< draft-ietf-calext-ical-relations-09.txt   draft-ietf-calext-ical-relations-10.txt >
Network Working Group M. Douglass Network Working Group M. Douglass
Internet-Draft Bedework Internet-Draft Bedework
Updates: 5545 (if approved) 23 January 2022 Updates: 5545 (if approved) 27 February 2022
Intended status: Standards Track Intended status: Standards Track
Expires: 27 July 2022 Expires: 31 August 2022
Support for iCalendar Relationships Support for iCalendar Relationships
draft-ietf-calext-ical-relations-09 draft-ietf-calext-ical-relations-10
Abstract Abstract
This specification updates the iCalendar RELATED-TO property by This specification updates the iCalendar RELATED-TO property defined
adding new relation types and introduces new iCalendar properties in RFC5545 by adding new relation types and introduces new iCalendar
LINK, CONCEPT and REFID to allow better linking and grouping of properties LINK, CONCEPT and REFID to allow better linking and
iCalendar components and related data. grouping of iCalendar components and related data.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on 27 July 2022. This Internet-Draft will expire on 31 August 2022.
Copyright Notice Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/ Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
skipping to change at page 2, line 14 skipping to change at page 2, line 14
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Structured iCalendar relationships . . . . . . . . . . . 3 1.1. Structured iCalendar relationships . . . . . . . . . . . 3
1.2. Grouped iCalendar relationships . . . . . . . . . . . . . 3 1.2. Grouped iCalendar relationships . . . . . . . . . . . . . 3
1.3. Concept relationships . . . . . . . . . . . . . . . . . . 3 1.3. Concept relationships . . . . . . . . . . . . . . . . . . 3
1.4. Linked relationships . . . . . . . . . . . . . . . . . . 4 1.4. Linked relationships . . . . . . . . . . . . . . . . . . 4
1.5. Caching and offline use . . . . . . . . . . . . . . . . . 5 1.5. Caching and offline use . . . . . . . . . . . . . . . . . 5
1.6. Conventions Used in This Document . . . . . . . . . . . . 5 1.6. Conventions Used in This Document . . . . . . . . . . . . 5
2. Reference Types . . . . . . . . . . . . . . . . . . . . . . . 5 2. LINK Property Reference Types . . . . . . . . . . . . . . . . 5
3. Link Relation Types . . . . . . . . . . . . . . . . . . . . . 6 3. Link Relation Types . . . . . . . . . . . . . . . . . . . . . 6
4. New temporal RELTYPE Parameter values . . . . . . . . . . . . 6 4. New temporal RELTYPE Parameter values . . . . . . . . . . . . 6
5. Additional New RELTYPE Parameter Values . . . . . . . . . . . 7 5. Additional New RELTYPE Parameter Values . . . . . . . . . . . 8
6. New Property Parameters . . . . . . . . . . . . . . . . . . . 8 6. New Property Parameters . . . . . . . . . . . . . . . . . . . 8
6.1. Link Relation . . . . . . . . . . . . . . . . . . . . . . 8 6.1. Link Relation . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Gap . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6.2. Gap . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. New Value Data Types . . . . . . . . . . . . . . . . . . . . 9 7. New Value Data Types . . . . . . . . . . . . . . . . . . . . 10
8. New Properties . . . . . . . . . . . . . . . . . . . . . . . 9 8. New Properties . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Concept . . . . . . . . . . . . . . . . . . . . . . . . . 9 8.1. Concept . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.2. Link . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8.2. Link . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.3. Refid . . . . . . . . . . . . . . . . . . . . . . . . . . 12 8.3. Refid . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9. Updates to RFC 5545 . . . . . . . . . . . . . . . . . . . . . 13 9. Updates to RFC 5545 . . . . . . . . . . . . . . . . . . . . . 14
9.1. RELATED-TO . . . . . . . . . . . . . . . . . . . . . . . 13 9.1. RELATED-TO . . . . . . . . . . . . . . . . . . . . . . . 14
10. Security Considerations . . . . . . . . . . . . . . . . . . . 15 10. Security Considerations . . . . . . . . . . . . . . . . . . . 17
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
11.1. iCalendar Property Registrations . . . . . . . . . . . . 16 11.1. iCalendar Property Registrations . . . . . . . . . . . . 17
11.2. iCalendar Property Parameter Registrations . . . . . . . 16 11.2. iCalendar Property Parameter Registrations . . . . . . . 18
11.3. iCalendar Value Data Type Registrations . . . . . . . . 16 11.3. iCalendar Value Data Type Registrations . . . . . . . . 18
11.4. iCalendar RELTYPE Value Registrations . . . . . . . . . 17 11.4. iCalendar RELTYPE Value Registrations . . . . . . . . . 18
11.5. New Reference Type Registration . . . . . . . . . . . . 17 11.5. New Reference Type Registration . . . . . . . . . . . . 19
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.1. Informative References . . . . . . . . . . . . . . . . . 18 13.1. Informative References . . . . . . . . . . . . . . . . . 20
13.2. Normative References . . . . . . . . . . . . . . . . . . 18 13.2. Normative References . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 19 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction 1. Introduction
iCalendar entities defined in [RFC5545] often need to be related to iCalendar entities defined in [RFC5545] often need to be related to
each other or to associated meta-data. The specifications below each other or to associated meta-data. The specifications below
support relationships of the following forms: support relationships of the following forms:
Structured iCalendar: iCalendar entities can be related to each Structured iCalendar: iCalendar entities can be related to each
other in some structured way, for example as parent, sibling, other in some structured way, for example as parent, sibling,
before, after. before, after.
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as a group. CATEGORIES are often used for this purpose but are as a group. CATEGORIES are often used for this purpose but are
problematic for application developers due to their lack of problematic for application developers due to their lack of
consistency and use as a free-form tag. consistency and use as a free-form tag.
