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rfc:rfc7986

Internet Engineering Task Force (IETF) C. Daboo Request for Comments: 7986 Apple Inc. Updates: 5545 October 2016 Category: Standards Track ISSN: 2070-1721

                    New Properties for iCalendar

Abstract

 This document defines a set of new properties for iCalendar data and
 extends the use of some existing properties to the entire iCalendar
 object.

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
 http://www.rfc-editor.org/info/rfc7986.

Copyright Notice

 Copyright (c) 2016 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
 (http://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.

Daboo Standards Track [Page 1] RFC 7986 iCalendar Property Extensions October 2016

Table of Contents

 1. Introduction ....................................................3
 2. Conventions Used in This Document ...............................3
 3. Backwards-Compatible Extension Properties .......................3
 4. Modifications to Calendar Components ............................4
 5. Properties ......................................................5
    5.1. NAME Property ..............................................5
    5.2. DESCRIPTION Property .......................................6
    5.3. UID Property ...............................................7
    5.4. LAST-MODIFIED Property .....................................8
    5.5. URL Property ...............................................8
    5.6. CATEGORIES Property ........................................8
    5.7. REFRESH-INTERVAL Property ..................................9
    5.8. SOURCE Property ...........................................10
    5.9. COLOR Property ............................................10
    5.10. IMAGE Property ...........................................11
    5.11. CONFERENCE Property ......................................13
 6. Property Parameters ............................................14
    6.1. DISPLAY Property Parameter ................................14
    6.2. EMAIL Property Parameter ..................................15
    6.3. FEATURE Property Parameter ................................16
    6.4. LABEL Property Parameter ..................................17
 7. Security Considerations ........................................18
 8. Privacy Considerations .........................................18
 9. IANA Considerations ............................................19
    9.1. Property Registrations ....................................19
    9.2. Parameter Registrations ...................................20
    9.3. Property Parameter Value Registries .......................20
 10. References ....................................................21
    10.1. Normative References .....................................21
    10.2. Informative References ...................................22
 Acknowledgments ...................................................23
 Author's Address ..................................................23

Daboo Standards Track [Page 2] RFC 7986 iCalendar Property Extensions October 2016

1. Introduction

 The iCalendar [RFC5545] data format is used to represent calendar
 data and is used with the iCalendar Transport-Independent
 Interoperability Protocol (iTIP) [RFC5546] to handle scheduling
 operations between calendar users. iCalendar is in widespread use,
 and in accordance with provisions in that specification, extension
 elements have been added by various vendors to the data format in
 order to support and enhance capabilities.  This specification
 collects a number of these ad hoc extensions and uses the new IANA
 registry capability defined in [RFC5545] to register standard
 variants with clearly defined definitions and semantics.  In
 addition, some new elements are introduced for features that vendors
 have recently been requesting.

2. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 [RFC2119].
 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].

3. Backwards-Compatible Extension Properties

 iCalendar defines properties that can have different value types
 indicated by a "VALUE" parameter.  The definition of a property
 specifies a "default" value type that is assumed to be used when no
 "VALUE" parameter is present.  However, this poses a problem to the
 iCalendar parser/generator software that does not know about the
 default values for new properties.  For example, if a new property
 "FOO" were defined with a default value type of URI and a URI value
 with a comma was used, an iCalendar generator not aware of this fact
 would likely treat the property value as "TEXT" and apply backslash
 escaping to the comma in the value, effectively making it an invalid
 URI value.
 To avoid this problem, this specification recommends that all
 properties not defined in [RFC5545] always include a "VALUE"
 parameter if the type is other than "TEXT".  That is, in the example
 above, the "FOO" property would have a "VALUE=URI" parameter.  This
 allows iCalendar parser/generator software to track the correct types
 of unknown properties.

Daboo Standards Track [Page 3] RFC 7986 iCalendar Property Extensions October 2016

 New properties defined in this specification use the term "no
 default" in the "Value Type" definition to indicate that the "VALUE"
 parameter has to be included.

