Where is the stable, open source CardDAV & CalDAV server, equivalent to Courier IMAP/POP? All I can see are Zimbra, some other half-finished Exchange replaceme

Where is the stable, open source CardDAV & CalDAV server, equivalent to Courier IMAP/POP? All I can see are Zimbra, some other half-finished Exchange replacements or PHP monstrosities, or an unmaintained mod_caldav for Apache. All I want is another piece of the Courier suite that integrates with my existing infrastructure, and uses the same modular authentication, TLS certificates etc. I don't want to maintain lots of different stacks in lots of different languages each with their own peculiar auth schemes. Perhaps I should just move the whole damn setup to Zimbra, but at that point it might as well just be Exchange.