idnits 2.17.1 draft-ietf-tzdist-caldav-timezone-ref-05.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- -- The draft header indicates that this document updates RFC4791, but the abstract doesn't seem to directly say this. It does mention RFC4791 though, so this could be OK. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year (Using the creation date from RFC4791, updated by this document, for RFC5378 checks: 2004-06-24) -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (October 13, 2015) is 3112 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group C. Daboo 3 Internet-Draft Apple 4 Updates: 4791 (if approved) October 13, 2015 5 Intended status: Standards Track 6 Expires: April 15, 2016 8 CalDAV: Time Zones by Reference 9 draft-ietf-tzdist-caldav-timezone-ref-05 11 Abstract 13 This document defines an update to the CalDAV calendar access 14 protocol (RFC 4791) to allow clients and servers to exchange 15 iCalendar data without the need to send full time zone data. 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 http://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 April 15, 2016. 34 Copyright Notice 36 Copyright (c) 2015 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 (http://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. Time Zones by Reference . . . . . . . . . . . . . . . . . . . 3 54 3.1. New Server Behavior . . . . . . . . . . . . . . . . . . . 3 55 4. New Client Behavior . . . . . . . . . . . . . . . . . . . . . 7 56 5. New WebDAV Properties . . . . . . . . . . . . . . . . . . . . 8 57 5.1. CALDAV:timezone-service-set . . . . . . . . . . . . . . . 8 58 5.2. CALDAV:calendar-timezone-id . . . . . . . . . . . . . . . 8 59 6. XML Element Definitions . . . . . . . . . . . . . . . . . . . 9 60 6.1. CALDAV:calendar-query XML Element . . . . . . . . . . . . 9 61 6.2. CALDAV:timezone-id XML Element . . . . . . . . . . . . . 9 62 7. Additional Message Header Fields . . . . . . . . . . . . . . 10 63 7.1. CalDAV-Timezones Request Header Field . . . . . . . . . . 10 64 8. Security Considerations . . . . . . . . . . . . . . . . . . . 10 65 9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 10 66 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 67 10.1. CalDAV-Timezones . . . . . . . . . . . . . . . . . . . . 11 68 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11 69 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 70 12.1. Normative References . . . . . . . . . . . . . . . . . . 11 71 12.2. Informative References . . . . . . . . . . . . . . . . . 12 72 Appendix A. Change History (To be removed by RFC Editor before 73 publication) . . . . . . . . . . . . . . . . . . . . 12 74 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 76 1. Introduction 78 The CalDAV [RFC4791] Calendar Access protocol allows clients to 79 access calendar data stored on a server in the iCalendar [RFC5545] 80 data format. In iCalendar, calendar data that uses local time in any 81 of its date and/or time values is specified as a date-time value in 82 combination with a time zone identifier ("TZID" property parameter). 83 The time zone identifier refers to a time zone definition (a 84 "VTIMEZONE" component) that has all of the rules required to 85 determine local time UTC offsets for the corresponding time zone. In 86 many cases, these "VTIMEZONE" components can be larger, octet-wise, 87 than the events or tasks which make use of them. However, iCalendar 88 currently requires all iCalendar objects ("VCALENDAR" components) 89 that refer to a time zone via its identifier to also include the 90 corresponding "VTIMEZONE" component. This leads to inefficiencies in 91 the CalDAV protocol because large amounts of "VTIMEZONE" data are 92 continuously being exchanged, and for the most part these time zone 93 definitions are unchanging. This is of particular problem to mobile 94 or limited devices, with limited network bandwidth, cpu processing, 95 and power constraints. 97 A set of standard time zone definitions are available at the IANA 98 hosted time zone database [RFC6557]. That database provides the 99 "raw" data for time zone definitions, and those can be converted into 100 iCalendar "VTIMEZONE" components for use in iCalendar applications, 101 as well as converted into other formats for use by other applications 102 (e.g., "zoneinfo" files often found on Unix-based operating systems). 