Support for Series in iCalendar
draft-ietf-calext-icalendar-series-02
The information below is for an old version of the document.
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|---|---|---|---|
| Author | Michael Douglass | ||
| Last updated | 2020-10-14 | ||
| Replaces | draft-douglass-icalendar-series | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Additional resources | Mailing list discussion | ||
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draft-ietf-calext-icalendar-series-02
Network Working Group M. Douglass
Internet-Draft 14 October 2020
Intended status: Standards Track
Expires: 17 April 2021
Support for Series in iCalendar
draft-ietf-calext-icalendar-series-02
Abstract
This document updates [RFC5545] by defining a new repeating set of
events known as a series. This differs from recurrences in that each
instance is a separate entity with a parent relationship to a
specified template entity.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 17 April 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
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Table of Contents
1. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terms and definitions . . . . . . . . . . . . . . . . . . . . 3
4. Overrides and iCalendar recurrences . . . . . . . . . . . . . 3
4.1. Changing the master start or the recurrence rules . . . . 3
4.2. Splitting recurrences . . . . . . . . . . . . . . . . . . 4
5. Series . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5.1. Modifying series patterns and splitting . . . . . . . . . 5
5.2. The series master . . . . . . . . . . . . . . . . . . . . 6
5.3. The series instances . . . . . . . . . . . . . . . . . . 6
6. Redefined Relation Type Value . . . . . . . . . . . . . . . . 6
7. New Property Parameters . . . . . . . . . . . . . . . . . . . 9
7.1. Split . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.2. Lookahead count . . . . . . . . . . . . . . . . . . . . . 10
7.3. Lookahead period . . . . . . . . . . . . . . . . . . . . 10
8. New Properties . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.2. Generating Series members . . . . . . . . . . . . . . . . 11
8.3. Series UID . . . . . . . . . . . . . . . . . . . . . . . 13
8.4. Series-exception-date . . . . . . . . . . . . . . . . . . 13
8.5. Series-date . . . . . . . . . . . . . . . . . . . . . . . 15
8.6. Series-id . . . . . . . . . . . . . . . . . . . . . . . . 16
8.7. Last series ID . . . . . . . . . . . . . . . . . . . . . 18
8.8. Series Rule . . . . . . . . . . . . . . . . . . . . . . . 21
9. Redefined RELATED-TO Property . . . . . . . . . . . . . . . . 22
9.1. RELATED-TO . . . . . . . . . . . . . . . . . . . . . . . 22
10. Backwards compatibility . . . . . . . . . . . . . . . . . . . 24
11. CalDAV extensions . . . . . . . . . . . . . . . . . . . . . . 25
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
12.1. iCalendar Property Registrations . . . . . . . . . . . . 25
12.2. iCalendar Property Parameter Registrations . . . . . . . 26
12.3. iCalendar RELTYPE Value Registrations . . . . . . . . . 26
13. Normative references . . . . . . . . . . . . . . . . . . . . 26
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 26
1. Acknowledgements
The authors would like to thank the members of the Calendaring and
Scheduling Consortium (CalConnect) for contributing their ideas and
support.
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2. Introduction
Since iCalendar was first defined there has been only one way to
express a repeating set of events - the recurrence. This defined a
master event, a set of rules for computing the instances and a way of
overriding certain instances.
This approach works well enough in certain situations but has many
problems which need to be addressed.
This specification introduces a new approach to repeating patterns of
entities which avoids some of the problems.
3. Terms and definitions
No terms and definitions are listed in this document.
4. Overrides and iCalendar recurrences
The recurrence rules specify how instances are to be computed. These
rules provide a set of keys - the RECURRENCE-ID - and an instance can
be created with the calculated start date/time and a copy of the
duration (or calculated end date/time).
The specification allows for overrides. These are handled by
supplying a complete replacement for the instance with a RECURRENCE-
ID property matching that of the instance being overridden. This may
change any of the properties (except the UID) - including start, end
or duration.