4. Modifications to Calendar Components

 This section details changes to the syntax defined in iCalendar
 [RFC5545].  New elements are defined in subsequent sections.
 calprops =/ *(
              ;
              ; The following are OPTIONAL,
              ; but MUST NOT occur more than once.
              ;
              uid / last-mod / url /
              refresh / source / color
              ;
              ; The following are OPTIONAL,
              ; and MAY occur more than once.
              ;
              name / description / categories /
              image
              ;
              )
 eventprop =/ *(
               ;
               ; The following are OPTIONAL,
               ; but MUST NOT occur more than once.
               ;
               color /
               ;
               ; The following are OPTIONAL,
               ; and MAY occur more than once.
               ;
               conference / image
               ;
               )
 todoprop =/ *(
              ;
              ; The following are OPTIONAL,
              ; but MUST NOT occur more than once.
              ;
              color /
              ;
              ; The following are OPTIONAL,
              ; and MAY occur more than once.

Daboo Standards Track [Page 4] RFC 7986 iCalendar Property Extensions October 2016

              ;
              conference / image
              ;
              )
 jourprop =/ *(
              ;
              ; The following are OPTIONAL,
              ; but MUST NOT occur more than once.
              ;
              color /
              ;
              ; The following are OPTIONAL,
              ; and MAY occur more than once.
              ;
              image
              ;
              )

5. Properties

5.1. NAME Property

 Property Name:  NAME
 Purpose:  This property specifies the name of the calendar.
 Value Type:  TEXT
 Property Parameters:  IANA, non-standard, alternate text
    representation, and language property parameters can be specified
    on this property.
 Conformance:  This property can be specified multiple times in an
    iCalendar object.  However, each property MUST represent the name
    of the calendar in a different language.
 Description:  This property is used to specify a name of the
    iCalendar object that can be used by calendar user agents when
    presenting the calendar data to a user.  Whilst a calendar only
    has a single name, multiple language variants can be specified by
    including this property multiple times with different "LANGUAGE"
    parameter values on each.

Daboo Standards Track [Page 5] RFC 7986 iCalendar Property Extensions October 2016

 Format Definition:  This property is defined by the following
    notation:
 name       = "NAME" nameparam ":" text CRLF
 nameparam  = *(
               ;
               ; The following are OPTIONAL,
               ; but MUST NOT occur more than once.
               ;
               (";" altrepparam) / (";" languageparam) /
               ;
               ; The following is OPTIONAL,
               ; and MAY occur more than once.
               ;
               (";" other-param)
               ;
               )
 Example:  The following is an example of this property:
 NAME:Company Vacation Days

5.2. DESCRIPTION Property

 This specification modifies the definition of the "DESCRIPTION"
 property to allow it to be defined in an iCalendar object.  The
 following additions are made to the definition of this property,
 originally specified in Section 3.8.1.5 of [RFC5545].
 Purpose:  This property specifies the description of the calendar.
 Conformance:  This property can be specified multiple times in an
    iCalendar object.  However, each property MUST represent the
    description of the calendar in a different language.
 Description:  This property is used to specify a lengthy textual
    description of the iCalendar object that can be used by calendar
    user agents when describing the nature of the calendar data to a
    user.  Whilst a calendar only has a single description, multiple
    language variants can be specified by including this property
    multiple times with different "LANGUAGE" parameter values on each.

Daboo Standards Track [Page 6] RFC 7986 iCalendar Property Extensions October 2016

5.3. UID Property

 This specification modifies the definition of the "UID" property to
 allow it to be defined in an iCalendar object.  The following
 additions are made to the definition of this property, originally
 specified in Section 3.8.4.7 of [RFC5545].
 Purpose:  This property specifies the persistent, globally unique
    identifier for the iCalendar object.  This can be used, for
    example, to identify duplicate calendar streams that a client may
    have been given access to.  It can be used in conjunction with the
    "LAST-MODIFIED" property also specified on the "VCALENDAR" object
    to identify the most recent version of a calendar.
 Conformance:  This property can be specified once in an iCalendar
    object.
 The description of the "UID" property in [RFC5545] contains some
 recommendations on how the value can be constructed.  In particular,
 it suggests use of host names, IP addresses, and domain names to
 construct the value.  However, this is no longer considered good
 practice, particularly from a security and privacy standpoint, since
 use of such values can leak key information about a calendar user or
 their client and network environment.  This specification updates
 [RFC5545] by stating that "UID" values MUST NOT include any data that
 might identify a user, host, domain, or any other security- or
 privacy-sensitive information.  It is RECOMMENDED that calendar user
 agents now generate "UID" values that are hex-encoded random
 Universally Unique Identifier (UUID) values as defined in
 Sections 4.4 and 4.5 of [RFC4122].
 The following is an example of such a property value:
 UID:5FC53010-1267-4F8E-BC28-1D7AE55A7C99
 Additionally, if calendar user agents choose to use other forms of
 opaque identifiers for the "UID" value, they MUST have a length less
 than 255 octets and MUST conform to the "iana-token" ABNF syntax
 defined in Section 3.1 of [RFC5545].