103 A new Time Zone Data Distribution Service [I-D.ietf-tzdist-service] 104 protocol is available that allows iCalendar applications to retrieve 105 these standard time zone definitions in a timely and accurate 106 fashion, instead of relying on possibly infrequent system updates of 107 time zone data that frequently result in mismatched calendar data and 108 resulting missed meetings between calendar users. Another benefit of 109 the Time Zone Data Distribution Service is that it provides a single 110 "reference" for standard time zone data that CalDAV clients and 111 servers can make use of to "agree" on standard time zone definitions, 112 and thus eliminate the need to exchange the data for those. 114 This specification defines a new mode of operation for CalDAV clients 115 and servers which allow them to exchange iCalendar data without the 116 need to send "VTIMEZONE" components for known, standard time zone 117 definitions. This can significantly reduce the amount of data that 118 needs to be sent between client and server, giving rise to 119 performance and efficiency improvements for each of them. 121 2. Conventions Used in This Document 123 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 124 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 125 "OPTIONAL" in this document are to be interpreted as described in 126 [RFC2119]. 128 Other notations used in this memo follow the notations of [RFC4791]. 130 3. Time Zones by Reference 132 Note that this specification only defines changes to iCalendar data 133 sent or received via the CalDAV protocol (both [RFC4791] and 134 [RFC6638], and extensions). These changes do not apply to other 135 means of exchanging calendar data, such as iTIP [RFC5546] based 136 scheduling mechanisms (e.g., iMIP [RFC6047]), or other methods. 138 3.1. New Server Behavior 140 3.1.1. Server Advertised Capability 142 A server that supports this specification MUST include "calendar-no- 143 timezone" as a field in the DAV response header field from an OPTIONS 144 request on a calendar home collection (see Section 6.2.1 of 146 [RFC4791]) or calendar collection (see Section 4.2 of [RFC4791]). 147 Clients MUST check for the presence of that field in the DAV response 148 header field before changing their behavior as per Section 4. 150 3.1.2. Associated Time Zone Data Distribution Service 152 A CalDAV server supporting this specification MUST have one or more 153 associated time zone distribution services [I-D.ietf-tzdist-service] 154 that provide data for the set of time zones known to the server and 155 expected to be used by clients. A CalDAV server advertises the set 156 of time zone distribution services it makes use of via a 157 CALDAV:timezone-service-set WebDAV property (see Section 5.1) defined 158 on calendar home collections. Clients can use the time zone data 159 distribution services listed in this property to fetch current time 160 zone definitions for the time zone identifiers in iCalendar data 161 retrieved from the server. This allows clients to keep their "built- 162 in" time zone definitions up to date. It also allows clients to use 163 an "on-demand" model for populating their local time zone definition 164 cache, only fetching a time zone definition when it is first seen in 165 calendar data, potentially allowing for savings on storage space by 166 eliminating the need to store time zone data that is not currently 167 being used. 169 When making use of the time zone data distribution services 170 advertised by a CalDAV server, clients MUST follow all the 171 requirements of the time zone data distribution service protocol 172 [I-D.ietf-tzdist-service], taking care to refresh time zone data in a 173 timely fashion. 175 3.1.3. Time Zones in CalDAV Responses 177 Servers MUST support the HTTP "CalDAV-Timezones" request header field 178 (see Section 7.1). If the "CalDAV-Timezones" request header field 179 has the value "T" on any HTTP request that returns iCalendar data, 180 then the server MUST include all the appropriate "VTIMEZONE" 181 components in the iCalendar data (all the ones that are referenced by 182 "TZID" property parameters). If the "CalDAV-Timezones" request 183 header field has the value "F" on any HTTP request that returns 184 iCalendar data, then the server MUST NOT return any "VTIMEZONE" 185 components if the time zone identifier matches one provided by any of 186 the advertised time zone distribution servers (see Section 3.1.2). 