scope:

This document defines a Session Initiation Protocol (SIP) event package for registrations. Through its REGISTER method, SIP allows a user agent to create, modify, and delete registrations. Registrations can also be altered by administrators in order to enforce policy. As a result, these registrations represent a piece of state in the network that can change dynamically. There are many cases where a user agent would like to be notified of changes in this state. This event package defines a mechanism by which those user agents can request and obtain such notifications.