If a long lived recurrence is heavily overridden it becomes very
cumbersome. The master plus overrides is considered a single
resource in most circumstances (iTip allows the delivery of a single
instance in certain situations).
Simple meetings can become heavily modified recurrences through
adding the weeks agenda to the description, changing of attendees
etc.
There are approaches being considered to mitigate some of these
issues which mostly involve only storing changes but recurrences are
still awkward to deal with.
4.1. Changing the master start or the recurrence rules
This can lead to some very difficult problems to resolve. In the
case of a heavily modified meeting it may be difficult to impossible
to determine which override applies to the newly modified event.
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For example, a weekly book-reading is moved from Monday to Friday.
There are weeks of scheduled events in the future. Do we move them
all forward to the next instance or skip one and move them back? If
it becomes bi-weekly rather than weekly do we drop every other or
just space them out more?
To be sure - some of these problems are not totally resolved by a
series approach but they become more tractable.
4.2. Splitting recurrences
The [RFC5545] THISANDFUTURE range is poorly supported. Splitting is
what a number of implementations use to avoid changing overrides in
the past.
The recurring event is split into 2, one being the truncated original
the other being a new recurring event starting at the time of the
THISANDFUTURE override.
There is left the problem of relating the two, this can be
accomplished by use of the RELATED-TO property but that is not
standardized.
5. Series
A series is a, generally regularly, repeating sets of events or tasks
each instance of which is usually, but not always, different in some
respect. Examples may be a library running an after-school reading
program which usually, takes place at the same time each week but
always differs in the book or author being studied.
In recurrences an instances is a calculated 'virtual' object, unless
overridden. It has the same UID as the master and a RECURRENCE-ID
which is always one of the calculated set.
In a series, a specified number of instances are created ahead of
time each with their own unique UID. They are all related to the
master using a SERIES-MASTER relation type defined in this
specification. Each instance acts as an individual component as far
as retrieval and searching is concerned.
Each instance and master is identified as a member of the full series
by the SERIES-UID property. The value of this property is the same
in all members of the series even when splits have occurred.
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As instances are created a LAST-SERIES-ID property is added or
updated in the master to indicate which instance was last created.
When there are SXDATE properties this property value may represent an
instance which cannot be created. It merely represents the latest
calculated date.
This property allows generated instances to be deleted without the
addition of SXDATE properties to the master. The SXDATE only
indicates future instances which MUST NOT be created.
As time goes on more instances are created either by the server or by
a client when it inspects the current state of the series. The
number of instances may be based on time or a count.
For example, an organization may allow rooms to be booked only 4
weeks ahead. Thus a series may be set up which has that 4 week set
of events in the future. Each will have the room as an attendee
ensuring that at least the room is booked at the regular time.
5.1. Modifying series patterns and splitting
If it becomes necessary to modify the series rules or the master
start then the series is always split at the point of the
modification.
When a series is split the previous master is modifed to truncate the
current series at the last generated instance and a parameter
SPLIT=YES is added to the series rule to indicate that this master is
now split.
The split may result in a number of instances related to the old
series but overlapping the new. It is up to the implementation to
decide what should be done with these but this usually requires a
degree of interaction with a human (or very intelligent robot). The
application may offer to copy them into the corresponding new
instances - if these can be easily determined, offer to delete all of
them or let the user manually copy information and delete.
The new series master is related to the old master by the new series
master having a RELATED-TO property with RELTYPE=SERIES-MASTER
pointing at the previous master. In that way a backwards chain of
series masters may be created
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5.2. The series master
A series master is identified in much the same way as a recurrence
master. It will contain an SRULE and 0 or more SDATE properties or 1
or more SDATE properties. Additionally it may contain 0 or more
SXDATE properties to exclude instances.
As noted above, if the series was split it may contain a RELATED-TO
property with RELTYPE=SERIES-MASTER and a value of the previous
series master.
