path: root/doc/rfc/rfc9253.txt

Internet Engineering Task Force (IETF)                       M. Douglass
Request for Comments: 9253                                      Bedework
Updates: 5545                                                August 2022
Category: Standards Track                                               
ISSN: 2070-1721


                  Support for iCalendar Relationships

Abstract

   This specification updates the iCalendar RELATED-TO property defined
   in RFC 5545 by adding new relation types and introduces new iCalendar
   properties (LINK, CONCEPT, and REFID) to allow better linking and
   grouping of iCalendar components and related data.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9253.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Structured iCalendar Relationships
     1.2.  Grouped iCalendar Relationships
     1.3.  Concept Relationships
     1.4.  Linked Relationships
     1.5.  Caching and Offline Use
     1.6.  Conventions Used in This Document
   2.  LINK Property Reference Types
   3.  Link Relation Types
   4.  New Temporal RELTYPE Parameter Values
   5.  Additional New RELTYPE Parameter Values
   6.  New Property Parameters
     6.1.  Link Relation
     6.2.  Gap
   7.  New Value Data Types
   8.  New Properties
     8.1.  Concept
     8.2.  Link
     8.3.  Refid
   9.  Updates to RFC 5545
     9.1.  RELATED-TO
   10. Security Considerations
   11. IANA Considerations
     11.1.  iCalendar Property Registrations
     11.2.  iCalendar Property Parameter Registrations
     11.3.  iCalendar Value Data Type Registrations
     11.4.  iCalendar RELTYPE Value Registrations
   12. References
     12.1.  Normative References
     12.2.  Informative References
   Acknowledgements
   Author's Address

1.  Introduction

   iCalendar entities defined in [RFC5545] often need to be related to
   each other or to associated metadata.  The specifications below
   support relationships of the following forms:

   Structured iCalendar:  iCalendar entities can be related to each
      other in some structured way, for example, as parent, sibling,
      before, or after.

   Grouped iCalendar:  iCalendar entities can be related to each other
      as a group.  CATEGORIES are often used for this purpose but are
      problematic for application developers due to their lack of
      consistency and use as a free-form tag.

   Linked:  Entities can be linked to other entities, such as vCards,
      through a URI and associated REL and FMTTYPE parameters.

1.1.  Structured iCalendar Relationships

   The iCalendar [RFC5545] RELATED-TO property has no support for
   temporal relationships as used by project management tools.

   The RELTYPE parameter is extended to take new values defining
   temporal relationships, a GAP parameter is defined to provide lead
   and lag values, and RELATED-TO is extended to allow URI values.
   These changes allow the RELATED-TO property to define a richer set of
   relationships useful for project management.

1.2.  Grouped iCalendar Relationships

   This specification defines a new REFID property, which allows
   arbitrary groups of entities to be associated with the same key
   value.

   REFID is used to identify a key allowing the association of
   components that are all related to the referring, aggregating
   component and the retrieval of components based on this key.  For
   example, this may be used to identify the tasks associated with a
   given project without having to communicate the task structure of the
   project.  A further example is the grouping of all sub-tasks
   associated with the delivery of a specific package in a package
   delivery system.

   As such, the presence of a REFID property imparts no meaning to the
   component.  It is merely a key to allow retrieval.  This is distinct
   from categorization, which, while allowing grouping, also adds
   meaning to the component to which it is attached.

1.3.  Concept Relationships

   The name CONCEPT is used by the Simple Knowledge Organization System,
   as defined in [W3C.REC-skos-reference-20090818].  The term "concept"
   more accurately defines what we often mean by a category.  It's not
   the text string that is important but the meaning attached to it.
   For example, the term "football" can mean very different sports.

   The introduction of CONCEPT allows a more structured approach to
   categorization, with the possibility of namespaced and path-like
   values.  Unlike REFID, the CONCEPT property imparts some meaning.  It
   is assumed that the value of this property will reference a well-
   defined category.

   The current CATEGORIES property defined in [RFC5545] is used as a
   free-form 'tagging' field.  These values have some meaning to those
   who apply them but not necessarily to any consumer.  As such, it is
   difficult to establish formal relationships between components based
   on their category.

