Network Working Group                                        M. Douglass
Internet-Draft                                                  Bedework
Updates: 5545 (if approved)                                22 March 2022
Intended status: Standards Track
Expires: 23 September 2022


                  Support for iCalendar Relationships
                  draft-ietf-calext-ical-relations-11

Abstract

   This specification updates the iCalendar RELATED-TO property defined
   in RFC5545 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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 23 September 2022.

Copyright Notice

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   document authors.  All rights reserved.

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   provided without warranty as described in the Revised BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Structured iCalendar relationships  . . . . . . . . . . .   3
     1.2.  Grouped iCalendar relationships . . . . . . . . . . . . .   3
     1.3.  Concept relationships . . . . . . . . . . . . . . . . . .   3
     1.4.  Linked relationships  . . . . . . . . . . . . . . . . . .   4
     1.5.  Caching and offline use . . . . . . . . . . . . . . . . .   5
     1.6.  Conventions Used in This Document . . . . . . . . . . . .   5
   2.  LINK Property Reference Types . . . . . . . . . . . . . . . .   5
   3.  Link Relation Types . . . . . . . . . . . . . . . . . . . . .   6
   4.  New temporal RELTYPE Parameter values . . . . . . . . . . . .   6
   5.  Additional New RELTYPE Parameter Values . . . . . . . . . . .   8
   6.  New Property Parameters . . . . . . . . . . . . . . . . . . .   8
     6.1.  Link Relation . . . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Gap . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  New Value Data Types  . . . . . . . . . . . . . . . . . . . .  10
   8.  New Properties  . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Concept . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.2.  Link  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.3.  Refid . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Updates to RFC 5545 . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  RELATED-TO  . . . . . . . . . . . . . . . . . . . . . . .  15
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     11.1.  iCalendar Property Registrations . . . . . . . . . . . .  17
     11.2.  iCalendar Property Parameter Registrations . . . . . . .  18
     11.3.  iCalendar Value Data Type Registrations  . . . . . . . .  18
     11.4.  iCalendar RELTYPE Value Registrations  . . . . . . . . .  19
     11.5.  New Reference Type Registration  . . . . . . . . . . . .  19
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     13.1.  Informative References . . . . . . . . . . . . . . . . .  20
     13.2.  Normative References . . . . . . . . . . . . . . . . . .  20
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   iCalendar entities defined in [RFC5545] often need to be related to
   each other or to associated meta-data.  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, after.

   Grouped iCalendar:  iCalendar entities can be related to each other




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      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 categorisation 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
   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.






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   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 [RFC5545] CATEGORY property 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 CATEGORY 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
   meta-data 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 [RFC4791]' (CalDAV)
   calendar collections.

   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 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.





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   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 web site.  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 which 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 meta-data such as a web page.

   However, to facilitate offline display the link type may identify
   important pieces of data which 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.




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   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

   [RFC8288] defines two forms of relation type: 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 the
   target of the relationship.  This is indicated by the direction of
   the arrow in the diagrams below.





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   Also each relationship may be modified by the addition of a GAP
   parameter to the relationship which 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 (B) cannot be completed until the design of the api
      (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  |<-+
                          ============



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                    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:  Indicates that the referenced calendar component is
      the first in a series the referencing calendar component is part
      of.

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

   RELTYPE=DEPENDS-ON:  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:  Establishes a reference from the current component to
      components with a REFID property which matches the value given in
      the associated RELATED-TO property.

   RELTYPE=CONCEPT:  Establishes a reference from the current component
      to components with a CONCEPT property which 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






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6.1.  Link Relation

   Parameter name:  LINKREL

   Purpose:  To specify the relationship of data referenced by a LINK
      property.

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

        linkrelparam = "LINKREL" "="
                       ("SOURCE"      ; Link to source of this component
                       / DQUOTE uri DQUOTE
                       / iana-token)   ; Other IANA registered type

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

      In addition to the value defined here 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=SOURCE:  identifies the source of the event information.

   Registration:  These relation types are registered in [RFC8288]

6.2.  Gap

   Parameter name:  GAP

   Purpose:  To specify 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





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      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 carpet  |
                                           ===============

               Figure 5: Finish to start relationship with lag

      For an example of lead time, in constructing a two storey 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





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      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 which 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 Library of Congress, or an open source of
      categorisation data.

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

        concept        =  "CONCEPT" conceptparam ":"
                              uri CRLF

        conceptparam = *(";" other-param)




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   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      = ; the elements herein may appear in any order,
                          ; and the order is not significant.

                          (";" "VALUE" "=" ("XML-REFERENCE" /
                                            "URI" /
                                            "UID"))
                          1*(";" linkrelparam)
                          1*(";" fmttypeparam)
                          1*(";" labelparam)
                          1*(";" languageparam)
                          *(";" other-param)





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      This property is a serialisation 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:  Maps to the "title" attribute defined in section 3.4.1 of
         [RFC8288].

      LANGUAGE:  Maps to the "hreflang" attribute defined in section
         3.4.1 of [RFC8288].

      LINKREL:  Maps to the link relation type defined in section 2.1 of
         [RFC8288].

      FMTTYPE:  Maps to the "type" attribute defined in section 3.4.1 of
         [RFC8288].

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

   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]))




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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)


      The current link registry

   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.



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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



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      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 changed in a corresponding way.  Similarly, if a
      PARENT calendar component is cancelled 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.









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        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 pf [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 [RFC4918] - Section 20.6.  Additionally
   note that the reference may be invalid or become so over time.

   The CONCEPT and redefined RELATED-TO property 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.









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






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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

11.5.  New Reference Type Registration

   The following link relation values have been added to the Reference
   Types Registry defined in Section 6.2.2 of [RFC8288].

                    +========+=========+=============+
                    | Name   | Status  | Reference   |
                    +========+=========+=============+
                    | SOURCE | Current | Section 6.1 |
                    +--------+---------+-------------+

                                 Table 5









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12.  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, Ken Murchison

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

13.  References

13.1.  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>.

13.2.  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>.




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   [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", World Wide Web Consortium
              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", World Wide Web Consortium 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., Daniel, R., and E. Maler, "XPointer xpointer()
              Scheme", World Wide Web Consortium WD WD-xptr-xpointer-
              20021219, 19 December 2002,
              <http://www.w3.org/TR/2002/WD-xptr-xpointer-20021219>.

Author's Address

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






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