The master will also contain a LAST-SERIES-ID if any instances have
been calculated and perhaps generated.
It is important to note that the series master is the first member of
the series. Thus the first instance always occurs AFTER the series
master.
5.3. The series instances
A series instance is identified by having a SERIES-ID property which
is calculated in the same manner as a RECURRENCE-ID. It MUST also
contain a RELATED-TO property with RELTYPE=SERIES-MASTER and a value
being the UID of the series master.
As noted above, if the series was split it may contain a RELATED-TO
property with RELTYPE=SERIES-MASTER and a value being the UID of the
previous series master.
6. Redefined Relation Type Value
Relationship parameter type values are defined in [RFC5545]. This
specification augments that parameter to include the new relationship
values SERIES-MASTER.
Format Definition
This property parameter is respecified as follows:
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reltypeparam = "RELTYPE" "="
("PARENT" ; Parent relationship - Default
/ "CHILD" ; Child relationship
/ "SIBLING" ; Sibling relationship
/ "DEPENDS-ON" ; refers to previous task
/ "REFID" ; Relationship based on REFID
/ "STRUCTURED-CATEGORY"
; Relationship based on STRUCTURED-CATEGORY
/ "FINISHTOSTART" ; Temporal relationship
/ "FINISHTOFINISH" ; Temporal relationship
/ "STARTTOFINISH" ; Temporal relationship
/ "STARTTOSTART" ; Temporal relationship
/ "SERIES-MASTER" ; link to the master component
/ iana-token ; Some other IANA-registered
; iCalendar relationship type
/ x-name) ; A non-standard, experimental
; relationship type
Figure 1
Description
This parameter can be specified on a property that references another
related calendar component. The parameter may specify the
hierarchical relationship type of the calendar component referenced
by the property when the value is PARENT, CHILD or SIBLING. If this
parameter is not specified on an allowable property, the default
relationship type is PARENT. Applications MUST treat x-name and
iana-token values they don't recognize the same way as they would the
PARENT value.
This parameter defines the temporal relationship when the value is
one of the project management standard relationships FINISHTOSTART,
FINISHTOFINISH, STARTTOFINISH or STARTTOSTART. This property will be
present in the predecessor entity and will refer to the successor
entity. The GAP parameter specifies the lead or lag time between the
predecessor and the successor. In the description of each temporal
relationship below we refer toTask-A which contains and controls the
relationship and Task-B the target of the relationship.
RELTYPE=PARENT See [RFC5545].
RELTYPE=CHILD See [RFC5545].
RELTYPE=SIBLING See [RFC5545].
RELTYPE=DEPENDS-ON Indicates that the current calendar component
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depends on the referenced calendar component in some manner. For
example a task may be blocked waiting on the other, referenced,
task.
RELTYPE=REFID Establishes a reference from the current component to
components with a REFID property which matches the value given in
the associated RELATED-TO property.
RELTYPE=SERIES-MASTER Indicates that the current calendar component
is based on the referenced calendar component. The value is a
UID.
RELTYPE=STRUCTURED-CATEGORY Establishes a reference from the current
component to components with a STRUCTURED-CATEGORY property which
matches the value given in the associated RELATED- TO property.
RELTYPE=FINISHTOSTART
Task-B cannot start until Task-A finishes. For example, when sanding
is complete, painting can begin.
============
| Task-A |--+
============ |
|
V
============
| Task-B |
============
Figure 2: Finish to start relationship
RELTYPE=FINISHTOFINISH
Task-B cannot finish before Task-A is finished, that is the end of
Task-A defines the end of Task-B. For example, we start the
potatoes, then the meat then the peas but they should all be cooked
at the same time.
============
| Task-A |--+
============ |
|
============ |
| Task-B |<-+
============
Figure 3: Finish to finish relationship
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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 when the game
starts (Task-A).
