path: root/doc/rfc/rfc9073.txt

Internet Engineering Task Force (IETF)                       M. Douglass
Request for Comments: 9073                                      Bedework
Updates: 5545                                                August 2021
Category: Standards Track                                               
ISSN: 2070-1721


                Event Publishing Extensions to iCalendar

Abstract

   This specification updates RFC 5545 by introducing a number of new
   iCalendar properties and components that are of particular use for
   event publishers and in social networking.

   This specification also defines a new "STRUCTURED-DATA" property for
   iCalendar (RFC 5545) to allow for data that is directly pertinent to
   an event or task to be included with the calendar data.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9073.

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.

Table of Contents

   1.  Introduction
     1.1.  Conventions Used in This Document
     1.2.  Terms Used in This Document
   2.  Components and Properties
   3.  Typed References
     3.1.  Use Cases
       3.1.1.  Piano Concert Performance
       3.1.2.  Itineraries
         3.1.2.1.  Reserving Facilities
   4.  Modifications to Calendar Components
   5.  New Property Parameters
     5.1.  Order
     5.2.  Schema
     5.3.  Derived
   6.  New Properties
     6.1.  Location Type
     6.2.  Participant Type
     6.3.  Resource Type
     6.4.  Calendar Address
     6.5.  Styled-Description
     6.6.  Structured-Data
   7.  New Components
     7.1.  Participant
       7.1.1.  Schedulable Participant
     7.2.  Location
     7.3.  Resource
   8.  Extended Examples
     8.1.  Example 1
     8.2.  Example 2
   9.  Security Considerations
     9.1.  URIs
     9.2.  Malicious Content
     9.3.  HTML Content
   10. Privacy Considerations
     10.1.  Tracking
     10.2.  Revealing Locations
   11. IANA Considerations
     11.1.  Additional iCalendar Registrations
       11.1.1.  Properties
       11.1.2.  Parameters
       11.1.3.  Components
     11.2.  Participant Types and Resource Types Registries
       11.2.1.  Participant Types
       11.2.2.  Resource Types
   12. Normative References
   Acknowledgements
   Author's Address

1.  Introduction

   The currently existing iCalendar standard [RFC5545] lacks useful
   methods for referencing additional, external information relating to
   calendar components.  Additionally, there is no standard way to
   provide rich-text descriptions or metadata associated with the event.

   Current practice is to embed this information as links in the
   description or to add nonstandard properties, as defined in
   Section 3.8.8.2 of [RFC5545].

   This document updates [RFC5545] to define a number of properties and
   components referencing such external information that can provide
   additional information about an iCalendar component.  The intent is
   to allow the interchange of such information between applications or
   systems (e.g., between clients, between client and server, and
   between servers).  Formats, such as vCard [RFC6350], are likely to be
   most useful to the receivers of such events as they may be used in
   other applications -- such as address books.

1.1.  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 is the ABNF notation of [RFC5234] as
   used by iCalendar [RFC5545].  Any syntax elements shown below that
   are not explicitly defined in this specification come from iCalendar
   [RFC5545].

1.2.  Terms Used in This Document

   Event:  When the word 'event' (perhaps with a capitalized 'E') is
      used, we are referring to gatherings, formal or informal (for
      example, a sports event, a party, or a concert).

   Social Calendaring:  Historically, calendar data and scheduling has
      been heavily biased towards meetings in a corporate environment.
      Some of the features defined in this document are to support a
      more informal, i.e., social, model.  For example, we may want to
      record who is participating in a public event.

2.  Components and Properties

   Previous extensions to the calendaring standards have been largely
   restricted to the addition of properties or parameters.  This is
   partly because iCalendar libraries had trouble handling components
   nested deeper than those defined in [RFC5545].

   In a break with this 'convention', this specification defines a
   number of components rather than properties.  This is a better match
   for the way [W3C.REC-xml-20081126] and JSON [RFC8259] handle such
   structures and allows richer definitions.

   It also allows for the addition of extra properties inside the
   components and resolves some of the problems of trying to add
   detailed information as a parameter.

3.  Typed References

   The properties and components defined here can all reference external
   metadata, which may be used by applications to provide further
   information to users.  By providing type information, clients and
   servers are able to discover interesting references and make use of
   them, perhaps for indexing or presenting additional, related
   information for the user.

   As always, clients should exercise caution in following references to
   external data.

   The "LOCATION" property [RFC5545] provides only an unstructured
   single text value for specifying the location where an event (or
   task) will occur.  This is inadequate for use cases where structured
   location information (e.g., address, region, country, or postal code)
   is required or preferred and limits widespread adoption of iCalendar
   in those settings.

   Using the "VLOCATION" component, rich information about multiple
   locations can be communicated in a "STRUCTURED-DATA" property;
   examples include address, region, country, postal code, parking
   availability, nearby restaurants, and the venue, among others.
   Servers and clients can retrieve the objects when storing the event
   and use them to index by geographic location.

   When a calendar client receives a calendar component, it can search
   the set of locations looking for those of particular interest.  The
   "LOCATION-TYPE" property and "FMTTYPE" parameter applied to the
   "STRUCTURED-DATA" property, if supplied, can be used to help the
   selection.

   The "PARTICIPANT" component is designed to handle common use cases in
   event publication.  It is generally important to provide information
   about the organizers of such events.  Sponsors wish to be referenced
   in a prominent manner.  In social calendaring, it is often important
   to identify the active participants (e.g,, a school sports team) and
   the inactive participants (e.g., the parents) in the event.

   The "PARTICIPANT" component can be used to provide useful extra data
   about an attendee.  For example, a location inside the PARTICIPANT
   gives the actual location of a remote attendee.  (But see the note
   about privacy.)

   Alternatively, the "PARTICIPANT" component can be used to provide a
   reference -- perhaps the address for mailing lists.

3.1.  Use Cases

   The main motivation for these changes has been event publication, but
   there are opportunities for use elsewhere.  The following use cases
   will describe some possible scenarios.

3.1.1.  Piano Concert Performance

   In putting together a concert, there are many participants: piano
   tuner, performer, stage hands, etc.  In addition, there are sponsors
   and various contacts to be provided.  There will also be a number of
   related locations.  A number of events can be created, all of which
   relate to the performance in different ways.

