stage a particular specifi- cation is at present. CHAPTER The Internet Multimedia Conferencing Architecture Chapter 3 Now that we have stipulated how the Internet Engineering Task Force (IETF) works and some general characteristics found in all the protocols designed by the IETF community, Session Initiation Protocol (SIP) among them, we can turn our attention to the context in which SIP is used. We will first see which protocols are used below SIP in the transport layer and which functionality they provide. We'll also discuss what motivated the Internet Engineering Steering Group (IESG) to charter the SIP working group in the first place, in order to understand where SIP is intended to work. We'll focus on the Internet multimedia conferencing architecture. This architecture includes a set of Internet protocols that together provide multimedia services in the Internet environment. SIP is part of this architecture, which proves to be one of SIP's strengths. As such, it interacts smoothly with the rest of the protocols and exploits their functionality; that is, SIP is one tool inside the toolkit that this architecture represents. The Internet Layered Architecture We have said that one of the most important strengths of the Internet is its suitability to be used as a service creation platform. The IETF adheres to a layered approach to create services. The Internet layered model consists of four layers that are implemented on top of the physical layer (Figure 3-1), which are commonly known as the Transmission Control Protocol/Internet Protocol (TCP/IP) protocol suite. A transport layer is implemented on top of the common IP layer and application layer protocols make use of the different transport protocols. A particular application aiming to provide a certain service picks up the appli- cation layer protocols required to implement the service. Let's say we want to build a Web browser that enables users to surf the Internet and to read e-mail. To do so, we will have to pick up two applica- tion layer protocols. We cho