5Internet Engineering Task Force (IETF) M. Douglass
6Request for Comments: 9253 Bedework
7Updates: 5545 August 2022
8Category: Standards Track
12 Support for iCalendar Relationships
16 This specification updates the iCalendar RELATED-TO property defined
17 in RFC 5545 by adding new relation types and introduces new iCalendar
18 properties (LINK, CONCEPT, and REFID) to allow better linking and
19 grouping of iCalendar components and related data.
23 This is an Internet Standards Track document.
25 This document is a product of the Internet Engineering Task Force
26 (IETF). It represents the consensus of the IETF community. It has
27 received public review and has been approved for publication by the
28 Internet Engineering Steering Group (IESG). Further information on
29 Internet Standards is available in Section 2 of RFC 7841.
31 Information about the current status of this document, any errata,
32 and how to provide feedback on it may be obtained at
33 https://www.rfc-editor.org/info/rfc9253.
37 Copyright (c) 2022 IETF Trust and the persons identified as the
38 document authors. All rights reserved.
40 This document is subject to BCP 78 and the IETF Trust's Legal
41 Provisions Relating to IETF Documents
42 (https://trustee.ietf.org/license-info) in effect on the date of
43 publication of this document. Please review these documents
44 carefully, as they describe your rights and restrictions with respect
45 to this document. Code Components extracted from this document must
46 include Revised BSD License text as described in Section 4.e of the
47 Trust Legal Provisions and are provided without warranty as described
48 in the Revised BSD License.
53 1.1. Structured iCalendar Relationships
54 1.2. Grouped iCalendar Relationships
55 1.3. Concept Relationships
56 1.4. Linked Relationships
57 1.5. Caching and Offline Use
58 1.6. Conventions Used in This Document
59 2. LINK Property Reference Types
60 3. Link Relation Types
61 4. New Temporal RELTYPE Parameter Values
62 5. Additional New RELTYPE Parameter Values
63 6. New Property Parameters
66 7. New Value Data Types
71 9. Updates to RFC 5545
73 10. Security Considerations
74 11. IANA Considerations
75 11.1. iCalendar Property Registrations
76 11.2. iCalendar Property Parameter Registrations
77 11.3. iCalendar Value Data Type Registrations
78 11.4. iCalendar RELTYPE Value Registrations
80 12.1. Normative References
81 12.2. Informative References
87 iCalendar entities defined in [RFC5545] often need to be related to
88 each other or to associated metadata. The specifications below
89 support relationships of the following forms:
91 Structured iCalendar: iCalendar entities can be related to each
92 other in some structured way, for example, as parent, sibling,
95 Grouped iCalendar: iCalendar entities can be related to each other
96 as a group. CATEGORIES are often used for this purpose but are
97 problematic for application developers due to their lack of
98 consistency and use as a free-form tag.
100 Linked: Entities can be linked to other entities, such as vCards,
101 through a URI and associated REL and FMTTYPE parameters.
1031.1. Structured iCalendar Relationships
105 The iCalendar [RFC5545] RELATED-TO property has no support for
106 temporal relationships as used by project management tools.
108 The RELTYPE parameter is extended to take new values defining
109 temporal relationships, a GAP parameter is defined to provide lead
110 and lag values, and RELATED-TO is extended to allow URI values.
111 These changes allow the RELATED-TO property to define a richer set of
112 relationships useful for project management.
1141.2. Grouped iCalendar Relationships
116 This specification defines a new REFID property, which allows
117 arbitrary groups of entities to be associated with the same key
120 REFID is used to identify a key allowing the association of
121 components that are all related to the referring, aggregating
122 component and the retrieval of components based on this key. For
123 example, this may be used to identify the tasks associated with a
124 given project without having to communicate the task structure of the
125 project. A further example is the grouping of all sub-tasks
126 associated with the delivery of a specific package in a package
129 As such, the presence of a REFID property imparts no meaning to the
130 component. It is merely a key to allow retrieval. This is distinct
131 from categorization, which, while allowing grouping, also adds
132 meaning to the component to which it is attached.
1341.3. Concept Relationships
136 The name CONCEPT is used by the Simple Knowledge Organization System,
137 as defined in [W3C.REC-skos-reference-20090818]. The term "concept"
138 more accurately defines what we often mean by a category. It's not
139 the text string that is important but the meaning attached to it.
