The IMS: IP Multimedia Concepts And Services, Second Edition

4.10. Watcher information event template package

Users subscribe to receive information about the state of a particular resource using an event package (referred to as the "main package" in this section). These subscribers can be referred to as "watchers". A watcher information template package allows a user to gain knowledge about watchers and the state of their subscriptions to the main package. A watcher information template package is identified with the token "winfo".

The primary use of this template package is for presence. Users subscribe to this template package to see who is subscribing to their presence information and the state of that subscription.

The information carried to the watcher information subscriber contains two important items: the status of each subscription made by the watchers of the main package and the event that caused the transition from the previous status to the current one.

The states of the watcher information package are described in [RFC3857] as follows:

• Init – no state is allocated for a subscription.

• Terminated – a policy exists that forbids a watcher from subscribing to the main event package.

• Active – a policy exists that authorizes a watcher to subscribe to the main package.

• Pending – no policy exists for that watcher.

• Waiting – similar to pending, but tells the template package subscriber that a user has attempted to subscribe to the main package and that the subscription expired before a policy was created.