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New challenging service scenarios are integrating wireless portable devices with limited and heterogeneous capabilities. They are expected to access both traditional and novel (context-dependent) Internet services. This not only calls for novel infrastructures to support the integration of mobile devices with the fixed network, but also stresses the necessity of negotiation-time tailoring and provision-time adaptation of quality of service (QoS). The paper presents a flexible and dynamic middleware for the management of multimedia QoS to wireless devices that can roam during service provisioning. The middleware exploits mobile agents that act as device shadow proxies over the fixed network to transparently follow the device movements between wireless localities, to negotiate the proper QoS level, and to dynamically adapt multimedia flows depending on device profiles and user preferences. In particular, the paper focuses on how the middleware achieves the on-line visibility of the device change of locality in a portable way over different implementations of different wireless technologies, i.e., IEEE 802.11 and Bluetooth. The first experimental results show that, notwithstanding the application-level approach, the middleware reorganization time is compatible with the requirements imposed by the addressed multimedia scenario.