RFC 4660: Functional Description of Event Notification Filtering
['RFC 6665: SIP-Specific Event Notification', 'RFC 4660: Functional Description of Event Notification Filtering', 'Session Initiation Protocol', 'Request for Comments', 'RFC 7621: A Clarification on the Use of Globally Routable User Agent URIs (GRUUs) in the SIP Event Notification Framework']