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Versions: (draft-douglass-ical-relations)  00 01         Standards Track
          02 03 04 05 06 07                                             
Network Working Group                                        M. Douglass
Internet-Draft                                                  Bedework
Updates: 5545 (if approved)                                July 30, 2021
Intended status: Standards Track
Expires: January 31, 2022


                  Support for iCalendar Relationships
                  draft-ietf-calext-ical-relations-07

Abstract

   This specification updates RELATED-TO defined in iCalendar (RFC5545)
   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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 31, 2022.

Copyright Notice

   Copyright (c) 2021 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified 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.  Reference Types . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Link Relation Types . . . . . . . . . . . . . . . . . . . . .   5
   4.  New temporal RELTYPE Parameter values . . . . . . . . . . . .   6
   5.  Additional New RELTYPE Parameter Values . . . . . . . . . . .   7
   6.  New Property Parameters . . . . . . . . . . . . . . . . . . .   8
     6.1.  Link Relation . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  Gap . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  New Value Data Types  . . . . . . . . . . . . . . . . . . . .   9
   8.  New Properties  . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Concept . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.2.  Link  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.3.  Refid . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Redefined RELATED-TO Property . . . . . . . . . . . . . . . .  12
     9.1.  RELATED-TO  . . . . . . . . . . . . . . . . . . . . . . .  12
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     11.1.  iCalendar Property Registrations . . . . . . . . . . . .  14
     11.2.  iCalendar Property Parameter Registrations . . . . . . .  15
     11.3.  iCalendar Value Data Type Registrations  . . . . . . . .  15
     11.4.  iCalendar RELTYPE Value Registrations  . . . . . . . . .  15
     11.5.  New Reference Type Registration  . . . . . . . . . . . .  16
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   13. Normative References  . . . . . . . . . . . . . . . . . . . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   iCalendar entities 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
      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.



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   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 currently existing iCalendar [RFC5545] RELATED-TO property has no
   support for temporal relationships as used by standard 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 related to the same object and retrieval of a
   component based on this key.  Two examples of how this may be used
   are to identify the tasks associated with a given project without
   having to communicate the task structure of the project, and to group
   all tasks associated to 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.CR-skos-reference-20090317].  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.




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   The current [RFC5545] CATEGORY property is used as a free form
   'tagging' field.  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
   the LINK header defined in [RFC8288]

   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.

   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.

   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 SHOULD NOT be treated as just another attachment.
   The ATTACH property is being extended to handle server-side
   management and stripping of inline data.  Clients may choose to




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   handle attachments differently as they are often an integral part of
   the message - for example, the agenda.  See [RFC8607]

1.5.  Caching and offline use

   To facilitate offline display the link type may identify important
   pieces of data which should be downloaded in advance.

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

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.

2.  Reference Types

   The actual reference value can take three forms specified by the type
   parameter

   URI:  The default type.  This is a URI referring to the target.

   UID:  This allows for linking within a single collection and the
      value MUST be another component within that collection.

   REFERENCE:  An XPointer.  In an XML environment it may be necessary
      to refer to an external XML artifact.  The XPointer is defined in
      [W3C.WD-xptr-xpointer-20021219] and allows addressing portions of
      XML documents.

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].







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4.  New temporal RELTYPE Parameter values

   This section defines the usual temporal relationships for use with
   the RELTYPE parameter redefined 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 or lag 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.

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


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

                  Figure 1: Finish to start relationship

   RELTYPE=FINISHTOFINISH:  Task-B cannot finish before Task-A is
      finished, that is the end of Task-A defines the end of Task-B.
      For example, we start the potatoes, then the meat then the peas
      but they should all be cooked at the same time.


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

                  Figure 2: Finish to finish relationship





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   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 when 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:  Indicates that the referenced calendar component is
      the first in a series the referenced 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.






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   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.

6.  New Property Parameters

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 = "REL" "="
                    ("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.  In addition to the values defined here any value
      defined in [RFC8288] may be used.  There is no default relation
      type.

   REL=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 temporaly related components.

   Format Definition:

   This parameter is defined by the following notation:

     gapparam      = "GAP" "=" dur-value

   Description:  This parameter MAY be specified on the RELATED-TO
      property, and defines the duration of time between the predecessor



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      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 "hang the drapes" then task A may be related to task B
      with RELTYPE=FINISHTOSTART with a gap long enough for the paint to
      dry.

      An example of lead time might be to relate a 1 week task A to the
      end of task B with RELTYPE=STARTTOFINISH and a negative gap of 1
      week so they finish at the same time.

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.

   REFERENCE  VALUE=REFERENCE indicates that the associated value is an
      XPointer referencing an associated XML artifact.

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.






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      Within the "VEVENT", "VTODO", or "VJOURNAL" calendar components,
      more than one formal category can be specified by using multiple
      CONCEPT properties.

      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)


   Example:

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

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

8.2.  Link

   Property name:  LINK

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

   Value type:  URI, TEXT or REFERENCE

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





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   Conformance:  This property MAY be specified 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 ":"
                         ( text /  ; for VALUE=REFERENCE
                           uri /  ; for VALUE=URI
                           text ) ; for VALUE=TEXT
                       CRLF

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

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


   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:http://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:
      http://example.com/tasks/01234567-abcd1234.ics







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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 MAY be specified multiple 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.  Redefined RELATED-TO Property

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.



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   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.

      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.  It is up to the target
      calendar system to maintain any property implications of this
      relationship.

   Format Definition:



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   This property is defined by the following notation:

      related    = "RELATED-TO" relparam ":"
                               ( uid /  ; 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:
      http://example.com/caldav/user/jb/cal/
      19960401-080045-4000F192713.ics

10.  Security Considerations

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

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

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]







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                   +----------+---------+-------------+
                   | Property | Status  | Reference   |
                   +----------+---------+-------------+
                   | CONCEPT  | Current | Section 8.1 |
                   | LINK     | Current | Section 8.2 |
                   | REFID    | Current | Section 8.3 |
                   +----------+---------+-------------+

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 |
                   | REL       | Current | Section 6.1 |
                   +-----------+---------+-------------+

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 |
                 +-----------------+---------+-----------+
                 | REFERENCE       | Current | Section 7 |
                 | UID             | Current | Section 7 |
                 +-----------------+---------+-----------+

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]












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                +-------------------+---------+-----------+
                | 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 |
                | REFID             | Current | Section 5 |
                | STARTTOFINISH     | Current | Section 4 |
                | STARTTOSTART      | Current | Section 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 |
                    +--------+---------+-------------+

12.  Acknowledgements

   The author would like to thank the members of 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.  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>.

   [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>.





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Internet-Draft           iCalendar Relationships               July 2021


   [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>.

   [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>.

   [W3C.CR-skos-reference-20090317]
              Bechhofer, S. and A. Miles, "SKOS Simple Knowledge
              Organization System Reference", World Wide Web Consortium
              CR CR-skos-reference-20090317, March 2009,
              <http://www.w3.org/TR/2009/CR-skos-reference-20090317>.

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

Author's Address

   Michael Douglass
   Bedework
   226 3rd Street
   Troy, NY  12180
   USA

   Email: mdouglass@bedework.com
   URI:   http://bedework.com












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