Support for iCalendar RelationshipsBedework226 3rd StreetTroyNY12180United States of Americamdouglass@bedework.comhttps://bedework.com
art
calextiCalendarlinkrelated-torelationships
This specification updates the iCalendar RELATED-TO property
defined in RFC 5545
by adding new relation types and introduces new iCalendar
properties (LINK, CONCEPT, and REFID) to allow better
linking and grouping of iCalendar components and related data.
Status of This Memo
This 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
.
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Table of Contents
. Introduction
. Structured iCalendar Relationships
. Grouped iCalendar Relationships
. Concept Relationships
. Linked Relationships
. Caching and Offline Use
. Conventions Used in This Document
. LINK Property Reference Types
. Link Relation Types
. New Temporal RELTYPE Parameter Values
. Additional New RELTYPE Parameter Values
. New Property Parameters
. Link Relation
. Gap
. New Value Data Types
. New Properties
. Concept
. Link
. Refid
. Updates to RFC 5545
. RELATED-TO
. Security Considerations
. IANA Considerations
. iCalendar Property Registrations
. iCalendar Property Parameter Registrations
. iCalendar Value Data Type Registrations
. iCalendar RELTYPE Value Registrations
. References
. Normative References
. Informative References
Acknowledgements
Author's Address
Introduction
iCalendar entities defined in often
need to be related to each other or to
associated metadata. The specifications below support
relationships of the following forms:
Structured iCalendar:
iCalendar entities can be related to each other in some structured
way, for example, as parent, sibling, before, or after.
Grouped iCalendar:
iCalendar entities can be related to each other as a group.
CATEGORIES are often used for this purpose but are problematic
for application developers due to their lack of consistency
and use as a free-form tag.
Linked:
Entities can be linked to other entities, such as vCards,
through a URI and associated REL and FMTTYPE parameters.
Structured iCalendar Relationships
The iCalendar
RELATED-TO property has no support for temporal relationships
as used by project management tools.
The RELTYPE parameter is extended to take new values
defining temporal relationships, a GAP parameter is defined to provide
lead and lag values, and RELATED-TO is extended to allow URI values.
These changes allow the RELATED-TO property to define a richer set of
relationships useful for project management.
Grouped iCalendar Relationships
This specification defines a new REFID property, which
allows arbitrary groups of entities to be associated with the same
key value.
REFID is used to identify a key allowing the association of
components that are all related to the referring, aggregating
component and the retrieval of components based on this key.
For example, this may be used to identify the tasks associated
with a given project without having to communicate the task
structure of the project. A further example is the grouping
of all sub-tasks associated with the delivery of a specific
package in a package delivery system.
As such, the presence of a REFID property imparts no meaning
to the component. It is merely a key to allow retrieval. This is
distinct from categorization, which, while allowing grouping,
also adds meaning to the component to which it is attached.
Concept Relationships
The name CONCEPT is used by the Simple Knowledge Organization
System, as defined in .
The term "concept" more accurately defines what we often
mean by a category. It's not the text string that is important
but the meaning attached to it. For example, the term
"football" can mean very different sports.
The introduction of CONCEPT allows a more structured
approach to categorization, with the possibility of namespaced
and path-like values. Unlike REFID, the CONCEPT property imparts some
meaning. It is assumed that the value of this property will
reference a well-defined category.
The current CATEGORIES property defined in is used
as a free-form 'tagging' field. These values have some meaning
to those who apply them but not necessarily to any consumer.
As such, it is difficult to establish
formal relationships between components based on their category.
Rather than attempt to add semantics to the CATEGORIES property,
it seems best to continue its usage as an informal tag and
establish a new CONCEPT property with more constraints.
Linked Relationships
The currently existing iCalendar standard lacks
a general purpose method for referencing additional, external information
relating to calendar components.
This document proposes a method for referencing typed external
information that can provide additional information about an iCalendar
component. This new LINK property is closely aligned to
, which defines the generic concept
of Web Linking, as well as its expression in the HTTP LINK header
field.
The LINK property defines a typed reference or relation to external
metadata or related resources. By providing type and format information
as parameters, clients and servers are able to discover interesting
references and make use of them, perhaps for indexing or the
presentation of interesting links for the user.
Calendar components are often grouped into collections to represent a
calendar or a series of tasks, for example, Calendaring Extensions to
WebDAV (CalDAV) calendar collections .
It is also often necessary to reference calendar components
in other collections. For example, a VEVENT might refer to
a VTODO from which it was derived. The PARENT, SIBLING, and CHILD
relationships defined for the
RELATED-TO property only allow for a unique identifier (UID), which is inadequate for many
purposes. Allowing other value types for those relationships may help
but would cause
backward-compatibility issues. The LINK property can link components
in different collections or even on different servers.