Daboo Standards Track [Page 7] RFC 7986 iCalendar Property Extensions October 2016

5.4. LAST-MODIFIED Property

 This specification modifies the definition of the "LAST-MODIFIED"
 property to allow it to be defined in an iCalendar object.  The
 following additions are made to the definition of this property,
 originally specified in Section 3.8.7.3 of [RFC5545].
 Purpose:  This property specifies the date and time that the
    information associated with the calendar was last revised.
 Conformance:  This property can be specified once in an iCalendar
    object.

5.5. URL Property

 This specification modifies the definition of the "URL" property to
 allow it to be defined in an iCalendar object.  The following
 additions are made to the definition of this property, originally
 specified in Section 3.8.4.6 of [RFC5545].
 Purpose:  This property may be used to convey a location where a more
    dynamic rendition of the calendar information can be found.
 Conformance:  This property can be specified once in an iCalendar
    object.

5.6. CATEGORIES Property

 This specification modifies the definition of the "CATEGORIES"
 property to allow it to be defined in an iCalendar object.  The
 following additions are made to the definition of this property,
 originally specified in Section 3.8.1.2 of [RFC5545].
 Purpose:  This property defines the categories for an entire
    calendar.
 Conformance:  This property can be specified multiple times in an
    iCalendar object.
 Description:  When multiple properties are present, the set of
    categories that apply to the iCalendar object are the union of all
    the categories listed in each property value.

Daboo Standards Track [Page 8] RFC 7986 iCalendar Property Extensions October 2016

5.7. REFRESH-INTERVAL Property

 Property Name:  REFRESH-INTERVAL
 Purpose:  This property specifies a suggested minimum interval for
    polling for changes of the calendar data from the original source
    of that data.
 Value Type:  DURATION -- no default
 Property Parameters:  IANA and non-standard property parameters can
    be specified on this property.
 Conformance:  This property can be specified once in an iCalendar
    object.
 Description:  This property specifies a positive duration that gives
    a suggested minimum polling interval for checking for updates to
    the calendar data.  The value of this property SHOULD be used by
    calendar user agents to limit the polling interval for calendar
    data updates to the minimum interval specified.
 Format Definition:  This property is defined by the following
    notation:
 refresh      = "REFRESH-INTERVAL" refreshparam
                  ":" dur-value CRLF
                  ;consisting of a positive duration of time.
 refreshparam = *(
                 ;
                 ; The following is REQUIRED,
                 ; but MUST NOT occur more than once.
                 ;
                 (";" "VALUE" "=" "DURATION") /
                 ;
                 ; The following is OPTIONAL,
                 ; and MAY occur more than once.
                 ;
                 (";" other-param)
                 ;
                 )
 Example:  The following is an example of this property:
 REFRESH-INTERVAL;VALUE=DURATION:P1W

Daboo Standards Track [Page 9] RFC 7986 iCalendar Property Extensions October 2016

5.8. SOURCE Property

 Property Name:  SOURCE
 Purpose:  This property identifies a URI where calendar data can be
    refreshed from.
 Value Type:  URI -- no default
 Property Parameters:  IANA and non-standard property parameters can
    be specified on this property.
 Conformance:  This property can be specified once in an iCalendar
    object.
 Description:  This property identifies a location where a client can
    retrieve updated data for the calendar.  Clients SHOULD honor any
    specified "REFRESH-INTERVAL" value when periodically retrieving
    data.  Note that this property differs from the "URL" property in
    that "URL" is meant to provide an alternative representation of
    the calendar data rather than the original location of the data.
 Format Definition:  This property is defined by the following
    notation:
 source       = "SOURCE" sourceparam ":" uri CRLF
 sourceparam = *(";" other-param)
 Example:  The following is an example of this property:
 SOURCE;VALUE=URI:https://example.com/holidays.ics

5.9. COLOR Property

 Property Name:  COLOR
 Purpose:  This property specifies a color used for displaying the
    calendar, event, todo, or journal data.
 Value Type:  TEXT
 Property Parameters:  IANA and non-standard property parameters can
    be specified on this property.
 Conformance:  This property can be specified once in an iCalendar
    object or in "VEVENT", "VTODO", or "VJOURNAL" calendar components.

