ion Extension Current Work References 5.1 Introduction SIP, the Session Initiation Protocol [1], is a signaling protocol for Internet conferencing, telephony, presence, events notification, and instant messaging. SIP can be used in any applications where session initiation is required, which includes event subscription and notification. Presence information is a status indicator that conveys the ability and willingness of a potential communication partner. The ability to request notification of events proves useful in many types of SIP services for which cooperation between end nodes is required. The SIP specification has been extended to support a notification mechanism, by which SIP nodes can request notification from remote nodes indicating that certain events have occurred, allowing subscription to present information and convey information on the presence of users [2]. SIP nodes can request notification from remote nodes indicating that certain events have occurred and convey the presence information. Presence information conveys the ability and willingness of a user to communicate across a set of devices. A presence service is a system that accepts, stores, and distributes presence information to interested parties (called watchers). A presence protocol is a protocol for providing a presence service over the Internet or any IP (Internet Protocol) networks. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security In this chapter, we first introduce the Session Initiation Protocol (SIP)- Specific Event Notification mechanism, followed by an explanation of what the presence information is. We will then see, based on Presence Informa- tion Data Format, how the notification mechanism works to convey presence information. Then we will introduce the event list notification mechanism that allows for requesting and conveying notifications and presence information for lists of resources. Finally, we will describe the current work, service, and appli- cation of notification mechanism an