When publishing events, it is useful to be able to refer back to the
source of that information. The actual event may have been consumed from
a feed or an ics file on a website. A LINK property can provide
a reference to the originator of the event.
Beyond the need to relate elements temporally, project
management tools often need to be able to specify the relationships
between the various events and tasks that make up a project.
The LINK property provides such a mechanism.
The LINK property MUST NOT be treated as just another
attachment. The ATTACH property defined in
has been extended by
to handle server-side management and stripping of inline data and to
provide additional data about the attachment (size, filename, etc.).
Additionally, clients may choose to handle attachments differently
from the LINK property, as attachments are often an integral part
of the message, for example, the agenda.
Caching and Offline Use
In general, the calendar entity should be self explanatory
without the need to download referenced metadata, such as a
web page.
However, to facilitate offline display, the link type may identify
important pieces of data that should be downloaded in
advance.
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 when, and only when, they appear in all capitals,
as shown here.
The notation used in this memo to (re-)define iCalendar elements is
the ABNF notation of , as used by
. Any syntax
elements shown below that are not explicitly defined in this
specification come from iCalendar [RFC5545].
LINK Property Reference Types
The reference value in the LINK property defined below can take
three forms specified by the VALUE parameter:
URI:
This is a URI referring to the target.
UID:
This allows for linking within a single collection of calendar
components, and the value MUST refer to another component
within the same collection.
XML-REFERENCE:
In an XML environment, it may be necessary to refer to a
fragment of an external XML artifact. This value is a URI
with an XPointer anchor value. The XPointer is defined in
, and its
use as an anchor is defined in .
Note that UID references may need updating on import. An example
is data to be imported from a file containing VTODO and VEVENT components,
with a VTODO referring to VEVENT components by UID. When imported
into a CalDAV system, the VTODO components are typically placed in a different
collection from the VEVENT components. This would require the
UID reference to be replaced with a URI.
Link Relation Types
Two forms of relation types are defined in : registered and
extension. Registered relation types are added to the "Link
Relations" registry, as specified in .
Extension relation types, defined in ,
are specified as unique URIs that are not registered in the registry.
The relation types defined in will
be registered with IANA in
accordance with the specifications in .
New Temporal RELTYPE Parameter Values
This section defines the usual temporal relationships
for use with the RELTYPE parameter defined in
:
FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH, or STARTTOSTART.
The RELATED-TO property with one or
more of these temporal relationships will be present in the
predecessor entity and will refer to the successor entity.
The GAP parameter (see )
specifies the lead (a negative value) or lag (a positive value)
time between the predecessor and the successor.
In the description of each temporal relationship
below, we refer to Task-A, which contains and controls the relationship, and
Task-B, which is the target of the relationship. This is indicated by
the direction of the arrows in the diagrams below.
Also, each relationship may be modified by the addition of a
GAP parameter to the relationship that applies to the targeted
component.
RELTYPE=FINISHTOSTART:
Task-B cannot start until Task-A finishes.
For example, when painting is complete, carpet laying can begin.
RELTYPE=FINISHTOFINISH:
Task-B can only be completed after Task-A
is finished. The related tasks may run in parallel before
completion.
For example, in the development of two related pieces of
software (e.g., the API and the implementation), the design
of the implementation (Task-B) cannot be completed until the
design of the API (Task-A) has been completed.
RELTYPE=STARTTOFINISH:
The start of Task-A (which occurs after Task-B)
controls the finish of Task-B.
For example, ticket sales (Task-B) end after the game
starts (Task-A).
RELTYPE=STARTTOSTART:
The start of Task-A triggers the start of Task-B,
that is, Task-B can start anytime after Task-A starts.
Additional New RELTYPE Parameter Values
This section defines the additional relationships below:
RELTYPE=FIRST:
This indicates that the referenced calendar component is the
first in a series the referencing calendar component is
part of.
RELTYPE=NEXT:
This indicates that the referenced calendar component is the
next in a series the referencing calendar component is
part of.
RELTYPE=DEPENDS-ON:
This indicates that the current calendar component depends on
the referenced calendar component in some manner. For example,
a task may be blocked waiting on the other, referenced, task.
RELTYPE=REFID:
This establishes a reference from the current component to
components with a REFID property that matches the value
given in the associated RELATED-TO property.
RELTYPE=CONCEPT:
This establishes a reference from the current component to
components with a CONCEPT property that matches the value
given in the associated RELATED-TO property.