   Rather than attempt to add semantics to the CATEGORIES property, it
   seems best to continue its usage as an informal tag and establish a
   new CONCEPT property with more constraints.

1.4.  Linked Relationships

   The currently existing iCalendar standard [RFC5545] lacks a general
   purpose method for referencing additional, external information
   relating to calendar components.

   This document proposes a method for referencing typed external
   information that can provide additional information about an
   iCalendar component.  This new LINK property is closely aligned to
   [RFC8288], which defines the generic concept of Web Linking, as well
   as its expression in the HTTP LINK header field.

   The LINK property defines a typed reference or relation to external
   metadata or related resources.  By providing type and format
   information as parameters, clients and servers are able to discover
   interesting references and make use of them, perhaps for indexing or
   the presentation of interesting links for the user.

   Calendar components are often grouped into collections to represent a
   calendar or a series of tasks, for example, Calendaring Extensions to
   WebDAV (CalDAV) calendar collections [RFC4791].

   It is also often necessary to reference calendar components in other
   collections.  For example, a VEVENT might refer to a VTODO from which
   it was derived.  The PARENT, SIBLING, and CHILD relationships defined
   for the RELATED-TO property only allow for a unique identifier (UID),
   which is inadequate for many purposes.  Allowing other value types
   for those relationships may help but would cause backward-
   compatibility issues.  The LINK property can link components in
   different collections or even on different servers.

   When publishing events, it is useful to be able to refer back to the
   source of that information.  The actual event may have been consumed
   from a feed or an ics file on a website.  A LINK property can provide
   a reference to the originator of the event.

   Beyond the need to relate elements temporally, project management
   tools often need to be able to specify the relationships between the
   various events and tasks that make up a project.  The LINK property
   provides such a mechanism.

   The LINK property MUST NOT be treated as just another attachment.
   The ATTACH property defined in [RFC5545] has been extended by
   [RFC8607] to handle server-side management and stripping of inline
   data and to provide additional data about the attachment (size,
   filename, etc.).

   Additionally, clients may choose to handle attachments differently
   from the LINK property, as attachments are often an integral part of
   the message, for example, the agenda.

1.5.  Caching and Offline Use

   In general, the calendar entity should be self explanatory without
   the need to download referenced metadata, such as a web page.

   However, to facilitate offline display, the link type may identify
   important pieces of data that should be downloaded in advance.

1.6.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The notation used in this memo to (re-)define iCalendar elements is
   the ABNF notation of [RFC5234], as used by [RFC5545].  Any syntax
   elements shown below that are not explicitly defined in this
   specification come from iCalendar [RFC5545].

2.  LINK Property Reference Types

   The reference value in the LINK property defined below can take three
   forms specified by the VALUE parameter:

   URI:  This is a URI referring to the target.

   UID:  This allows for linking within a single collection of calendar
      components, and the value MUST refer to another component within
      the same collection.

   XML-REFERENCE:  In an XML environment, it may be necessary to refer
      to a fragment of an external XML artifact.  This value is a URI
      with an XPointer anchor value.  The XPointer is defined in
      [W3C.WD-xptr-xpointer-20021219], and its use as an anchor is
      defined in [W3C.REC-xptr-framework-20030325].

   Note that UID references may need updating on import.  An example is
   data to be imported from a file containing VTODO and VEVENT
   components, with a VTODO referring to VEVENT components by UID.  When
   imported into a CalDAV system, the VTODO components are typically
   placed in a different collection from the VEVENT components.  This
   would require the UID reference to be replaced with a URI.

3.  Link Relation Types

   Two forms of relation types are defined in [RFC8288]: registered and
   extension.  Registered relation types are added to the "Link
   Relations" registry, as specified in Section 2.1.1 of [RFC8288].
   Extension relation types, defined in Section 2.1.2 of [RFC8288], are
   specified as unique URIs that are not registered in the registry.

   The relation types defined in Section 6.1 will be registered with
   IANA in accordance with the specifications in [RFC8288].

4.  New Temporal RELTYPE Parameter Values

   This section defines the usual temporal relationships for use with
   the RELTYPE parameter defined in Section 3.2.15 of [RFC5545]:
   FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH, or STARTTOSTART.

   The [RFC5545] RELATED-TO property with one or more of these temporal
   relationships will be present in the predecessor entity and will
   refer to the successor entity.

