One barrier to deployment of large-scale QoS networks is scalability of QoS routing. Two major sources of overhead incurred by QoS routing are frequent network state updates and QoS-based path computation. Researchers have proposed an impressive array of techniques to reduce these overheads. The goal of this paper is to survey scalability techniques, in the context of a taxonomy intended to illustrate the relationships between different approaches and highlight areas of open research. We also include selected performance results from our own work, to demonstrate the inherent tension between scalability and performance. Our results are obtained using simulations of networks with up to 200 nodes; such large topologies are critical for fully evaluating scalability techniques
