The document proposes using MPEG-21 metadata to enable cross-layer optimizations for improving quality of experience. It presents a three-step approach: (1) describing relationships between quality metrics across network layers in a Cross-Layer Model, (2) instantiating the model using MPEG-21 metadata descriptions of usage environment, constraints and adaptations, and (3) implementing a decision engine to optimize adaptations based on the model and descriptions. An example shows how MPEG-21 could be used to optimize scalable video streaming across spatial, temporal and quality layers.