   The GAP parameter (see Section 6.2) specifies the lead (a negative
   value) or lag (a positive value) time between the predecessor and the
   successor.

   In the description of each temporal relationship below, we refer to
   Task-A, which contains and controls the relationship, and Task-B,
   which is the target of the relationship.  This is indicated by the
   direction of the arrows in the diagrams below.

   Also, each relationship may be modified by the addition of a GAP
   parameter to the relationship that applies to the targeted component.

   RELTYPE=FINISHTOSTART:  Task-B cannot start until Task-A finishes.
      For example, when painting is complete, carpet laying can begin.

                  ============
                  |  Task-A  |
                  ============
                             |
                             V
                             ============
                             |  Task-B  |
                             ============

                    Figure 1: Finish-to-Start Relationship

   RELTYPE=FINISHTOFINISH:  Task-B can only be completed after Task-A is
      finished.  The related tasks may run in parallel before
      completion.

      For example, in the development of two related pieces of software
      (e.g., the API and the implementation), the design of the
      implementation (Task-B) cannot be completed until the design of
      the API (Task-A) has been completed.

                          ==================
                          |     Task-A     |--+
                          ==================  |
                                              |
                                ============  |
                                |  Task-B  |<-+
                                ============

                   Figure 2: Finish-to-Finish Relationship

   RELTYPE=STARTTOFINISH:  The start of Task-A (which occurs after Task-
      B) controls the finish of Task-B.  For example, ticket sales
      (Task-B) end after the game starts (Task-A).

                                 ============
                              +--|  Task-A  |
                              |  ============
                              +---------+
                          ============  |
                          |  Task-B  |<-+
                          ============

                    Figure 3: Start-to-Finish Relationship

   RELTYPE=STARTTOSTART:  The start of Task-A triggers the start of
      Task-B, that is, Task-B can start anytime after Task-A starts.

                          ============
                       +--|  Task-A  |
                       |  ============
                       |
                       |  ============
                       +->|  Task-B  |
                          ============

                    Figure 4: Start-to-Start Relationship

5.  Additional New RELTYPE Parameter Values

   This section defines the additional relationships below:

   RELTYPE=FIRST:  This indicates that the referenced calendar component
      is the first in a series the referencing calendar component is
      part of.

   RELTYPE=NEXT:  This indicates that the referenced calendar component
      is the next in a series the referencing calendar component is part
      of.

   RELTYPE=DEPENDS-ON:  This indicates that the current calendar
      component depends on the referenced calendar component in some
      manner.  For example, a task may be blocked waiting on the other,
      referenced, task.

   RELTYPE=REFID:  This establishes a reference from the current
      component to components with a REFID property that matches the
      value given in the associated RELATED-TO property.

   RELTYPE=CONCEPT:  This establishes a reference from the current
      component to components with a CONCEPT property that matches the
      value given in the associated RELATED-TO property.

   Note that the relationship types of PARENT, CHILD, and SIBLING
   establish a hierarchical relationship.  The new types of FIRST and
   NEXT are an ordering relationship.

6.  New Property Parameters

6.1.  Link Relation

   Parameter name:  LINKREL

   Purpose:  This property specifies the relationship of data referenced
      by a LINK property.

   Format Definition:  This parameter is defined by the following
      notation:

      linkrelparam = "LINKREL" "="
                      (DQUOTE uri DQUOTE
                     / iana-token)   ; Other IANA registered type

   Description:  This parameter MUST be specified on all LINK properties
      and define the type of reference.  This allows programs consuming
      this data to automatically scan for references they support.
      There is no default relation type.

      Any link relation in the link registry established by [RFC8288],
      or new link relations, may be used.  It is expected that link
      relation types seeing significant usage in calendaring will have
      the calendaring usage described in an RFC.

   LINKREL=latest-version:  This identifies the latest version of the
      event information.

   Registration:  These relation types are registered in [RFC8288].

6.2.  Gap

   Parameter name:  GAP

   Purpose:  This property specifies the length of the gap, positive or
      negative, between two components with a temporal relationship.

