This thesis has been carried out in the field of Computer Networks. We have looked after two challenging and popular multimedia applications over the Internet, namely Real-time Multimedia Applications, considering Multimedia Conferencing and Voice over IP (VoIP) services in general, and Content-Oriented Applications, that are multimedia web services such as User Generated Contents (UGC) platforms and Online Social Networks. We have conducted our studies starting from a thorough review of both functional and non-functional issues related to the above mentioned application families.

We first consider a number of different facets of Real-time Multimedia Applications, dealing with architectural, performance and security issues. We embrace an engineering approach, by involving modeling, implementation and simulation. We try and adhere to standard architectures by also actively contributing to them, arriving at the definition and completion of a fullyfledged standards-compliant web conferencing platform. We propose original contributions to face performance and security issues. With respect to the former aspect, we address scalability and performance analysis by leveraging formal methods. As to the latter point, we design an effective profiling system for VoIP users which is able to detect an interesting class of VoIP threats represented by “social” attacks.