CardDAV

CardDAV is an Internet standard that provides access to contact data stored on a remote server. It extends the WebDAV protocol by defining how address books and vCards are stored, retrieved, and manipulated over HTTP. The specification is RFC 6352, published by the IETF in 2011, and it relies on the WebDAV framework to expose address book resources and support common HTTP methods such as GET, PUT, DELETE, and PROPFIND, as well as WebDAV-specific REPORT requests for querying.

Data model and format: CardDAV uses the vCard data format (RFC 6350), with vCard entries representing individual contacts. Address books are collections of vCard resources, organized under a user’s address book home set. Clients can discover available address books, create new vCards, update existing ones, and delete cards. The standard supports multiple address books per user and can implement access control via WebDAV ACLs to control who can read or modify contact data.

Operations and features: Typical CardDAV operations include listing address books, fetching specific vCards, and performing batch-style queries to retrieve multiple entries efficiently. The protocol is designed to support synchronization and incremental updates, enabling clients to keep local contact data in sync with the server. Authentication and security are provided by the underlying WebDAV/HTTP layer, commonly over TLS, with various supported methods.

Adoption and implementations: CardDAV is widely supported by major platforms and server implementations. Client applications on macOS, iOS, Linux, and other systems commonly implement CardDAV for contact synchronization. Server implementations include Radicale, Nextcloud, ownCloud, and SOGo, among others, reflecting CardDAV’s role as a interoperable, platform-agnostic solution for contact management.