Linked: Entities can be linked to other entities such as vcards Linked: Entities can be linked to other entities such as vcards
through a URI and associated REL and FMTTYPE parameters. through a URI and associated REL and FMTTYPE parameters.
1.1. Structured iCalendar relationships 1.1. Structured iCalendar relationships
The iCalendar [RFC5545] RELATED-TO property has no support for The iCalendar [RFC5545] RELATED-TO property has no support for
temporal relationships as used by standard project management tools. temporal relationships as used by project management tools.
The RELTYPE parameter is extended to take new values defining The RELTYPE parameter is extended to take new values defining
temporal relationships, a GAP parameter is defined to provide lead temporal relationships, a GAP parameter is defined to provide lead
and lag values, and RELATED-TO is extended to allow URI values. 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 These changes allow the RELATED-TO property to define a richer set of
relationships useful for project management. relationships useful for project management.
1.2. Grouped iCalendar relationships 1.2. Grouped iCalendar relationships
This specification defines a new REFID property which allows This specification defines a new REFID property which allows
arbitrary groups of entities to be associated with the same key arbitrary groups of entities to be associated with the same key
value. value.
REFID is used to identify a key allowing the association of REFID is used to identify a key allowing the association of
components that are related to the same object and retrieval of a components that are all related to the referring, aggregating
component based on this key. Two examples of how this may be used component and the retrieval of components based on this key. For
are to identify the tasks associated with a given project without example, this may be used to identify the tasks associated with a
having to communicate the task structure of the project, and to group given project without having to communicate the task structure of the
all tasks associated to a specific package in a package delivery project. A further example is the grouping of all sub-tasks
system. 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 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 component. It is merely a key to allow retrieval. This is distinct
from categorisation which, while allowing grouping also adds meaning from categorisation which, while allowing grouping also adds meaning
to the component to which it is attached. to the component to which it is attached.
1.3. Concept relationships 1.3. Concept relationships
The name CONCEPT is used by the Simple Knowledge Organization System The name CONCEPT is used by the Simple Knowledge Organization System
defined in [W3C.REC-skos-reference-20090818]. The term "concept" defined in [W3C.REC-skos-reference-20090818]. The term "concept"
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the text string that is important but the meaning attached to it. the text string that is important but the meaning attached to it.
For example, the term "football" can mean very different sports. For example, the term "football" can mean very different sports.
The introduction of CONCEPT allows a more structured approach to The introduction of CONCEPT allows a more structured approach to
categorization, with the possibility of namespaced and path-like categorization, with the possibility of namespaced and path-like
values. Unlike REFID the CONCEPT property imparts some meaning. It values. Unlike REFID the CONCEPT property imparts some meaning. It
is assumed that the value of this property will reference a well is assumed that the value of this property will reference a well
defined category. defined category.
The current [RFC5545] CATEGORY property is used as a free form The current [RFC5545] CATEGORY property is used as a free form
'tagging' field. As such it is difficult to establish formal 'tagging' field. These values have some meaning to those who apply
relationships between components based on their category. 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 CATEGORY property it Rather than attempt to add semantics to the CATEGORY property it
seems best to continue its usage as an informal tag and establish a seems best to continue its usage as an informal tag and establish a
new CONCEPT property with more constraints. new CONCEPT property with more constraints.
1.4. Linked relationships 1.4. Linked relationships
The currently existing iCalendar standard [RFC5545] lacks a general The currently existing iCalendar standard [RFC5545] lacks a general
purpose method for referencing additional, external information purpose method for referencing additional, external information
relating to calendar components. relating to calendar components.
This document proposes a method for referencing typed external This document proposes a method for referencing typed external
information that can provide additional information about an information that can provide additional information about an
iCalendar component. This new LINK property is closely aligned to iCalendar component. This new LINK property is closely aligned to
the LINK header defined in [RFC8288] [RFC8288] 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 The LINK property defines a typed reference or relation to external
meta-data or related resources. By providing type and format meta-data or related resources. By providing type and format
information as parameters, clients and servers are able to discover information as parameters, clients and servers are able to discover
interesting references and make use of them, perhaps for indexing or interesting references and make use of them, perhaps for indexing or
the presentation of interesting links for the user. 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 [RFC4791]' (CalDAV)
calendar collections.
It is also often necessary to reference calendar components in other It is also often necessary to reference calendar components in other
collections. For example, a VEVENT might refer to a VTODO from which collections. For example, a VEVENT might refer to a VTODO from which
it was derived. The PARENT, SIBLING and CHILD relationships defined it was derived. The PARENT, SIBLING and CHILD relationships defined
for the RELATED-TO property only allow for a UID which is inadequate for the RELATED-TO property only allow for a UID which is inadequate
for many purposes. Allowing other value types for those for many purposes. Allowing other value types for those
relationships may help but would cause backward compatibility issues. relationships may help but would cause backward compatibility issues.
The LINK property can link components in different collections or The LINK property can link components in different collections or
even on different servers. even on different servers.
When publishing events it is useful to be able to refer back to the 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 source of that information. The actual event may have been consumed
from a feed or an ics file on a web site. A LINK property can from a feed or an ics file on a web site. A LINK property can
provide a reference to the originator of the event. provide a reference to the originator of the event.
Beyond the need to relate elements temporally, project management Beyond the need to relate elements temporally, project management
tools often need to be able to specify the relationships between the tools often need to be able to specify the relationships between the
various events and tasks which make up a project. The LINK property various events and tasks which make up a project. The LINK property
provides such a mechanism. provides such a mechanism.
The LINK property MUST NOT be treated as just another attachment. The LINK property MUST NOT be treated as just another attachment.