   There may be an iCalendar Transport-independent Interoperability
   Protocol (iTIP) [RFC5546] meeting request for the piano tuner, who
   will arrive before the performance.  Other members of staff may also
   receive meeting requests.

   An event can also be created for publication, which will have a
   "PARTICIPANT" component for the pianist providing a reference to
   vCard information ([RFC6350]) about the performer.  This event would
   also hold information about parking, local subway stations, and the
   venue itself.  In addition, there may be sponsorship information for
   sponsors of the event and perhaps paid sponsorship properties,
   essentially advertising local establishments.

3.1.2.  Itineraries

   These additions also provide opportunities for the travel industry.
   When booking a flight, the "PARTICIPANT" component can be used to
   provide references to businesses at the airports and to rental car
   businesses at the destination.

   The embedded location information can guide the traveler around the
   airport itself or to their final destination.  The contact
   information can provide detailed information about the booking agent,
   airlines, car hire companies, and hotel.

3.1.2.1.  Reserving Facilities

   For a meeting, the size of a room and the equipment needed depends,
   to some extent, on the number of attendees actually in the room.

   A meeting may have many attendees, none of which are co-located.  The
   current "ATTENDEE" property does not allow for the addition of such
   metadata.  The "PARTICIPANT" component allows attendees to specify
   their location.

4.  Modifications to Calendar Components

   The following changes to the syntax defined in iCalendar [RFC5545]
   are made here.  New elements are defined in subsequent sections.

   ; Addition of PARTICIPANT, VLOCATION, and VRESOURCE
   ; as valid components
   eventc     = "BEGIN" ":" "VEVENT" CRLF
                eventprop *alarmc *participantc *locationc *resourcec
                "END" ":" "VEVENT" CRLF

   ; Addition of properties STYLED-DESCRIPTION and STRUCTURED-DATA
   eventprop  =/ *styleddescription
                 *sdataprop

   ; Addition of PARTICIPANT, VLOCATION, and VRESOURCE
   ; as valid components
   todoc      = "BEGIN" ":" "VTODO" CRLF
                todoprop *alarmc *participantc *locationc *resourcec
                "END" ":" "VTODO" CRLF

   ; Addition of properties STYLED-DESCRIPTION and STRUCTURED-DATA
   todoprop =/ *styleddescription
               *sdataprop

   ; Addition of PARTICIPANT, VLOCATION, and VRESOURCE
   ; as valid components
   journalc   = "BEGIN" ":" "VJOURNAL" CRLF
                jourprop *participantc *locationc *resourcec
                "END" ":" "VJOURNAL" CRLF

   ; Addition of properties STYLED-DESCRIPTION and STRUCTURED-DATA
   jourprop =/ *styleddescription
               *sdataprop

   ; Addition of PARTICIPANT, VLOCATION, and VRESOURCE
   ; as valid components
   freebusyc  = "BEGIN" ":" "VFREEBUSY" CRLF
                fbprop *participantc *locationc *resourcec
                "END" ":" "VFREEBUSY" CRLF

   ; Addition of property STYLED-DESCRIPTION
   fbprop     =/ *styleddescription

5.  New Property Parameters

5.1.  Order

   Parameter name:  ORDER

   Purpose:  This parameter defines ordering for the associated
      property.

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

      orderparam    = "ORDER" "=" integer
                      ;           Must be greater than or equal to 1

   Description:  The "ORDER" parameter is OPTIONAL and is used to
      indicate the relative ordering of the corresponding instance of a
      property.  Its value MUST be an integer greater than or equal to 1
      that specifies the order, with 1 being the first in the ordering.

      When the parameter is absent, the default MUST be to interpret the
      property instance as being ordered last, that is, the property
      will appear after any other instances of the same property with
      any value of ORDER.

      When any "ORDER" parameters have the same value, all the
      associated properties appear as a group within which there is no
      defined order.

      Note that the value of this parameter is to be interpreted only in
      relation to values assigned to other corresponding instances of
      the same property in the same entity.

      This parameter MUST NOT be applied to a property that does not
      allow multiple instances.

   Example uses:  The ORDER may be applied to the "PARTICIPANT-TYPE"
      property to indicate the relative importance of the participant,
      for example, as a sponsor or a performer.  For example, ORDER=1
      could define the principal performer or soloist.

5.2.  Schema

   Parameter Name:  SCHEMA

   Purpose:  This parameter specifies the schema used for the content of
      a "STRUCTURED-DATA" property value.

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

      schemaparam    = "SCHEMA" "=" DQUOTE uri DQUOTE

   Description:  This property parameter SHOULD be specified on
      "STRUCTURED-DATA" properties.  When present, it provides
      identifying information about the nature of the content of the
      corresponding "STRUCTURED-DATA" property value.  This can be used
      to supplement the media type information provided by the "FMTTYPE"
      parameter on the corresponding property.