============
+--| Task-A |
| ============
|
============ |
| Task-B |<-+
============
Figure 4: Start to finish relationship
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-A |
| ============
|
| ============
+->| Task-B |
============
Figure 5: Start to start relationship
7. New Property Parameters
7.1. Split
Parameter name SPLIT
Purpose To indicate a series has been split.
Format Definition
This parameter is defined by the following notation:
splitparam = "SPLIT" "="
("YES" ; The series is split
/ "NO" ; The series is not split (default)
/ x-name ; Experimental reference type
/ iana-token) ; Other IANA registered type
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Figure 6
Description This parameter MAY be specified on the SRULE property to
indicate that the series has been split with SPLIT=YES. Once
split is is probably inappropriate to modify the series further.
7.2. Lookahead count
Parameter name LOOKAHEAD-COUNT
Purpose To specify the number of series instances that should be
generated in advance.
Format Definition
This parameter is defined by the following notation:
lookahead-countparam = "LOOKAHEAD-COUNT" "=" 1*DIGIT
Figure 7
Description
This parameter MAY be specified on the SRULE property to indicate how
many series instances should be generated in advance.
An implementation is free to apply its own limts but MUST NOT
generate more than those defined by this parameter and/or the
LOOKAHEAD-PERIOD parameter.
If both the LOOKAHEAD-PERIOD and LOOKAHEAD-COUNT arameters are
supplied the result should be limited by both.
For example, if the LOOKAHEAD-PERIOD parameter would cause 8
instances to be generated but LOOKAHEAD-COUNT specifies 4 then only 4
instances will be generated.
7.3. Lookahead period
Parameter name LOOKAHEAD-PERIOD
Purpose To specify a maximum period for which series instances
should be generated in advance.
Format Definition
This parameter is defined by the following notation:
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lookahead-periodparam = "LOOKAHEAD-PERIOD" "="
DQUOTE dur-value DQUOTE
Figure 8
Description
This parameter MAY be specified on the SRULE property to indicate how
far in advance series instances should be generated.
An implementation is free to apply its own limts but MUST NOT
generate more than those defined by this parameter and/or the
LOOKAHEAD-COUNT parameter.
If both the LOOKAHEAD-PERIOD and LOOKAHEAD-COUNT arameters are
supplied the result should be limited by both.
For example, if the LOOKAHEAD-PERIOD parameter would cause 8
instances to be generated but LOOKAHEAD-COUNT specifies 4 then only 4
instances will be generated.
The value is a quoted duration.
8. New Properties
8.1. General
The SERIES-ID, LAST-SERIES-ID, SDATE and SXDATE properties are
identical in form and in the parameters they take.
All must conform in form to the DTSTART property of the master
component. Only the SDATE may specify a time which is not part of
the calculated series.
The SRULE property value is identical in form to the RRULE property
defined in [RFC5545]. The LOOKAHEAD-COUNT and LOOKAHEAD-PERIOD
parameters indicate how many instances should be generated in
advance.
8.2. Generating Series members
An agent, either the server or a client, will periodically extend the
set of instances. The number of such generated instances is limited
by:
Elements of the rule The UNTIL or COUNT parts of the rule define
when the series terminates. Thus a COUNT=100 specifies a maximum
of 100 series members.
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Lookahead count This specifies how many series members can exist
from the current date/time. Thus a LOOKAHEAD-COUNT=4 means a
maximum of 4 generated instances.
Lookahead period This specifies how far into the future series
members can be generated. Thus a LOOKAHEAD-PERIOD="PT2M" means a
maximum period of 2 months.
System limits This client or server SHOULD also apply limits to
prevent a series from generating an overlarge set of members.
The starting point for the calculation is the DTSTART of the master
component or the LAST-SERIES-ID if it exists in the master. In both
cases the instance represented by that date is NOT generated as part
of the instance set and the actual instance may have been excluded by
an SXDATE property but the starting date is still valid.
