In a data network congestion control in a virtualized environment is enforced in packet flows to and from virtual machines in a host. A hypervisor and network interface hardware in the host are trusted components. Enforcement comprises estimating congestion states in the data network attributable to respective packet flows, recognizing a new packet that belongs to one of the data packet flows, and using one or more of the trusted components and to make a determination based on the congestion states that the new packet belongs to a congestion-producing packet flow. A congestion-control policy is applied by one or more of the trusted components to the new packet responsively to the determination.