   Example:
        STRUCTURED-DATA;FMTTYPE=application/ld+json;
         SCHEMA="https://schema.org/FlightReservation";
         ENCODING=BASE64;VALUE=BINARY:ICAgIDxzY3JpcHQgdHlwZT0iYXBwb
         GljYXRpb24vbGQranNvbiI+CiAgICB7CiAgICAgICJAY29
         udGV4dCI6ICJodHRwOi8vc2NoZW1hLm9yZyIsCiAgICAgICJAdHlwZSI
         6ICJGbGlnaHRSZXNlcnZhdGlvbiIsCiAgICAgICJyZXNlcnZhdGlvbkl
         kIjogIlJYSjM0UCIsCiAgICAgICJyZXNlcnZhdGlvblN0YXR1cyI6ICJ
         odHRwOi8vc2NoZW1hLm9yZy9SZXNlcnZhdGlvbkNvbmZpcm1lZCIsCiA
         gICAgICJwYXNzZW5nZXJQcmlvcml0eVN0YXR1cyI6ICJGYXN0IFRyYWN
         rIiwKICAgICAgInBhc3NlbmdlclNlcXVlbmNlTnVtYmVyIjogIkFCQzE
         yMyIsCiAgICAgICJzZWN1cml0eVNjcmVlbmluZyI6ICJUU0EgUHJlQ2h
         lY2siLAogICAgICAidW5kZXJOYW1lIjogewogICAgICAgICJAdHlwZSI
         6ICJQZXJzb24iLAogICAgICAgICJuYW1lIjogIkV2YSBHcmVlbiIKICA
         gICAgfSwKICAgICAgInJlc2VydmF0aW9uRm9yIjogewogICAgICAgICJ
         AdHlwZSI6ICJGbGlnaHQiLAogICAgICAgICJmbGlnaHROdW1iZXIiOiA
         iVUExMTAiLAogICAgICAgICJwcm92aWRlciI6IHsKICAgICAgICAgICJ
         AdHlwZSI6ICJBaXJsaW5lIiwKICAgICAgICAgICJuYW1lIjogIkNvbnR
         pbmVudGFsIiwKICAgICAgICAgICJpYXRhQ29kZSI6ICJDTyIsCiAgICA
         gICAgICAiYm9hcmRpbmdQb2xpY3kiOiAiaHR0cDovL3NjaGVtYS5vcmc
         vWm9uZUJvYXJkaW5nUG9saWN5IgogICAgICAgIH0sCiAgICAgICAgInN
         lbGxlciI6IHsKICAgICAgICAgICJAdHlwZSI6ICJBaXJsaW5lIiwKICA
         gICAgICAgICJuYW1lIjogIlVuaXRlZCIsCiAgICAgICAgICAiaWF0YUN
         vZGUiOiAiVUEiCiAgICAgICAgfSwKICAgICAgICAiZGVwYXJ0dXJlQWl
         ycG9ydCI6IHsKICAgICAgICAgICJAdHlwZSI6ICJBaXJwb3J0IiwKICA
         gICAgICAgICJuYW1lIjogIlNhbiBGcmFuY2lzY28gQWlycG9ydCIsCiA
         gICAgICAgICAiaWF0YUNvZGUiOiAiU0ZPIgogICAgICAgIH0sCiAgICA
         gICAgImRlcGFydHVyZVRpbWUiOiAiMjAxNy0wMy0wNFQyMDoxNTowMC0
         wODowMCIsCiAgICAgICAgImFycml2YWxBaXJwb3J0IjogewogICAgICA
         gICAgIkB0eXBlIjogIkFpcnBvcnQiLAogICAgICAgICAgIm5hbWUiOiA
         iSm9obiBGLiBLZW5uZWR5IEludGVybmF0aW9uYWwgQWlycG9ydCIsCiA
         gICAgICAgICAiaWF0YUNvZGUiOiAiSkZLIgogICAgICAgIH0sCiAgICA
         gICAgImFycml2YWxUaW1lIjogIjIwMTctMDMtMDVUMDY6MzA6MDAtMDU
         6MDAiCiAgICAgIH0KICAgIH0KICAgIDwvc2NyaXB0Pg==

5.3.  Derived

   Parameter Name:  DERIVED

   Purpose:  This parameter specifies that the value of the associated
      property is derived from some other property value or values.

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

      derivedparam    = "DERIVED" "=" ("TRUE" / "FALSE")
      ; Default is FALSE

   Description:  This property parameter MAY be specified on any
      property when the value is derived from some other property or
      properties.  When present with a value of TRUE, clients MUST NOT
      update the property.

      As an example, if a "STYLED-DESCRIPTION" property is present with
      FMTTYPE="application/rtf", then there may be an additional
      "STYLED-DESCRIPTION" property with FMTTYPE="text/html" and
      DERIVED=TRUE, as well as a value created from the rtf value.

   Example:
      STYLED-DESCRIPTION;FMTTYPE=text/html;
       DERIVED=TRUE:<html>...</html>

6.  New Properties

   This specification makes use of the "NAME" property, which is defined
   in [RFC7986].

6.1.  Location Type

   Property Name:  LOCATION-TYPE

   Purpose:  This property specifies the type(s) of a location.

   Value Type:  The value type for this property is TEXT.  The allowable
      values are defined below.

   Description:  This property MAY be specified in "VLOCATION"
      components and provides a way to differentiate multiple locations.
      For example, it allows event producers to provide location
      information for the venue and the parking.

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

      loctype      = "LOCATION-TYPE" loctypeparam ":"
                     text *("," text)
                     CRLF

      loctypeparam   = *(";" other-param)

      Multiple values may be used if the location has multiple purposes,
      for example, a hotel and a restaurant.

      Values for this parameter are taken from the values defined in
      Section 3 of [RFC4589].  New location types SHOULD be registered
      in the manner laid down in Section 5 of [RFC4589].

6.2.  Participant Type

   Property Name:  PARTICIPANT-TYPE

   Purpose:  This property specifies the type of participant.

   Value Type:  The value type for this property is TEXT.  The allowable
      values are defined below.

   Property Parameters:  Nonstandard parameters can be specified on this
      property.

   Conformance:  This property MUST be specified once within a
      "PARTICIPANT" component.

   Description:  This property defines the type of participation in
      events or tasks.  Participants can be individuals or
      organizations, for example, a soccer team, the spectators, or the
      musicians.

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

      participanttype   = "PARTICIPANT-TYPE" partvalueparam ":"
                           partvalue CRLF

      partvalue    = ("ACTIVE"
                     / "INACTIVE"
                     / "SPONSOR"
                     / "CONTACT"
                     / "BOOKING-CONTACT"
                     / "EMERGENCY-CONTACT"
                     / "PUBLICITY-CONTACT"
                     / "PLANNER-CONTACT"
                     / "PERFORMER"
                     / "SPEAKER"
                     / iana-token)     ; Other IANA-registered
                                       ; values

      partvalueparam   = *(";" other-param)

   Example:  The following is an example of this property.

      PARTICIPANT-TYPE:SPEAKER

   The registered values for the "PARTICIPANT-TYPE" property have the
   meanings described here:

   ACTIVE:  A participant taking an active role -- for example, a team
      member.

   INACTIVE:  A participant taking an inactive role -- for example, an
      audience member.

   SPONSOR:  A sponsor of the event.  The "ORDER" parameter may be used
      with this participant type to define the relative order of
      multiple sponsors.

   CONTACT:  Contact information for the event.  The "ORDER" parameter
      may be used with this participant type to define the relative
      order of multiple contacts.

   BOOKING-CONTACT:  Contact information for reservations or payment.

   EMERGENCY-CONTACT:  Contact in case of emergency.

