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Two-layer hierarchy unstructured peer-to-peer (P2P) systems, comprising an upper layer of superpeers and an underlying layer of ordinary peers, are commonly used to improve the performance of large-scale P2P systems. However, the optimal superpeer network design involves several requirements including superpeer degree, network diameter, scalability, load balancing, and flooding performance. A perfect difference graph has desirable properties to satisfy the above design rationale of superpeers overlay network. This paper proposes a two-layer hierarchical unstructured P2P system in which a perfect difference graph (PDG) is used to dynamically construct and maintain the superpeer overlay topology. In addition, the broadcasting performance of the P2P system is enhanced through the use of a PDG-based forwarding algorithm, which ensures that each superpeer receives just one lookup query flooding message. The theoretical results show that the proposed system improves existing superpeer hierarchical unstructured P2P systems in terms of a smaller network diameter, fewer lookup flooding messages, and a reduced average delay, and the experimental results show that the proposed two-layer hierarchy P2P system performs very well in the dynamic network environment.