Data sharing across components, infrastructures, and companies is increasing, enabling diverse types of data processing. At the same time, data providers want to maintain control over their data to protect their privacy or intellectual property. Usage control mechanisms enable restrictions on data processing, for example, to specific processing purposes or timeframes for data use. While existing work addresses various aspects of enforcing usage control policies, it remains unclear how close existing solutions actually come to the goal of enforcing those policies with technical means. This paper details functional and security requirements for proactive data usage control enforcement. We present different levels for the technical enforcement capabilities of components, that each add value for data providers and consumers. We assess the current status of technical solutions and identify main challenges for data usage control enforcement.