   PUBLICITY-CONTACT:  Contact for publicity.

   PLANNER-CONTACT:  Contact for the event planner or organizer.

   PERFORMER:  A performer -- for example, the soloist or the
      accompanist.  The "ORDER" parameter may be used with this
      participant type to define the relative order of multiple
      performers.  For example, ORDER=1 could define the principal
      performer or soloist.

   SPEAKER:  Speaker at an event.

6.3.  Resource Type

   Property Name:  RESOURCE-TYPE

   Purpose:  This property specifies the type of resource.

   Value Type:  The value type for this property is TEXT.  The allowable
      values are defined below.

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

      restypeprop   = "RESOURCE-TYPE" restypeparam ":"
                      restypevalue CRLF

      restypevalue  = ("ROOM"
                     / "PROJECTOR"
                     / "REMOTE-CONFERENCE-AUDIO"
                     / "REMOTE-CONFERENCE-VIDEO"
                     / iana-token)     ; Other IANA-registered
                     ; values

      restypeparam   = *(";" other-param)

   Description:  This property MAY be specified in "VRESOURCE"
      components and provides a way to differentiate multiple resources.

      The registered values are described below.  New resource types
      SHOULD be registered in the manner laid down in this
      specification.

   ROOM:  A room for the event/meeting.

   PROJECTOR:  Projection equipment.

   REMOTE-CONFERENCE-AUDIO:  Audio remote conferencing facilities.

   REMOTE-CONFERENCE-VIDEO:  Video remote conferencing facilities.

6.4.  Calendar Address

   Property Name:  CALENDAR-ADDRESS

   Purpose:  This property specifies the calendar address for a
      participant.

   Value Type:  CAL-ADDRESS

   Property Parameters:  IANA-registered or nonstandard property
      parameters can be specified on this property.

   Conformance:  This property MAY be specified once within a
      "PARTICIPANT" component.

   Description:  This property provides a calendar user address for the
      participant.  If there is an "ATTENDEE" property with the same
      value, then the participant is schedulable.

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

      calendaraddress   = "CALENDAR-ADDRESS" caladdressparam ":"
                     cal-address CRLF

      caladdressparam   = *(";" other-param)

6.5.  Styled-Description

   Property Name:  STYLED-DESCRIPTION

   Purpose:  This property provides for one or more rich-text
      descriptions to replace that provided by the "DESCRIPTION"
      property.

   Value Type:  There is no default value type for this property.  The
      value type can be set to URI or TEXT.  Other text-based value
      types can be used when defined in the future.  Clients MUST ignore
      any properties with value types they do not understand.

   Property Parameters:  IANA-registered, nonstandard, id, alternate
      text representation, format type, derived, and language property
      parameters can be specified on this property.

   Conformance:  The property can be specified multiple times in the
      "VEVENT", "VTODO", "VJOURNAL", "VFREEBUSY", "PARTICIPANT", or
      "VALARM" calendar components.

      If it does appear more than once, there MUST be exactly one
      instance of the property with no "DERIVED" parameter or
      DERIVED=FALSE.  All others MUST have DERIVED=TRUE.

      Additionally, if there is one or more "STYLED-DESCRIPTION"
      property, then the "DESCRIPTION" property should either be absent
      or have the parameter DERIVED=TRUE.

   Description:  This property supports rich-text descriptions, for
      example, HTML.  Event publishers typically wish to provide more
      and better-formatted information about the event.

      This property is used in the "VEVENT" and "VTODO" components to
      capture lengthy textual descriptions associated with the activity.
      This property is used in the "VJOURNAL" calendar component to
      capture one or more textual journal entries.  This property is
      used in the "VALARM" calendar component to capture the display
      text for a DISPLAY category of alarm and to capture the body text
      for an EMAIL category of alarm.  In the "PARTICIPANT" component,
      it provides a detailed description of the participant.

      VALUE=TEXT is used to provide rich text inline as the property
      value.

      VALUE=URI is used to provide a link to rich-text content, which is
      expected to be displayed inline as part of the event.

      In either case, the "DESCRIPTION" property should be absent or
      contain a plain-text rendering of the styled text.

      Applications MAY attempt to guess the media type of the resource
      via inspection of its content if and only if the media type of the
      resource is not given by the "FMTTYPE" parameter.  If the media
      type remains unknown, calendar applications SHOULD treat it as
      type "text/html" and process the content as defined in
      [W3C.REC-html51-20171003].

      Multiple "STYLED-DESCRIPTION" properties may be used to provide
      different formats or different language variants.  However, all
      but one MUST have DERIVED=TRUE.

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

      styleddescription = "STYLED-DESCRIPTION" styleddescparam ":"
                         styleddescval CRLF

      styleddescparam   = *(
                         ; The following is REQUIRED
                         ; but MUST NOT occur more than once.
                         ;
                         (";" "VALUE" "=" ("URI" / "TEXT")) /
                         ;
                         ; The following are OPTIONAL
                         ; but MUST NOT occur more than once.
                         ;
                         (";" altrepparam) / (";" languageparam) /
                         (";" fmttypeparam) / (";" derivedparam) /
                         ;
                         ; The following is OPTIONAL
                         ; and MAY occur more than once.
                         ;
                         (";" other-param)
                         )

      styleddescval   = ( uri / text )
      ;Value MUST match value type

   Example:  The following is an example of this property.  It points to
      an HTML description.

      STYLED-DESCRIPTION;VALUE=URI:http://example.org/desc001.html

6.6.  Structured-Data

   Property Name:  STRUCTURED-DATA

   Purpose:  This property specifies ancillary data associated with the
      calendar component.

   Value Type:  There is no default value type for this property.  The
      value type can be set to TEXT, BINARY, or URI.

   Property Parameters:  IANA-registered, nonstandard, inline encoding,
      and value data type property parameters can be specified on this
      property.  The format type and schema parameters can be specified
      on this property and MUST be present for text or inline binary
      encoded content information.

   Conformance:  This property can be specified multiple times in an
      iCalendar object.  Typically, it would be used in the "VEVENT",
      "VTODO", or "VJOURNAL" calendar components.