The ATTACH property defined in [RFC5545] is being extended to handle The ATTACH property defined in [RFC5545] has been extended by
server-side management and stripping of inline data. Clients may [RFC8607] to handle server-side management and stripping of inline
choose to handle attachments differently from the LINK property as data and to provide additional data about the attachment (size,
they are often an integral part of the message - for example, the filename etc).
agenda.
For more information on managed attachments see [RFC8607] 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.
1.5. Caching and offline use 1.5. Caching and offline use
To facilitate offline display the link type may identify important
pieces of data which should be downloaded in advance.
In general, the calendar entity should be self explanatory without In general, the calendar entity should be self explanatory without
the need to download referenced meta-data such as a web page. the need to download referenced meta-data such as a web page.
However, to facilitate offline display the link type may identify
important pieces of data which should be downloaded in advance.
1.6. Conventions Used in This Document 1.6. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY" and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY" and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
2. Reference Types The notation used in this memo to (re-)define iCalendar elements is
the ABNF notation of [RFC5234] as used by [RFC5545]. Any syntax
elements shown below that are not explicitly defined in this
specification come from iCalendar [RFC5545].
The actual reference value can take three forms specified by the type 2. LINK Property Reference Types
parameter
URI: The default type. This is a URI referring to the target. The reference value in the LINK property defined below can take three
forms specified by the VALUE parameter:
UID: This allows for linking within a single collection and the URI: This is a URI referring to the target.
value MUST be another component within that collection.
REFERENCE: In an XML environment it may be necessary to refer to a UID: This allows for linking within a single collection of calendar
fragment of an external XML artifact. This value is a URI with an components and the value MUST refer to another component within
XPointer anchor value. The XPointer is defined in the same collection.
[W3C.WD-xptr-xpointer-20021219] and it's use as an anchor is
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
[W3C.WD-xptr-xpointer-20021219] and its use as an anchor is
defined in [W3C.REC-xptr-framework-20030325] defined in [W3C.REC-xptr-framework-20030325]
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.
3. Link Relation Types 3. Link Relation Types
[RFC8288] defines two forms of relation type: registered and [RFC8288] defines two forms of relation type: registered and
extension. Registered relation types are added to the Link Relations extension. Registered relation types are added to the Link Relations
registry as specified in Section 2.1.1 of [RFC8288]. Extension registry as specified in Section 2.1.1 of [RFC8288]. Extension
relation types, defined in Section 2.1.2 of [RFC8288], are specified relation types, defined in Section 2.1.2 of [RFC8288], are specified
as unique URIs that are not registered in the registry. as unique URIs that are not registered in the registry.
The relation types defined in Section 6.1 will be registered with The relation types defined in Section 6.1 will be registered with
IANA in accordance with the specifications in [RFC8288]. IANA in accordance with the specifications in [RFC8288].
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4. New temporal RELTYPE Parameter values 4. New temporal RELTYPE Parameter values
This section defines the usual temporal relationships for use with This section defines the usual temporal relationships for use with
the RELTYPE parameter defined in Section 3.2.15 of [RFC5545]: the RELTYPE parameter defined in Section 3.2.15 of [RFC5545]:
FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH or STARTTOSTART. FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH or STARTTOSTART.
The [RFC5545] RELATED-TO property with one or more of these temporal The [RFC5545] RELATED-TO property with one or more of these temporal
relationships will be present in the predecessor entity and will relationships will be present in the predecessor entity and will
refer to the successor entity. refer to the successor entity.
The GAP parameter (see Section 6.2) specifies the lead or lag time The GAP parameter (see Section 6.2) specifies the lead (a negative
between the predecessor and the successor. In the description of value) or lag (a positive value) time between the predecessor and the
each temporal relationship below we refer to Task-A, which contains successor.
and controls the relationship, and Task-B the target of the
relationship. In the description of each temporal relationship below we refer to
Task-A, which contains and controls the relationship, and Task-B the
target of the relationship. This is indicated by the direction of
the arrow in the diagrams below.
Also each relationship may be modified by the addition of a GAP
parameter to the relationship which applies to the targeted
component.
RELTYPE=FINISHTOSTART: Task-B cannot start until Task-A finishes. RELTYPE=FINISHTOSTART: Task-B cannot start until Task-A finishes.
For example, when sanding is complete, painting can begin. For example, when painting is complete, carpet-laying can begin.
============ ============
| Task-A |--+ | Task-A |
============ | ============
| |
V V
============ ============
| Task-B | | Task-B |
============ ============
Figure 1: Finish to start relationship Figure 1: Finish to start relationship
RELTYPE=FINISHTOFINISH: Task-B can only be completed after Task-A is RELTYPE=FINISHTOFINISH: Task-B can only be completed after Task-A is
finished. The related tasks may run in parallel before finished. The related tasks may run in parallel before
completion. completion.
For example, if the goal is to prepare a meal, we start the For example, in the development of two related pieces of software,
potatoes, then the meat then the peas but they should all be e.g. the api and the implementation, the design of the
cooked at the same time. implementation (B) cannot be completed until the design of the api
(A) has been completed.
================== ==================
| Task-A |--+ | Task-A |--+
================== | ================== |
| |
============ | ============ |
| Task-B |<-+ | Task-B |<-+
============ ============
Figure 2: Finish to finish relationship Figure 2: Finish to finish relationship
RELTYPE=STARTTOFINISH: The start of Task-A (which occurs after Task- RELTYPE=STARTTOFINISH: The start of Task-A (which occurs after Task-
B) controls the finish of Task-B. For example, ticket sales B) controls the finish of Task-B. For example, ticket sales
(Task-B) end when the game starts (Task-A). (Task-B) end after the game starts (Task-A).