The starting date/time property defines the first instance in the
next batch of series members. Note that the starting property value
MUST match the pattern of the series rule, if specified. For
example, if the rule specifies every Wednesday the starting date MUST
be a Wednesday.
The end date/time of the set will be provided by the UNTIL part of
the rule, the LOOKAHEAD-PERIOD or by a system maxima.
A set of date/time values can be generated within those contraints.
As each date/time value is generated it can be ignored if it is one
of the SXDATE values.
Generation of values can terminate when the size of the result
exceeds that given by the COUNT rule element, the LOOKAHEAD-COUNT
value or any system limit.
Any SDATE values that fall within the current range and are not in
the set of SXDATE values can be added and the result truncated again
to match the size limits.
Finally, any date/time values that have already been generated and
are present as SERIES-ID values should be removed from the set. What
remains is the new set of members to extend the current series.
The last of those values becomes the new value for the LAST-SERIES-ID
property in the series master.
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As noted above the "SXDATE" property can be used to exclude the value
specified in the master. This leads to a complication as the master
needs to be preserved as a container for the values which define the
series. This is flagged by adding a DELETED-MASTER element to the
SERIES-STATUS property.
8.3. Series UID
Property name SERIES-UID
Purpose This property defines the persistent, globally unique
identifier for the full series.
Value Type TEXT
Property Parameters IANA and non-standard property parameters can be
specified on this property.
Conformance This property MUST be specified in any "VEVENT",
"VTODO", and "VJOURNAL" calendar components acting as a series
master or series instance.
Description The SERIES-UID MUST be globally unique. This value
SHOULD be generated by following the recommendations in [RFC7986].
Format Definition
This property is defined by the following notation:
seruid = "SERIES-UID" seruidparam ":" text CRLF
seruidparam = *(";" other-param)
Figure 9
EXAMPLE
The following is an example of this property:
SERIES-UID:123e4567-e89b-12d3-a456-426655440000
Figure 10
8.4. Series-exception-date
Property name SXDATE
Purpose This property defines the list of DATE-TIME exceptions for
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series of events, to-dos or journal entries.
Value Type The default value type for this property is DATE-TIME.
The value type can be set to DATE.
Property Parameters IANA, non-standard, value data type, and time
zone identifier property parameters can be specified on this
property.
Conformance This property can be specified in "VEVENT", "VTODO", and
"VJOURNAL" calendar components acting as the series master.
Description The exception dates, if specified, are used when
computing the instances of the series. They specify date/time
values which are to be removed from the set of possible series
instances.
Format Definition
This property is defined by the following notation:
sxdate = "SXDATE" sxdtparam ":" sxdtval *("," sxdtval) CRLF
sxdtparam = *(
;
; The following are OPTIONAL,
; but MUST NOT occur more than once.
;
(";" "VALUE" "=" ("DATE-TIME" / "DATE")) /
;
(";" tzidparam) /
;
; The following is OPTIONAL,
; and MAY occur more than once.
;
(";" other-param)
;
)
sxdtval = date-time / date
;Value MUST match value type
Figure 11
EXAMPLE
The following is an example of this property:
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SXDATE:19960402T010000Z,19960403T010000Z,19960404T010000Z
Figure 12
8.5. Series-date
Property name SDATE
Purpose This property defines the list of DATE-TIME values for
series of events, to-dos or journal entries.
Value Type The default value type for this property is DATE-TIME.
The value type can be set to DATE.
Property Parameters IANA, non-standard, value data type, and time
zone identifier property parameters can be specified on this
property.
Conformance This property can be specified in "VEVENT", "VTODO", and
"VJOURNAL" calendar components acting as the series master.
Description This property can appear along with the "SRULE" property
to define a extra series occurrences. When they both appear in a
series master component, the instances are defined by the union of
occurrences defined by both the "SDATE" and "SRULE".
Format Definition
This property is defined by the following notation:
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sdate = "SDATE" sdtparam ":" sdtval *("," sdtval) CRLF
sdtparam = *(
;
; The following are OPTIONAL,
; but MUST NOT occur more than once.
