Support for Series in iCalendar
draft-ietf-calext-icalendar-series-03

Document Type Active Internet-Draft (calext WG)
Author Michael Douglass 
Last updated 2021-04-12
Replaces draft-douglass-icalendar-series
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Network Working Group                                        M. Douglass
Internet-Draft                                             12 April 2021
Intended status: Standards Track                                        
Expires: 14 October 2021

                    Support for Series in iCalendar
                 draft-ietf-calext-icalendar-series-03

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.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 14 October 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   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. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
     11.1.  iCalendar Property Registrations . . . . . . . . . . . .  25
     11.2.  iCalendar Property Parameter Registrations . . . . . . .  25
     11.3.  iCalendar RELTYPE Value Registrations  . . . . . . . . .  26
   12. Normative References  . . . . . . . . . . . . . . . . . . . .  26
   13. Informative References  . . . . . . . . . . . . . . . . . . .  27
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Acknowledgements

   The author would like to thank the members of the Calendaring and
   Scheduling Consortium technical committees and the following
   individuals for contributing their ideas, support and comments:

   The author would also like to thank the Calendaring and Scheduling
   Consortium for advice with this specification.

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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

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 [RFC5545] 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.  This can help if
   the changes are minor but heavily modified instances are still a
   problem.

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?

   While these problems are not totally resolved by a series approach,
   they become more tractable.

4.2.  Splitting recurrences

   The [RFC5545] THISANDFUTURE range is poorly supported.  Splitting is
   the approach 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  SRULE

   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... == 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.

11.  IANA Considerations

11.1.  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

11.2.  iCalendar Property Parameter Registrations

   The following iCalendar property parameter names have been added to
   the iCalendar Parameters Registry defined in [RFC5545].

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               +==================+=========+=============+
               | Parameter        | Status  | Reference   |
               +==================+=========+=============+
               | LOOKAHEAD-COUNT  | Current | Section 7.2 |
               +------------------+---------+-------------+
               | LOOKAHEAD-PERIOD | Current | Section 7.3 |
               +------------------+---------+-------------+
               | SPLIT            | Current | Section 7.1 |
               +------------------+---------+-------------+

                                 Table 2

11.3.  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

12.  Normative References

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

   [RFC2518]  Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.
              Jensen, "HTTP Extensions for Distributed
              Authoring — WEBDAV", RFC 2518, IETF RFC 2518,
              DOI 10.17487/RFC2518, February 1999,
              <https://www.rfc-editor.org/info/rfc2518>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", RFC 3986,
              IETF RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4791]  Daboo, C., Desruisseaux, B., and L. Dusseault,
              "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
              IETF RFC 4791, DOI 10.17487/RFC4791, March 2007,
              <https://www.rfc-editor.org/info/rfc4791>.

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   [RFC5545]  Desruisseaux, B., Ed., "Internet Calendaring and
              Scheduling Core Object Specification (iCalendar)", RFC
              5545, IETF RFC 5545, DOI 10.17487/RFC5545, September 2009,
              <https://www.rfc-editor.org/info/rfc5545>.

   [RFC5546]  Daboo, C., Ed., "iCalendar Transport-Independent
              Interoperability Protocol (iTIP)", RFC 5546, IETF RFC
              5546, DOI 10.17487/RFC5546, December 2009,
              <https://www.rfc-editor.org/info/rfc5546>.

   [RFC6047]  Melnikov, A., Ed., "iCalendar Message-Based
              Interoperability Protocol (iMIP)", RFC 6047, IETF RFC
              6047, DOI 10.17487/RFC6047, December 2010,
              <https://www.rfc-editor.org/info/rfc6047>.

   [RFC6638]  Daboo, C. and B. Desruisseaux, "Scheduling Extensions to
              CalDAV", RFC 6638, IETF RFC 6638, DOI 10.17487/RFC6638,
              June 2012, <https://www.rfc-editor.org/info/rfc6638>.

   [RFC7986]  Daboo, C., "New Properties for iCalendar", RFC 7986,
              IETF RFC 7986, DOI 10.17487/RFC7986, October 2016,
              <https://www.rfc-editor.org/info/rfc7986>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", RFC 8174, IETF RFC 8174,
              DOI 10.17487/RFC8174, May 2017,
              <https://www.rfc-editor.org/info/rfc8174>.

13.  Informative References

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", RFC 3552, IETF RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918, IETF RFC
              4918, DOI 10.17487/RFC4918, June 2007,
              <https://www.rfc-editor.org/info/rfc4918>.

   [RFC5378]  Bradner, S., Ed. and J. Contreras, Ed., "Rights
              Contributors Provide to the IETF Trust", RFC 5378,
              IETF RFC 5378, DOI 10.17487/RFC5378, November 2008,
              <https://www.rfc-editor.org/info/rfc5378>.

Author's Address

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   Michael Douglass

   Email: mikeadouglass@gmail.com

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