187 However, the server MUST return the appropriate "VTIMEZONE" component 188 for each time zone with an identifier not available on the advertised 189 time zone distribution servers. This behaviour applies to all HTTP 190 requests on CalDAV resources that return iCalendar data either 191 directly (such as a "GET" request on a calendar object resource), or 192 embedded in a "structured" response such as a DAV:multistatus 193 returned by a "REPORT" or "PROPFIND" request. 195 Observation and experiments have shown that, in the vast majority of 196 cases, CalDAV clients have typically ignored time zone definitions in 197 data received from servers, and instead make use of their own "built- 198 in" definitions for the corresponding time zone identifier. This 199 means that it is reasonable for CalDAV servers to unilaterally decide 200 not to send "VTIMEZONE" components for standard time zones that 201 clients are expected to have "built-in" (i.e., IANA time zones). 202 Thus, in the absence of a "CalDAV-Timezones" request header field, 203 servers advertizing the "calendar-no-timezone" capability MAY opt to 204 not send standard "VTIMEZONE" components. Servers that do that will 205 need to provide an administrator configuration setting to override 206 the new default behavior based on client "User-Agent" request header 207 field field values, or other suitable means of identifying the client 208 software in use. 210 3.1.4. Time Zones in CalDAV Requests 212 In addition to servers not sending time zone definitions to clients 213 in iCalendar data, this specification also allows clients to not 214 include time zone definitions when sending iCalendar data to the 215 server, as per Section 4. This behaviour applies to all HTTP 216 requests on CalDAV resources that include iCalendar data either 217 directly in the request body (such as a "PUT" request on a calendar 218 object resource), or embedded in a "structured" request body such as 219 a one used by "PROPPATCH" request. 221 Note that, as per Section 4, clients might send time zone definitions 222 for time zones that are not advertised by any of the time zone 223 services associated with the server. In that case, servers have 224 various choices: 226 1. Servers can preserve the original time zone definitions in the 227 iCalendar data sent by the client, so that those can be returned 228 to that or other clients who subsequently request iCalendar data. 230 2. Servers can refuse to accept any unknown/non-standard time zones, 231 in which case they MUST reject the HTTP request containing such 232 data using a WebDAV precondition code of CALDAV:valid-timezone. 234 3. Servers can, with appropriate knowledge, map the unknown/non- 235 standard time zone to a standard time zone definition that 236 accurately matches the one supplied by the client. In doing so, 237 servers will need to re-write the iCalendar data to make use of 238 the new standard time zone identifier chosen by the mapping 239 procedure. Any subsequent request to fetch the calendar data 240 would see the new time zone identifier in the calendar data. 241 Note there is one important situation where this re-mapping is 242 not appropriate: an attendee's copy of an event. In that case 243 the original time zone definition needs to be preserved as the 244 organizer's calendar user agent will expect to see that in any 245 iTIP [RFC5546] replies sent by the attendee. 247 3.1.5. Support of Time Zone Identifiers in WebDAV Properties 249 CalDAV defines a CALDAV:calendar-timezone WebDAV property that is 250 used by clients to set a default time zone for the server to use when 251 doing time-based queries on calendar data (see Section 5.3.2 of 252 [RFC4791]). The content of that WebDAV property is an iCalendar 253 "VTIMEZONE" component. This specification defines a new 254 CALDAV:calendar-timezone-id WebDAV property that allows the default 255 time zone to be set via its time zone identifier, rather than 256 providing the full "VTIMEZONE" component (see Section 5.2). This 257 WebDAV property MUST be present on all resources that also support 258 the CALDAV:calendar-timezone WebDAV property. Its value MUST match 259 the value of the "TZID" iCalendar property in the "VTIMEZONE" 260 component in the CALDAV:calendar-timezone WebDAV property on the same 261 resource. The server MUST accept clients setting either the 262 CALDAV:calendar-timezone or the CALDAV:calendar-timezone-id, and it 263 MUST adjust the value of the alternate property to reflect any 264 changes. i.e., if a client sets the CALDAV:calendar-timezone-id 265 WebDAV property value to "America/New_York", then the server will 266 return the full "VTIMEZONE" data for that time zone in the 267 CALDAV:calendar-timezone WebDAV property. 