Open Calendar Sharing and Scheduling with CalDAV

Open Calendar Sharing and Scheduling with CalDAV

Standards Track

Editor : Jim Whitehead, [email protected] .edu

Building on a decade of work on calendar standards, the CalDAV protocol promises to unlock the potential of widespread calendar interoperability. It permits calendar sharing over the Web and reduces the coordination cost of scheduling meetings across and within organizational boundaries.The protocol extends the Web Distributed Authoring and Versioning (WebDAV) protocol — itself, a simple extension of HTTP — to provide services for calendar maintenance, queries, event scheduling, and security.

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onsider for a moment the effort it takes to schedule a meeting that includes people from multiple organizations. Currently, doing so involves phone calls and multiple rounds of email, usually coordinated by one person, to build consensus for a time that works for most people; the coordinator must also delicately handle cases in which the chosen time excludes one or more participants. For meetings in which everyone comes from the same organization, participants can solve scheduling issues largely via calendar sharing and scheduling software. To schedule a meeting with Oracle Calendar or Microsoft Exchange/Outlook, for example, you log in to a calendar server and then issue search requests for available times among a group of meeting participants. Once the application finds a free time slot, it sends a meeting request and then handles acceptance or rejection replies. Organizations that adopt this tech-

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nology can see dramatic benefits as meeting-coordination time drops from tens to single-digit minutes — as long as everybody uses client software that supports the chosen enterprise application. Unfortunately, these applications work only within existing organizations. Consider what happens when Jim, who works outside the organization, wants to schedule a meeting with Larry and his coworkers, who are inside. Because he doesn’t have an account on the internal calendar system, Jim must send an email to Larry, who then takes on the coordination task to find meeting slots that work for his coworkers. Larry must then negotiate with Jim for a time slot that works for members of Jim’s organization. Even if Larry and Jim both had access to calendar systems for their respective organizations, the process would remain tedious and prone to coordination breakdowns. To efficiently schedule meetings, users need an interoperability protocol that lets

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Published by the IEEE Computer Society

Lisa Dusseault Open Source Applications Foundation Jim Whitehead University of California, Santa Cruz

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Standards Track

a diverse range of calendar clients and servers communicate over the Internet. With such a protocol, we could use existing tools to schedule cross-organization meetings as efficiently as within-organization meetings. This protocol would also make it easier to support calendar access and scheduling functionality on a broad range of devices by providing an open, consistent, standardized way to retrieve calendar data.

History of Calendar Interoperability Protocol developers have long recognized the need for a calendar access and scheduling protocol. A July 1996 press release from Netscape Communications heralded the formation of a working group dedicated to developing standards for calendaring and scheduling on the Internet.1 That group became the IETF’s Calendaring and Scheduling (CalSch) working group, which operated from October 1996 through September 2004. CalSch initially divided its work into three main lines of development: • a data model and textual representation for calendar events (which generated the iCalendar specification), • the transport of calendar information via email and LDAP (which resulted in the iCalendar Message-Based Interoperability Protocol [iMIP]), and • a general-purpose specification for calendar access and scheduling (which became the Calendar Access Protocol [CAP]). A key problem in developing interoperable calendar applications is determining a standard way to represent calendar items, including those that repeat over time (a meeting held every Monday at 11 a.m., for instance). CalSch built on earlier work by the Versit consortium, which developed an initial calendaring and scheduling specification called vCalendar (www.imc.org/pdi/). After two years of refining that work, CalSch produced RFC 2445,2 which is now in widespread use. The iCalendar data format it defines shares vCalendar’s non-XML format for representing attribute-value pairs. CalSch also developed the iCalendar TransportIndependent Interoperability Protocol (iTIP) for calendar retrieval and scheduling operations.3 This document described conceptually how to perform calendar-related operations, but it didn’t provide a concrete, on-the-wire protocol. The iMIP specification,4 which describes how to perform iTIP

