Event Publishing Extensions to iCalendar
draft-ietf-calext-eventpub-extensions-18

Document Type Active Internet-Draft (calext WG)
Author Michael Douglass 
Last updated 2021-01-29 (latest revision 2021-01-13)
Replaces draft-douglass-calendar-extension
Stream Internent Engineering Task Force (IETF)
Intended RFC status Proposed Standard
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Stream WG state Submitted to IESG for Publication (wg milestone: Jun 2019 - Send Event Publicati... )
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Details
Network Working Group                                        M. Douglass
Internet-Draft                                                  Bedework
Updates: 5545 (if approved)                             January 13, 2021
Intended status: Standards Track
Expires: July 17, 2021

                Event Publishing Extensions to iCalendar
                draft-ietf-calext-eventpub-extensions-18

Abstract

   This specification updates RFC5545 by introducing a number of new
   iCalendar properties and components which are of particular use for
   event publishers and in social networking.

   This specification also defines a new STRUCTURED-DATA property for
   iCalendar RFC5545 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 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 July 17, 2021.

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

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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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   3
     1.2.  Terms Used in This Document . . . . . . . . . . . . . . .   3
   2.  Components and properties . . . . . . . . . . . . . . . . . .   4
   3.  Typed References  . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Piano Concert Performance . . . . . . . . . . . . . .   5
       3.1.2.  Itineraries . . . . . . . . . . . . . . . . . . . . .   5
         3.1.2.1.  Reserving facilities  . . . . . . . . . . . . . .   6
   4.  Modifications to Calendar Components  . . . . . . . . . . . .   6
   5.  New Property Parameters . . . . . . . . . . . . . . . . . . .   7
     5.1.  Order . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Schema  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Derived . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  New Properties  . . . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  Location Type . . . . . . . . . . . . . . . . . . . . . .  10
     6.2.  Participant Type  . . . . . . . . . . . . . . . . . . . .  11
     6.3.  Resource Type . . . . . . . . . . . . . . . . . . . . . .  13
     6.4.  Calendar Address  . . . . . . . . . . . . . . . . . . . .  14
     6.5.  Styled-Description  . . . . . . . . . . . . . . . . . . .  14
     6.6.  Structured-Data . . . . . . . . . . . . . . . . . . . . .  16
   7.  New Components  . . . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Participant . . . . . . . . . . . . . . . . . . . . . . .  18
       7.1.1.  Schedulable Participant . . . . . . . . . . . . . . .  22
     7.2.  Location  . . . . . . . . . . . . . . . . . . . . . . . .  22
     7.3.  Resource  . . . . . . . . . . . . . . . . . . . . . . . .  23
   8.  Extended examples . . . . . . . . . . . . . . . . . . . . . .  25
     8.1.  Example 1 . . . . . . . . . . . . . . . . . . . . . . . .  25
     8.2.  Example 2 . . . . . . . . . . . . . . . . . . . . . . . .  26
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
     9.1.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  27
     9.2.  Malicious Content . . . . . . . . . . . . . . . . . . . .  28
     9.3.  HTML Content  . . . . . . . . . . . . . . . . . . . . . .  28
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  28
     10.1.  Tracking . . . . . . . . . . . . . . . . . . . . . . . .  28
     10.2.  Revealing Locations  . . . . . . . . . . . . . . . . . .  28
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
     11.1.  Additional iCalendar Registrations . . . . . . . . . . .  29
       11.1.1.  Properties . . . . . . . . . . . . . . . . . . . . .  29
       11.1.2.  Parameters . . . . . . . . . . . . . . . . . . . . .  29
       11.1.3.  Components . . . . . . . . . . . . . . . . . . . . .  30
     11.2.  New Registration Tables  . . . . . . . . . . . . . . . .  30

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       11.2.1.  Participant Types  . . . . . . . . . . . . . . . . .  30
       11.2.2.  Resource Types . . . . . . . . . . . . . . . . . . .  31
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  31
   13. Normative References  . . . . . . . . . . . . . . . . . . . .  31
   Appendix A.  Open issues  . . . . . . . . . . . . . . . . . . . .  33
   Appendix B.  Change log . . . . . . . . . . . . . . . . . . . . .  33
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  36

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 meta-data associated with the
   event.

   Current practice is to embed this information as links in the
   description or to add non-standard properties as defined in [RFC5545]
   section 3.8.8.2.

   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 interchange of such information between applications or
   systems (e.g., between clients, between client and server, and
   between servers).  Formats such as vCard [RFC2426] 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.

1.2.  Terms Used in This Document

   Event:  When the (perhaps with a capitalised 'E') word 'event' 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.

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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 'tradition' 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
   meta-data 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 the presenting of additional related
   information for the user.

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

   The [RFC5545] LOCATION property 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, 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, for
   example, address, region, country, postal code as well as other
   information such as parking availability, nearby restaurants and the
   venue.  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 STRUCTURED-DATA FMTTYPE parameter, if
   supplied, can be used to help the selection.

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   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 in the event, for example a
   school sports team, and the inactive participants, for example the
   parents.

   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 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
   [RFC2426] information 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 car hire
   businesses at the destination.

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   The embedded location information can guide the traveler at the
   airport or to their final destination.  The contact information can
   provide detailed information about the booking agent, the airlines,
   car hire companies and the 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
   meta-data.  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.

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      ; 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, 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, 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:  To define ordering for the associated property.

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   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:  To specify 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

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      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:  To specify that the value of the associated property is
      derived from some other property value or values.

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   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 and
      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:  To specify 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:

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   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
      [RFC4589] section 3.  New location types SHOULD be registered in
      the manner laid down in section 5 of that specification.

6.2.  Participant Type

   Property name:  PARTICIPANT-TYPE

   Purpose:  To specify the type of participant.

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

   Property Parameters:  Non-standard 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:

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   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

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   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:  To specify 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.

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6.4.  Calendar Address

   Property name:  CALENDAR-ADDRESS

   Purpose:  To specify the calendar address for a participant.

   Value type:  CAL-ADDRESS

   Property Parameters:  IANA-registered, or non-standard 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, non-standard, 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.

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      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 be either 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" 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:

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   This property is defined by the following notation:

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

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

                         (";" "VALUE" "=" ("URI" / "TEXT"))

                         [";" altrepparam]
                         [";" languageparam]
                         [";" fmttypeparam]
                         [";" derivedparam]

                         *(";" 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, non-standard, 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 "VEVENT", "VTODO"
      or "VJOURNAL" calendar components.

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   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 it is 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 non-calendaring 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:

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   This property is defined by the following notation:

      sdataprop    = "STRUCTURED-DATA" sdataparam ":"
                        sdataval CRLF

      sdataparam     = ; all parameter elements may appear in any order,
                       ; and the order is not significant.
                       (sdataparamtext / sdataparambin / sdataparamuri)
                       *(";" other-param)

      sdataparamtext = ";VALUE=TEXT"
                       ";" fmttypeparam
                       ";" schemaparam

      sdataparambin  = ";VALUE=BINARY"
                       ";ENCODING=BASE64"
                       ";" fmttypeparam
                       ";" schemaparam

      sdataparamuri  = ";VALUE=URI"
                       [";" fmttypeparam]
                       [";" schemaparam]

      sdataval       = ( binary / text /uri )
                       ; value MUST match value type

   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.

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   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 meta-data related to the
      attendee.

   Format Definition:

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   This component is defined by the following notation:

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

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

                     uid
                     participanttype

                     [calendaraddress]
                     [created]
                     [description]
                     [dtstamp]
                     [geo]
                     [last-mod]
                     [priority]
                     [seq]
                     [status]
                     [summary]
                     [url]

                     *attach
                     *categories
                     *comment
                     *contact
                     *location
                     *rstatus
                     *related
                     *resources
                     *strucloc
                     *strucres
                     *styleddescription
                     *sdataprop
                     *iana-prop

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

   Privacy Issues:  When a LOCATION 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

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      without the participant's express permission.  For further
      comments see Section 10

   Example:

   The following is an example of this component.  It contains a
   STRUCTURED-DATA property which 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

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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 wuth existing clients and servers when
   used to schedule events and tasks the ATTENDEE property MUST be used
   to specify the sheduling 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

   o  It contains a CALENDAR-ADDRESS property

   o  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.  A LOCATION property within the PARTICIPANT
   component might allow better selection of meeting times when
   participants are in different timezones.

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.

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

                     uid

                     [name]
                     [description]
                     [geo]
                     [loctype]

                     *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

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

                     uid

                     [name]
                     [description]
                     [geo]
                     [restype]

                     *sdataprop
                     *iana-prop

      The NAME property is defined in [RFC7986]

   Example:

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   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

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   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

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   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

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   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.

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   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 new iCalendar properties to be
   added to the registry defined in Section 8.2.3 of [RFC5545]:

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

11.1.2.  Parameters

   This document defines the following new iCalendar property parameters
   to be added to the registry defined in Section 8.2.4 of [RFC5545]:

          +--------------------+---------+----------------------+
          | Property Parameter | Status  | Reference            |
          +--------------------+---------+----------------------+
          | ORDER              | Current | RFCXXXX, Section 5.1 |
          | SCHEMA             | Current | RFCXXXX, Section 5.2 |
          +--------------------+---------+----------------------+

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11.1.3.  Components

   This document defines the following new iCalendar components to be
   added to the registry defined in Section 8.3.1 of [RFC5545]:

             +-------------+---------+----------------------+
             | Component   | Status  | Reference            |
             +-------------+---------+----------------------+
             | PARTICIPANT | Current | RFCXXXX, Section 7.1 |
             | VLOCATION   | Current | RFCXXXX, Section 7.2 |
             | VRESOURCE   | Current | RFCXXXX, Section 7.3 |
             +-------------+---------+----------------------+

11.2.  New Registration Tables

   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 [3].  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

   The following table has been used to initialize the participant types
   registry.