============ ============
+--| Task-A | +--| Task-A |
| ============ | ============
| +---------+
============ | ============ |
| Task-B |<-+ | Task-B |<-+
============ ============
Figure 3: Start to finish relationship Figure 3: Start to finish relationship
RELTYPE=STARTTOSTART: The start of Task-A triggers the start of RELTYPE=STARTTOSTART: The start of Task-A triggers the start of
Task-B, that is Task-B can start anytime after Task-A starts. Task-B, that is Task-B can start anytime after Task-A starts.
============ ============
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+->| Task-B | +->| Task-B |
============ ============
Figure 4: Start to start relationship Figure 4: Start to start relationship
5. Additional New RELTYPE Parameter Values 5. Additional New RELTYPE Parameter Values
This section defines the additional relationships below: This section defines the additional relationships below:
RELTYPE=FIRST: Indicates that the referenced calendar component is RELTYPE=FIRST: Indicates that the referenced calendar component is
the first in a series the referenced calendar component is part the first in a series the referencing calendar component is part
of.
RELTYPE=NEXT: Indicates that the referenced calendar component is
the next in a series the referencing calendar component is part
of. of.
RELTYPE=DEPENDS-ON: Indicates that the current calendar component RELTYPE=DEPENDS-ON: Indicates that the current calendar component
depends on the referenced calendar component in some manner. For depends on the referenced calendar component in some manner. For
example a task may be blocked waiting on the other, referenced, example a task may be blocked waiting on the other, referenced,
task. task.
RELTYPE=REFID: Establishes a reference from the current component to RELTYPE=REFID: Establishes a reference from the current component to
components with a REFID property which matches the value given in components with a REFID property which matches the value given in
the associated RELATED-TO property. the associated RELATED-TO property.
RELTYPE=CONCEPT: Establishes a reference from the current component RELTYPE=CONCEPT: Establishes a reference from the current component
to components with a CONCEPT property which matches the value to components with a CONCEPT property which matches the value
given in the associated RELATED-TO property. given in the associated RELATED-TO property.
6. New Property Parameters 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.
6. New Property Parameters
6.1. Link Relation 6.1. Link Relation
Parameter name: LINKREL Parameter name: LINKREL
Purpose: To specify the relationship of data referenced by a LINK Purpose: To specify the relationship of data referenced by a LINK
property. property.
Format Definition: This parameter is defined by the following Format Definition: This parameter is defined by the following
notation: notation:
linkrelparam = "LINKREL" "=" linkrelparam = "LINKREL" "="
("SOURCE" ; Link to source of this component ("SOURCE" ; Link to source of this component
/ DQUOTE uri DQUOTE / DQUOTE uri DQUOTE
/ iana-token) ; Other IANA registered type / iana-token) ; Other IANA registered type
Description: This parameter MUST be specified on all LINK Description: This parameter MUST be specified on all LINK
properties, and defines the type of reference. This allows properties, and defines the type of reference. This allows
programs consuming this data to automatically scan for references programs consuming this data to automatically scan for references
they support. There is no default relation type. they support. There is no default relation type.
In addition to the values defined here any value defined in In addition to the value defined here any link relation in the
[RFC8288] may be used. However these uses SHOULD be documented in link registry established by [RFC8288], or new link relations, may
an RFC updating both [RFC5545] and [RFC8288] be used.
LINKREL=SOURCE: identifies the source of the event information. LINKREL=SOURCE: identifies the source of the event information.
Registration: These relation types are registered in [RFC8288] Registration: These relation types are registered in [RFC8288]
6.2. Gap 6.2. Gap
Parameter name: GAP Parameter name: GAP
Purpose: To specify the length of the gap, positive or negative, Purpose: To specify the length of the gap, positive or negative,
between two temporaly related components. between two components with a temporal relationship.
Format Definition: This parameter is defined by the following Format Definition: This parameter is defined by the following
notation: notation where dur-value is defined in section 3.3.6 of [RFC5545].
:
gapparam = "GAP" "=" dur-value gapparam = "GAP" "=" dur-value
Description: This parameter MAY be specified on the RELATED-TO Description: This parameter MAY be specified on the RELATED-TO
property, and defines the duration of time between the predecessor property, and defines the duration of time between the predecessor
and successor in an interval. When positive it defines the lag and successor in an interval. When positive it defines the lag
time between a task and its logical successor. When negative it time between a task and its logical successor. When negative it
defines the lead time. defines the lead time.
An example of lag time might be if task A is "paint the room" and An example of lag time might be if task A is "paint the room" and
task B is "hang the drapes" then task A may be related to task B task B is "lay the carpets" then task A may be related to task B
with RELTYPE=FINISHTOSTART with a gap long enough for the paint to with RELTYPE=FINISHTOSTART with a gap of 1 day - long enough for
dry. the paint to dry.
An example of lead time might be to relate a 1 week task A to the ====================
end of task B with RELTYPE=STARTTOFINISH and a negative gap of 1 | Paint the room |--+
week so they finish at the same time. ==================== |
|(lag of one day)
|
| ===============
+->| lay carpet |
===============
Figure 5: Finish to start relationship with lag
For an example of lead time, in constructing a two storey building
the electrical work must be done before painting. However the
painter can move in to the first floor as the electricians move
upstairs.
=====================
| Electrical work |--+
===================== |
+-------------+
|(lead of estimated time)
| ==================
+->| Painting |
==================
Figure 6: Finish to start relationship with lead
7. New Value Data Types 7. New Value Data Types
This specification defines the following new value types to be used This specification defines the following new value types to be used
with the VALUE property parameter: with the VALUE property parameter:
UID VALUE=UID indicates that the associated value is the UID for a UID VALUE=UID indicates that the associated value is the UID for a
component. component.