Note that the relationship types of PARENT, CHILD, and SIBLING
establish a hierarchical relationship. The new types of FIRST
and NEXT are an ordering relationship.
New Property ParametersLink Relation
Parameter name:
LINKREL
Purpose:
This property specifies the relationship of data referenced by a LINK property.
Format Definition:
This parameter is defined by the following notation:
linkrelparam = "LINKREL" "="
(DQUOTE uri DQUOTE
/ iana-token) ; Other IANA registered type
Description:
This parameter MUST be specified on all LINK properties and
define the type of reference. This allows programs consuming this
data to automatically scan for references they support. There is no default
relation type.Any link relation
in the link registry established by ,
or new link relations, may be used.
It is expected that link relation types seeing significant
usage in calendaring will have the calendaring usage
described in an RFC.
LINKREL=latest-version:
This identifies the latest version of the event information.
Registration:
These relation types are registered in .
Gap
Parameter name:
GAP
Purpose:
This property specifies the length of the gap, positive or negative,
between two components with a temporal relationship.
Format Definition:
This parameter is defined by the following notation, where
dur-value is defined in . :
gapparam = "GAP" "=" dur-value
Description:
This parameter MAY be specified on the RELATED-TO property and
defines the duration of time between the predecessor and successor
in an interval. When positive, it defines the lag time between
a task and its logical successor. When negative, it defines
the lead time.
An example of lag time might be if Task-A is "paint the room"
and Task-B is
"lay the carpets". Then, Task-A may be related to Task-B with
RELTYPE=FINISHTOSTART with a gap of 1 day -- long enough for the paint to
dry.
For an example of lead time, in constructing a two-story
building, the electrical work must be done before painting.
However, the painter can move in to the first floor as the
electricians move upstairs.
New Value Data Types
This specification defines the following new value types to
be used with the VALUE property parameter:
UID:
VALUE=UID indicates that the associated value is the UID for a
component.
XML-REFERENCE:
VALUE=XML-REFERENCE indicates that the associated value
references an associated XML artifact and is a URI
with an XPointer anchor value. The XPointer is defined in
, and its
use as an anchor is defined in .
New PropertiesConcept
Property name:
CONCEPT
Purpose:
This property defines the formal categories for a calendar
component.
Value type:
URI
Property Parameters:
IANA and non-standard parameters can be specified on this property.
Conformance:
This property can be specified zero or more times in any
iCalendar component.
Description:
This property is used to specify formal categories or
classifications of the calendar component. The values
are useful in searching for a calendar component of a
particular type and category.This categorization is distinct from the more informal
"tagging" of components provided by the existing CATEGORIES
property. It is expected that the value of the CONCEPT
property will reference an external resource that
provides information about the categorization.In addition, a structured URI value allows for
hierarchical categorization of events.Possible category resources are the various proprietary
systems, for example, the Library of Congress, or an open
source of categorization data.
Format Definition:
This property is defined by the following notation:
concept = "CONCEPT" conceptparam ":"
uri CRLF
conceptparam = *(";" other-param)
Example:
The following is an example of this property. It points to a
server acting as the source for the calendar object.
CONCEPT:https://example.com/event-types/arts/music
Link
Property name:
LINK
Purpose:
This property provides a reference to external information
related to a component.
Value type:
URI, UID, or XML-REFERENCE
Property Parameters:
The VALUE parameter is required. Non-standard, link relation type,
format type, label, and language parameters can also be
specified on this property. The LABEL parameter
is defined in .
Conformance:
This property can be specified zero or more times in any
iCalendar component.
Description:
When used in a component, the value of this property points to
additional information related to the component. For example,
it may reference the originating web server.
Format Definition:
This property is defined by the following notation:
link = "LINK" linkparam ":"
( uri / ; for VALUE=XML-REFERENCE
uri / ; for VALUE=URI
text ) ; for VALUE=UID
CRLF
linkparam = (";" "VALUE" "=" ("XML-REFERENCE" /
"URI" /
"UID"))
1*(";" linkrelparam)
1*(";" fmttypeparam)
1*(";" labelparam)
1*(";" languageparam)
*(";" other-param)
; the elements herein may appear in any order,
; and the order is not significant.
This property is a serialization of the model in
, where the link target is
carried in the property value, the link context is the
containing calendar entity, and the link relation type
and any target attributes are carried in iCalendar property
parameters.
The LINK property parameters
map to attributes as follows:
LABEL:
This parameter maps to the "title" attribute defined in .
LANGUAGE:
This parameter maps to the "hreflang" attribute defined in .
LINKREL:
This parameter maps to the link relation type defined in .
FMTTYPE:
This parameter maps to the "type" attribute defined in .
