An impairment unit, method, and machine readable storage media for emulating network impairments. An impairment unit may include a network interface to receive network traffic from a network and to transmit impaired network traffic to the network. An impairment engine may generate the impaired traffic based on the received network traffic, the impaired network traffic including at least some duplicate packets. The impairment engine may determine a bandwidth available for transmission of duplicate packets based on the received network traffic and may constrain the total length of the duplicate packets to fit within the available bandwidth.