A standard for group calendaring and scheduling from the IETF. It enables calendaring data to be transmitted and automatically entered into the recipient's schedule. In addition a request for a meeting with free and busy times might be sent, and the recipient's software could book the time automatically and send back the response. Proposed as a standard to replace proprietary specifications such as in Outlook/Exchange, Lotus Notes and Groupwise, iCalendar messages are files with an .ical, .ics, .ifb or icalendar extension. The IETF Calendaring and Scheduling Standards Simplification group, dubbed "calsify," works on interoperability between different scheduling systems. The Successor to vCalendar iCalendar superseded the vCalendar standard developed by Versit, then turned over to the Internet Mail Consortium (IMC) and later the IETF. It added more detail to vCalendar events and defined a protocol for transporting it.