http://www.rfc-editor.org/rfc/rfc4235.txt
RFC 4235: An INVITE-Initiated Dialog Event Package for the Session Initiation Protocol (SIP), J. Rosenberg, H. Schulzrinne, R. Mahy2005-11-01T23:00:00-00:00This document defines a dialog event package for the SIP Events architecture, along with a data format used in notifications for this package. The dialog package allows users to subscribe to another user and to receive notification of the changes in state of INVITE-initiated dialog usages in which the subscribed-to user is involved. [STANDARDS-TRACK]