140 For example, the term "football" can mean very different sports.
142 The introduction of CONCEPT allows a more structured approach to
143 categorization, with the possibility of namespaced and path-like
144 values. Unlike REFID, the CONCEPT property imparts some meaning. It
145 is assumed that the value of this property will reference a well-
148 The current CATEGORIES property defined in [RFC5545] is used as a
149 free-form 'tagging' field. These values have some meaning to those
150 who apply them but not necessarily to any consumer. As such, it is
151 difficult to establish formal relationships between components based
154 Rather than attempt to add semantics to the CATEGORIES property, it
155 seems best to continue its usage as an informal tag and establish a
156 new CONCEPT property with more constraints.
1581.4. Linked Relationships
160 The currently existing iCalendar standard [RFC5545] lacks a general
161 purpose method for referencing additional, external information
162 relating to calendar components.
164 This document proposes a method for referencing typed external
165 information that can provide additional information about an
166 iCalendar component. This new LINK property is closely aligned to
167 [RFC8288], which defines the generic concept of Web Linking, as well
168 as its expression in the HTTP LINK header field.
170 The LINK property defines a typed reference or relation to external
171 metadata or related resources. By providing type and format
172 information as parameters, clients and servers are able to discover
173 interesting references and make use of them, perhaps for indexing or
174 the presentation of interesting links for the user.
176 Calendar components are often grouped into collections to represent a
177 calendar or a series of tasks, for example, Calendaring Extensions to
178 WebDAV (CalDAV) calendar collections [RFC4791].
180 It is also often necessary to reference calendar components in other
181 collections. For example, a VEVENT might refer to a VTODO from which
182 it was derived. The PARENT, SIBLING, and CHILD relationships defined
183 for the RELATED-TO property only allow for a unique identifier (UID),
184 which is inadequate for many purposes. Allowing other value types
185 for those relationships may help but would cause backward-
186 compatibility issues. The LINK property can link components in
187 different collections or even on different servers.
189 When publishing events, it is useful to be able to refer back to the
190 source of that information. The actual event may have been consumed
191 from a feed or an ics file on a website. A LINK property can provide
192 a reference to the originator of the event.
194 Beyond the need to relate elements temporally, project management
195 tools often need to be able to specify the relationships between the
196 various events and tasks that make up a project. The LINK property
197 provides such a mechanism.
199 The LINK property MUST NOT be treated as just another attachment.
200 The ATTACH property defined in [RFC5545] has been extended by
201 [RFC8607] to handle server-side management and stripping of inline
202 data and to provide additional data about the attachment (size,
205 Additionally, clients may choose to handle attachments differently
206 from the LINK property, as attachments are often an integral part of
207 the message, for example, the agenda.
2091.5. Caching and Offline Use
211 In general, the calendar entity should be self explanatory without
212 the need to download referenced metadata, such as a web page.
214 However, to facilitate offline display, the link type may identify
215 important pieces of data that should be downloaded in advance.
2171.6. Conventions Used in This Document
219 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
220 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
221 "OPTIONAL" in this document are to be interpreted as described in
222 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
223 capitals, as shown here.
225 The notation used in this memo to (re-)define iCalendar elements is
226 the ABNF notation of [RFC5234], as used by [RFC5545]. Any syntax
227 elements shown below that are not explicitly defined in this
228 specification come from iCalendar [RFC5545].
2302. LINK Property Reference Types
232 The reference value in the LINK property defined below can take three
233 forms specified by the VALUE parameter:
235 URI: This is a URI referring to the target.
237 UID: This allows for linking within a single collection of calendar
238 components, and the value MUST refer to another component within
241 XML-REFERENCE: In an XML environment, it may be necessary to refer
242 to a fragment of an external XML artifact. This value is a URI
243 with an XPointer anchor value. The XPointer is defined in
244 [W3C.WD-xptr-xpointer-20021219], and its use as an anchor is
245 defined in [W3C.REC-xptr-framework-20030325].
247 Note that UID references may need updating on import. An example is
248 data to be imported from a file containing VTODO and VEVENT
249 components, with a VTODO referring to VEVENT components by UID. When
250 imported into a CalDAV system, the VTODO components are typically
251 placed in a different collection from the VEVENT components. This
252 would require the UID reference to be replaced with a URI.