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operations via email, has seen some success, including several vendor implementations and some cross-vendor interoperability. However, iMIP isn’t commonly used today by typical calendaring or email applications. CAP provides services that calendar applications can use to access calendar items from remote servers, search for open time periods in another person’s calendar (known as free/busy queries), and schedule meetings. (See www.calsch.org/ ietf/drafts.html for a complete list of CalSch’s documents, drafts, and issues.) In versions 00 through 05 of CAP (released between August 1999 and July 2001), CalSch developed an entirely new protocol that was distinct from all existing application-layer protocols, although it borrowed somewhat from the Post-Office Protocol (POP) for its interaction style. In versions 06 through 11 (November 2001 through July 2003), the working group used the Blocks Extensible Exchange Protocol (BEEP) for its marshalling syntax and messaging behavior.5 CalSch made no further progress on CAP, and the IETF closed the working group in September 2004. After four years of development, CAP was dead. As CAP development was slowly progressing, several implementers were routing around the working group to release functional Internet calendars. In 2002, Apple Computer released its iCal personal calendar application, which supports Internet-based calendar sharing. With iCal, a user can publish a calendar to a server running Web Distributed Authoring and Versioning (WebDAV),6 from which anyone else can view and download events. Although Apple designed the application to integrate with its WebDAV-based .Mac service, iCal works with any WebDAV server. A few opensource clients adopted iCal-over-WebDAV as the de facto first open calendaring standard. In WebDAV,6 Apple made an interesting choice. The protocol extends HTTP to include overwrite prevention (locking), namespace operations (list collection, move, copy, create collection), and metadata (properties). Combined with HTTP’s capabilities for reading, writing, and deleting Web resources, WebDAV provides all the features necessary for remote publishing and sharing of calendars. Given that Apple was already using WebDAV for access to its .Mac Internet disk service, it was able to piggyback calendar sharing on top of the existing WebDAV server infrastructure — a far more attractive option than implementing and fielding server infrastructure for a new protocol

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CalDAV

such as CAP. Apple showed, in a very public way, that calendars could be treated like any other Web resource accessible via HTTP. Although Apple’s iCal provides useful capabilities for individuals to publish and share their calendars, it does have drawbacks for corporate use. It isn’t easy to search for free/busy times across a large set of people or to find other people’s calendars. Due to synchronization issues, iCal also makes it very tricky to have someone else manage your calendar for you. These problems largely stem from protocol-level shortcomings. Although native WebDAV easily supports individual publishing and sharing, it doesn’t provide any support for calendar locating, searching, or workflow scheduling. Yet, Apple’s success with iCal-over-WebDAV raised the question of whether specialized calendar support could be added to WebDAV, rather than requiring a new protocol like CAP. Ideally, we would like a calendar access protocol to support standard within-organization meeting scheduling, as well as collaborative calendar sharing such as a family might use (Figure 1). Additionally, we want to make it much easier to schedule a meeting between participants from multiple organizations (Figure 2). Supporting these scenarios is the motivation for the Calendaring and Scheduling Extensions to WebDAV (CalDAV) protocol.7

Daughter

Father

Mother CalDAV

CalDAV Family calendar March

CalDAV server (at ISP)

Figure 1. Collaborative editing scenario with a shared calendar. This family uses a shared calendar maintained on a CalDAV server run by their ISP. The father uses a laptop computer at work and home to update the calendar; the mother uses a PDA with wireless access. Their daughter keeps in synch with the rest of the family by viewing and updating the calendar from her cell phone.

Open Source Applications Foundation

• Calendar maintenance. Users can create multiple personal calendars (one each for work, conference time slots, home, and so on) via a new mkcalendar method. • Calendar queries. People can search other people’s calendars for free/busy times, or they can discover who is participating in a given meeting. Calendar applications can use queries to discover when to-do list items are due, and a flexible new report type supports a wide range of calendar queries. • Calendar security. Users can control how much of their calendars are visible to others, and who has permission to change them, by using CalDAV extensions to the WebDAV access control protocol. Using the same framework, a separate draft will build on the core CalDAV specification to provide optional scheduling workflow functionality. This

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University of California, Santa Cruz iMIP

Lisa

Jim

CalDAV in a Nutshell The base CalDAV protocol provides three main features:

CalDAV

CalDAV CalDAV

CalDAV CalDAV Lisa's calendar

Jim's calendar

March

March

OSAF CalDAV server

UCSC CalDAV server

Figure 2. Scheduling across an organizational boundary. Lisa and Jim work for two separate organizations and need to schedule a meeting. Using her CalDAV calendar client, Lisa first searches their calendars for free/busy times on the day she’d like to meet. She then sends a meeting invitation, which appears in Jim’s event inbox. Jim accepts the meeting and updates the calendars to show this. will let people using calendar applications make and reply to meeting invitations with a new schedule method along with inbox and outbox collections, and invitation fanout (replication) rules. Meeting participants can be from a single

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Standards Track

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