;
(";" "VALUE" "=" ("DATE-TIME" / "DATE" / "PERIOD")) /
(";" tzidparam) /
;
; The following is OPTIONAL,
; and MAY occur more than once.
;
(";" other-param)
;
)
sdtval = date-time / date
;Value MUST match value type
Figure 13
EXAMPLE
The following are examples of this property:
SDATE:19970714T123000Z
SDATE;TZID=America/New_York:19970714T083000
SDATE;VALUE=PERIOD:19960403T020000Z/19960403T040000Z,
19960404T010000Z/PT3H
SDATE;VALUE=DATE:19970101,19970120,19970217,19970421
19970526,19970704,19970901,19971014,19971128,19971129,19971225
Figure 14
8.6. Series-id
Property name SERIES-ID
Purpose This property is used in conjunction with the "UID" and
"SEQUENCE" properties to identify a specific instance of a
"VEVENT", "VTODO", or "VJOURNAL" calendar component in a series.
The property value is the original value of the "DTSTART" property
of the series instance before any changes occur.
Value type The default value type is DATE-TIME. The value type can
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be set to a DATE value type. This property MUST have the same
value type as the "DTSTART" property contained within the series
component. Furthermore, this property MUST be specified as a date
with local time if and only if the "DTSTART" property contained
within the series component is specified as a date with local
time.
Property Parameters IANA, non-standard, value data type and time
zone identifier parameters can be specified on this property.
Conformance This property can be specified zero or more times in any
iCalendar component.
Description
The SERIES-ID is the originally calculated value of the DTSTART
property based on the master identified by the RELATED-TO property
with a RELTYPE=SERIES-MASTER parameter.
The full series of components can only be retrieved by searching
for all components with a matching RELATED-TO property.
Figure 15
If the value of the "DTSTART" property is a DATE type value, then
the value MUST be the calendar date for the series instance.
Figure 16
The DATE-TIME value is set to the time when the original series
instance would occur; meaning that if the intent is to change a
Friday meeting to Thursday, the DATE-TIME is still set to the
original Friday meeting.
Figure 17
The "SERIES-ID" property is used in conjunction with the "UID" and
"SEQUENCE" properties to identify a particular instance of an
event, to-do, or journal in the series. For a given pair of "UID"
and "SEQUENCE" property values, the "SERIES-ID" value for a series
instance is fixed.
Figure 18
Format Definition
This property is defined by the following notation:
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serid = "SERIES-ID" sidparam ":" sidval CRLF
sidparam = *(
;
; The following are OPTIONAL,
; but MUST NOT occur more than once.
;
(";" "VALUE" "=" ("DATE-TIME" / "DATE")) /
(";" tzidparam) /
;
; The following is OPTIONAL,
; and MAY occur more than once.
;
(";" other-param)
;
)
sidval = date-time / date
;Value MUST match value type
Figure 19
EXAMPLE
The following are examples of this property:
SERIES-ID;VALUE=DATE:19960401
SERIES-ID;TZID=America/New_York:20170120T120000
Figure 20
8.7. Last series ID
Property name LAST-SERIES-ID
Purpose
To specify the last calculated instance of the series. When new
instances are created they MUST have a SERIES-ID after the value of
this property.
In all respects this property is identical to SERIES-ID and is in
fact a copy of the SERIES-ID which would be present in the last
created instance (assuming it is not suppressed by an SXDATE).
Value type DATE or DATE_TIME (the default). This has the same
requirements as SERIES-ID.
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Property Parameters IANA, non-standard, value data type and time
zone identifier parameters can be specified on this property.