Daboo Standards Track [Page 10] RFC 7986 iCalendar Property Extensions October 2016

 Description:  This property specifies a color that clients MAY use
    when presenting the relevant data to a user.  Typically, this
    would appear as the "background" color of events or tasks.  The
    value is a case-insensitive color name taken from the CSS3 set of
    names, defined in Section 4.3 of [W3C.REC-css3-color-20110607].
 Format Definition:  This property is defined by the following
    notation:
 color          = "COLOR" colorparam ":" text CRLF
                   ; Value is CSS3 color name
 colorparam     = *(";" other-param)
 Example:  The following is an example of this property:
 COLOR:turquoise

5.10. IMAGE Property

 Property Name:  IMAGE
 Purpose:  This property specifies an image associated with the
    calendar or a calendar component.
 Value Type:  URI or BINARY -- no default.  The value MUST be data
    with a media type of "image" or refer to such data.
 Property Parameters:  IANA, non-standard, display, inline encoding,
    and value data type property parameters can be specified on this
    property.  The format type parameter can be specified on this
    property and is RECOMMENDED for inline binary-encoded content
    information.
 Conformance:  This property can be specified multiple times in an
    iCalendar object or in "VEVENT", "VTODO", or "VJOURNAL" calendar
    components.
 Description:  This property specifies an image for an iCalendar
    object or a calendar component via a URI or directly with inline
    data that can be used by calendar user agents when presenting the
    calendar data to a user.  Multiple properties MAY be used to
    specify alternative sets of images with, for example, varying
    media subtypes, resolutions, or sizes.  When multiple properties
    are present, calendar user agents SHOULD display only one of them,
    picking one that provides the most appropriate image quality, or
    display none.  The "DISPLAY" parameter is used to indicate the
    intended display mode for the image.  The "ALTREP" parameter,

Daboo Standards Track [Page 11] RFC 7986 iCalendar Property Extensions October 2016

    defined in [RFC5545], can be used to provide a "clickable" image
    where the URI in the parameter value can be "launched" by a click
    on the image in the calendar user agent.
 Format Definition:  This property is defined by the following
    notation:
 image      = "IMAGE" imageparam
              (
                (
                  ";" "VALUE" "=" "URI"
                  ":" uri
                ) /
                (
                  ";" "ENCODING" "=" "BASE64"
                  ";" "VALUE" "=" "BINARY"
                  ":" binary
                )
              )
              CRLF
 imageparam = *(
               ;
               ; The following is OPTIONAL for a URI value,
               ; RECOMMENDED for a BINARY value,
               ; and MUST NOT occur more than once.
               ;
               (";" fmttypeparam) /
               ;
               ; The following are OPTIONAL,
               ; and MUST NOT occur more than once.
               ;
               (";" altrepparam) / (";" displayparam) /
               ;
               ; The following is OPTIONAL,
               ; and MAY occur more than once.
               ;
               (";" other-param)
               ;
               )
 Example:  The following is an example of this property:
 IMAGE;VALUE=URI;DISPLAY=BADGE;FMTTYPE=image/png:h
  ttp://example.com/images/party.png

Daboo Standards Track [Page 12] RFC 7986 iCalendar Property Extensions October 2016

5.11. CONFERENCE Property

 Property Name:  CONFERENCE
 Purpose:  This property specifies information for accessing a
    conferencing system.
 Value Type:  URI -- no default.
 Property Parameters:  IANA, non-standard, feature, and label property
    parameters can be specified on this property.
 Conformance:  This property can be specified multiple times in a
    "VEVENT" or "VTODO" calendar component.
 Description:  This property specifies information for accessing a
    conferencing system for attendees of a meeting or task.  This
    might be for a telephone-based conference number dial-in with
    access codes included (such as a tel: URI [RFC3966] or a sip: or
    sips: URI [RFC3261]), for a web-based video chat (such as an http:
    or https: URI [RFC7230]), or for an instant messaging group chat
    room (such as an xmpp: URI [RFC5122]).  If a specific URI for a
    conferencing system is not available, a data: URI [RFC2397]
    containing a text description can be used.
    A conference system can be a bidirectional communication channel
    or a uni-directional "broadcast feed".
    The "FEATURE" property parameter is used to describe the key
    capabilities of the conference system to allow a client to choose
    the ones that give the required level of interaction from a set of
    multiple properties.
    The "LABEL" property parameter is used to convey additional
    details on the use of the URI.  For example, the URIs or access
    codes for the moderator and attendee of a teleconference system
    could be different, and the "LABEL" property parameter could be
    used to "tag" each "CONFERENCE" property to indicate which is
    which.
    The "LANGUAGE" property parameter can be used to specify the
    language used for text values used with this property (as per
    Section 3.2.10 of [RFC5545]).

