What SIP method is used to initially set up a watcher observing another person's presence state?

The method used to initially set up a watcher observing another person's presence state is called Subscribe. In the context of the Session Initiation Protocol (SIP) and Presence Information Data Format (PIDF), a watcher is a user who wants to know the presence status of another user, often referred to as the "presentity."

When a watcher subscribes to a presentity's presence information, they send a Subscribe request to the presentity's server. This request establishes a monitoring relationship, allowing the watcher to receive updates about the presentity's status. The server then responds with a Notify message that contains the presentity's current presence information. This interaction is fundamental to the SIP-based presence system, enabling real-time status sharing among users.

Other methods listed, such as Notify, Create, and Register, serve different purposes within the SIP framework. Notify, for instance, is used to inform subscribers of changes in presence state, while Register is involved in registering a user's location with a SIP server. Create does not pertain to SIP methods relevant to presence and is therefore not applicable in this context.