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Versions: (draft-douglass-ical-relations) 00 01 02 03

Network Working Group                                        M. Douglass
Internet-Draft                                       Spherical Cow Group
Updates: 5545 (if approved)                             October 11, 2017
Intended status: Standards Track
Expires: April 14, 2018


                  Support for iCalendar Relationships
                  draft-ietf-calext-ical-relations-03

Abstract

   This specification updates RELATED-TO defined in [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 April 14, 2018.

Copyright Notice

   Copyright (c) 2017 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
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   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.  Redefined Relation Type Value . . . . . . . . . . . . . . . .   5
   5.  New Property Parameters . . . . . . . . . . . . . . . . . . .   8
     5.1.  Rel . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Gap . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  New Value Data Types  . . . . . . . . . . . . . . . . . . . .   9
   7.  New Properties  . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Concept . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.2.  Link  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.3.  Refid . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   8.  Redefined RELATED-TO Property . . . . . . . . . . . . . . . .  13
     8.1.  RELATED-TO  . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     10.1.  iCalendar Property Registrations . . . . . . . . . . . .  16
     10.2.  iCalendar Property Parameter Registrations . . . . . . .  16
     10.3.  iCalendar Value Data Type Registrations  . . . . . . . .  16
     10.4.  iCalendar RELTYPE Value Registrations  . . . . . . . . .  17
     10.5.  New Reference Type Registration  . . . . . . . . . . . .  17
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   12. Normative References  . . . . . . . . . . . . . . . . . . . .  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   iCalendar entities often need to be related to each other or to
   associated meta-data.  These relationships can take the following
   forms

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

   Grouped iCalendar:  iCalendar entities are related to each other as a
      group.  CATEGORIES are often used for this purpose but are
      problematic for application developers.

   Linked:  Entities are linked to each other through typed references.



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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 tasks
   that are related to the same object and retrieval of a task based on
   this key.  This may be, for example, to identify the tasks associated
   with a given project without having to communicate the task structure
   of the project, or, for example, in a package delivery system all
   tasks associated to a specific package.

   As such, the presence of a REFID property imparts no meaning to the
   event.  It is merely a key to allow retrieval.  This is distinct from
   categorisation which, while allowing grouping also adds meaning to
   the entity 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].  This more accurately
   defines what we mean by a catgeory.  It's not the words but the
   meaning.

   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.  As such it is difficult to establish formal
   relationships between components based on their category.





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   Rather than attempt to add semantics to the current property it
   seeems best to continue its usage as an informal tag and establish a
   new 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 [RFC5988]

   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 often necessary to relate calendar components.  The current
   RELATED-TO property only allows for a UID which is inadequate for
   many purposes.  Allowing other types may help but might raise a
   number of 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
   handle attachments differently as they are often an integral part of
   the message - for example, the agenda.  See
   [I-D.daboo-caldav-attachments]








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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
   [RFC2119].

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 is assumed to 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

   [RFC5988] defines two form of relation types, registered and
   extension.  Registered relation types are added to a registry defined
   by [RFC5988] while extension relation types are specified as unique
   unregistered URIs, (at least unregistered in the [RFC5988] registry).

   The relation types defined here will be registered with IANA in
   accordance with the specifications in [RFC5988].

4.  Redefined Relation Type Value

   Relationship parameter type values are defined in section 3.2.15. of
   [RFC5545].  This specification redefines that type to include the new
   temporal relationship values FINISHTOSTART, FINISHTOFINISH,
   STARTTOFINISH and STARTTOSTART.  It also adds the DEPENDS-ON value to
   provide a link to a component upon which the current component
   depends.



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   Format Definition:

   This property parameter is defined by the following notation:

     reltypeparam   = "RELTYPE" "="
                      ("PARENT"    ; Parent relationship - Default
                     / "CHILD"     ; Child relationship
                     / "SIBLING"   ; Sibling relationship
                     / "DEPENDS-ON" ; refers to previous task
                     / "REFID"     ; Relationship based on REFID
                     / "CONCEPT"
                             ; Relationship based on CONCEPT
                     / "FINISHTOSTART" ;  Temporal relationship
                     / "FINISHTOFINISH" ;  Temporal relationship
                     / "STARTTOFINISH" ;  Temporal relationship
                     / "STARTTOSTART" ;  Temporal relationship
                     / iana-token  ; Some other IANA-registered
                                   ; iCalendar relationship type
                     / x-name)     ; A non-standard, experimental
                                   ; relationship type

   Description:  This parameter can be specified on a property that
      references another related calendar component.  The parameter may
      specify the hierarchical relationship type of the calendar
      component referenced by the property when the value is PARENT,
      CHILD or SIBLING.  If this parameter is not specified on an
      allowable property, the default relationship type is PARENT.
      Applications MUST treat x-name and iana-token values they don't
      recognize the same way as they would the PARENT value.

      This parameter defines the temporal relationship when the value is
      one of the project management standard relationships
      FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH or STARTTOSTART.
      This property will be present in the predecessor entity and will
      refer to the successor entity.  The GAP parameter 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=PARENT:  See [RFC5545] section 3.2.15.

   RELTYPE=CHILD:  See [RFC5545] section 3.2.15.

   RELTYPE=SIBLING:  See [RFC5545] section 3.2.15.

   RELTYPE=DEPENDS-ON:  Indicates that the current calendar component
      depends on the referenced calendar component in some manner.  For



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      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.

   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

   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).