Daboo Standards Track [Page 13] RFC 7986 iCalendar Property Extensions October 2016

 Format Definition:  This property is defined by the following
    notation:
 conference = "CONFERENCE" confparam  ":" uri CRLF
 confparam  = *(
               ;
               ; The following is REQUIRED,
               ; but MUST NOT occur more than once.
               ;
               (";" "VALUE" "=" "URI") /
               ;
               ; The following are OPTIONAL,
               ; and MUST NOT occur more than once.
               ;
               (";" featureparam) / (";" labelparam) /
               (";" languageparam ) /
               ;
               ; The following is OPTIONAL,
               ; and MAY occur more than once.
               ;
               (";" other-param)
               ;
               )
 Example:  The following are examples of this property:
 CONFERENCE;VALUE=URI;FEATURE=PHONE,MODERATOR;
  LABEL=Moderator dial-in:tel:+1-412-555-0123,,,654321
 CONFERENCE;VALUE=URI;FEATURE=PHONE;
  LABEL=Attendee dial-in:tel:+1-412-555-0123,,,555123
 CONFERENCE;VALUE=URI;FEATURE=PHONE;
  LABEL=Attendee dial-in:tel:+1-888-555-0456,,,555123
 CONFERENCE;VALUE=URI;FEATURE=CHAT;
  LABEL=Chat room:xmpp:chat-123@conference.example.com
 CONFERENCE;VALUE=URI;FEATURE=AUDIO,VIDEO;
  LABEL=Attendee dial-in:https://chat.example.com/audio?id=123456

6. Property Parameters

6.1. DISPLAY Property Parameter

 Parameter Name:  DISPLAY
 Purpose:  To specify different ways in which an image for a calendar
    or component can be displayed.

Daboo Standards Track [Page 14] RFC 7986 iCalendar Property Extensions October 2016

 Format Definition:  This property parameter is defined by the
    following notation:
 displayparam = "DISPLAY" "=" displayval *("," displayval)
 displayval =  ("BADGE" /     ; image inline with the title of the
                              ; event
                "GRAPHIC" /   ; a full image replacement for the event
                              ; itself
                "FULLSIZE" /  ; an image that is used to enhance the
                              ; event
                "THUMBNAIL" / ; a smaller variant of "FULLSIZE" to be
                              ; used when space for the image is
                              ; constrained
                x-name /      ; Experimental type
                iana-token)   ; Other IANA-registered type
                              ;
                              ; Default is BADGE
 Description:  This property parameter MAY be specified on "IMAGE"
    properties.  In the absence of this parameter, the default value
    "BADGE" MUST be used.  The value determines how a client ought to
    present an image supplied in iCalendar data to the user.
    Values for this parameter are registered with IANA as per
    Section 9.3.1.  New values can be added to this registry following
    the procedure outlined in Section 8.2.1 of [RFC5545].
    Servers and clients MUST handle x-name and iana-token values they
    don't recognize by not displaying any image at all.
 Example:
 IMAGE;VALUE=URI;DISPLAY=BADGE,THUMBNAIL;FMTTYPE=image/png:https://exa
  mple.com/images/weather-cloudy.png

6.2. EMAIL Property Parameter

 Parameter Name:  EMAIL
 Purpose:  To specify an email address that is used to identify or
    contact an organizer or attendee.
 Format Definition:  This property parameter is defined by the
    following notation:
 emailparam = "EMAIL" "=" param-value

Daboo Standards Track [Page 15] RFC 7986 iCalendar Property Extensions October 2016

 Description:  This property parameter MAY be specified on "ORGANIZER"
    or "ATTENDEE" properties.  This property can be used in situations
    where the calendar user address value of the "ORGANIZER" and
    "ATTENDEE" properties is not likely to be an identifier that
    recipients of scheduling messages could use to match the calendar
    user with, for example, an address book entry.  The value of this
    property is an email address that can easily be matched by
    recipients.  Recipients can also use this value as an alternative
    means of contacting the calendar user via email.  If a recipient's
    calendar user agent allows the recipient to save contact
    information based on the "ORGANIZER" or "ATTENDEE" properties,
    those calendar user agents SHOULD use any "EMAIL" property
    parameter value for the email address of the contact over any
    mailto: calendar user address specified as the value of the
    property.  Calendar user agents SHOULD NOT include an "EMAIL"
    property parameter when its value matches the calendar user
    address specified as the value of the property.
 Example:
 ATTENDEE;CN=Cyrus Daboo;EMAIL=cyrus@example.com:mailto:opaque-toke
  n-1234@example.com

