  As WiFi grows ever more popular, airtime contention becomes an increasing
problem. One way to alleviate this is through network policy enforcement.
Unfortunately, WiFi lacks protocol support for configuring policies for its
usage, and since network-wide coordination cannot generally be ensured,
enforcing policy is challenging. However, as we have shown in previous work, an
access point can influence the behaviour of connected devices by changing its
scheduling of transmission opportunities, which can be used to achieve airtime
fairness. In this work, we show that this mechanism can be extended to
successfully enforce airtime usage policies in WiFi networks. We implement this
as an extension our previous airtime fairness work, and present PoliFi, the
resulting policy enforcement system. Our evaluation shows that PoliFi makes it
possible to express a range of useful policies. These include prioritisation of
specific devices; balancing groups of devices for sharing between different
logical networks or network slices; and limiting groups of devices to implement
guest networks or other low-priority services. We also show how these can be
used to improve the performance of a real-world DASH video streaming
application.