269 If a client attempts to update the CALDAV:calendar-timezone-id with a 270 value that does not correspond to a time zone that is known to the 271 server, the server MUST reject the property update using a 272 CALDAV:valid-timezone pre-condition error. In such cases, clients 273 MAY repeat the request using the CALDAV:calendar-timezone instead, 274 and provide the full iCalendar data for the time zone being set. 276 3.1.6. Support of Time Zone Identifiers in CALDAV:calendar-query REPORT 278 CalDAV calendar query reports support a CALDAV:timezone XML element 279 that is used by clients to set a specific time zone for the server to 280 use when doing time-based queries on calendar data (see Sections 7.3 281 and 9.8 of [RFC4791]). The content of that XML element is an 282 iCalendar "VTIMEZONE" component. This specification defines a new 283 CALDAV:timezone-id XML element, that can be used as an alternative to 284 the CALDAV:timezone XML element, that allows a specific time zone to 285 be set via its time zone identifier, rather than providing the full 286 "VTIMEZONE" component (see Section 6.2). Servers MUST support 287 clients providing a time zone identifier for use in a calendar query 288 REPORT using this new element. 290 If a client attempts use a CALDAV:timezone-id XML element with a 291 value that does not correspond to a time zone that is known to the 292 server, the server MUST reject the request with a CALDAV:valid- 293 timezone pre-condition error. In such cases, clients MAY repeat the 294 request using the CALDAV:timezone XML element instead, and provide 295 the full iCalendar data for the time zone being used. 297 4. New Client Behavior 299 When a server advertises the "calendar-no-timezone" field in a DAV 300 response header field (as per Section 3.1.1): 302 1. Clients SHOULD include an HTTP "CalDAV-Timezones" request header 303 field with a value of "F" to ensure that the CalDAV server does 304 not include "VTIMEZONE" components in any iCalendar data returned 305 in a response (see Section 3.1.3), for those time zones whose 306 identifier is one provided by any of the advertised time zone 307 distribution servers (see Section 3.1.2). In this case, clients 308 will have to retrieve the missing standard time zone definitions 309 either from their own cache of standard time zones, or from the 310 set of time zone distribution servers advertised by the CalDAV 311 server (see Section 3.1.2). 313 2. Clients can include an HTTP "CalDAV-Timezones" request header 314 field with a value of "T" to ensure that the CalDAV server does 315 include all "VTIMEZONE" components in any iCalendar data returned 316 in a response (see Section 3.1.3). 318 3. Clients can expect servers not to include standard time zone 319 definitions in any iCalendar data they receive from the server, 320 if there is no "CalDAV-Timezones" request header field in the 321 HTTP request. Clients MUST retrieve standard time zone 322 definitions either from its own cache of standard time zones, or 323 from the set of time zone distribution servers advertised by the 324 CalDAV server (see Section 3.1.2). 326 4. Clients SHOULD remove standard time zone definitions from 327 iCalendar data they send to the server, provided the 328 corresponding time zone identifier is one available on any of the 329 server's advertised time zone distribution servers (see 330 Section 3.1.2). 332 5. Clients MUST send time zone definitions in iCalendar data for any 333 time zone identifiers not available via any of the server's 334 advertised time zone distribution servers. Clients MUST be 335 prepared for the server to reject such data or map the time zone 336 to one in the set of standard time zones provided by the server's 337 associated time zone services (as per Section 3.1.4). 339 6. Clients SHOULD make use of the CALDAV:calendar-timezone-id WebDAV 340 property (see Section 3.1.5) and CalDAV:timezone-id XML element 341 (see Section 3.1.6) for specifying default and specific time 342 zones to use in calendar queries executed by the server. 