 a special SIP entity that handles registration from SIP user agents but does not route SIP messages. It receives registrations from users, extracts information about their current locations (such as IP address, port, and user name), and then stores the information in local databases. A registrar is very often a logical entity only and is usually collocated with proxy servers. A redirect server is yet another optional SIP component. A redirect server receives requests and looks up the intended recipient of a request in the location database created by a registrar. It then creates a list of current locations of the user and sends this list to the request originator. The redirect server does not route SIP messages; rather, it returns a redirect to the user agent for direct routing. SIP and Presence Systems Presence is defined as the ability, willingness, desire, and capability of a user to communicate across media end devices and even time and space. Presence systems collect and distribute presence information to interested par- ties such as other users or applications. Policy-that is, who is allowed to see what and when-is central to presence. The value of presence is based on the richness of the data it has access to. Presence is the ability to see in real-time where someone is, how that person prefers to be reached, and even what the person is doing. Today, only one in seven business calls is completed successfully. Instead, we get busy signals or are routed to voicemail. Users spend a lot of time trying many different numbers for the same person and calling many numbers in succession, trying to reach someone. Presence, therefore, has great value in improving the pro- ductivity of an organization because it can see ahead of time whether a call will succeed. It can tell the caller which is the best way to reach a user, and it can allow a caller to quickly see who among a set of candidates is available for a call. Accuracy is paramount to presence. The greater the accuracy, the greater the value of presence. The w