 globe to another service provider and maintain not only their URL "number", but also their personal tailored services and prefer- ences. The end user gains control over all possible preferences, depending on various parameters such as who the other party is, what network he is on and what devices he is using, as well as time of day, subject, and other variables. The new dimension in communications called "presence" enables users for the first time to indulge in "polite calling" by first sensing presence and pref- erences of the other party, before making a call. In its turn, presence can trigger location- and time-dependent user preferences. Users may want to be contacted in different ways, depending on their location and type of network access. E-commerce will also benefit from IP communications. Extremely com- plex telecom applications, as found in call centers, have become even more com- plex when integrated with e-mail and web applications for e-commerce. Such applications, however, are quite straightforward to implement using SIP, due to its common structure with the web and e-mail. For example, both call routing and e-mail routing to agents-based on various criteria such as queue length, skill set, time of day, customer ID, the web page the customer is looking at, and customer history-can be reduced to simple XML scripts when using SIP and another IETF standard, the Call Processing Language (CPL). These examples are in no way exhaustive, but are mentioned here as a way of introduction. This book starts with a short summary of the Internet, the World Wide Web, and its core protocols and addressing. Though familiar to many readers, these chapters provide useful focus on issues for the topics ahead. The introduc- tion to SIP is made easy and understandable by examples that illustrate the pro- tocol architecture and message details. Finally, in the core of the book, a methodical and complete explanation of SIP is provided. We refer the reader to the Table of Contents for a better overview and navigation throug