Hierarchical Fair Service Curve (HFSC) is based on a QoS and a CBQ algorithm which is used mostly on Unix operating systems. It is the first QoS algorithm to simultaneously support all three of real-time, adaptive best-effort, and link-sharing services.
Also used in NetBSD 5.0, FreeBSD 8.0 and OpenBSD 4.6; and the router firmware DD-WRT.