2543. Link Relation Types
256 Two forms of relation types are defined in [RFC8288]: registered and
257 extension. Registered relation types are added to the "Link
258 Relations" registry, as specified in Section 2.1.1 of [RFC8288].
259 Extension relation types, defined in Section 2.1.2 of [RFC8288], are
260 specified as unique URIs that are not registered in the registry.
262 The relation types defined in Section 6.1 will be registered with
263 IANA in accordance with the specifications in [RFC8288].
2654. New Temporal RELTYPE Parameter Values
267 This section defines the usual temporal relationships for use with
268 the RELTYPE parameter defined in Section 3.2.15 of [RFC5545]:
269 FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH, or STARTTOSTART.
271 The [RFC5545] RELATED-TO property with one or more of these temporal
272 relationships will be present in the predecessor entity and will
273 refer to the successor entity.
275 The GAP parameter (see Section 6.2) specifies the lead (a negative
276 value) or lag (a positive value) time between the predecessor and the
279 In the description of each temporal relationship below, we refer to
280 Task-A, which contains and controls the relationship, and Task-B,
281 which is the target of the relationship. This is indicated by the
282 direction of the arrows in the diagrams below.
284 Also, each relationship may be modified by the addition of a GAP
285 parameter to the relationship that applies to the targeted component.
287 RELTYPE=FINISHTOSTART: Task-B cannot start until Task-A finishes.
288 For example, when painting is complete, carpet laying can begin.
299 Figure 1: Finish-to-Start Relationship
301 RELTYPE=FINISHTOFINISH: Task-B can only be completed after Task-A is
302 finished. The related tasks may run in parallel before
305 For example, in the development of two related pieces of software
306 (e.g., the API and the implementation), the design of the
307 implementation (Task-B) cannot be completed until the design of
308 the API (Task-A) has been completed.
318 Figure 2: Finish-to-Finish Relationship
320 RELTYPE=STARTTOFINISH: The start of Task-A (which occurs after Task-
321 B) controls the finish of Task-B. For example, ticket sales
322 (Task-B) end after the game starts (Task-A).
332 Figure 3: Start-to-Finish Relationship
334 RELTYPE=STARTTOSTART: The start of Task-A triggers the start of
335 Task-B, that is, Task-B can start anytime after Task-A starts.
345 Figure 4: Start-to-Start Relationship
3475. Additional New RELTYPE Parameter Values
349 This section defines the additional relationships below:
351 RELTYPE=FIRST: This indicates that the referenced calendar component
352 is the first in a series the referencing calendar component is
355 RELTYPE=NEXT: This indicates that the referenced calendar component
356 is the next in a series the referencing calendar component is part
359 RELTYPE=DEPENDS-ON: This indicates that the current calendar
360 component depends on the referenced calendar component in some
361 manner. For example, a task may be blocked waiting on the other,
364 RELTYPE=REFID: This establishes a reference from the current
365 component to components with a REFID property that matches the
366 value given in the associated RELATED-TO property.
368 RELTYPE=CONCEPT: This establishes a reference from the current
369 component to components with a CONCEPT property that matches the
370 value given in the associated RELATED-TO property.
372 Note that the relationship types of PARENT, CHILD, and SIBLING
373 establish a hierarchical relationship. The new types of FIRST and
374 NEXT are an ordering relationship.
3766. New Property Parameters
380 Parameter name: LINKREL
382 Purpose: This property specifies the relationship of data referenced
385 Format Definition: This parameter is defined by the following
388 linkrelparam = "LINKREL" "="
390 / iana-token) ; Other IANA registered type
392 Description: This parameter MUST be specified on all LINK properties
393 and define the type of reference. This allows programs consuming
394 this data to automatically scan for references they support.
395 There is no default relation type.
397 Any link relation in the link registry established by [RFC8288],
398 or new link relations, may be used. It is expected that link
399 relation types seeing significant usage in calendaring will have
400 the calendaring usage described in an RFC.
402 LINKREL=latest-version: This identifies the latest version of the
405 Registration: These relation types are registered in [RFC8288].
411 Purpose: This property specifies the length of the gap, positive or
412 negative, between two components with a temporal relationship.
