Editing SIMPLE (instant messaging protocol) (section)

===Presence===
The SIMPLE presence specifications can be broken up into:
* The '''core protocol machinery'''. This provides the actual SIP extensions for subscriptions, notifications and publications. RFC 6665 defines the SUBSCRIBE and NOTIFY methods. SUBSCRIBE allows users to subscribe to an event on a server, the server responds with NOTIFY whenever the event occurs. RFC 3856 defines how to make use of SUBSCRIBE/NOTIFY for presence. Two models are defined: an end-to-end model in which each User Agent handles presence subscriptions itself; and a centralized model. The latter introduces the concept of a presence server; all subscriptions are handled by this server. The message PUBLISH (RFC 3903) allows User Agents to inform the presence server about their subscription states.
* '''Presence documents'''. The presence information is coded in [[XML]] documents, that are carried in the bodies of the respective SIP messages. RFC 3863 and RFC 4479 describe this procedure, RFC 4480 (RPID), RFC 4481, RFC 4482 (CPID) and various drafts describe contents and formats of the presence documents.
* '''Privacy, policy and provisioning'''. If the centralized model is used, the User Agents need a way to define who may subscribe to which amount of their presence information. RFC 4745 and RFC 5025 define a framework for authorization policies controlling access to application-specific data. The [[XML Configuration Access Protocol|XCAP]] (RFC 4825), carried by HTTP, allows User Agents to communicate their presence rules to a XCAP server, which rules the information exposed by the presence server. RFC 3857 and RFC 3858 define a subscription event "watcher info". User Agents may subscribe to this event to be informed who is subscribing to their presence information.