   Format Definition:  This parameter is defined by the following
      notation, where dur-value is defined in Section 3.3.6 of
      [RFC5545]. :

        gapparam      = "GAP" "=" dur-value

   Description:  This parameter MAY be specified on the RELATED-TO
      property and defines the duration of time between the predecessor
      and successor in an interval.  When positive, it defines the lag
      time between a task and its logical successor.  When negative, it
      defines the lead time.

      An example of lag time might be if Task-A is "paint the room" and
      Task-B is "lay the carpets".  Then, Task-A may be related to
      Task-B with RELTYPE=FINISHTOSTART with a gap of 1 day -- long
      enough for the paint to dry.

                  ====================
                  |  paint the room  |--+
                  ====================  |
                                        |(lag of one day)
                                        |
                                        |  ===================
                                        +->| lay the carpet  |
                                           ===================

               Figure 5: Finish-to-Start Relationship with Lag

      For an example of lead time, in constructing a two-story building,
      the electrical work must be done before painting.  However, the
      painter can move in to the first floor as the electricians move
      upstairs.

                  =====================
                  |  electrical work  |--+
                  =====================  |
                           +-------------+
                           |(lead of estimated time)
                           |  ==================
                           +->|    painting    |
                              ==================

               Figure 6: Finish-to-Start Relationship with Lead

7.  New Value Data Types

   This specification defines the following new value types to be used
   with the VALUE property parameter:

   UID:  VALUE=UID indicates that the associated value is the UID for a
      component.

   XML-REFERENCE:  VALUE=XML-REFERENCE indicates that the associated
      value references an associated XML artifact and is a URI with an
      XPointer anchor value.  The XPointer is defined in
      [W3C.WD-xptr-xpointer-20021219], and its use as an anchor is
      defined in [W3C.REC-xptr-framework-20030325].

8.  New Properties

8.1.  Concept

   Property name:  CONCEPT

   Purpose:  This property defines the formal categories for a calendar
      component.

   Value type:  URI

   Property Parameters:  IANA and non-standard parameters can be
      specified on this property.

   Conformance:  This property can be specified zero or more times in
      any iCalendar component.

   Description:  This property is used to specify formal categories or
      classifications of the calendar component.  The values are useful
      in searching for a calendar component of a particular type and
      category.

      This categorization is distinct from the more informal "tagging"
      of components provided by the existing CATEGORIES property.  It is
      expected that the value of the CONCEPT property will reference an
      external resource that provides information about the
      categorization.

      In addition, a structured URI value allows for hierarchical
      categorization of events.

      Possible category resources are the various proprietary systems,
      for example, the Library of Congress, or an open source of
      categorization data.

   Format Definition:  This property is defined by the following
      notation:

        concept        =  "CONCEPT" conceptparam ":"
                              uri CRLF

        conceptparam = *(";" other-param)

   Example:  The following is an example of this property.  It points to
      a server acting as the source for the calendar object.

        CONCEPT:https://example.com/event-types/arts/music

8.2.  Link

   Property name:  LINK

   Purpose:  This property provides a reference to external information
      related to a component.

   Value type:  URI, UID, or XML-REFERENCE

   Property Parameters:  The VALUE parameter is required.  Non-standard,
      link relation type, format type, label, and language parameters
      can also be specified on this property.  The LABEL parameter is
      defined in [RFC7986].

   Conformance:  This property can be specified zero or more times in
      any iCalendar component.

   Description:  When used in a component, the value of this property
      points to additional information related to the component.  For
      example, it may reference the originating web server.

   Format Definition:  This property is defined by the following
      notation:

         link           = "LINK" linkparam ":"
                            ( uri /  ; for VALUE=XML-REFERENCE
                              uri /  ; for VALUE=URI
                              text ) ; for VALUE=UID
                          CRLF

         linkparam      = (";" "VALUE" "=" ("XML-REFERENCE" /
                                      "URI" /
                                      "UID"))
                          1*(";" linkrelparam)
                          1*(";" fmttypeparam)
                          1*(";" labelparam)
                          1*(";" languageparam)
                          *(";" other-param)
                          ; the elements herein may appear in any order,
                          ; and the order is not significant.

      This property is a serialization of the model in [RFC8288], where
      the link target is carried in the property value, the link context
      is the containing calendar entity, and the link relation type and
      any target attributes are carried in iCalendar property
      parameters.