   Description:  The existing properties in iCalendar cover key elements
      of events and tasks, such as start time, end time, location,
      summary, etc.  However, different types of events often have other
      specific "fields" that are useful to include in the calendar data.
      For example, an event representing an airline flight could include
      the airline, flight number, departure and arrival airport codes,
      check-in and gate-closing times, etc.  As another example, a
      sporting event might contain information about the type of sport,
      the home and away teams, the league the teams are in, information
      about nearby parking, etc.

      This property is used to specify ancillary data in some structured
      format, either directly (inline) as a "TEXT" or "BINARY" value or
      as a link via a "URI" value.

      Rather than define new iCalendar properties for the variety of
      event types that might occur, it would be better to leverage
      existing schemas for such data.  For example, schemas available at
      <https://schema.org> include different event types.  By using
      standard schemas, interoperability can be improved between
      calendar clients and noncalendaring systems that wish to generate
      or process the data.

      This property allows the direct inclusion of ancillary data whose
      schema is defined elsewhere.  This property also includes
      parameters to clearly identify the type of the schema being used
      so that clients can quickly and easily spot what is relevant
      within the calendar data and present that to users or process it
      within the calendaring system.

      iCalendar does support an "ATTACH" property, which can be used to
      include documents or links to documents within the calendar data.
      However, that property does not allow data to be included as a
      "TEXT" value (a feature that "STRUCTURED-DATA" does allow), plus
      attachments are often treated as "opaque" data to be processed by
      some other system rather than the calendar client.  Thus, the
      existing "ATTACH" property is not sufficient to cover the specific
      needs of inclusion of schema data.  Extending the "ATTACH"
      property to support a new value type would likely cause
      interoperability problems.  Additionally, some implementations
      manage attachments by stripping them out and replacing with a link
      to the resource.  Thus, a new property to support inclusion of
      schema data is warranted.

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

      sdataprop    = "STRUCTURED-DATA" sdataparam
                       (
                         ";" "VALUE" "=" "TEXT"
                         ":" text
                       ) /
                       (
                         ";" "ENCODING" "=" "BASE64"
                         ";" "VALUE" "=" "BINARY"
                         ";" binary
                       ) /
                       (
                         ";" "VALUE" "=" "URI"
                         ":" uri
                       )
                       CRLF

      sdataparam  = *(
                       ;
                       ; The following is OPTIONAL for a URI value,
                       ; REQUIRED for a TEXT or BINARY value,
                       ; and MUST NOT occur more than once.
                       ;
                       (";" fmttypeparam) /
                       (";" schemaparam) /
                       ;
                       ; The following is OPTIONAL
                       ; and MAY occur more than once.
                       ;
                       (";" other-param)
                       ;
                    )

   Example:  The following is an example of this property.

      STRUCTURED-DATA;FMTTYPE=application/ld+json;
       SCHEMA="https://schema.org/SportsEvent";
       VALUE=TEXT:{\n
         "@context": "http://schema.org"\,\n
         "@type": "SportsEvent"\,\n
         "homeTeam": "Pittsburgh Pirates"\,\n
         "awayTeam": "San Francisco Giants"\n
       }\n

7.  New Components

7.1.  Participant

   Component name:  PARTICIPANT

   Purpose:  This component provides information about a participant in
      an event or task.

   Conformance:  This component can be specified multiple times in a
      "VEVENT", "VTODO", "VJOURNAL", or "VFREEBUSY" calendar component.

   Description:  This component provides information about a participant
      in a calendar component.  A participant may be an attendee in a
      scheduling sense, and the "ATTENDEE" property may be specified in
      addition.  Participants can be individuals or organizations, for
      example, a soccer team, the spectators, or the musicians.

      "STRUCTURED-DATA" properties, if present, may refer to definitions
      of the participant -- such as a vCard.

      The "CALENDAR-ADDRESS" property, if present, will provide a cal-
      address.  If an "ATTENDEE" property has the same value, the
      participant is considered schedulable.  The "PARTICIPANT"
      component can be used to contain additional metadata related to
      the attendee.

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

      participantc = "BEGIN" ":" "PARTICIPANT" CRLF
                     partprop *locationc *resourcec
                     "END" ":" "PARTICIPANT" CRLF

      partprop     = *(
                     ;
                     ; The following are REQUIRED
                     ; but MUST NOT occur more than once.
                     ;
                     participanttype / uid /
                     ;
                     ; The following are OPTIONAL
                     ; but MUST NOT occur more than once.
                     ;
                     calendaraddress / created / description / dtstamp /
                     geo / last-mod / priority / seq /
                     status / summary / url /
                     ;
                     ; The following are OPTIONAL
                     ; and MAY occur more than once.
                     ;
                     attach / categories / comment
                     contact / location / rstatus / related /
                     resources / strucloc / strucres /
                     styleddescription / sdataprop / iana-prop
                     ;
                     )

   Note:  When the "PRIORITY" property is supplied, it defines the
      ordering of "PARTICIPANT" components with the same value for the
      "PARTICIPANT-TYPE" property.

   Privacy Issues:  When a "LOCATION" property is supplied, it provides
      information about the location of a participant at a given time or
      times.  This may represent an unacceptable privacy risk for some
      participants.  User agents MUST NOT broadcast this information
      without the express permission of the participants whose location
      would be exposed.  For further comments, see Section 10.

   Example:  The following is an example of this component.  It contains
      a "STRUCTURED-DATA" property that points to a vCard providing
      information about the event participant.

      BEGIN:PARTICIPANT
      UID: em9lQGZvb2GFtcGxlLmNvbQ
      PARTICIPANT-TYPE:PERFORMER
      STRUCTURED-DATA;VALUE=URI:
       http://dir.example.com/vcard/aviolinist.vcf
      END:PARTICIPANT

   Example:  The following is an example for the primary contact.

      BEGIN:PARTICIPANT
      UID: em9lQGZvb2GFtcGxlLmNvbQ
      STRUCTURED-DATA;VALUE=URI;
       http://dir.example.com/vcard/contacts/contact1.vcf
      PARTICIPANT-TYPE:CONTACT
      DESCRIPTION:A contact
      END:PARTICIPANT

   Example:  The following is an example for a participant with contact
      and location.