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

11.2.2.  Resource Types

   The following table has been used to initialize the resource types
   registry.

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

12.  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, Scott Otis.

13.  Normative References

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

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   [RFC2426]  Dawson, F. and T. Howes, "vCard MIME Directory Profile",
              RFC 2426, DOI 10.17487/RFC2426, September 1998,
              <https://www.rfc-editor.org/info/rfc2426>.

   [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>.

   [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>.

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

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   [W3C.REC-xml-20081126]
              Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "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>.

Appendix A.  Open issues

   None at the moment

Appendix B.  Change log

   To be deleted on publication

   calext-v18 2021-??-??  MD

   o  Fix incorrect participant type property name in PARTICIPANT.

   o  Allow parameters on LOCATION-TYPE.

   calext-v17 2021-01-03 MD

   o  Remove STRUCTURED-LOCATION property, add VLOCATION component.

   o  Remove STRUCTURED-RESOURCE property, add VRESOURCE component.

   o  Make LOCATION-TYPE multi-valued property for location.

   o  Make RESOURCE-TYPE multi-valued property for resource.

   o  Tidy up abnf.

   calext-v16 2019-10-09 MD

   o  Make LOCTYPE multi-valued.

   o  Add all ATTENDEE scheduling parameters to CALENDAR-ADDRESS.

   calext-v15 2019-10-08 MD

   o  Address various DICUSS points.

   calext-v14 2019-06-11 MD

   o  Definition of event and social calendaring.

   o  Remove redefinition of SOURCE - use STRUCTURED-DATA.

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   calext-v13 2019-05-26 MD

   o  Respond to various issues.

   calext-v12 2019-02-28 MD

   o  Fix styled-description example.  Respond to various AD issues.
      Some typos.

   calext-v11 2019-02-27 MD

   o  Add DERIVED parameter for styled-description, RELATED parameter
      for structured-location

   calext-v09 2018-08-30 MD

   o  Sorted out inconsistencies in refs to 5546

   calext-v08 2018-07-06 MD

   o  Add some text for equal ORDER values

   o  Switched scheduleaddress to calendaraddress in participant abnf.
      Also added more properties

   o  Fixed PARTICIPANT abnf

   calext-v04 2017-10-11 MD

   o  Change SCHEDULE-ADDRESS to CALENDAR-ADDRESS

   o  Explicitly broaden scope of SOURCE

   o  Add initial registry for RESTYPE and move new tables into separate
      section.

   o  Fix PARTTYPE/PARTICIPANT-TYPE inconsistency

   calext-v03 2017-10-09 MD

   o  Mostly typographical and other minor changes

   calext-v02 2017-04-20 MD

   o  Add SCHEDULE-ADDRESS property

   o  PARTICIPANT becomes a component rather than a property.  Turn many
      of the former parameters into properties.

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   o  Use existing ATTENDEE property for scheduling.

   calext-v01 2017-02-18 MD

   o  Change ASSOCIATE back to PARTICIPANT

   o  PARTICIPANT becomes a component rather than a property.  Turn many
      of the former parameters into properties.

   calext-v00 2016-08-??  MD

   o  Name changed - taken up by calext working group

   v06 2016-06-26 MD

   o  Fix up abnf

   o  change ref to ietf from daboo

   o  take out label spec - use Cyrus spec

   v05 2016-06-14 MD

   o  Remove GROUP and HASH. they can be dealt with elsewhere if desired

   o  Change ORDER to integer >= 1.

   o  Incorporate Structured-Data into this specification.

   v04 2014-02-01 MD

   o  Added updates attribute.

   o  Minor typos.

   o  Resubmitted mostly to refresh the draft.

   v03 2013-03-06 MD

   o  Replace PARTICIPANT with ASSOCIATE plus related changes.

   o  Added section showing modifications to components.

   o  Replace ID with GROUP and modify HASH.

   o  Replace TITLE param with LABEL.

   o  Fixed STYLED-DESCRIPTION in various ways, correct example.

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   v02 2012-11-02 MD

   o  Collapse sections with description of properties and the use cases
      into a section with sub-sections.

   o  New section to describe relating properties.

   o  Remove idref and upgrade hash to have the reference

   o  No default value types on properties..

   v01 2012-10-18 MD Many changes.

   o  SPONSOR and STRUCTURED-CONTACT are now in PARTICIPANT

   o  Add a STRUCTURED-RESOURCE property

   o  STYLED-DESCRIPTION to handle rich text

   o  Much more...

   2011-01-07

   o  Remove MEDIA - it's going in the Cyrus RFC

   o  Rename EXTENDED-... to STRUCTURED-...

   o  Add TYPE parameter to SPONSOR

   v00 2007-10-19 MD Initial version

Author's Address

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

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

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