      The LINK property parameters map to [RFC8288] attributes as
      follows:

      LABEL:  This parameter maps to the "title" attribute defined in
         Section 3.4.1 of [RFC8288].

      LANGUAGE:  This parameter maps to the "hreflang" attribute defined
         in Section 3.4.1 of [RFC8288].

      LINKREL:  This parameter maps to the link relation type defined in
         Section 2.1 of [RFC8288].

      FMTTYPE:  This parameter maps to the "type" attribute defined in
         Section 3.4.1 of [RFC8288].

      There is no mapping for "title*", "anchor", "rev", or "media"
      [RFC8288].

   Example:  The following is an example of this property, which
      provides a reference to the source for the calendar object.

        LINK;LINKREL=SOURCE;LABEL=Venue;VALUE=URI:
         https://example.com/events

   Example:  The following is an example of this property, which
      provides a reference to an entity from which this one was derived.
      The link relation is a vendor-defined value.

        LINK;LINKREL="https://example.com/linkrel/derivedFrom";
         VALUE=URI:
         https://example.com/tasks/01234567-abcd1234.ics

   Example:  The following is an example of this property, which
      provides a reference to a fragment of an XML document.  The link
      relation is a vendor-defined value.

        LINK;LINKREL="https://example.com/linkrel/costStructure";
         VALUE=XML-REFERENCE:
         https://example.com/xmlDocs/bidFramework.xml
         #xpointer(descendant::CostStruc/range-to(
         following::CostStrucEND[1]))

8.3.  Refid

   Property name:  REFID

   Purpose:  This property value acts as a key for associated iCalendar
      entities.

   Value type:  TEXT

   Property Parameters:  Non-standard parameters can be specified on
      this property.

   Conformance:  This property can be specified zero or more times in
      any iCalendar component.

   Description:  The value of this property is free-form text that
      creates an identifier for associated components.  All components
      that use the same REFID value are associated through that value
      and can be located or retrieved as a group.  For example, all of
      the events in a travel itinerary would have the same REFID value,
      so as to be grouped together.

   Format Definition:  This property is defined by the following
      notation:

        refid      = "REFID" refidparam ":" text CRLF


        refidparam      = *(";" other-param)

   Example:  The following is an example of this property.

        REFID:itinerary-2014-11-17

9.  Updates to RFC 5545

   This specification updates the RELATED-TO property defined in
   Section 3.8.4.5 of [RFC5545].  The contents of Section 9.1 replace
   that section.

   The RELTYPE parameter is extended to take new values defining
   temporal relationships, a GAP parameter is defined to provide lead
   and lag values, and RELATED-TO is extended to allow URI values.
   These changes allow the RELATED-TO property to define a richer set of
   relationships useful for project management.

9.1.  RELATED-TO

   Property name:  RELATED-TO

   Purpose:  This property is used to represent a relationship or
      reference between one calendar component and another.  The
      definition here extends the definition in Section 3.8.4.5 of
      [RFC5545] by allowing URI or UID values and a GAP parameter.

   Value Type:  URI, UID, or TEXT

   Property Parameters:  Relationship type, IANA, and non-standard
      property parameters can be specified on this property.

   Conformance:  This property MAY be specified in any iCalendar
      component.

   Description:  By default or when VALUE=UID is specified, the property
      value consists of the persistent, globally unique identifier of
      another calendar component.  This value would be represented in a
      calendar component by the UID property.

      By default, the property value points to another calendar
      component that has a PARENT relationship to the referencing
      object.  The RELTYPE property parameter is used to either
      explicitly state the default PARENT relationship type to the
      referenced calendar component or to override the default PARENT
      relationship type and specify either a CHILD or SIBLING
      relationship or a temporal relationship.

      The PARENT relationship indicates that the calendar component is a
      subordinate of the referenced calendar component.  The CHILD
      relationship indicates that the calendar component is a superior
      of the referenced calendar component.  The SIBLING relationship
      indicates that the calendar component is a peer of the referenced
      calendar component.

      To preserve backwards compatibility, the value type MUST be UID
      when the PARENT, SIBLING, or CHILD relationships are specified.

      The FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH, or STARTTOSTART
      relationships define temporal relationships, as specified in the
      RELTYPE parameter definition.

