Calendaring Extensions to WebDAV, or CalDAV, is an Internet standard allowing a client to access scheduling information on a remote server. It extends WebDAV (HTTP-based protocol for data manipulation) specification and uses iCalendar format for the data. The protocol is defined by RFC 4791. It allows multiple client access to the same information thus allowing cooperative planning and information sharing. Many server and client applications support the protocol.
The CalDAV specification was first published in 2003 by Lisa Dusseault as an Internet Draft submitted to the Internet Engineering Task Force (IETF), and it quickly gained support from several calendaring software vendors. In January 2005 the first interoperability event was organized by the CalConnect consortium. Since March 2007, the CalDAV specification is described in the RFC 4791. CalDAV is designed for implementation by any collaborative software, client or server, that needs to maintain, access or share collections of events. It is being developed as an open standard to foster interoperability between software from different implementors.
The architecture of CalDAV (partially inherited from the underlying specifications) organizes the data (events, tasks, free-busy info, notes) in directories (collections), where multiple items (resources) reside. The resources and collections can be accessed by one or more users, using standard HTTP and DAV semantics to detect conflicting changes, or to provide locking.
For access control the concept of ACLs are used, so each operation (view, edit, delete etc.) can be denied or granted per user. Therefore the specification requires that CalDAV servers must support "WebDAV Access Control Protocol" (RFC 3744). The calendar resources must use iCalendar format, which allows the server to understand and process the data. Parsing the iCalendar items is necessary, because the server has to support a number of calendaring-specific operations such as doing free-busy time reports and expansion of recurring events. With this functionality, a user may synchronize his or her own calendar to a CalDAV server, and share it among multiple devices or with other users. The protocol also supports non-personal calendars, such as calendars for sites or organizations.
Some developers have criticized CalDAV's complexity, claiming that the difficulty of implementing it will lead to implementations that can exhibit small bugs when talking to each other, much as has happened with IMAP for mail. However, several developers who went ahead with implementations have said it is not hard to get something working quickly, and many organizations have committed to providing CalDAV products and serious momentum is now building behind having it become a viable standard for calendaring and scheduling across the internet.
On August 7, 2006, Apple Computer announced that Mac OS X 10.5 "Leopard" would include iCal 3.0, an application that supports the CalDAV access and scheduling standards. Mac OS X Server 10.5 "Leopard Server" includes iCal Server, which implements the CalDAV access and scheduling protocols.. The iCal Server has been released under an open source license as the Darwin Calendar Server. on March 17, 2009, Apple Computer announced that CalDAV would be included in the iPhone 3.0 SDK.
Bedework, an open-source enterprise calendar system, implements the CalDAV access and scheduling protocols , as do a variety of free CalDAV server software (with varying levels of adherence to specifications), including DAViCal, Cosmo (the server-side companion to Chandler), Zimbra, openCRX and several others.
Oracle Beehive, a unified communication and collaboration software solution, supports a number of open standards including CalDAV [5 ]. This allows Beehive to work with a number of calendaring clients including Apple iCal, Mozilla Lightning, and Mozilla Sunbird [6 ].
The Mozilla Calendar Project applications (Lightning, a plugin for Thunderbird and Sunbird, a standalone version) also support CalDAV calendars. Other freely available client software include Evolution, Mulberry, Chandler and eM Client.
Synchronica, a developer of mobile push email and synchronization solutions announced that their Synchronica Mobile Gateway and Synchronica Mobile Backup products are both fully compatible with the CalDAV standard, allowing compatibility across a wide range of calendar applications.
Atmail provides a complete client & server Calendaring solution - Based on the CalDAV protocol, Atmail provides a complete CalDAV client, full server implementation, and support for a wide range of desktop clients and mobile devices.