6.3. FEATURE Property Parameter

 Parameter Name:  FEATURE
 Purpose:  To specify a feature or features of a conference or
    broadcast system.
 Format Definition:  This property parameter is defined by the
    following notation:
 featureparam = "FEATURE" "=" featuretext *("," featuretext)
 featuretext  =  ("AUDIO" /     ; Audio capability
                  "CHAT" /      ; Chat or instant messaging
                  "FEED" /      ; Blog or Atom feed
                  "MODERATOR" / ; Moderator dial-in code
                  "PHONE" /     ; Phone conference
                  "SCREEN" /    ; Screen sharing
                  "VIDEO" /     ; Video capability
                  x-name /      ; Experimental type
                  iana-token)   ; Other IANA-registered type

Daboo Standards Track [Page 16] RFC 7986 iCalendar Property Extensions October 2016

 Description:  This property parameter MAY be specified on the
    "CONFERENCE" property.  Multiple values can be specified.  The
    "MODERATOR" value is used to indicate that the property value is
    specific to the owner/initiator of the conference and contains a
    URI that "activates" the system (e.g., a "moderator" access code
    for a phone conference system that is different from the "regular"
    access code).
 Example:
 CONFERENCE;VALUE=URI;FEATURE=AUDIO:rtsp://audio.example.com/
  event
 CONFERENCE;VALUE=URI;FEATURE=AUDIO,VIDEO:https://video-chat.exam
  ple.com/;group-id=1234

6.4. LABEL Property Parameter

 Parameter Name:  LABEL
 Purpose:  To provide a human-readable label.
 Format Definition:  This property parameter is defined by the
    following notation:
 labelparam = "LABEL" "=" param-value
 Description:  This property parameter MAY be specified on the
    "CONFERENCE" property.  It is anticipated that other extensions to
    iCalendar will reuse this property parameter on new properties
    that they define.  As a result, clients MUST expect to find this
    property parameter present on many different properties.  It
    provides a human-readable label that can be presented to calendar
    users to allow them to discriminate between properties that might
    be similar or provide additional information for properties that
    are not self-describing.  The "LANGUAGE" property parameter can be
    used to specify the language of the text in the parameter value
    (as per Section 3.2.10 of [RFC5545]).
 Example:
 CONFERENCE;VALUE=URI;FEATURE=VIDEO;
  LABEL="Web video chat, access code=76543";
  :https://video-chat.example.com/;group-id=1234

Daboo Standards Track [Page 17] RFC 7986 iCalendar Property Extensions October 2016

7. Security Considerations

 Several of the new properties or parameters defined by this
 specification allow reference to "external" URIs.  Care MUST be taken
 when accessing data at external URIs as malicious content could be
 present.  Clients SHOULD ensure that suitable permission is granted
 by calendar users before such URIs are dereferenced.
 The "REFRESH-INTERVAL" property could be used by an attacker to make
 a client carry out rapid requests to the server hosting the calendar
 by specifying a very short duration (e.g., one second).  This could
 lead to resource consumption on the client or server and to denial-
 of-service attacks against the server.  Clients MUST ensure that they
 throttle requests to the server to a reasonable rate.  In most cases,
 updating a public calendar once per day would suffice.  If the
 "REFRESH-INTERVAL" is any less than that, clients SHOULD warn the
 calendar user and allow them to override it with a longer value.
 The "CONFERENCE" property can include a "FEATURE" property parameter
 with a "MODERATOR" value.  In some cases, the access code used by the
 owner/initiator of a conference might be private to an individual,
 and clients and servers MUST ensure that such properties are not sent
 to attendees of a scheduled component.
 Both the "COLOR" and "IMAGE" properties are likely to be used by
 calendar users to express their own personal view of the calendar
 data.  In addition, these properties could be used by attackers to
 produce a confusing display in a calendar user agent.  When such
 properties are encountered in calendar data that has come from other
 calendar users (e.g., via a scheduling message, "public" calendar
 subscription, etc.), it is advisable for the client to give the
 receiving calendar user the option to remove (or adjust) these
 properties as the data is imported into their calendar system.
 This specification changes the recommendations on how "UID" property
 values are constructed to minimize leaking any information that might
 be security sensitive.
 Security considerations in [RFC5545] and [RFC5546] MUST also be
 adhered to.