REFERENCE VALUE=REFERENCE indicates that the associated value XML-REFERENCE VALUE=XML-REFERENCE indicates that the associated
references an associated XML artifact and is a URI with an value references an associated XML artifact and is a URI with an
XPointer anchor value. The XPointer is defined in XPointer anchor value. The XPointer is defined in
[W3C.WD-xptr-xpointer-20021219] and it's use as an anchor is [W3C.WD-xptr-xpointer-20021219] and its use as an anchor is
defined in [W3C.REC-xptr-framework-20030325]. defined in [W3C.REC-xptr-framework-20030325].
8. New Properties 8. New Properties
8.1. Concept 8.1. Concept
Property name: CONCEPT Property name: CONCEPT
Purpose: This property defines the formal categories for a calendar Purpose: This property defines the formal categories for a calendar
component. component.
skipping to change at page 10, line 13 skipping to change at page 11, line 27
specified on this property. specified on this property.
Conformance: This property can be specified zero or more times in Conformance: This property can be specified zero or more times in
any iCalendar component. any iCalendar component.
Description: This property is used to specify formal categories or Description: This property is used to specify formal categories or
classifications of the calendar component. The values are useful classifications of the calendar component. The values are useful
in searching for a calendar component of a particular type and in searching for a calendar component of a particular type and
category. category.
Within the "VEVENT", "VTODO", or "VJOURNAL" calendar components,
more than one formal category can be specified by using multiple
CONCEPT properties.
This categorization is distinct from the more informal "tagging" This categorization is distinct from the more informal "tagging"
of components provided by the existing CATEGORIES property. It is of components provided by the existing CATEGORIES property. It is
expected that the value of the CONCEPT property will reference an expected that the value of the CONCEPT property will reference an
external resource which provides information about the external resource which provides information about the
categorization. categorization.
In addition, a structured URI value allows for hierarchical In addition, a structured URI value allows for hierarchical
categorization of events. categorization of events.
Possible category resources are the various proprietary systems, Possible category resources are the various proprietary systems,
skipping to change at page 10, line 41 skipping to change at page 11, line 51
notation: notation:
concept = "CONCEPT" conceptparam ":" concept = "CONCEPT" conceptparam ":"
uri CRLF uri CRLF
conceptparam = *(";" other-param) conceptparam = *(";" other-param)
Example: The following is an example of this property. It points to Example: The following is an example of this property. It points to
a server acting as the source for the calendar object. a server acting as the source for the calendar object.
CONCEPT:http://example.com/event-types/arts/music CONCEPT:https://example.com/event-types/arts/music
8.2. Link 8.2. Link
Property name: LINK Property name: LINK
Purpose: This property provides a reference to external information Purpose: This property provides a reference to external information
related to a component. related to a component.
Value type: URI, TEXT or REFERENCE Value type: URI, UID or XML-REFERENCE
Property Parameters: The VALUE parameter is required. Non-standard, Property Parameters: The VALUE parameter is required. Non-standard,
reference type or format type parameters can also be specified on link relation type, format type, label and language parameters can
this property. The LABEL parameter is defined in [RFC7986] also be specified on this property. The LABEL parameter is
defined in [RFC7986].
Conformance: This property can be specified zero or more times in Conformance: This property can be specified zero or more times in
any iCalendar component. any iCalendar component.
Description: When used in a component the value of this property Description: When used in a component the value of this property
points to additional information related to the component. For points to additional information related to the component. For
example, it may reference the originating web server. example, it may reference the originating web server.
Format Definition: This property is defined by the following Format Definition: This property is defined by the following
notation: notation:
link = "LINK" linkparam ":" link = "LINK" linkparam ":"
( uri / ; for VALUE=REFERENCE ( uri / ; for VALUE=XML-REFERENCE
uri / ; for VALUE=URI uri / ; for VALUE=URI
text ) ; for VALUE=TEXT text ) ; for VALUE=UID
CRLF CRLF
linkparam = ; the elements herein may appear in any order, linkparam = ; the elements herein may appear in any order,
; and the order is not significant. ; and the order is not significant.
(";" "VALUE" "=" ("REFERENCE" / (";" "VALUE" "=" ("XML-REFERENCE" /
"URI" / "URI" /
"TEXT")) "UID"))
1*(";" linkrelparam) 1*(";" linkrelparam)
(";" fmttypeparam) 1*(";" fmttypeparam)
(";" labelparam) 1*(";" labelparam)
(";" langparam) 1*(";" languageparam)
*(";" other-param) *(";" other-param)
This property is a serialisation of the model in [RFC8288], where This property is a serialisation of the model in [RFC8288], where
the link target is carried in the property value, the link context the link target is carried in the property value, the link context
is the containing calendar entity, and the link relation type and is the containing calendar entity, and the link relation type and
any target attributes are carried in iCalendar property any target attributes are carried in iCalendar property
parameters. parameters.
The LINK property parameters map to [RFC8288] attributes as The LINK property parameters map to [RFC8288] attributes as
follows: follows:
LABEL: Maps to the "title" attribute defined in section 3.4.1 of LABEL: Maps to the "title" attribute defined in section 3.4.1 of
[RFC8288]. [RFC8288].
LANG: Maps to the "hreflang" attribute defined in section 3.4.1 LANGUAGE: Maps to the "hreflang" attribute defined in section
of [RFC8288]. 3.4.1 of [RFC8288].
LINKREL: Maps to the link relation type defined in section 2.1 of LINKREL: Maps to the link relation type defined in section 2.1 of
[RFC8288]. [RFC8288].
FMTTYPE: Maps to the "type" attribute defined in section 3.4.1 of FMTTYPE: Maps to the "type" attribute defined in section 3.4.1 of
[RFC8288]. [RFC8288].
There is no mapping for [RFC8288] "title*", "anchor", "rev" or There is no mapping for [RFC8288] "title*", "anchor", "rev" or
"media". "media".