There is no mapping for "title*",
"anchor", "rev", or "media" .
Example:
The following is an example of this property, which
provides a reference to the source for the calendar object.
LINK;LINKREL=SOURCE;LABEL=Venue;VALUE=URI:
https://example.com/events
Example:
The following is an example of this property, which
provides a reference to an entity from which this one
was derived. The link relation is a vendor-defined value.
LINK;LINKREL="https://example.com/linkrel/derivedFrom";
VALUE=URI:
https://example.com/tasks/01234567-abcd1234.ics
Example:
The following is an example of this property, which
provides a reference to a fragment of an XML document.
The link relation is a vendor-defined value.
LINK;LINKREL="https://example.com/linkrel/costStructure";
VALUE=XML-REFERENCE:
https://example.com/xmlDocs/bidFramework.xml
#xpointer(descendant::CostStruc/range-to(
following::CostStrucEND[1]))
Refid
Property name:
REFID
Purpose:
This property value acts as a key for associated
iCalendar entities.
Value type:
TEXT
Property Parameters:
Non-standard parameters can be specified on this property.
Conformance:
This property can be specified zero or more times in any
iCalendar component.
Description:
The value of this property is free-form text that
creates an identifier for associated components. All components
that use the same REFID value are associated through that value
and can be located or retrieved as a group. For example, all of
the events in a travel itinerary would have the same REFID value,
so as to be grouped together.
Format Definition:
This property is defined by the following notation:
refid = "REFID" refidparam ":" text CRLF
refidparam = *(";" other-param)
Example:
The following is an example of this property.
REFID:itinerary-2014-11-17
Updates to RFC 5545
This specification updates the RELATED-TO property defined
in . The contents
of replace that section.
The RELTYPE parameter is extended to take new values defining
temporal relationships, a GAP parameter is defined to provide lead
and lag values, and RELATED-TO is extended to allow URI values.
These changes allow the RELATED-TO property to define a richer set of
relationships useful for project management.
RELATED-TO
Property name:
RELATED-TO
Purpose:
This property is used to represent a relationship or
reference between one calendar component and another. The definition
here extends the definition in by allowing URI or UID values and a GAP parameter.
Value Type:
URI, UID, or TEXT
Property Parameters:
Relationship type, IANA, and non-standard
property parameters can be specified on this property.
Conformance:
This property MAY be specified in any iCalendar component.
Description:
By default or when VALUE=UID is specified, the property value
consists of the persistent, globally
unique identifier of another calendar component. This value would
be represented in a calendar component by the UID property.By default, the property value points to another calendar
component that has a PARENT relationship to the referencing
object. The RELTYPE property parameter is used to either
explicitly state the default PARENT relationship type to the
referenced calendar component or to override the default PARENT
relationship type and specify either a CHILD or SIBLING
relationship or a temporal relationship.The PARENT relationship indicates that the calendar
component is a subordinate of the referenced calendar component.
The CHILD relationship indicates that the calendar component is a
superior of the referenced calendar component. The SIBLING
relationship indicates that the calendar component is a peer of
the referenced calendar component.To preserve backwards compatibility, the value type MUST
be UID when the PARENT, SIBLING, or CHILD
relationships are specified.The FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH, or STARTTOSTART
relationships define temporal relationships, as specified in the
RELTYPE parameter definition.The FIRST and NEXT define ordering relationships between
calendar components.The DEPENDS-ON relationship indicates that the current
calendar component depends on
the referenced calendar component in some manner. For example,
a task may be blocked waiting on the other, referenced, task.The REFID and CONCEPT relationships establish a reference
from the current component to the referenced component.Changes to a calendar component referenced by this property can
have an implicit impact on the related calendar component. For
example, if a group event changes its start or end date or time,
then the related, dependent events will need to have their start
and end dates and times changed in a corresponding way. Similarly, if a
PARENT calendar component is canceled or deleted, then there is
an implied impact to the related CHILD calendar components. This
property is intended only to provide information on the
relationship of calendar components.Deletion of the target component, for example, the target
of a FIRST, NEXT, or temporal relationship, can result in broken
links.It is up to the target calendar system to maintain any
property implications of these relationships.
Format Definition:
This property is defined by the following notation:
related = "RELATED-TO" relparam ":"
( text / ; for VALUE=UID
uri / ; for VALUE=URI
text ) ; for VALUE=TEXT or default
CRLF
relparam = ; the elements herein may appear in any order,
; and the order is not significant.
[";" "VALUE" "=" ("UID" /
"URI" /
"TEXT")]
[";" reltypeparam]
[";" gapparam]
*(";" other-param)
Example:
The following are examples of this property.