8. Privacy Considerations

 Several of the new properties or parameters defined by this
 specification allow reference to "external" URIs.  Access to those
 URIs could be tracked, leading to loss of privacy.  Clients SHOULD
 ensure that suitable permission is granted by calendar users before
 such URIs are dereferenced.  In particular, calendar publishers

Daboo Standards Track [Page 18] RFC 7986 iCalendar Property Extensions October 2016

 wishing to help protect the privacy of their subscribers MUST use
 HTTP with Transport Layer Security [RFC7230] ("https:" URIs instead
 of "http:" URIs) for access to calendar data or ancillary data such
 as images.
 In general, for their own privacy protection, users have to rely on
 the privacy policies of any conferencing system being accessed via
 the "CONFERENCE" property.  It is entirely possible for such systems
 to uniquely identify and log the activity and participation (or lack
 thereof) of calendar users in the conference.  Calendar user agents
 SHOULD track which conferencing systems are used and warn users the
 first time a new one is about to be used.  This is particularly
 important if the client automatically "dials in" to the conference
 when the event start time occurs.
 By giving different calendar users different values for the "REFRESH-
 INTERVAL" property, it is possible for a publisher of calendar data
 to uniquely identify each refresh from each calendar users' clients
 and thereby track user activity and IP address over time.  To address
 this, clients SHOULD add or subtract some random amount of time from
 the published "REFRESH-INTERVAL" value when doing actual refreshes.
 This specification changes the recommendations on how "UID" property
 values are constructed to minimize leaking any information that might
 be privacy sensitive.
 Privacy considerations in [RFC5545] and [RFC5546] MUST also be
 adhered to.

9. IANA Considerations

9.1. Property Registrations

 This document defines the following new iCalendar properties.  IANA
 has registered the new properties in the "Properties" registry
 defined in Section 8.3.2 of [RFC5545].  IANA has also added a
 reference to this document where the properties originally defined in
 RFC 5545 have been updated by this document.

Daboo Standards Track [Page 19] RFC 7986 iCalendar Property Extensions October 2016

 +------------------+---------+--------------------------------------+
 | Property         | Status  | Reference                            |
 +------------------+---------+--------------------------------------+
 | NAME             | Current | RFC 7986, Section 5.1                |
 | DESCRIPTION      | Current | RFC 5545, Section 3.8.1.5; RFC 7986, |
 |                  |         | Section 5.2                          |
 | UID              | Current | RFC 5545, Section 3.8.4.7; RFC 7986, |
 |                  |         | Section 5.3                          |
 | LAST-MODIFIED    | Current | RFC 5545, Section 3.8.7.3 RFC 7986,  |
 |                  |         | Section 5.4                          |
 | URL              | Current | RFC 5545, Section 3.8.4.6; RFC 7986, |
 |                  |         | Section 5.5                          |
 | CATEGORIES       | Current | RFC 5545, Section 3.8.1.2; RFC 7986, |
 |                  |         | Section 5.6                          |
 | REFRESH-INTERVAL | Current | RFC 7986, Section 5.7                |
 | SOURCE           | Current | RFC 7986, Section 5.8                |
 | COLOR            | Current | RFC 7986, Section 5.9                |
 | IMAGE            | Current | RFC 7986, Section 5.10               |
 | CONFERENCE       | Current | RFC 7986, Section 5.11               |
 +------------------+---------+--------------------------------------+

9.2. Parameter Registrations

 This document defines the following new iCalendar property
 parameters.  IANA has registered these in the "Parameters" registry
 defined in Section 8.3.3 of [RFC5545].
       +--------------------+---------+-----------------------+
       | Property Parameter | Status  | Reference             |
       +--------------------+---------+-----------------------+
       | DISPLAY            | Current | RFC 7986, Section 6.1 |
       | EMAIL              | Current | RFC 7986, Section 6.2 |
       | FEATURE            | Current | RFC 7986, Section 6.3 |
       | LABEL              | Current | RFC 7986, Section 6.4 |
       +--------------------+---------+-----------------------+

9.3. Property Parameter Value Registries

 IANA has created two new registries for iCalendar elements: the
 "Display Types" registry and the "Feature Types" registry.
 Additional codes MAY be used, provided the process and template
 described in Sections 8.2.1 and 8.2.6 of [RFC5545] are used to
 register them.

