DCMP: A distributed cycle minimization protocol for peer-to-peer networks

Broadcast-based Peer-to-Peer (P2P) networks, including flat (e.g., Gnutella) and two-layer super-peer implementations (e.g., Kazaa), are extremely popular nowadays due to their simplicity, ease of deployment and versatility. The unstructured network topology, however, contains many cyclic paths which introduce numerous duplicate messages in the system. While such messages can be identified and ignored, they still consume a large proportion of the bandwidth and other resources, causing bottlenecks in the entire network. In this paper we describe DCMP, a dynamic, fully decentralized protocol which reduces significantly the duplicate messages by eliminating unnecessary cycles. As queries are transmitted through the peers, DCMP identifies the problematic paths and attempts to break the cycles, while maintaining the connectivity of the network. In order to preserve the fault resilience and load balancing properties of unstructured P2P systems, DCMP avoids creating a hierarchical organization. Instead, it applies cycle elimination symmetrically around some powerful peers to keep the average path length small. The overall structure is constructed fast with very low overhead. With the information collected during this process, distributed maintenance is performed efficiently even if peers quit the system without notification. The experimental results from our simulator and the prototype implementation on PlanetLab, confirm that DCMP improves significantly the scalability of unstructured P2P systems without sacrificing their desirable properties. Moreover, due to its simplicity, DCMP can be easily implemented in various existing P2P systems and is orthogonal to the search algorithms.