Conformance This property MAY be specified 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:
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last-series-i = "LAST-SERIES-ID" lastseriesidparam /
(
";" "VALUE" "=" "TEXT"
":" text
)
(
";" "VALUE" "=" "REFERENCE"
":" text
)
(
";" "VALUE" "=" "URI"
":" uri
)
CRLF
lastseriesidparam = *(
; the following is MANDATORY
; and MAY occur more than once
(";" relparam) /
; the following are MANDATORY
; but MUST NOT occur more than once
(";" fmttypeparam) /
(";" labelparam) /
; labelparam is defined in ...
; the following is OPTIONAL
; and MAY occur more than once
(";" xparam)
)
Figure 21
EXAMPLE
The following is an example of this property. It points to a server
acting as the source for the calendar object.
LINK;REL=SOURCE;LABEL=The Egg:http://example.com/events
Figure 22
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8.8. Series Rule
Property name RRULE
Purpose This property defines a rule or repeating pattern for a
series of events, to-dos or journal entries.
Value Type RECUR
Property Parameters IANA, non-standard, look-ahead count or date
property parameters can be specified on this property.
Conformance This property can be specified in any "VEVENT", "VTODO",
and "VJOURNAL" calendar component, but it SHOULD NOT be specified
more than once.
Description
The series rule, if specified, is used in computing the instances to
be generated for the series. These are generated by considering the
master "DTSTART" property along with the "SRULE", "SDATE", and
"SXDATE" properties contained within the series master. The
"DTSTART" property defines the first instance in the recurrence set
which is represented by that master event.
Unlike the RRULE the "DTSTART" property MUST be synchronized with the
series rule, if specified. For example, if the DTSTARTS species a
date on Wednesday but the SRULE specifies every Tuesday then a server
or client MUSt reject the component.
The final series is represented by gathering all of the start DATE-
TIME values generated by any of the specified "SRULE" and "SDATE"
properties, and then excluding any start DATE-TIME values specified
by "SXDATE" properties. This implies that start DATE- TIME values
specified by "SXDATE" properties take precedence over those specified
by inclusion properties (i.e., "SDATE" and "SRULE"). Where duplicate
instances are generated by the "SRULE" and "SDATE" properties, only
one instance is considered. Duplicate instances are ignored.
The "DTSTART" property specified within the master iCalendar object
defines the first instance of the recurrence. In most cases, a
"DTSTART" property of DATE-TIME value type used with a series rule,
should be specified as a date with local time and time zone reference
to make sure all the recurrence instances start at the same local
time regardless of time zone changes.
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If the duration of the series component is specified with the "DTEND"
or "DUE" property, then the same exact duration will apply to all the
members of the generated series. Else, if the duration of the series
master component is specified with the "DURATION" property, then the
same nominal duration will apply to all the members of the generated
series and the exact duration of each instance will depend on its
specific start time. For example, series instances of a nominal
duration of one day will have an exact duration of more or less than
24 hours on a day where a time zone shift occurs. The duration of a
specific instance may be modified in an exception component or simply
by using an "SDATE" property of PERIOD value type.
Format Definition
This property is defined by the following notation:
srule = "SRULE" srulparam ":" recur CRLF
sruleparam = *(
; the following are OPTIONAL
; but MUST NOT occur more than once
(";" lookahead-countparam) /
(";" lookahead-periodparam) /
; the following is OPTIONAL
; and MAY occur more than once
(";" xparam)
)
Figure 23
EXAMPLE
TODO - Say they are pretty much the same as RRULE but extra params
9. Redefined RELATED-TO Property
9.1. 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 [RFC5545] by including a
section on RELTYPE=SERIES-MASTER.
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Value type URI, UID or TEXT
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.
The FINISHTOSTART, FINISHTOFINISH, STARTTOFINISH or STARTTOSTART
relationships define temporal relationships as specified in the
reltype parameter definition.
The SERIES-MASTER relationship when included in a series instance
refers to the master of that series. When included in a series
master it refers to a previous master in a chain of spilt series.
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 changed in a corresponding way. Similarly, if a
PARENT calendar component is cancelled 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. It is up to the target
calendar system to maintain any property implications of this
relationship.