414 Format Definition: This parameter is defined by the following
415 notation, where dur-value is defined in Section 3.3.6 of
418 gapparam = "GAP" "=" dur-value
420 Description: This parameter MAY be specified on the RELATED-TO
421 property and defines the duration of time between the predecessor
422 and successor in an interval. When positive, it defines the lag
423 time between a task and its logical successor. When negative, it
424 defines the lead time.
426 An example of lag time might be if Task-A is "paint the room" and
427 Task-B is "lay the carpets". Then, Task-A may be related to
428 Task-B with RELTYPE=FINISHTOSTART with a gap of 1 day -- long
429 enough for the paint to dry.
432 | paint the room |--+
433 ==================== |
436 | ===================
437 +->| lay the carpet |
440 Figure 5: Finish-to-Start Relationship with Lag
442 For an example of lead time, in constructing a two-story building,
443 the electrical work must be done before painting. However, the
444 painter can move in to the first floor as the electricians move
447 =====================
448 | electrical work |--+
449 ===================== |
451 |(lead of estimated time)
456 Figure 6: Finish-to-Start Relationship with Lead
4587. New Value Data Types
460 This specification defines the following new value types to be used
461 with the VALUE property parameter:
463 UID: VALUE=UID indicates that the associated value is the UID for a
466 XML-REFERENCE: VALUE=XML-REFERENCE indicates that the associated
467 value references an associated XML artifact and is a URI with an
468 XPointer anchor value. The XPointer is defined in
469 [W3C.WD-xptr-xpointer-20021219], and its use as an anchor is
470 defined in [W3C.REC-xptr-framework-20030325].
476 Property name: CONCEPT
478 Purpose: This property defines the formal categories for a calendar
483 Property Parameters: IANA and non-standard parameters can be
484 specified on this property.
486 Conformance: This property can be specified zero or more times in
487 any iCalendar component.
489 Description: This property is used to specify formal categories or
490 classifications of the calendar component. The values are useful
491 in searching for a calendar component of a particular type and
494 This categorization is distinct from the more informal "tagging"
495 of components provided by the existing CATEGORIES property. It is
496 expected that the value of the CONCEPT property will reference an
497 external resource that provides information about the
500 In addition, a structured URI value allows for hierarchical
501 categorization of events.
503 Possible category resources are the various proprietary systems,
504 for example, the Library of Congress, or an open source of
507 Format Definition: This property is defined by the following
510 concept = "CONCEPT" conceptparam ":"
513 conceptparam = *(";" other-param)
515 Example: The following is an example of this property. It points to
516 a server acting as the source for the calendar object.
518 CONCEPT:https://example.com/event-types/arts/music
524 Purpose: This property provides a reference to external information
525 related to a component.
527 Value type: URI, UID, or XML-REFERENCE
529 Property Parameters: The VALUE parameter is required. Non-standard,
530 link relation type, format type, label, and language parameters
531 can also be specified on this property. The LABEL parameter is
532 defined in [RFC7986].
534 Conformance: This property can be specified zero or more times in
535 any iCalendar component.
537 Description: When used in a component, the value of this property
538 points to additional information related to the component. For
539 example, it may reference the originating web server.
541 Format Definition: This property is defined by the following
544 link = "LINK" linkparam ":"
545 ( uri / ; for VALUE=XML-REFERENCE
546 uri / ; for VALUE=URI
547 text ) ; for VALUE=UID
550 linkparam = (";" "VALUE" "=" ("XML-REFERENCE" /
556 1*(";" languageparam)
558 ; the elements herein may appear in any order,
559 ; and the order is not significant.
561 This property is a serialization of the model in [RFC8288], where
562 the link target is carried in the property value, the link context
563 is the containing calendar entity, and the link relation type and
564 any target attributes are carried in iCalendar property
567 The LINK property parameters map to [RFC8288] attributes as
570 LABEL: This parameter maps to the "title" attribute defined in
571 Section 3.4.1 of [RFC8288].
573 LANGUAGE: This parameter maps to the "hreflang" attribute defined
574 in Section 3.4.1 of [RFC8288].