Example: The following is an example of this property which provides Example: The following is an example of this property which provides
a reference to the source for the calendar object. a reference to the source for the calendar object.
LINK;LINKREL=SOURCE;LABEL=Venue;VALUE=URI:http://example.com/events LINK;LINKREL=SOURCE;LABEL=Venue;VALUE=URI:
https://example.com/events
Example: The following is an example of this property which provides Example: The following is an example of this property which provides
a reference to an entity from which this one was derived. The a reference to an entity from which this one was derived. The
link relation is a vendor defined value link relation is a vendor defined value.
LINK;LINKREL="https://example.com/linkrel/derivedFrom";VALUE=URI: LINK;LINKREL="https://example.com/linkrel/derivedFrom";
http://example.com/tasks/01234567-abcd1234.ics VALUE=URI:
https://example.com/tasks/01234567-abcd1234.ics
Example: The following is an example of this property which provides Example: The following is an example of this property which provides
a reference to a fragment of an XML document. The link relation a reference to a fragment of an XML document. The link relation
is a vendor defined value is a vendor defined value.
LINK;LINKREL="https://example.com/linkrel/costStructure"; LINK;LINKREL="https://example.com/linkrel/costStructure";
VALUE=REFERENCE: VALUE=XML-REFERENCE:
http://example.com/xmlDocs/bidFramework.xml https://example.com/xmlDocs/bidFramework.xml
#xpointer(descendant::CostStruc/range-to(following::CostStrucEND[1])) #xpointer(descendant::CostStruc/range-to(
following::CostStrucEND[1]))
8.3. Refid 8.3. Refid
Property name: REFID Property name: REFID
Purpose: This property value acts as a key for associated iCalendar Purpose: This property value acts as a key for associated iCalendar
entities. entities.
Value type: TEXT Value type: TEXT
skipping to change at page 13, line 26 skipping to change at page 14, line 36
The current link registry The current link registry
Example: The following is an example of this property. Example: The following is an example of this property.
REFID:itinerary-2014-11-17 REFID:itinerary-2014-11-17
9. Updates to RFC 5545 9. Updates to RFC 5545
This specification updates the RELATED-TO property defined in This specification updates the RELATED-TO property defined in
Section 3.8.4.5 of [RFC5545]. Section 3.8.4.5 of [RFC5545]. The contents of Section 9.1 replace
that section.
The RELTYPE parameter is extended to take new values defining The RELTYPE parameter is extended to take new values defining
temporal relationships, a GAP parameter is defined to provide lead temporal relationships, a GAP parameter is defined to provide lead
and lag values, and RELATED-TO is extended to allow URI values. 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 These changes allow the RELATED-TO property to define a richer set of
relationships useful for project management. relationships useful for project management.
9.1. RELATED-TO 9.1. RELATED-TO
Property name: RELATED-TO Property Name: RELATED-TO
Purpose: This property is used to represent a relationship or Purpose: This property is used to represent a relationship or
reference between one calendar component and another. The reference between one calendar component and another. The
definition here extends the definition in Section 3.8.4.5 of definition here extends the definition in Section 3.8.4.5 of
[RFC5545] by allowing URI or UID values and a GAP parameter. [RFC5545] by allowing URI or UID values and a GAP parameter.
Value type: URI, UID or TEXT Value Type: URI, UID or TEXT
Property Parameters: Relationship type, IANA and non-standard Property Parameters: Relationship type, IANA and non-standard
property parameters can be specified on this property. property parameters can be specified on this property.
Conformance: This property MAY be specified in any iCalendar Conformance: This property MAY be specified in any iCalendar
component. component.
Description: By default or when VALUE=UID is specified, the property Description: By default or when VALUE=UID is specified, the property
value consists of the persistent, globally unique identifier of value consists of the persistent, globally unique identifier of
another calendar component. This value would be represented in a another calendar component. This value would be represented in a
skipping to change at page 14, line 32 skipping to change at page 15, line 43
indicates that the calendar component is a peer of the referenced indicates that the calendar component is a peer of the referenced
calendar component. calendar component.
To preserve backwards compatibility the value type MUST be UID To preserve backwards compatibility the value type MUST be UID
when the PARENT, SIBLING or CHILD relationships are specified. when the PARENT, SIBLING or CHILD relationships are specified.
The FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH or STARTTOSTART The FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH or STARTTOSTART
relationships define temporal relationships as specified in the relationships define temporal relationships as specified in the
reltype parameter definition. 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 Changes to a calendar component referenced by this property can
have an implicit impact on the related calendar component. For have an implicit impact on the related calendar component. For
example, if a group event changes its start or end date or time, example, if a group event changes its start or end date or time,
then the related, dependent events will need to have their start then the related, dependent events will need to have their start
and end dates changed in a corresponding way. Similarly, if a and end dates changed in a corresponding way. Similarly, if a
PARENT calendar component is cancelled or deleted, then there is PARENT calendar component is cancelled or deleted, then there is
an implied impact to the related CHILD calendar components. This an implied impact to the related CHILD calendar components. This
property is intended only to provide information on the property is intended only to provide information on the
relationship of calendar components. It is up to the target relationship of calendar components.
calendar system to maintain any property implications of this
relationship. 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 Format Definition: This property is defined by the following
notation: notation:
related = "RELATED-TO" relparam ":" related = "RELATED-TO" relparam ":"
( uid / ; for VALUE=UID ( text / ; for VALUE=UID
uri / ; for VALUE=URI uri / ; for VALUE=URI
text ) ; for VALUE=TEXT or default text ) ; for VALUE=TEXT or default
CRLF CRLF
relparam = ; the elements herein may appear in any order, relparam = ; the elements herein may appear in any order,
; and the order is not significant. ; and the order is not significant.