Daboo Standards Track [Page 20] RFC 7986 iCalendar Property Extensions October 2016

9.3.1. Display Types Registry

 The following table has been used to initialize the "Display Types"
 registry.
          +--------------+---------+-----------------------+
          | Display Type | Status  | Reference             |
          +--------------+---------+-----------------------+
          | BADGE        | Current | RFC 7986, Section 6.1 |
          | GRAPHIC      | Current | RFC 7986, Section 6.1 |
          | FULLSIZE     | Current | RFC 7986, Section 6.1 |
          | THUMBNAIL    | Current | RFC 7986, Section 6.1 |
          +--------------+---------+-----------------------+

9.3.2. Feature Types Registry

 The following table has been used to initialize the "Feature Types"
 registry.
          +--------------+---------+-----------------------+
          | Feature Type | Status  | Reference             |
          +--------------+---------+-----------------------+
          | AUDIO        | Current | RFC 7986, Section 6.3 |
          | CHAT         | Current | RFC 7986, Section 6.3 |
          | FEED         | Current | RFC 7986, Section 6.3 |
          | MODERATOR    | Current | RFC 7986, Section 6.3 |
          | PHONE        | Current | RFC 7986, Section 6.3 |
          | SCREEN       | Current | RFC 7986, Section 6.3 |
          | VIDEO        | Current | RFC 7986, Section 6.3 |
          +--------------+---------+-----------------------+

10. References

10.1. 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,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
            Unique IDentifier (UUID) URN Namespace", RFC 4122,
            DOI 10.17487/RFC4122, July 2005,
            <http://www.rfc-editor.org/info/rfc4122>.

Daboo Standards Track [Page 21] RFC 7986 iCalendar Property Extensions October 2016

 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234,
            DOI 10.17487/RFC5234, January 2008,
            <http://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,
            <http://www.rfc-editor.org/info/rfc5545>.
 [RFC5546]  Daboo, C., Ed., "iCalendar Transport-Independent
            Interoperability Protocol (iTIP)", RFC 5546,
            DOI 10.17487/RFC5546, December 2009,
            <http://www.rfc-editor.org/info/rfc5546>.
 [W3C.REC-css3-color-20110607]
            Celik, T., Lilley, C., and D. Baron, "CSS Color Module
            Level 3", World Wide Web Consortium Recommendation
            REC-css3-color-20110607, June 2011,
            <https://www.w3.org/TR/2011/REC-css3-color-20110607>.

10.2. Informative References

 [RFC2397]  Masinter, L., "The "data" URL scheme", RFC 2397,
            DOI 10.17487/RFC2397, August 1998,
            <http://www.rfc-editor.org/info/rfc2397>.
 [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
            A., Peterson, J., Sparks, R., Handley, M., and E.
            Schooler, "SIP: Session Initiation Protocol", RFC 3261,
            DOI 10.17487/RFC3261, June 2002,
            <http://www.rfc-editor.org/info/rfc3261>.
 [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
            RFC 3966, DOI 10.17487/RFC3966, December 2004,
            <http://www.rfc-editor.org/info/rfc3966>.
 [RFC5122]  Saint-Andre, P., "Internationalized Resource Identifiers
            (IRIs) and Uniform Resource Identifiers (URIs) for the
            Extensible Messaging and Presence Protocol (XMPP)",
            RFC 5122, DOI 10.17487/RFC5122, February 2008,
            <http://www.rfc-editor.org/info/rfc5122>.
 [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
            Protocol (HTTP/1.1): Message Syntax and Routing",
            RFC 7230, DOI 10.17487/RFC7230, June 2014,
            <http://www.rfc-editor.org/info/rfc7230>.

Daboo Standards Track [Page 22] RFC 7986 iCalendar Property Extensions October 2016

Acknowledgments

 Thanks to the following individuals for feedback: Bernard
 Desruisseaux, Mike Douglass, Lucia Fedorova, Ken Murchison, Arnaud
 Quillaud, and Dave Thewlis.
 This specification came about via discussions at the Calendaring and
 Scheduling Consortium.

Author's Address

 Cyrus Daboo
 Apple Inc.
 1 Infinite Loop
 Cupertino, CA  95014
 United States of America
 Email: cyrus@daboo.name
 URI:   http://www.apple.com/

Daboo Standards Track [Page 23]

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