      BEGIN:PARTICIPANT
      UID: em9lQGZvb2GFtcGxlLmNdrt
      STRUCTURED-DATA;VALUE=URI;
       http://dir.example.com/vcard/contacts/my-card.vcf
      PARTICIPANT-TYPE:SPEAKER
      DESCRIPTION:A participant
      BEGIN:VLOCATION
      UID:123456-abcdef-98765432
      NAME:My home location
      STRUCTURED-DATA;VALUE=URI:
       http://dir.example.com/addresses/my-home.vcf
      END:VLOCATION
      END:PARTICIPANT

7.1.1.  Schedulable Participant

   A "PARTICIPANT" component may represent someone or something that
   needs to be scheduled, as defined for ATTENDEE in [RFC5545] and
   [RFC5546].  The "PARTICIPANT" component may also represent someone or
   something that is NOT to receive scheduling messages.

   For backwards compatibility with existing clients and servers when
   used to schedule events and tasks, the "ATTENDEE" property MUST be
   used to specify the scheduling parameters as defined for that
   property.

   For other, future uses, the "CALENDAR-ADDRESS" property MUST be used
   to specify those parameters.

   A "PARTICIPANT" component is defined to be schedulable if:

   *  it contains a "CALENDAR-ADDRESS" property and

   *  that property value is the same as the value for an "ATTENDEE"
      property.

   If both of these conditions apply, then the participant defined by
   the value of the URL property will take part in scheduling
   operations, as defined in [RFC5546].

   An appropriate use for the "PARTICIPANT" component in scheduling
   would be to store "SEQUENCE" and "DTSTAMP" properties associated with
   replies from each "ATTENDEE" property.  A "LOCATION" property within
   the "PARTICIPANT" component might allow better selection of meeting
   times when participants are in different time zones.

7.2.  Location

   Component name:  VLOCATION

   Purpose:  This component provides rich information about the location
      of an event using the structured data property or, optionally, a
      plain-text typed value.

   Conformance:  This component can be specified multiple times in a
      "VEVENT", "VTODO", "VJOURNAL", "VFREEBUSY", or "PARTICIPANT"
      calendar component.

   Description:  There may be a number of locations associated with an
      event.  This component provides detailed information about a
      location.

      When used in a component, the value of this property provides
      information about the event venue or of related services, such as
      parking, dining, stations, etc.

      "STRUCTURED-DATA" properties, if present, may refer to
      representations of the location -- such as a vCard.

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

      locationc    = "BEGIN" ":" "VLOCATION" CRLF
                     locprop
                     "END" ":" "VLOCATION" CRLF

      locprop      = *(
                     ;
                     ; The following are REQUIRED
                     ; but MUST NOT occur more than once.
                     ;
                     uid /
                     ;
                     ; The following are OPTIONAL
                     ; but MUST NOT occur more than once.
                     ;
                     description / geo / loctype / name
                     ;
                     ; The following are OPTIONAL
                     ; and MAY occur more than once.
                     ;
                     sdataprop / iana-prop
                  )

      The "NAME" property is defined in [RFC7986].

   Example:  The following is an example of this component.  It points
      to a venue.

      BEGIN:VLOCATION
      UID:123456-abcdef-98765432
      NAME:The venue
      STRUCTURED-DATA;VALUE=URI:
       http://dir.example.com/venues/big-hall.vcf
      END:VLOCATION

7.3.  Resource

   Component name:  VRESOURCE

   Purpose:  This component provides a typed reference to external
      information about a resource or, optionally, a plain-text typed
      value.  Typically, a resource is anything that might be required
      or used by a calendar entity and possibly has a directory entry.

   Conformance:  This component can be specified multiple times in a
      "VEVENT", "VTODO", "VJOURNAL", "VFREEBUSY", or "PARTICIPANT"
      calendar component.

   Description:  When used in a component, this component provides
      information about resources used for the event, such as rooms,
      projectors, and conferencing capabilities.

      The RESOURCE-TYPE value registry provides a place in which
      resource types may be registered.

      "STRUCTURED-DATA" properties, if present, may refer to
      representations of the resource -- such as a vCard.

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

      resourcec    = "BEGIN" ":" "VRESOURCE" CRLF
                     resprop
                     "END" ":" "VRESOURCE" CRLF

      resprop      = *(
                     ;
                     ; The following are REQUIRED
                     ; but MUST NOT occur more than once.
                     ;
                     uid /
                     ;
                     ; The following are OPTIONAL
                     ; but MUST NOT occur more than once.
                     ;
                     description / geo / name / restype /
                     ;
                     ; The following are OPTIONAL
                     ; and MAY occur more than once.
                     ;
                     sdataprop / iana-prop
                   )

      The "NAME" property is defined in [RFC7986].

   Example:  The following is an example of this component.  It refers
      to a projector.

      BEGIN:VRESOURCE
      UID:456789-abcdef-98765432
      NAME:The projector
      RESOURCE-TYPE:projector
      STRUCTURED-DATA;VALUE=URI:http://dir.example.com/projectors/3d.vcf
      END:VRESOURCE

8.  Extended Examples

   The following are some examples of the use of the properties defined
   in this specification.  They include additional properties defined in
   [RFC7986], which includes "IMAGE".

8.1.  Example 1

   The following is an example of a "VEVENT" describing a concert.  It
   includes location information for the venue itself, as well as
   references to parking and restaurants.

   BEGIN:VEVENT
   CREATED:20200215T145739Z
   DESCRIPTION: Piano Sonata No 3\n
    Piano Sonata No 30
   DTSTAMP:20200215T145739Z
   DTSTART;TZID=America/New_York:20200315T150000Z
   DTEND;TZID=America/New_York:20200315T163000Z
   LAST-MODIFIED:20200216T145739Z
   SUMMARY:Beethoven Piano Sonatas
   UID:123456
   IMAGE;VALUE=URI;DISPLAY=BADGE;FMTTYPE=image/png:h
    ttp://example.com/images/concert.png
   BEGIN:PARTICIPANT
   PARTICIPANT-TYPE:SPONSOR
   UID:dG9tQGZvb2Jhci5xlLmNvbQ
   STRUCTURED-DATA;VALUE=URI:http://example.com/sponsor.vcf
   END:PARTICIPANT
   BEGIN:PARTICIPANT
   PARTICIPANT-TYPE:PERFORMER:
   UID:em9lQGZvb2GFtcGxlLmNvbQ
   STRUCTURED-DATA;VALUE=URI:http://www.example.com/people/johndoe.vcf
   END:PARTICIPANT
   BEGIN:VLOCATION
   UID:123456-abcdef-98765432
   NAME:The venue
   STRUCTURED-DATA;VALUE=URI:http://dir.example.com/venues/big-hall.vcf
   END:VLOCATION
   BEGIN:VLOCATION
   UID:123456-abcdef-87654321
   NAME:Parking for the venue
   STRUCTURED-DATA;VALUE=URI:http://dir.example.com/venues/parking.vcf
   END:VLOCATION
   END:VEVENT

8.2.  Example 2

   The following is an example of a "VEVENT" describing a meeting.  One
   of the attendees is a remote participant.

