Streaming applications in distributed systems are facing different problems at different layers of the Open Systems Interconnection (OSI) stack. Requirements regarding bandwidth, delay, jitter, video and terminal characteristics (temporal and space scalability) and user's preferences come together to influence the architecture of multimedia delivery services with particular quality-of-services (QoS). However the Internet architecture is only capable to support best-effort services, with varying network conditions during multimedia delivery. Currently, a multitude of protection and adaptation strategies exist at different OSI layers, however these strategies are independent of each other. To improve the perceived media quality delivered to end users over heterogeneous networks, QoS strategies should combine mechanisms at different layers, including application layer, transport layer and data link layer. In this paper, we formalize the problem of multimedia streaming application over heterogeneous networks, then discuss the challenges and finally present a putative architecture.