The provision of novel Internet services has both to specify and to maintain differentiated Quality-of-Service (QoS) levels. Services should tailor to different user QoS preferences together with the differentiated quality properties deriving from servers and access points and devices, from workstations connected with high-capacity networks to wearable devices exploiting limited-capacity wireless links. The paper claims that service provision with negotiated and controlled QoS over best-effort networks calls for a support infrastructure that activates intermediate nodes along the path between clients and servers. In fact, the paper proposes MASQ, an active middleware solution for the QoS management of Video-on-Demand (VoD) streaming. At negotiation time, MASQ exploits code mobility to establish an active path between the requesting client and the VoD server chosen to tailor VoD flows based on user profiles and device properties. At provision time, MASQ dynamically controls the offered QoS level to adapt locally when and where network resource availability changes. MASQ significantly benefits from dynamic and flexible programmability stemming from the employment of high-level policies.