Format Definition This property is defined by the following
notation:
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related = "RELATED-TO" relparam ( ":" text ) /
(
";" "VALUE" "=" "UID"
":" uid
)
(
";" "VALUE" "=" "URI"
":" uri
)
CRLF
relparam = *(
;
; The following are OPTIONAL,
; but MUST NOT occur more than once.
;
(";" reltypeparam) /
(";" gapparam) /
;
; The following is OPTIONAL,
; and MAY occur more than once.
;
(";" other-param)
;
)
Figure 24
EXAMPLE
The following are examples of this property.
RELATED-TO;RELTYPE=SERIES-MASTER:19960401-080045-4000F192713
Figure 25
10. Backwards compatibility
Any clients following the approach specified in [RFC5545] are
expected to ignore any properties or parameters they don't recognize.
For such clients the series appears to be an unconnected set of
components. They all have their own unique UIDS. If the client
updates an instance this should be identical in effect to an update
carried out by a client aware of the new properties.
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Updates MUST preserve the SERIES-ID, LAST-SERIES-ID, SRULE, SDATE and
SXDATE properties. A client which does not do so is in violation of
[RFC5545].
TODO - More text needed here...
11. CalDAV extensions
This specification may extend Caldav by adding reports to return all
members of a series given the series master UID. This could be
handled by the current query mechanism but it is likely to be
sufficiently frequently used that a special query is appropriate.
It is also likely we will want a CalDAV operation to split a series
and generate the additional members of the series as a single atomic
operation. == Security Considerations
Clients and servers should take care to limit the number of generated
instances to a reasonable value. This can be a relatively small
value.
12. IANA Considerations
12.1. iCalendar Property Registrations
The following iCalendar property names have been added to the
iCalendar Properties Registry defined in [RFC5545].
+================+=========+=============+
| Property | Status | Reference |
+================+=========+=============+
| LAST-SERIES-ID | Current | Section 8.7 |
+----------------+---------+-------------+
| SERIES-ID | Current | Section 8.6 |
+----------------+---------+-------------+
| SERIES-UID | Current | Section 8.3 |
+----------------+---------+-------------+
| SDATE | Current | Section 8.5 |
+----------------+---------+-------------+
| SRULE | Current | Section 8.8 |
+----------------+---------+-------------+
| SXDATE | Current | Section 8.4 |
+----------------+---------+-------------+
Table 1
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12.2. iCalendar Property Parameter Registrations
The following iCalendar property parameter names have been added to
the iCalendar Parameters Registry defined in [RFC5545].
+==================+=========+=============+
| Parameter | Status | Reference |
+==================+=========+=============+
| LOOKAHEAD-COUNT | Current | Section 7.2 |
+------------------+---------+-------------+
| LOOKAHEAD-PERIOD | Current | Section 7.3 |
+------------------+---------+-------------+
| SPLIT | Current | Section 7.1 |
+------------------+---------+-------------+
Table 2
12.3. iCalendar RELTYPE Value Registrations
The following iCalendar "RELTYPE" values have been added to the
iCalendar Relationship Types Registry defined in [RFC5545].
+===================+=========+===========+
| Relationship Type | Status | Reference |
+===================+=========+===========+
| SERIES-ID | Current | Section 5 |
+-------------------+---------+-----------+
Table 3
13. Normative references
[RFC5545] Desruisseaux, B., Ed., "Internet Calendaring and
Scheduling Core Object Specification (iCalendar)", IETF
RFC 5545, IETF RFC 5545, DOI 10.17487/RFC5545, September
2009, <https://www.rfc-editor.org/info/rfc5545>.
[RFC7986] Daboo, C., "New Properties for iCalendar", IETF RFC 7986,
IETF RFC 7986, DOI 10.17487/RFC7986, October 2016,
<https://www.rfc-editor.org/info/rfc7986>.
Author's Address
Michael Douglass
Email: mikeadouglass@gmail.com
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