[";" "VALUE" "=" ("UID" / [";" "VALUE" "=" ("UID" /
"URI" / "URI" /
"TEXT")] "TEXT")]
[";" reltypeparam] [";" reltypeparam]
[";" gapparam] [";" gapparam]
*(";" other-param) *(";" other-param)
Example: The following are examples of this property. Example: The following are examples of this property.
RELATED-TO:jsmith.part7.19960817T083000.xyzMail@example.com RELATED-TO:jsmith.part7.19960817T083000.xyzMail@example.com
RELATED-TO:19960401-080045-4000F192713-0052@example.com RELATED-TO:19960401-080045-4000F192713-0052@example.com
RELATED-TO;VALUE=URI;RELTYPE=STARTTOFINISH: RELATED-TO;VALUE=URI;RELTYPE=STARTTOFINISH:
http://example.com/caldav/user/jb/cal/ https://example.com/caldav/user/jb/cal/
19960401-080045-4000F192713.ics 19960401-080045-4000F192713.ics
10. Security Considerations 10. Security Considerations
All of the security considerations of section 7 pf [RFC5545] apply to
this specification.
Applications using the LINK property need to be aware of the risks Applications using the LINK property need to be aware of the risks
entailed in using the URIs provided as values. See section 7 of entailed in using the URIs provided as values. See section 7 of
[RFC3986] for a discussion of the security considerations relating to [RFC3986] for a discussion of the security considerations relating to
URIs. URIs.
In particular note section 7.1 "Reliability and Consistency" of In particular note section 7.1 "Reliability and Consistency" of
[RFC3986] which points out the lack of a stability guarantee for [RFC3986] which points out the lack of a stability guarantee for
referenced resources. referenced resources.
When the value is a REFERENCE type the targeted data is an XML 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 document or portion thereof. Consumers need to be aware of the
security issues related to XML processing - in particular those security issues related to XML processing - in particular those
related to XML entities. See [RFC4918] - Section 20.6. Additionally related to XML entities. See [RFC4918] - Section 20.6. Additionally
note that the reference may be invalid or become so over time. note that the reference may be invalid or become so over time.
The CONCEPT and redefined RELATED-TO property have the same issues in The CONCEPT and redefined RELATED-TO property have the same issues in
that values may be URIs. that values may be URIs.
Extremely large values for the GAP parameter may lead to unexpected
behavior.
11. IANA Considerations 11. IANA Considerations
11.1. iCalendar Property Registrations 11.1. iCalendar Property Registrations
The following iCalendar property names have been added to the The following iCalendar property names have been added to the
iCalendar Properties Registry defined in Section 8.3.2 of [RFC5545] iCalendar Properties Registry defined in Section 8.3.2 of [RFC5545].
IANA has also added a reference to this document where the properties IANA has also added a reference to this document where the properties
originally defined in [RFC5545] have been updated by this document. originally defined in [RFC5545] have been updated by this document.
+============+=========+========================+ +============+=========+========================+
| Property | Status | Reference | | Property | Status | Reference |
+============+=========+========================+ +============+=========+========================+
| CONCEPT | Current | Section 8.1 | | CONCEPT | Current | Section 8.1 |
+------------+---------+------------------------+ +------------+---------+------------------------+
| LINK | Current | Section 8.2 | | LINK | Current | Section 8.2 |
+------------+---------+------------------------+ +------------+---------+------------------------+
skipping to change at page 16, line 32 skipping to change at page 18, line 9
+------------+---------+------------------------+ +------------+---------+------------------------+
| RELATED-TO | Current | [RFC5545], Section 9.1 | | RELATED-TO | Current | [RFC5545], Section 9.1 |
+------------+---------+------------------------+ +------------+---------+------------------------+
Table 1 Table 1
11.2. iCalendar Property Parameter Registrations 11.2. iCalendar Property Parameter Registrations
The following iCalendar property parameter names have been added to The following iCalendar property parameter names have been added to
the iCalendar Parameters Registry defined in Section 8.3.3 of the iCalendar Parameters Registry defined in Section 8.3.3 of
[RFC5545] [RFC5545].
+===========+=========+=============+ +===========+=========+=============+
| Parameter | Status | Reference | | Parameter | Status | Reference |
+===========+=========+=============+ +===========+=========+=============+
| GAP | Current | Section 6.2 | | GAP | Current | Section 6.2 |
+-----------+---------+-------------+ +-----------+---------+-------------+
| LINKREL | Current | Section 6.1 | | LINKREL | Current | Section 6.1 |
+-----------+---------+-------------+ +-----------+---------+-------------+
Table 2 Table 2
11.3. iCalendar Value Data Type Registrations 11.3. iCalendar Value Data Type Registrations
The following iCalendar property parameter names have been added to The following iCalendar property parameter names have been added to
the iCalendar Value Data Types Registry defined in Section 8.3.4 of the iCalendar Value Data Types Registry defined in Section 8.3.4 of
[RFC5545] [RFC5545].
+=================+=========+===========+ +=================+=========+===========+
| Value Data Type | Status | Reference | | Value Data Type | Status | Reference |
+=================+=========+===========+ +=================+=========+===========+
| REFERENCE | Current | Section 7 | | XML-REFERENCE | Current | Section 7 |
+-----------------+---------+-----------+ +-----------------+---------+-----------+
| UID | Current | Section 7 | | UID | Current | Section 7 |
+-----------------+---------+-----------+ +-----------------+---------+-----------+
Table 3 Table 3
11.4. iCalendar RELTYPE Value Registrations 11.4. iCalendar RELTYPE Value Registrations
The following iCalendar "RELTYPE" values have been added to the The following iCalendar "RELTYPE" values have been added to the
iCalendar Relationship Types Registry defined in Section 8.3.8 of iCalendar Relationship Types Registry defined in Section 8.3.8 of
[RFC5545] [RFC5545].
