Some embodiments provide a method for reducing congestion in a network stack that includes a series of components that send data packets through the network stack to a network. At a first component of the network stack, the method receives a data packet from a second component of the network stack. The method identifies a usage indicator value for a flow to which the data packet belongs. The usage indicator value is based on a comparison of a size of the flow to a size of a queue for a third component of the network stack. The method determines whether to send the data packet based on a comparison of the usage indicator value to a threshold usage value. The method sends the data packet to a next component of the network stack only when the usage indicator value is less than the threshold usage value.