344 5. New WebDAV Properties 346 5.1. CALDAV:timezone-service-set 348 Name: timezone-service-set 350 Namespace: urn:ietf:params:xml:ns:caldav 352 Purpose: Specifies one or more time zone data distribution servers 353 being used by the CalDAV server to provide standard time zone 354 data. 356 Conformance: This property SHOULD be defined on CalDAV calendar home 357 collection resources. If defined, it SHOULD NOT be returned by a 358 PROPFIND DAV:allprop request (as defined in Section 14.2 of 359 [RFC4918]). 361 Description: The CALDAV:timezone-service-set property lists one or 362 more time zone data distribution servers that the CalDAV server is 363 using to provide its set of time zone data. See Section 3.1.2 for 364 more details. 366 Definition: 368 369 DAV:href value: URI of a time zone data distribution service 370 as defined by this specification. 372 Example: 374 377 https://timezones.example.com 378 380 5.2. CALDAV:calendar-timezone-id 382 Name: calendar-timezone-id 384 Namespace: urn:ietf:params:xml:ns:caldav 386 Purpose: Specifies a time zone identifier for a calendar collection. 388 Conformance: This property SHOULD be defined on all resources where 389 the CALDAV:calendar-timezone property is also defined. If 390 defined, it SHOULD NOT be returned by a PROPFIND DAV:allprop 391 request (as defined in Section 14.2 of [RFC4918]). 393 Description: The CALDAV:calendar-timezone-id property is used as an 394 alternative to the CALDAV:calendar-timezone property (see 395 Section 5.3.2 of [RFC4791]). It allows clients to set the default 396 time zone using only a time zone identifier. It also indicates to 397 the client, the time zone identifier of the current default time 398 zone. See Section 3.1.5 for more details. 400 Definition: 402 403 PCDATA value: an time zone identifier. 405 Example: 407 US-Eastern< 409 /C:calendar-timezone-id> 411 6. XML Element Definitions 413 6.1. CALDAV:calendar-query XML Element 415 The CALDAV:calendar-query XML element, defined in Section 9.5 of 416 [RFC4791], is modified to allow use of the CALDAV:timezone-id XML 417 element as follows: 419 Definition: 421 426 6.2. CALDAV:timezone-id XML Element 428 Name: timezone-id 430 Namespace: urn:ietf:params:xml:ns:caldav 432 Purpose: Specifies the time zone identifier for a time zone 433 component to use when determining the results of a report. 435 Description: The CALDAV:timezone-id XML element is used as an 436 alternative to the CALDAV:timezone XML element (see Section 9.8 of 437 [RFC4791]) in calendar query reports, to allow a client to specify 438 a time zone using a time zone identifier rather than providing the 439 full iCalendar time zone data. See Section 3.1.6 for more 440 details. 442 Definition: 444 445 PCDATA value: an time zone identifier. 447 7. Additional Message Header Fields 449 7.1. CalDAV-Timezones Request Header Field 451 The "CalDAV-Timezones" request header field provides a way for a 452 client to indicate to the server whether it wants "VTIMEZONE" 453 components returned in any iCalendar data that is part of the HTTP 454 response. The value "T" indicates that the client wants time zone 455 data returned, the value "F" indicates that it does not. 457 CalDAV-Timezones = "T" / "F" 459 Example: 461 CalDAV-Timezones: F 463 8. Security Considerations 465 This specifications adds Time Zone Data Distribution Service 466 [I-D.ietf-tzdist-service] servers into the overall calendaring and 467 scheduling client/server architecture, as a critical component, and 468 thus adds a new vector of attack against such systems. As such, 469 administrators of CalDAV servers SHOULD ensure that any advertised 470 time zone distribution servers are protected by a level of security 471 commensurate with all the other components in the system. 473 Beyond the above point, this specification does not introduce any new 474 security concerns beyond those addressed in CalDAV [RFC4791], 475 iCalendar [RFC5545], and the Time Zone Data Distribution Service 476 [I-D.ietf-tzdist-service]. 478 9. Privacy Considerations 480 The privacy recommendations in Section 9 of the Time Zone Data 481 Distribution Service [I-D.ietf-tzdist-service] specification SHOULD 482 be used to ensure that details of clients' interactions with CalDAV 483 servers are not exposed to potential network observers. Note that 484 since events can be delivered to a calendar user from an outside 485 source (e.g., using iTIP [RFC5546]), and an attacker could create a 486 calendar event with, e.g., a fake or lesser-used time zone identifier 487 that could be used to monitor the calendar user's activity and 488 interaction with others, this specification increases the importance 489 of using the mitigations of privacy issues discussed in 490 [I-D.ietf-tzdist-service]. 