   BEGIN:VEVENT
   CREATED:20200215T145739Z
   DTSTAMP:20200215T145739Z
   DTSTART;TZID=America/New_York:20200315T150000Z
   DTEND;TZID=America/New_York:20200315T163000Z
   LAST-MODIFIED:20200216T145739Z
   SUMMARY:Conference planning
   UID:123456
   ORGANIZER:mailto:a@example.com
   ATTENDEE;PARTSTAT=ACCEPTED;CN=A:mailto:a@example.com
   ATTENDEE;RSVP=TRUE;CN=B:mailto:b@example.com
   BEGIN:PARTICIPANT
   PARTICIPANT-TYPE:ACTIVE:
   UID:v39lQGZvb2GFtcGxlLmNvbQ
   STRUCTURED-DATA;VALUE=URI:http://www.example.com/people/b.vcf
   LOCATION:At home
   END:PARTICIPANT
   END:VEVENT

9.  Security Considerations

   This specification extends [RFC5545] and makes further use of
   possibly linked data.  While calendar data is not unique in this
   regard, it is worth reminding implementors of some of the dangers and
   safeguards.

9.1.  URIs

   See [RFC3986] for a discussion of the security considerations
   relating to URIs.  Because of the issues discussed there and below,
   clients SHOULD NOT follow URIs and fetch content automatically and
   should only do so at the explicit request of the user.

   Fetching remote resources carries inherent risks.  Connections must
   only be allowed on well-known ports, using allowed protocols
   (generally just HTTP/HTTPS on their default ports).  The URL must be
   resolved externally and not allowed to access internal resources.
   Connecting to an external source reveals IP (and therefore generally
   location) information.

   A maliciously constructed iCalendar object may contain a very large
   number of URIs.  In the case of published calendars with a large
   number of subscribers, such objects could be widely distributed.
   Implementations should be careful to limit the automatic fetching of
   linked resources to reduce the risk of this being an amplification
   vector for a denial-of-service attack.

9.2.  Malicious Content

   For the "STRUCTURED-DATA" property, agents need to be aware that a
   client could attack underlying storage by sending extremely large
   values and could attack processing time by uploading a recurring
   event with a large number of overrides and then repeatedly adding,
   updating, and deleting structured data.

   Agents should set reasonable limits on storage size and number of
   instances and apply those constraints.  Calendar protocols should
   ensure there is a way to report on such limits being exceeded.

   Malicious content could be introduced into the calendar server by way
   of the "STRUCTURED-DATA" property and propagated to many end users
   via scheduling.  Servers SHOULD check this property for malicious or
   inappropriate content.  Upon detecting such content, servers SHOULD
   remove the property.

9.3.  HTML Content

   When processing HTML content, applications need to be aware of the
   many security and privacy issues, as described in the IANA
   Considerations section of [W3C.REC-html51-20171003].

10.  Privacy Considerations

10.1.  Tracking

   Properties with a "URI" value type can expose their users to privacy
   leaks, as any network access of the URI data can be tracked both by a
   network observer and by the entity hosting the remote resource.
   Clients SHOULD NOT automatically download data referenced by the URI
   without explicit instruction from users.

   To help alleviate some of the concerns, protocols and services could
   provide proxy services for downloading referenced data.

10.2.  Revealing Locations

   The addition of location information to the new participant component
   provides information about the location of participants at a given
   time.  This information MUST NOT be distributed to other participants
   without those participant's express permission.  Note that there may
   be a number of participants who may be unaware of their inclusion in
   the data.

   Agents processing and distributing calendar data must be aware that
   it has the property of providing information about a future time when
   a given individual may be at a particular location, which could
   enable targeted attacks against that individual.

   The same may be true of other information contained in the
   participant component.  In general, revealing only as much as is
   absolutely necessary should be the approach taken.

   For example, there may be some privacy considerations relating to the
   "ORDER" parameter, as it provides an indication of the organizer's
   perception of the relative importance of other participants.

11.  IANA Considerations

11.1.  Additional iCalendar Registrations

11.1.1.  Properties

   This document defines the following iCalendar properties that have
   been added to the "Properties" registry defined in Section 8.2.3 of
   [RFC5545]:

         +====================+=========+=======================+
         | Property           | Status  | Reference             |
         +====================+=========+=======================+
         | CALENDAR-ADDRESS   | Current | RFC 9073, Section 6.4 |
         +--------------------+---------+-----------------------+
         | LOCATION-TYPE      | Current | RFC 9073, Section 6.1 |
         +--------------------+---------+-----------------------+
         | PARTICIPANT-TYPE   | Current | RFC 9073, Section 6.2 |
         +--------------------+---------+-----------------------+
         | RESOURCE-TYPE      | Current | RFC 9073, Section 6.3 |
         +--------------------+---------+-----------------------+
         | STRUCTURED-DATA    | Current | RFC 9073, Section 6.6 |
         +--------------------+---------+-----------------------+
         | STYLED-DESCRIPTION | Current | RFC 9073, Section 6.5 |
         +--------------------+---------+-----------------------+

              Table 1: Additions to the Properties Registry

11.1.2.  Parameters

   This document defines the following iCalendar property parameters
   that have been added to the "Parameters" registry defined in
   Section 8.2.4 of [RFC5545]:

              +===========+=========+=======================+
              | Parameter | Status  | Reference             |
              +===========+=========+=======================+
              | ORDER     | Current | RFC 9073, Section 5.1 |
              +-----------+---------+-----------------------+
              | SCHEMA    | Current | RFC 9073, Section 5.2 |
              +-----------+---------+-----------------------+
              | DERIVED   | Current | RFC 9073, Section 5.3 |
              +-----------+---------+-----------------------+

