st one: SIP. My background was in telecommunications, SO I was familiar with the complex suite of protocols used for signaling in the Public Switched Telephone Network. It was readily apparent to me that SIP would be revolutionary in the telecommunications industry. Only a few weeks later I remember describing SIP to a colleague as the "SS7 of future telephony"-quite a bold statement for a protocol that almost no one had heard of, and that was not even yet a proposed standard! Nearly 2 years later, I have continued to work almost exclusively with SIP since that day in my position with WorldCom, giving seminars and teaching the protocol to others. This book grew out of those seminars and my work on vari- ous Internet-Drafts. This revolutionary protocol was also the discovery of a radical standards body-the Internet Engineering Task Force (IETF). Later, I attended my first IETF meeting, which was for me a career changing event. To interact with this dedicated band of engineers and developers, who have quietly taken the Internet from obscurity into one of the most important technological developments of the late 20th century, for the first time was truly exciting. Just a few short years later, SIP has taken the telecommunications industry by storm. The industry press contains announcement after announcement of SIP product and service support from established vendor startups, and from estab- lished carriers. As each new group and company joins the dialog, the protocol has been able to adapt and grow without becoming unwieldy or overly complex. In SIP: Understanding the Session Initiation Protocol the future, I believe that SIP, along with a TCP/IP stack, will find its way into practically every intelligent electronic device that has a need to communicate with the outside world. With my telecommunications background, it is not surprising that I rely on telephone examples and analogies throughout this book to explain and illus- trate SIP. This is also consistent with the probability that telecommunications is the first