492 10. IANA Considerations 494 The message header field below should be added to the Permanent 495 Message Header Field Registry (see [RFC3864]). 497 10.1. CalDAV-Timezones 499 Header field name: CalDAV-Timezones 501 Applicable protocol: http 503 Status: standard 505 Author/Change controller: IETF 507 Specification document(s): this specification (Section 7.1) 509 Related information: none 511 11. Acknowledgments 513 Thanks to Mike Douglass, Andrew McMillan, and Ken Murchison. This 514 specification came about via discussions at the Calendaring and 515 Scheduling Consortium. 517 12. References 519 12.1. Normative References 521 [I-D.ietf-tzdist-service] 522 Douglass, M. and C. Daboo, "Time Zone Data Distribution 523 Service", draft-ietf-tzdist-service-11 (work in progress), 524 July 2015. 526 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 527 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ 528 RFC2119, March 1997, 529 . 531 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 532 Procedures for Message Header Fields", BCP 90, RFC 3864, 533 DOI 10.17487/RFC3864, September 2004, 534 . 536 [RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault, 537 "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791, DOI 538 10.17487/RFC4791, March 2007, 539 . 541 [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed 542 Authoring and Versioning (WebDAV)", RFC 4918, DOI 543 10.17487/RFC4918, June 2007, 544 . 546 [RFC5545] Desruisseaux, B., Ed., "Internet Calendaring and 547 Scheduling Core Object Specification (iCalendar)", RFC 548 5545, DOI 10.17487/RFC5545, September 2009, 549 . 551 [RFC6638] Daboo, C. and B. Desruisseaux, "Scheduling Extensions to 552 CalDAV", RFC 6638, DOI 10.17487/RFC6638, June 2012, 553 . 555 12.2. Informative References 557 [RFC5546] Daboo, C., Ed., "iCalendar Transport-Independent 558 Interoperability Protocol (iTIP)", RFC 5546, DOI 10.17487/ 559 RFC5546, December 2009, 560 . 562 [RFC6047] Melnikov, A., Ed., "iCalendar Message-Based 563 Interoperability Protocol (iMIP)", RFC 6047, DOI 10.17487/ 564 RFC6047, December 2010, 565 . 567 [RFC6557] Lear, E. and P. Eggert, "Procedures for Maintaining the 568 Time Zone Database", BCP 175, RFC 6557, DOI 10.17487/ 569 RFC6557, February 2012, 570 . 572 Appendix A. Change History (To be removed by RFC Editor before 573 publication) 575 Changes in -05: 577 1. IESG Review: abstract now says "updates" instead of "extends". 579 2. IESG Review: 3.1.5, 3.1.6 section title tweaks. 581 3. IESG Review: add tzdist reference in Security Considerations. 583 4. IESG Review: fixed truncated sentence in bullet #3 in Section 4. 585 5. IESG Review: added privacy considerations section that references 586 the corresponding tzdist service section. 588 6. IESG Review: added text to security considerations section to 589 indicate tzdist servers need to be protected as well as the 590 caldav server. 592 Changes in -04: 594 1. AD Review: added Updates 4791 596 2. AD Review: reworded first bullet of Section 4 to indicate clients 597 have a choice of where to locate time zones 599 3. AD Review: added text about server providing an admin config 600 option to override new default behavior 602 4. AD Review: Replaced Prefer header with CalDAV-Timezones 604 Changes in -03: 606 1. Chair Review: minor editorial changes 608 2. Chair Review: expanded item #3 in Section 3.1.4 to clarify the 609 behavior and indicate when it would not be appropriate. 611 Changes in -02: 613 1. Ticket #27: added an HTTP Prefer header preference to allow 614 clients to explicitly state whether they want or do not want 615 VTIMEZONEs from the server. 617 Changes in -01: 619 1. Ticket #24: rewording intro to Section 3 to remove the MUST. 621 2. Ticket #26: rewording Section 3.1.1 to not provide advice about 622 how to handle limited devices. 624 Changes in -00: 626 1. Initial WG draft derived from draft-daboo-caldav-timezone-ref-01, 627 with some terminology changes to match WG name. 629 2. "timezone" -> "time zone" (https://tools.ietf.org/wg/tzdist/trac/ 630 ticket/6). 632 Author's Address 634 Cyrus Daboo 635 Apple Inc. 636 1 Infinite Loop 637 Cupertino , CA 95014 638 USA 640 Email: cyrus@daboo.name 641 URI: http://www.apple.com/