Introduction - If you have any usage issues, please Google them yourself
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.