Described are a system and method of providing quality of service assurances for transmitting packets over a channel capable of transmission at a nominal bandwidth. A plurality of classes is defined. Each class represents an aggregate behavior of packets and is allocated a nominal departure rate at which the packets of that class are transmitted when an available bandwidth of the channel is substantially at the nominal bandwidth. Each class is assured a minimum allocation of the available bandwidth for transmitting packets of that class if the available bandwidth of the channel is operating at less than the nominal bandwidth. The minimum allocations to each of the classes can be proportionally different than the nominal departure rates allocated to these classes.