Network Working Group E. Pot
Internet-Draft fruux GmbH
Expires: December 29, 2016 C. Daboo
E. York
Apple Inc.
June 27, 2016

CalDAV Calendar Sharing


This specification defines sharing calendars between users on a CalDAV server.

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Table of Contents

1. Introduction

Users of CalDAV [RFC4791] often require a mechanism to share a calendar with other users.

In the past this use-case has been fulfilled by non-standard means. This specification aims to describe a standard way for clients and servers to share calendars.

Sharing calendars is for the most part completely implemented using draft-pot-webdav-resource-sharing, but there are a few considerations specific to CalDAV to ensure that mechanisms such as scheduling still behaves as expected.

2. Conventions Used in This Document

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

When XML element types in the namespaces "DAV:" and "urn:ietf:params:xml:ns:caldav" are referenced in this document outside of the context of an XML fragment, the string "DAV:" and "CALDAV:" will be prefixed to the element type names respectively.

3. Calendar sharing

While the draft-pot-webdav-resource-sharing specification allows sharing of potentially any resource on a server, this specification only concerns itself with sharing calendar collections, as defined in CalDAV [RFC4791].

Sharing of resources other than calendar collections is not addressed in this specification.

4. Per-instance calendar data

Servers that support calendar sharing MUST support "per-instance" calendar data in calendar object resources stored in shared calendars. This allows each sharee and the sharer to store their own alarms and free busy transparency status without "interfering" with other users who also have access to the same calendar object resources.

For calendaring object resources in shared calendar collections, the server MUST treat the following iCalendar data objects as per-instance:

5. Scheduling

CalDAV Scheduling [RFC6638] defines how a CalDAV server carries out scheduling operations when calendar object resources are created, modified or deleted and include "ORGANIZER" and "ATTENDEE" iCalendar properties.

Normally when a server schedules events, the server will determine the intent of the user and the action to take by inspecting the "ATTENDEE" and "ORGANIZER" properties, and comparing this to the values of the CALDAV:calendar-user-address-set property defined on the principal resource of the user who owns the calendar-home in which the relevant calendar is contained. However, this can be problematic when a calendar is shared, and appears in multiple calendar collections.

For example, if both sharer and sharee appear as an ORGANIZER of an event, a HTTP DELETE might either cause an event to be cancelled for everyone, or just DECLINED by an attendee depending on who is considered to be the relevant principal performing the action.

This issue can be described more precisely as follows: if a user performs a scheduling operation on a scheduled calendar object, does the user act on behalf of theirselves, or on behalf of the sharer of the calendar?

We've identified that there's some use-cases where you'd want the user to act on behalf of theirselves, and some where the action should be on behalf of the owner. This specification supports both use-cases.

5.1. The CALDAV:calendar-user-address-set WebDAV property

Section 2.4.1 of [RFC6638] describes a CALDAV:calendar-user-address-set WebDAV property defined on WebDAV principals.

This specification extends its usage and allows it to also be defined on individual calendars.

Clients conforming to this specification should inspect every calendar for the existance of this property before doing scheduling operations, whether the calendar is shared or not. Only if the property is not defined on a calendar, it should fall back to the "CALDAV:calendar-user-address-set" property defined on the principal which owns the calendar-home in which the calendar is contained in.

This allows a CalDAV server to indicate to clients on behalf of whom the user-agent should perform scheduling operations.

5.2. Scheduling on read-only calendars

Clients and Servers should NOT allow a user to perform scheduling operations on calendar objects appearing in calendars for which they were not granted read-write access.

5.3. Example use-case: a secretary

A user might need a different user on a calendaring system to manage his or her calendars. This person might be a secretary acting on behalf of a busy individual.

In this particular use-case, when the secretary creates an event, or accepts an invitation, the action must be taken on behalf of the owner of the calendar.

Therefore, the CalDAV server should provide a CALDAV:calendar-user-address-set property on the sharee's instance of the calendar. This property should contain the same value as the CALDAV:calendar-user-address-set property set on the sharer's principal resource.