576 LINKREL: This parameter maps to the link relation type defined in
577 Section 2.1 of [RFC8288].
579 FMTTYPE: This parameter maps to the "type" attribute defined in
580 Section 3.4.1 of [RFC8288].
582 There is no mapping for "title*", "anchor", "rev", or "media"
585 Example: The following is an example of this property, which
586 provides a reference to the source for the calendar object.
588 LINK;LINKREL=SOURCE;LABEL=Venue;VALUE=URI:
589 https://example.com/events
591 Example: The following is an example of this property, which
592 provides a reference to an entity from which this one was derived.
593 The link relation is a vendor-defined value.
595 LINK;LINKREL="https://example.com/linkrel/derivedFrom";
597 https://example.com/tasks/01234567-abcd1234.ics
599 Example: The following is an example of this property, which
600 provides a reference to a fragment of an XML document. The link
601 relation is a vendor-defined value.
603 LINK;LINKREL="https://example.com/linkrel/costStructure";
605 https://example.com/xmlDocs/bidFramework.xml
606 #xpointer(descendant::CostStruc/range-to(
607 following::CostStrucEND[1]))
613 Purpose: This property value acts as a key for associated iCalendar
618 Property Parameters: Non-standard parameters can be specified on
621 Conformance: This property can be specified zero or more times in
622 any iCalendar component.
624 Description: The value of this property is free-form text that
625 creates an identifier for associated components. All components
626 that use the same REFID value are associated through that value
627 and can be located or retrieved as a group. For example, all of
628 the events in a travel itinerary would have the same REFID value,
629 so as to be grouped together.
631 Format Definition: This property is defined by the following
634 refid = "REFID" refidparam ":" text CRLF
637 refidparam = *(";" other-param)
639 Example: The following is an example of this property.
641 REFID:itinerary-2014-11-17
6439. Updates to RFC 5545
645 This specification updates the RELATED-TO property defined in
646 Section 3.8.4.5 of [RFC5545]. The contents of Section 9.1 replace
649 The RELTYPE parameter is extended to take new values defining
650 temporal relationships, a GAP parameter is defined to provide lead
651 and lag values, and RELATED-TO is extended to allow URI values.
652 These changes allow the RELATED-TO property to define a richer set of
653 relationships useful for project management.
657 Property name: RELATED-TO
659 Purpose: This property is used to represent a relationship or
660 reference between one calendar component and another. The
661 definition here extends the definition in Section 3.8.4.5 of
662 [RFC5545] by allowing URI or UID values and a GAP parameter.
664 Value Type: URI, UID, or TEXT
666 Property Parameters: Relationship type, IANA, and non-standard
667 property parameters can be specified on this property.
669 Conformance: This property MAY be specified in any iCalendar
672 Description: By default or when VALUE=UID is specified, the property
673 value consists of the persistent, globally unique identifier of
674 another calendar component. This value would be represented in a
675 calendar component by the UID property.
677 By default, the property value points to another calendar
678 component that has a PARENT relationship to the referencing
679 object. The RELTYPE property parameter is used to either
680 explicitly state the default PARENT relationship type to the
681 referenced calendar component or to override the default PARENT
682 relationship type and specify either a CHILD or SIBLING
683 relationship or a temporal relationship.
685 The PARENT relationship indicates that the calendar component is a
686 subordinate of the referenced calendar component. The CHILD
687 relationship indicates that the calendar component is a superior
688 of the referenced calendar component. The SIBLING relationship
689 indicates that the calendar component is a peer of the referenced
692 To preserve backwards compatibility, the value type MUST be UID
693 when the PARENT, SIBLING, or CHILD relationships are specified.
695 The FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH, or STARTTOSTART
696 relationships define temporal relationships, as specified in the
697 RELTYPE parameter definition.
699 The FIRST and NEXT define ordering relationships between calendar
702 The DEPENDS-ON relationship indicates that the current calendar
703 component depends on the referenced calendar component in some
704 manner. For example, a task may be blocked waiting on the other,
707 The REFID and CONCEPT relationships establish a reference from the
708 current component to the referenced component.
710 Changes to a calendar component referenced by this property can
711 have an implicit impact on the related calendar component. For
712 example, if a group event changes its start or end date or time,
713 then the related, dependent events will need to have their start
714 and end dates and times changed in a corresponding way.
715 Similarly, if a PARENT calendar component is canceled or deleted,
716 then there is an implied impact to the related CHILD calendar
717 components. This property is intended only to provide information
718 on the relationship of calendar components.
720 Deletion of the target component, for example, the target of a
721 FIRST, NEXT, or temporal relationship, can result in broken links.
723 It is up to the target calendar system to maintain any property
724 implications of these relationships.
726 Format Definition: This property is defined by the following
729 related = "RELATED-TO" relparam ":"
730 ( text / ; for VALUE=UID
731 uri / ; for VALUE=URI
732 text ) ; for VALUE=TEXT or default
735 relparam = ; the elements herein may appear in any order,
736 ; and the order is not significant.
737 [";" "VALUE" "=" ("UID" /
744 Example: The following are examples of this property.
746 RELATED-TO:jsmith.part7.19960817T083000.xyzMail@example.com
748 RELATED-TO:19960401-080045-4000F192713-0052@example.com
750 RELATED-TO;VALUE=URI;RELTYPE=STARTTOFINISH:
751 https://example.com/caldav/user/jb/cal/
752 19960401-080045-4000F192713.ics
75410. Security Considerations
756 All of the security considerations of Section 7 of [RFC5545] apply to
759 Applications using the LINK property need to be aware of the risks
760 entailed in using the URIs provided as values. See Section 7 of
761 [RFC3986] for a discussion of the security considerations relating to
764 In particular, note Section 7.1 (Reliability and Consistency) of
765 [RFC3986], which points out the lack of a stability guarantee for
766 referenced resources.
768 When the value is an XML-REFERENCE type, the targeted data is an XML
769 document or portion thereof. Consumers need to be aware of the
770 security issues related to XML processing -- in particular, those
771 related to XML entities. See Section 20.6 of [RFC4918].
772 Additionally, note that the reference may be invalid or become so
775 The CONCEPT and redefined RELATED-TO properties have the same issues
776 in that values may be URIs.
778 Extremely large values for the GAP parameter may lead to unexpected
78111. IANA Considerations
78311.1. iCalendar Property Registrations
785 The following iCalendar property names have been added to the
786 iCalendar "Properties" registry defined in Section 8.3.2 of
787 [RFC5545]. IANA has also added a reference to this document, where
788 the properties originally defined in [RFC5545] have been updated by
791 +============+=========+=============================+
792 | Property | Status | Reference |
793 +============+=========+=============================+
794 | CONCEPT | Current | Section 8.1 |
795 +------------+---------+-----------------------------+
796 | LINK | Current | Section 8.2 |
797 +------------+---------+-----------------------------+
798 | REFID | Current | Section 8.3 |
799 +------------+---------+-----------------------------+
800 | RELATED-TO | Current | [RFC5545], Section 3.8.4.5; |
801 | | | RFC 9253, Section 9.1 |
802 +------------+---------+-----------------------------+
80611.2. iCalendar Property Parameter Registrations
808 The following iCalendar property parameter names have been added to
809 the iCalendar "Parameters" registry defined in Section 8.3.3 of
812 +===========+=========+=============+
813 | Parameter | Status | Reference |
814 +===========+=========+=============+
815 | GAP | Current | Section 6.2 |
816 +-----------+---------+-------------+
817 | LINKREL | Current | Section 6.1 |
818 +-----------+---------+-------------+
82211.3. iCalendar Value Data Type Registrations
824 The following iCalendar property parameter names have been added to
825 the iCalendar "Value Data Types" registry defined in Section 8.3.4 of
828 +=================+=========+===========+
829 | Value Data Type | Status | Reference |
830 +=================+=========+===========+
831 | XML-REFERENCE | Current | Section 7 |
832 +-----------------+---------+-----------+
833 | UID | Current | Section 7 |
834 +-----------------+---------+-----------+
83811.4. iCalendar RELTYPE Value Registrations
840 The following iCalendar "RELTYPE" values have been added to the
841 iCalendar "Relationship Types" registry defined in Section 8.3.8 of
844 +===================+=========+===========+
845 | Relationship Type | Status | Reference |
846 +===================+=========+===========+
847 | CONCEPT | Current | Section 5 |
848 +-------------------+---------+-----------+
849 | DEPENDS-ON | Current | Section 5 |
850 +-------------------+---------+-----------+
851 | FINISHTOFINISH | Current | Section 4 |
852 +-------------------+---------+-----------+
853 | FINISHTOSTART | Current | Section 4 |
854 +-------------------+---------+-----------+
855 | FIRST | Current | Section 5 |
856 +-------------------+---------+-----------+
857 | NEXT | Current | Section 5 |
858 +-------------------+---------+-----------+
859 | REFID | Current | Section 5 |
860 +-------------------+---------+-----------+
861 | STARTTOFINISH | Current | Section 4 |
862 +-------------------+---------+-----------+
863 | STARTTOSTART | Current | Section 4 |
864 +-------------------+---------+-----------+
87012.1. Normative References
872 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
873 Requirement Levels", BCP 14, RFC 2119,
874 DOI 10.17487/RFC2119, March 1997,
875 <https://www.rfc-editor.org/info/rfc2119>.