      The FIRST and NEXT define ordering relationships between calendar
      components.

      The DEPENDS-ON relationship indicates that the current calendar
      component depends on the referenced calendar component in some
      manner.  For example, a task may be blocked waiting on the other,
      referenced, task.

      The REFID and CONCEPT relationships establish a reference from the
      current component to the referenced component.

      Changes to a calendar component referenced by this property can
      have an implicit impact on the related calendar component.  For
      example, if a group event changes its start or end date or time,
      then the related, dependent events will need to have their start
      and end dates and times changed in a corresponding way.
      Similarly, if a PARENT calendar component is canceled or deleted,
      then there is an implied impact to the related CHILD calendar
      components.  This property is intended only to provide information
      on the relationship of calendar components.

      Deletion of the target component, for example, the target of a
      FIRST, NEXT, or temporal relationship, can result in broken links.

      It is up to the target calendar system to maintain any property
      implications of these relationships.

   Format Definition:  This property is defined by the following
      notation:

         related    = "RELATED-TO" relparam ":"
                                  ( text / ; for VALUE=UID
                                    uri /  ; for VALUE=URI
                                    text ) ; for VALUE=TEXT or default
                      CRLF

         relparam   = ; the elements herein may appear in any order,
                      ; and the order is not significant.
                      [";" "VALUE" "=" ("UID" /
                                        "URI" /
                                        "TEXT")]
                      [";" reltypeparam]
                      [";" gapparam]
                      *(";" other-param)

   Example:  The following are examples of this property.

        RELATED-TO:jsmith.part7.19960817T083000.xyzMail@example.com

        RELATED-TO:19960401-080045-4000F192713-0052@example.com

        RELATED-TO;VALUE=URI;RELTYPE=STARTTOFINISH:
         https://example.com/caldav/user/jb/cal/
         19960401-080045-4000F192713.ics

10.  Security Considerations

   All of the security considerations of Section 7 of [RFC5545] apply to
   this specification.

   Applications using the LINK property need to be aware of the risks
   entailed in using the URIs provided as values.  See Section 7 of
   [RFC3986] for a discussion of the security considerations relating to
   URIs.

   In particular, note Section 7.1 (Reliability and Consistency) of
   [RFC3986], which points out the lack of a stability guarantee for
   referenced resources.

   When the value is an XML-REFERENCE type, the targeted data is an XML
   document or portion thereof.  Consumers need to be aware of the
   security issues related to XML processing -- in particular, those
   related to XML entities.  See Section 20.6 of [RFC4918].
   Additionally, note that the reference may be invalid or become so
   over time.

   The CONCEPT and redefined RELATED-TO properties have the same issues
   in that values may be URIs.

   Extremely large values for the GAP parameter may lead to unexpected
   behavior.

11.  IANA Considerations

11.1.  iCalendar Property Registrations

   The following iCalendar property names have been added to the
   iCalendar "Properties" registry defined in Section 8.3.2 of
   [RFC5545].  IANA has also added a reference to this document, where
   the properties originally defined in [RFC5545] have been updated by
   this document.

          +============+=========+=============================+
          | Property   | Status  | Reference                   |
          +============+=========+=============================+
          | CONCEPT    | Current | Section 8.1                 |
          +------------+---------+-----------------------------+
          | LINK       | Current | Section 8.2                 |
          +------------+---------+-----------------------------+
          | REFID      | Current | Section 8.3                 |
          +------------+---------+-----------------------------+
          | RELATED-TO | Current | [RFC5545], Section 3.8.4.5; |
          |            |         | RFC 9253, Section 9.1       |
          +------------+---------+-----------------------------+

                                 Table 1

11.2.  iCalendar Property Parameter Registrations

   The following iCalendar property parameter names have been added to
   the iCalendar "Parameters" registry defined in Section 8.3.3 of
   [RFC5545].

                   +===========+=========+=============+
                   | Parameter | Status  | Reference   |
                   +===========+=========+=============+
                   | GAP       | Current | Section 6.2 |
                   +-----------+---------+-------------+
                   | LINKREL   | Current | Section 6.1 |
                   +-----------+---------+-------------+

                                  Table 2

11.3.  iCalendar Value Data Type Registrations

   The following iCalendar property parameter names have been added to
   the iCalendar "Value Data Types" registry defined in Section 8.3.4 of
   [RFC5545].

