There is a message-based interoperability protocol for iCalendar: https://datatracker.ietf.org/doc/rfc5546/ https://datatracker.ietf.org/doc/rfc5546/ and https:

There is a message-based interoperability protocol for iCalendar: https://datatracker.ietf.org/doc/rfc5546/ https://datatracker.ietf.org/doc/rfc5546/ and https://datatracker.ietf.org/doc/rfc6047/ https://datatracker.ietf.org/doc/rfc6047/. This is what Google Mail + Calendar are using, including features like RSVP, etc. It's all there, just passing iCal objects around.