This paper identifies the requirements placed against a resource manager function for use in QoS-enabled IP networks. The structure of the Internet and the composition of the services motivate the development of on-line resource brokers that can make admission decisions in real time, based on the best up-to-date information available on the state of the network. The difficulties with traffic models and traffic characterisation stimulate the use of measurement-based techniques that bypass the modelling step and provide the resource requirements of the traffic directly. The paper introduces a novel method for the computation of the bandwidth requirement of Internet traffic flows using the many sources asymptotics. The estimator method significantly reduces the computational complexity of the calculations, allowing the real-time operation of the algorithm even in a measurement-based environment. The method delivers bandwidth-type quantity, the format preferred by Internet control applications. The savings arising from the reduced computational complexity compared to existing methods are demonstrated in a numerical example.