+===================+=========+===========+ +===================+=========+===========+
| Relationship Type | Status | Reference | | Relationship Type | Status | Reference |
+===================+=========+===========+ +===================+=========+===========+
| CONCEPT | Current | Section 5 | | CONCEPT | Current | Section 5 |
+-------------------+---------+-----------+ +-------------------+---------+-----------+
| DEPENDS-ON | Current | Section 5 | | DEPENDS-ON | Current | Section 5 |
+-------------------+---------+-----------+ +-------------------+---------+-----------+
| FINISHTOFINISH | Current | Section 4 | | FINISHTOFINISH | Current | Section 4 |
+-------------------+---------+-----------+ +-------------------+---------+-----------+
| FINISHTOSTART | Current | Section 4 | | FINISHTOSTART | Current | Section 4 |
+-------------------+---------+-----------+ +-------------------+---------+-----------+
| FIRST | Current | Section 5 | | FIRST | Current | Section 5 |
+-------------------+---------+-----------+ +-------------------+---------+-----------+
| NEXT | Current | Section 5 |
+-------------------+---------+-----------+
| REFID | Current | Section 5 | | REFID | Current | Section 5 |
+-------------------+---------+-----------+ +-------------------+---------+-----------+
| STARTTOFINISH | Current | Section 4 | | STARTTOFINISH | Current | Section 4 |
+-------------------+---------+-----------+ +-------------------+---------+-----------+
| STARTTOSTART | Current | Section 4 | | STARTTOSTART | Current | Section 4 |
+-------------------+---------+-----------+ +-------------------+---------+-----------+
Table 4 Table 4
11.5. New Reference Type Registration 11.5. New Reference Type Registration
The following link relation values have been added to the Reference The following link relation values have been added to the Reference
Types Registry defined in Section 6.2.2 of [RFC8288] Types Registry defined in Section 6.2.2 of [RFC8288].
+========+=========+=============+ +========+=========+=============+
| Name | Status | Reference | | Name | Status | Reference |
+========+=========+=============+ +========+=========+=============+
| SOURCE | Current | Section 6.1 | | SOURCE | Current | Section 6.1 |
+--------+---------+-------------+ +--------+---------+-------------+
Table 5 Table 5
12. Acknowledgements 12. Acknowledgements
The author would like to thank the members of the Calendaring and The author would like to thank the members of CalConnect, the
Scheduling Consortium technical committees and the following Calendaring and Scheduling Consortium technical committees and the
individuals for contributing their ideas, support and comments: following individuals for contributing their ideas, support and
comments:
Adrian Apthorp, Cyrus Daboo, Marten Gajda, Ken Murchison Adrian Apthorp, Cyrus Daboo, Marten Gajda, Ken Murchison
The author would also like to thank CalConnect, the Calendaring and The author would also like to thank CalConnect, the Calendaring and
Scheduling Consortium for advice with this specification. Scheduling Consortium for advice with this specification.
13. References 13. References
13.1. Informative References 13.1. Informative References
[RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault,
"Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
DOI 10.17487/RFC4791, March 2007,
<https://www.rfc-editor.org/info/rfc4791>.
[RFC8607] Daboo, C., Quillaud, A., and K. Murchison, Ed., [RFC8607] Daboo, C., Quillaud, A., and K. Murchison, Ed.,
"Calendaring Extensions to WebDAV (CalDAV): Managed "Calendaring Extensions to WebDAV (CalDAV): Managed
Attachments", RFC 8607, DOI 10.17487/RFC8607, June 2019, Attachments", RFC 8607, DOI 10.17487/RFC8607, June 2019,
<https://www.rfc-editor.org/info/rfc8607>. <https://www.rfc-editor.org/info/rfc8607>.
13.2. Normative References 13.2. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
skipping to change at page 19, line 5 skipping to change at page 20, line 36
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005, RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>. <https://www.rfc-editor.org/info/rfc3986>.
[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed [RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, Authoring and Versioning (WebDAV)", RFC 4918,
DOI 10.17487/RFC4918, June 2007, DOI 10.17487/RFC4918, June 2007,
<https://www.rfc-editor.org/info/rfc4918>. <https://www.rfc-editor.org/info/rfc4918>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5545] Desruisseaux, B., Ed., "Internet Calendaring and [RFC5545] Desruisseaux, B., Ed., "Internet Calendaring and
Scheduling Core Object Specification (iCalendar)", Scheduling Core Object Specification (iCalendar)",
RFC 5545, DOI 10.17487/RFC5545, September 2009, RFC 5545, DOI 10.17487/RFC5545, September 2009,
<https://www.rfc-editor.org/info/rfc5545>. <https://www.rfc-editor.org/info/rfc5545>.
[RFC7986] Daboo, C., "New Properties for iCalendar", RFC 7986, [RFC7986] Daboo, C., "New Properties for iCalendar", RFC 7986,
DOI 10.17487/RFC7986, October 2016, DOI 10.17487/RFC7986, October 2016,
<https://www.rfc-editor.org/info/rfc7986>. <https://www.rfc-editor.org/info/rfc7986>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
skipping to change at page 19, line 48 skipping to change at page 21, line 35
20021219, 19 December 2002, 20021219, 19 December 2002,
<http://www.w3.org/TR/2002/WD-xptr-xpointer-20021219>. <http://www.w3.org/TR/2002/WD-xptr-xpointer-20021219>.
Author's Address Author's Address
Michael Douglass Michael Douglass
Bedework Bedework
226 3rd Street 226 3rd Street
Troy, NY 12180 Troy, NY 12180
United States of America United States of America
Email: mdouglass@bedework.com Email: mdouglass@bedework.com
URI: http://bedework.com URI: https://bedework.com
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