5.4. Example use-case: a team calendar

On a calendaring server, there might be a calendar containing events which is shared to an team of several people.

This calendar might contain regular meetings for various members in the team. The team calendar is shared to everyone to keep people in the loop with who is meeting who.

When a new scheduling object is created by a user, and this user invites several attendees who are also members of the team, these members need to be able to update their attendence status. In this scenario, every sharee acts on behalf of theirselves in the context of the team calendar.

Therefore, the CALDAV:calendar-user-address-set property should for every instance of the shared calendar either be omitted, or match the sharee principal's value for CALDAV:calendar-user-address-set.

5.5. Effects on schedule-default-calendar-url

The schedule-default-calendar-url WebDAV property, which is defined in Section 9.2 of [RFC6638] defines in which CalDAV calendar new invitations should be delivered.

This specification restricts this property further. The value of schedule-default-calendar-url MUST NOT point to a calendar for which the CALDAV:calendar-user-address-set property is defined and does not match the value of the principal schedule-default-calendar-url is pointing to.

5.6. Effects on items delivered to the scheduling inbox

RFC6638 defines a "Scheduling Inbox Collection". This collection contains notifications of scheduling messages.

If a user is performing scheduling operations on behalf of another user, a CalDAV server MAY also choose to deliver scheduling notifications to sharees for calendars owned by a different user.

If a CalDAV server implements this, the CalDAV server MUST only deliver scheduling messages that relate to scheduling objects that appear on shared calendars.

5.7. Calendar objects appearing more than once in a calendar-home

Implementors of this specification should be aware a calendar object with a particular UID might appear more than once in a single users' calendar home.

An example of this is a situation where a user invites a user to an event, and then also invites the same user to share the calendar where the invitiation was created.

The event might also contain vastly different information. The user might for example only have been invited to a single instance of a recurring event.

Calendaring user agents MAY coelesce events that appear on multiple calendars via their user interface.

A server MUST NOT de-duplicate events.

5.8. Contribution towards free-busy

When calculating free-busy information, the CALDAV:calendar-user-address-set property must be considered.

The implication is that if the CALDAV:calendar-user-address-set is set on a calendar, and it doesn't match the CALDAV:calendar-user-address-set for whom the freebusy report is requested, then the CALDAV:calendar-user-address-set set on the calendar MUST be used to calculate the report.

However, generally shared calendars in which you schedule on behalf of a different user should not be considered, because they SHOULD have a "CALDAV:schedule-calendar-transp" property set with a value of "CALDAV:transparent".

6. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4791] Daboo, C., Desruisseaux, B. and L. Dusseault, "Calendaring Extensions to WebDAV (CalDAV)", RFC 4791, DOI 10.17487/RFC4791, March 2007.
[RFC4918] Dusseault, L., "HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)", RFC 4918, DOI 10.17487/RFC4918, June 2007.
[RFC6352] Daboo, C., "CardDAV: vCard Extensions to Web Distributed Authoring and Versioning (WebDAV)", RFC 6352, DOI 10.17487/RFC6352, August 2011.
[RFC6638] Daboo, C. and B. Desruisseaux, "Scheduling Extensions to CalDAV", RFC 6638, DOI 10.17487/RFC6638, June 2012.
[RFC7303] Thompson, H. and C. Lilley, "XML Media Types", RFC 7303, DOI 10.17487/RFC7303, July 2014.

Appendix A. Change History

Changes in -01:

  1. Added support for CALDAV:calendar-user-address-set on calendars.
  2. Add a large amount of detailed information about scheduling-related behavior.

Authors' Addresses

Evert Pot fruux GmbH Koenigsstrasse 32 Muenster, NRW 48143 Germany EMail: URI:
Cyrus Daboo Apple Inc. 1 Infinite Loop Cupertino, CA 95014 USA EMail: URI:
Eric York Apple Inc. 1 Infinite Loop Cupertino, CA 95014 USA URI: