Read this white paper from Microsoft – it describes the RTC Platform and the base protocols: SIP, RTP, SIMPLE.
This paper is written for IT professionals and developers interested in understanding the concepts, protocols, and technologies of real-time communications. It describes protocols such as the Internet Engineering Task Force (IETF) Session Initiation Protocol (SIP), SIP Instant Messaging and Presence Language Extensions (SIMPLE), and Real-time Transport Protocol (RTP). Microsoft uses these protocols and related technologies to provide a real-time communications (RTC) platform for corporate multi-modal communication, which includes voice and video communication, instant messaging, application sharing, and collaboration. Throughout this paper, voice communication and the way the Microsoft Windows XP operating system supports it are used to illustrate how the underlying technologies work.