Tourist : A Self - Adaptive Structured Overlay in Heterogeneous P 2 P networks

Structured overlays provide a basic substrate for many peer-to-peer systems. Existing structured overlays can be classified into two categories, O(logN)-hop ones and O(1)-hop ones. The former are suitable for large and dynamic systems, while the latter are suitable for small or stable ones. However, for the lack of adaptivity, it is difficult for a peer-to-peer system designer to choose from these two kinds of overlays because the eventual size and churn rate are not easy to predict in the design phase. To address this problem, we propose a self-adaptive structured overlay Tourist, which can adapt itself to the changing environment dynamically. On the one hand, Tourist can achieve 1-hop to 2-hop routing in most cases (e.g., in a 1,000,000-node system where nodes’ average lifetime is only 1 hour). On the other hand, when the system size is extremely large or the nodes’ churn rate is very high, Tourist can always guarantee O(logN)-hop routing for all the messages. Tourist nodes determine their routing table size autonomously: nodes with different capacities hold routing tables with different sizes. This makes Tourist sufficiently utilize all the nodes’ allowable bandwidth to achieve as high routing efficiency as possible. Tourist also allows each node to adjust its routing table size dynamically, which is the essential reason for the selfadaptivity. Simulation results show that in a common 1,000,000node system, Tourist can route all the messages within two hops and each node only pays no more than 1% bandwidth for its routing table maintenance. Only when the churn rate is very high (nodes’ average lifetime being less than 1.5 minutes) Tourist would turn to an O(logN)-hop overlay where there are some messages traversing more than two hops. Keywords-structured overlay; peer-to-peer; DHT routing