RELATED-TO:jsmith.part7.19960817T083000.xyzMail@example.com
RELATED-TO:19960401-080045-4000F192713-0052@example.com
RELATED-TO;VALUE=URI;RELTYPE=STARTTOFINISH:
https://example.com/caldav/user/jb/cal/
19960401-080045-4000F192713.ics
Security Considerations
All of the security considerations of apply to this specification.
Applications using the LINK property need to be aware of the risks
entailed in using the URIs provided as values. See for
a discussion of the security considerations relating to URIs.
In particular, note Section Reliability and Consistency of ,
which points out the lack of a stability
guarantee for referenced resources.
When the value is an XML-REFERENCE type, the targeted data is an XML document
or portion thereof. Consumers need to be aware of the security issues
related to XML processing -- in particular, those related to XML entities.
See . Additionally, note that
the reference may be invalid or become so over time.
The CONCEPT and redefined RELATED-TO properties have
the same issues in that values may be URIs.
Extremely large values for the GAP parameter may lead to
unexpected behavior.
IANA ConsiderationsiCalendar Property Registrations
The following iCalendar property names have been added to
the iCalendar "Properties" registry defined in . IANA has also added a
reference to this document, where the properties originally defined in
have been updated by this document.
Property
Status
Reference
CONCEPT
Current
LINK
Current
REFID
Current
RELATED-TO
Current
; RFC 9253,
iCalendar Property Parameter Registrations
The following iCalendar property parameter names have been added to
the iCalendar "Parameters" registry defined in .
Parameter
Status
Reference
GAP
Current
LINKREL
Current
iCalendar Value Data Type Registrations
The following iCalendar property parameter names have been added to
the iCalendar "Value Data Types" registry defined in .
Value Data Type
Status
Reference
XML-REFERENCE
Current
UID
Current
iCalendar RELTYPE Value Registrations
The following iCalendar "RELTYPE" values have been added to
the iCalendar "Relationship Types" registry defined in .
Relationship Type
Status
Reference
CONCEPT
Current
DEPENDS-ON
Current
FINISHTOFINISH
Current
FINISHTOSTART
Current
FIRST
Current
NEXT
Current
REFID
Current
STARTTOFINISH
Current
STARTTOSTART
Current
ReferencesNormative ReferencesKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Uniform Resource Identifier (URI): Generic SyntaxA Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK]HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)Web Distributed Authoring and Versioning (WebDAV) consists of a set of methods, headers, and content-types ancillary to HTTP/1.1 for the management of resource properties, creation and management of resource collections, URL namespace manipulation, and resource locking (collision avoidance).RFC 2518 was published in February 1999, and this specification obsoletes RFC 2518 with minor revisions mostly due to interoperability experience. [STANDARDS-TRACK]Augmented BNF for Syntax Specifications: ABNFInternet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK]Internet Calendaring and Scheduling Core Object Specification (iCalendar)This document defines the iCalendar data format for representing and exchanging calendaring and scheduling information such as events, to-dos, journal entries, and free/busy information, independent of any particular calendar service or protocol. [STANDARDS-TRACK]New Properties for iCalendarThis document defines a set of new properties for iCalendar data and extends the use of some existing properties to the entire iCalendar object.Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.Web LinkingThis specification defines a model for the relationships between resources on the Web ("links") and the type of those relationships ("link relation types").It also defines the serialisation of such links in HTTP headers with the Link header field.SKOS Simple Knowledge Organization System ReferenceXPointer FrameworkXPointer xpointer() SchemeInformative ReferencesCalendaring Extensions to WebDAV (CalDAV)This document defines extensions to the Web Distributed Authoring and Versioning (WebDAV) protocol to specify a standard way of accessing, managing, and sharing calendaring and scheduling information based on the iCalendar format. This document defines the "calendar-access" feature of CalDAV. [STANDARDS-TRACK]Calendaring Extensions to WebDAV (CalDAV): Managed AttachmentsThis specification adds an extension to the Calendaring Extensions to WebDAV (CalDAV) to allow attachments associated with iCalendar data to be stored and managed on the server.This specification documents existing code deployed by multiple vendors. It is published as an Informational specification rather than Standards Track due to its noncompliance with multiple best current practices of HTTP.Acknowledgements
The author would like to thank the members of CalConnect,
the Calendaring and
Scheduling Consortium technical committees, and the following
individuals for contributing their ideas, support, and comments:
, ,
, and
The author would also like to thank CalConnect and the Calendaring and
Scheduling Consortium for advice with this specification.
Author's AddressBedework226 3rd StreetTroyNY12180United States of Americamdouglass@bedework.comhttps://bedework.com