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                                        ============
                                     +--|  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.  New Property Parameters

5.1.  Rel

   Parameter name:  REL

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

   Format Definition:

   This parameter is defined by the following notation:

     relparam     = "REL" "="
                    ("SOURCE"      ; Link to source of this component
                    / DQUOTE uri DQUOTE
                    / x-name        ; Experimental reference type
                    / 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




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      defined in [RFC5988] 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 [RFC5988]

5.2.  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
      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.

6.  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.






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7.  New Properties

7.1.  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.

      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 derived from
      something like the dmoz.org data.

   Format Definition:











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   This property is defined by the following notation:

     concept        =  "CONCEPT" conceptparam ":"
                           uri CRLF

     conceptparam = *(
                   ;
                   ; The following is OPTIONAL,
                   ; and MAY occur more than once.
                   ;
                   (";" other-param)
                   ;
                   )


   Example:

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

     SCONCEPT:http://example.com/event-types/sports
     CONCEPT:http://example.com/event-types/arts/music
     CONCEPT:http://example.com/task-types/delivery

7.2.  Link

   Property name:  LINK

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

   Value type:  URI, TEXT or REFERENCE

   Property Parameters:  Non-standard, reference type or format type
      parameters can be specified on this property.

   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:







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   This property is defined by the following notation:

     link            = "LINK" linkparam  /
                       (
                         ";" "VALUE" "=" "TEXT"
                         ":" text
                       )
                       (
                         ";" "VALUE" "=" "REFERENCE"
                         ":" text
                       )
                       (
                         ";" "VALUE" "=" "URI"
                         ":" uri
                       )
                       CRLF


     linkparam       = *(

                     ; the following is MANDATORY
                     ; and MAY occur more than once

                     (";" relparam) /

                     ; the following are MANDATORY
                     ; but MUST NOT occur more than once

                     (";" fmttypeparam) /
                     (";" labelparam) /
                     ; labelparam is defined in ...

                     ; the following is OPTIONAL
                     ; and MAY occur more than once

                     (";" xparam)

                     )


   Example:

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

     LINK;REL=SOURCE;LABEL=The Egg:http://example.com/events





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7.3.  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 a text identifier that
      allows associated components to be located or retrieved as a
      whole.  For example all of the events in a travel itinerary would
      have the same REFID value.

   Format Definition:

   This property is defined by the following notation:

     refid      = "REFID" refidparam ":" text CRLF


     refidparam      = *(

                     ; the following is OPTIONAL
                     ; and MAY occur more than once

                     (";" xparam)

                     )


   Example:

   The following is an example of this property.

     REFID:itinerary-2014-11-17

8.  Redefined RELATED-TO Property







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8.1.  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.

      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



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      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:

   This property is defined by the following notation:

     related    = "RELATED-TO" relparam ( ":" text ) /
                  (
                    ";" "VALUE" "=" "UID"
                    ":" uid
                  )
                  (
                    ";" "VALUE" "=" "URI"
                    ":" uri
                  )
                  CRLF

     relparam   = *(
                 ;
                 ; The following are OPTIONAL,
                 ; but MUST NOT occur more than once.
                 ;
                 (";" reltypeparam) /
                 (";" gapparam) /
                 ;
                 ; The following is OPTIONAL,
                 ; and MAY occur more than once.
                 ;
                 (";" 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





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9.  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.

10.  IANA Considerations

10.1.  iCalendar Property Registrations

   The following iCalendar property names have been added to the
   iCalendar Properties Registry defined in Section 8.3.2 of [RFC5545]

                   +----------+---------+-------------+
                   | Property | Status  | Reference   |
                   +----------+---------+-------------+
                   | CONCEPT  | Current | Section 7.1 |
                   | LINK     | Current | Section 7.2 |
                   | REFID    | Current | Section 7.3 |
                   +----------+---------+-------------+

10.2.  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   |
                   +-----------+---------+-------------+
                   | REL       | Current | Section 5.1 |
                   | GAP       | Current | Section 5.2 |
                   +-----------+---------+-------------+

10.3.  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]









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                 +-----------------+---------+-----------+
                 | Value Data Type | Status  | Reference |
                 +-----------------+---------+-----------+
                 | UID             | Current | Section 6 |
                 | REFERENCE       | Current | Section 6 |
                 +-----------------+---------+-----------+

10.4.  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 |
                +-------------------+---------+-----------+
                | DEPENDS-ON        | Current | Section 4 |
                | REFID             | Current | Section 4 |
                | CONCEPT           | Current | Section 4 |
                | FINISHTOSTART     | Current | Section 4 |
                | FINISHTOFINISH    | Current | Section 4 |
                | STARTTOFINISH     | Current | Section 4 |
                | STARTTOSTART      | Current | Section 4 |
                +-------------------+---------+-----------+

10.5.  New Reference Type Registration

   The following link relation values have been added to the Reference
   Types Registry defined in Section 6.2.2 of [RFC5988]

                    +--------+---------+-------------+
                    | Name   | Status  | Reference   |
                    +--------+---------+-------------+
                    | SOURCE | Current | Section 5.1 |
                    +--------+---------+-------------+

11.  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 the Calendaring and Scheduling
   Consortium for advice with this specification.





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12.  Normative References

   [I-D.daboo-caldav-attachments]
              Daboo, C. and A. Quillaud, "CalDAV Managed Attachments",
              draft-daboo-caldav-attachments-03 (work in progress),
              February 2014.

   [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>.

   [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>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <https://www.rfc-editor.org/info/rfc5988>.

   [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.REC-xml-20060816]
              Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fourth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20060816, August 2006,
              <http://www.w3.org/TR/2006/REC-xml-20060816>.

   [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>.







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Author's Address

   Michael Douglass
   Spherical Cow Group
   226 3rd Street
   Troy, NY  12180
   USA

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









































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