877 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
878 Resource Identifier (URI): Generic Syntax", STD 66,
879 RFC 3986, DOI 10.17487/RFC3986, January 2005,
880 <https://www.rfc-editor.org/info/rfc3986>.
882 [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
883 Authoring and Versioning (WebDAV)", RFC 4918,
884 DOI 10.17487/RFC4918, June 2007,
885 <https://www.rfc-editor.org/info/rfc4918>.
887 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
888 Specifications: ABNF", STD 68, RFC 5234,
889 DOI 10.17487/RFC5234, January 2008,
890 <https://www.rfc-editor.org/info/rfc5234>.
892 [RFC5545] Desruisseaux, B., Ed., "Internet Calendaring and
893 Scheduling Core Object Specification (iCalendar)",
894 RFC 5545, DOI 10.17487/RFC5545, September 2009,
895 <https://www.rfc-editor.org/info/rfc5545>.
897 [RFC7986] Daboo, C., "New Properties for iCalendar", RFC 7986,
898 DOI 10.17487/RFC7986, October 2016,
899 <https://www.rfc-editor.org/info/rfc7986>.
901 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
902 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
903 May 2017, <https://www.rfc-editor.org/info/rfc8174>.
905 [RFC8288] Nottingham, M., "Web Linking", RFC 8288,
906 DOI 10.17487/RFC8288, October 2017,
907 <https://www.rfc-editor.org/info/rfc8288>.
909 [W3C.REC-skos-reference-20090818]
910 Miles, A. and S. Bechhofer, "SKOS Simple Knowledge
911 Organization System Reference", W3C Recommendation REC-
912 skos-reference-20090818, 18 August 2009,
913 <https://www.w3.org/TR/2009/REC-skos-reference-20090818>.
915 [W3C.REC-xptr-framework-20030325]
916 Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
917 Framework", W3C Recommendation REC-xptr-framework-
918 20030325, 25 March 2003,
919 <https://www.w3.org/TR/2003/REC-xptr-framework-20030325>.
921 [W3C.WD-xptr-xpointer-20021219]
922 DeRose, S., Maler, E., and R. Daniel, "XPointer xpointer()
923 Scheme", W3C WD WD-xptr-xpointer-20021219, 19 December
925 <http://www.w3.org/TR/2002/WD-xptr-xpointer-20021219>.
92712.2. Informative References
929 [RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault,
930 "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
931 DOI 10.17487/RFC4791, March 2007,
932 <https://www.rfc-editor.org/info/rfc4791>.
934 [RFC8607] Daboo, C., Quillaud, A., and K. Murchison, Ed.,
935 "Calendaring Extensions to WebDAV (CalDAV): Managed
936 Attachments", RFC 8607, DOI 10.17487/RFC8607, June 2019,
937 <https://www.rfc-editor.org/info/rfc8607>.
941 The author would like to thank the members of CalConnect, the
942 Calendaring and Scheduling Consortium technical committees, and the
943 following individuals for contributing their ideas, support, and
946 Adrian Apthorp, Cyrus Daboo, Marten Gajda, and Ken Murchison
948 The author would also like to thank CalConnect and the Calendaring
949 and Scheduling Consortium for advice with this specification.
957 United States of America
958 Email: mdouglass@bedework.com
959 URI: https://bedework.com