Which method is used by a watcher to get state updates from a Presentity?

The method used by a watcher to get state updates from a Presentity is the Subscribe method. In the context of the Session Initiation Protocol (SIP), a Presentity is an entity that has a certain presence state, such as online, offline, busy, etc.

When a watcher is interested in receiving updates about the state of a Presentity, they initiate a subscription to that Presentity’s presence information. This subscription is typically done by sending a SIP SUBSCRIBE request to the Presentity. The Presentity then acknowledges this subscription and provides updates regarding its state changes through NOTIFY messages.

The Subscribe method is essential because it establishes the communication channel necessary for a watcher to receive ongoing updates. Each time a change occurs in the Presentity's state, the Presentity sends a NOTIFY message to all interested watchers who have subscribed, thus keeping them informed of any relevant changes.

In contrast, the other methods listed serve different purposes. Notify is the method used by the Presentity to inform the watcher of state changes after a subscription is established. Query generally refers to a process of retrieving information rather than subscribing for updates. Publish is used by a Presentity to announce its state to interested watchers but does not pertain to the mechanism of receiving updates