database for IMAP flags. 1054 Imagine that another client has changed the state of a flag \Deleted 1055 on the message 101 and that the change updated the mod-sequence for 1056 the message. The server knows that the mod-sequence for the mailbox 1057 has changed; however, it also knows that: 1059 a. the client is not interested in \Deleted flag, as it hasn't 1060 included it in +FLAGS.SILENT operation; and 1062 b. the state of the flag $Processed hasn't changed (the server can 1063 determine this by comparing cached flag state with the state of 1064 the flag in the database). 1066 Therefore, the server doesn't have to report MODIFIED to the client. 1067 Instead, the server may set $Processed flag, update the mod-sequence 1068 for the message 101 once again and send an untagged FETCH response 1069 with new mod-sequence and flags: 1071 S: * 101 FETCH (MODSEQ (303011130956) 1072 FLAGS ($Processed \Deleted \Answered)) 1074 See also Section 3.10 for additional quality-of-implementation 1075 issues. 1077 6. Long Command Lines 1079 This document updates recommended line length limits specified in 1080 Section 3.2.1.5 of [RFC2683]. While the advice in the first 1081 paragraph of that section still applies ("use compact message/UID set 1082 representations"), the 1000 octet limit suggested in the second 1083 paragraph turned out to be quite problematic when the CONDSTORE 1084 extension is used. The updated recommendation is as follows: a 1085 client should limit the length of the command lines it generates to 1086 approximately 8192 octets (including all quoted strings but not 1087 including literals). 1089 7. Security Considerations 1091 It is believed that the Conditional STORE extension doesn't raise any 1092 new security concerns that are not already discussed in [RFC3501]. 1093 However, the availability of this extension may make it possible for 1094 IMAP4 to be used in critical applications it could not be used for 1095 previously, making correct IMAP server implementation and operation 1096 even more important. 1098 8. IANA Considerations 1100 IMAP4 capabilities are registered by publishing a standards track or 1101 IESG approved experimental RFC. The registry is currently located 1102 at: 1104 http://www.iana.org/assignments/imap4-capabilities 1106 This document defines the CONDSTORE IMAP capability. IANA has added 1107 it to the registry accordingly. 1109 9. Acknowledgements 1111 Thank you to Steve Hole for co-editing RFC 4551. 1113 Thank you to Dave Cridland for helping to convert the original text 1114 RFC to xml2rfc format. 1116 Some text was borrowed from "IMAP ANNOTATE Extension" [RFC5257] by 1117 Randall Gellens and Cyrus Daboo and from "ACAP -- Application 1118 Configuration Access Protocol" [RFC2244] by Chris Newman and John 1119 Myers. 1121 Many thanks to Randall Gellens for his thorough review of the 1122 document. 1124 The authors also acknowledge the feedback provided by Cyrus Daboo, 1125 Larry Greenfield, Chris Newman, Harrie Hazewinkel, Arnt Gulbrandsen, 1126 Timo Sirainen, Mark Crispin, Ned Freed, Ken Murchison, and Dave 1127 Cridland. 1129 10. References 1131 10.1. Normative References 1133 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1134 Requirement Levels", BCP 14, RFC 2119, March 1997. 1136 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 1137 4rev1", RFC 3501, March 2003. 1139 [RFC4466] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4 1140 ABNF", RFC 4466, April 2006. 1142 [RFC5161] Gulbrandsen, A. and A. Melnikov, "The IMAP ENABLE 1143 Extension", RFC 5161, March 2008. 1145 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 1146 Specifications: ABNF", STD 68, RFC 5234, January 2008. 1148 [RFC5256] Crispin, M. and K. Murchison, "Internet Message Access 1149 Protocol - SORT and THREAD Extensions", RFC 5256, June 1150 2008. 1152 10.2. Informative References 1154 [RFC1305] Mills, D., "Network Time Protocol (Version 3) 1155 Specification, Implementation", RFC 1305, March 1992. 1157 [RFC2180] Gahrns, M., "IMAP4 Multi-Accessed Mailbox Practice", RFC 1158 2180, July 1997. 1160 [RFC2244] Newman, C. and J. Myers, "ACAP -- Application 1161 Configuration Access Protocol", RFC 2244, November 1997. 1163 [RFC2683] Leiba, B., "IMAP4 Implementation Recommendations", RFC 1164 2683, September 1999. 1166 [RFC4314] Melnikov, A., "IMAP4 Access Control List (ACL) Extension", 1167 RFC 4314, December 2005. 1169 [RFC4731] Melnikov, A. and D. Cridland, "IMAP4 Extension to SEARCH 1170 Command for Controlling What Kind of Information Is 1171 Returned", RFC 4731, November 2006. 1173 [RFC5257] Daboo, C. and R. Gellens, "Internet Message Access 1174 Protocol - ANNOTATE Extension", RFC 5257, June 2008. 1176 [RFC5267] Cridland, D. and C. King, "Contexts for IMAP4", RFC 5267, 1177 July 2008. 1179 Appendix A. Changes since RFC 4551 1181 Fixed errata 3401, 3506 and 3509. 1183 Updated references. 1185 Incorporated some text from RFC 5161 (no semantic change.) 1187 Editorial corrections. 1189 Author's Address 1191 Alexey Melnikov 1192 Isode Ltd. 1193 5 Castle Business Village 1194 36 Station Road 1195 Hampton, Middlesex TW12 2BX 1196 UK 1198 Email: Alexey.Melnikov@isode.com