                 +=================+=========+===========+
                 | Value Data Type | Status  | Reference |
                 +=================+=========+===========+
                 | XML-REFERENCE   | Current | Section 7 |
                 +-----------------+---------+-----------+
                 | UID             | Current | Section 7 |
                 +-----------------+---------+-----------+

                                  Table 3

11.4.  iCalendar RELTYPE Value Registrations

   The following iCalendar "RELTYPE" values have been added to the
   iCalendar "Relationship Types" registry defined in Section 8.3.8 of
   [RFC5545].

                +===================+=========+===========+
                | Relationship Type | Status  | Reference |
                +===================+=========+===========+
                | CONCEPT           | Current | Section 5 |
                +-------------------+---------+-----------+
                | DEPENDS-ON        | Current | Section 5 |
                +-------------------+---------+-----------+
                | FINISHTOFINISH    | Current | Section 4 |
                +-------------------+---------+-----------+
                | FINISHTOSTART     | Current | Section 4 |
                +-------------------+---------+-----------+
                | FIRST             | Current | Section 5 |
                +-------------------+---------+-----------+
                | NEXT              | Current | Section 5 |
                +-------------------+---------+-----------+
                | REFID             | Current | Section 5 |
                +-------------------+---------+-----------+
                | STARTTOFINISH     | Current | Section 4 |
                +-------------------+---------+-----------+
                | STARTTOSTART      | Current | Section 4 |
                +-------------------+---------+-----------+

                                  Table 4

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918,
              DOI 10.17487/RFC4918, June 2007,
              <https://www.rfc-editor.org/info/rfc4918>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5545]  Desruisseaux, B., Ed., "Internet Calendaring and
              Scheduling Core Object Specification (iCalendar)",
              RFC 5545, DOI 10.17487/RFC5545, September 2009,
              <https://www.rfc-editor.org/info/rfc5545>.

   [RFC7986]  Daboo, C., "New Properties for iCalendar", RFC 7986,
              DOI 10.17487/RFC7986, October 2016,
              <https://www.rfc-editor.org/info/rfc7986>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

   [W3C.REC-skos-reference-20090818]
              Miles, A. and S. Bechhofer, "SKOS Simple Knowledge
              Organization System Reference", W3C Recommendation REC-
              skos-reference-20090818, 18 August 2009,
              <https://www.w3.org/TR/2009/REC-skos-reference-20090818>.

   [W3C.REC-xptr-framework-20030325]
              Grosso, P., Maler, E., Marsh, J., and N. Walsh, "XPointer
              Framework", W3C Recommendation REC-xptr-framework-
              20030325, 25 March 2003,
              <https://www.w3.org/TR/2003/REC-xptr-framework-20030325>.

   [W3C.WD-xptr-xpointer-20021219]
              DeRose, S., Maler, E., and R. Daniel, "XPointer xpointer()
              Scheme", W3C WD WD-xptr-xpointer-20021219, 19 December
              2002,
              <http://www.w3.org/TR/2002/WD-xptr-xpointer-20021219>.

12.2.  Informative References

   [RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
              "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
              DOI 10.17487/RFC4791, March 2007,
              <https://www.rfc-editor.org/info/rfc4791>.

   [RFC8607]  Daboo, C., Quillaud, A., and K. Murchison, Ed.,
              "Calendaring Extensions to WebDAV (CalDAV): Managed
              Attachments", RFC 8607, DOI 10.17487/RFC8607, June 2019,
              <https://www.rfc-editor.org/info/rfc8607>.

Acknowledgements

   The author would like to thank the members of CalConnect, the
   Calendaring and Scheduling Consortium technical committees, and the
   following individuals for contributing their ideas, support, and
   comments:

   Adrian Apthorp, Cyrus Daboo, Marten Gajda, and Ken Murchison

   The author would also like to thank CalConnect and the Calendaring
   and Scheduling Consortium for advice with this specification.

Author's Address

   Michael Douglass
   Bedework
   226 3rd Street
   Troy, NY 12180
   United States of America
   Email: mdouglass@bedework.com
   URI:   https://bedework.com