               Table 2: Additions to the Parameters Registry

11.1.3.  Components

   This document defines the following iCalendar components that have
   been added to the "Components" registry defined in Section 8.3.1 of
   [RFC5545]:

             +=============+=========+=======================+
             | Component   | Status  | Reference             |
             +=============+=========+=======================+
             | PARTICIPANT | Current | RFC 9073, Section 7.1 |
             +-------------+---------+-----------------------+
             | VLOCATION   | Current | RFC 9073, Section 7.2 |
             +-------------+---------+-----------------------+
             | VRESOURCE   | Current | RFC 9073, Section 7.3 |
             +-------------+---------+-----------------------+

               Table 3: Additions to the Components Registry

11.2.  Participant Types and Resource Types Registries

   This section defines new registration tables for PARTICIPANT-TYPE and
   RESOURCE-TYPE values.  These tables are updated using the same
   approaches laid down in Section 8.2.1 of [RFC5545].

   This document creates new IANA registries for participant and
   resource types.  IANA will maintain these registries and, following
   the policies outlined in [RFC8126], new tokens are assigned after
   Expert Review.  The Expert Reviewer will generally consult the IETF
   GEOPRIV Working Group mailing list or its designated successor.
   Updates or deletions of tokens from the registration follow the same
   procedures.  The Expert Review should be guided by a few common-sense
   considerations.  For example, tokens should not be specific to a
   country, region, organization, or company; they should be well
   defined and widely recognized.  The Expert's support of IANA will
   include providing IANA with the new token(s) when the update is
   provided only in the form of a schema and providing IANA with the new
   schema element(s) when the update is provided only in the form of a
   token.  To ensure widespread usability across protocols, tokens MUST
   follow the character set restrictions for XML Names
   [W3C.REC-xml-20040204].  Each registration must include the name of
   the token and a brief description similar to the ones offered herein
   for the initial registrations contained this document.

11.2.1.  Participant Types

          +===================+=========+=======================+
          | Participant Type  | Status  | Reference             |
          +===================+=========+=======================+
          | ACTIVE            | Current | RFC 9073, Section 6.2 |
          +-------------------+---------+-----------------------+
          | INACTIVE          | Current | RFC 9073, Section 6.2 |
          +-------------------+---------+-----------------------+
          | SPONSOR           | Current | RFC 9073, Section 6.2 |
          +-------------------+---------+-----------------------+
          | CONTACT           | Current | RFC 9073, Section 6.2 |
          +-------------------+---------+-----------------------+
          | BOOKING-CONTACT   | Current | RFC 9073, Section 6.2 |
          +-------------------+---------+-----------------------+
          | EMERGENCY-CONTACT | Current | RFC 9073, Section 6.2 |
          +-------------------+---------+-----------------------+
          | PUBLICITY-CONTACT | Current | RFC 9073, Section 6.2 |
          +-------------------+---------+-----------------------+
          | PLANNER-CONTACT   | Current | RFC 9073, Section 6.2 |
          +-------------------+---------+-----------------------+
          | PERFORMER         | Current | RFC 9073, Section 6.2 |
          +-------------------+---------+-----------------------+
          | SPEAKER           | Current | RFC 9073, Section 6.2 |
          +-------------------+---------+-----------------------+

             Table 4: Initial Contents of the Participant Types
                                  Registry

11.2.2.  Resource Types

       +=========================+=========+=======================+
       | Resource Type           | Status  | Reference             |
       +=========================+=========+=======================+
       | PROJECTOR               | Current | RFC 9073, Section 6.3 |
       +-------------------------+---------+-----------------------+
       | ROOM                    | Current | RFC 9073, Section 6.3 |
       +-------------------------+---------+-----------------------+
       | REMOTE-CONFERENCE-AUDIO | Current | RFC 9073, Section 6.3 |
       +-------------------------+---------+-----------------------+
       | REMOTE-CONFERENCE-VIDEO | Current | RFC 9073, Section 6.3 |
       +-------------------------+---------+-----------------------+

          Table 5: Initial Contents of the Resource Types Registry

12.  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>.

   [RFC4589]  Schulzrinne, H. and H. Tschofenig, "Location Types
              Registry", RFC 4589, DOI 10.17487/RFC4589, July 2006,
              <https://www.rfc-editor.org/info/rfc4589>.

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

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

   [RFC5546]  Daboo, C., Ed., "iCalendar Transport-Independent
              Interoperability Protocol (iTIP)", RFC 5546,
              DOI 10.17487/RFC5546, December 2009,
              <https://www.rfc-editor.org/info/rfc5546>.

   [RFC6350]  Perreault, S., "vCard Format Specification", RFC 6350,
              DOI 10.17487/RFC6350, August 2011,
              <https://www.rfc-editor.org/info/rfc6350>.

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

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [W3C.REC-html51-20171003]
              Faulkner, S., Ed., Eicholz, A., Ed., Leithead, T., Ed.,
              and A. Danilo, Ed., "HTML 5.1 2nd Edition", World Wide Web
              Consortium Recommendation REC-html51-20171003, October
              2017, <https://www.w3.org/TR/2017/REC-html51-20171003>.

   [W3C.REC-xml-20040204]
              Sperberg-McQueen, M., Maler, E., Bray, T., Paoli, J., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Third
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20040204, February 2004,
              <https://www.w3.org/TR/2004/REC-xml-20040204>.

   [W3C.REC-xml-20081126]
              Bray, T., Ed., Paoli, J., Ed., Sperberg-McQueen, M., Ed.,
              Maler, E., Ed., and F. Yergeau, Ed., "Extensible Markup
              Language (XML) 1.0 (Fifth Edition)", World Wide Web
              Consortium Recommendation REC-xml-20081126, November 2008,
              <https://www.w3.org/TR/2008/REC-xml-20081126>.

Acknowledgements

   The author would like to thank Chuck Norris of eventful.com for his
   work, which led to the development of this RFC.

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

   Cyrus Daboo, John Haug, Dan Mendell, Ken Murchison, and Scott Otis.

Author's Address

   Michael Douglass
   Bedework
   226 3rd Street
   Troy, NY 12180
   United States of America

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