Evaluation of Unstructured Overlay Maintenance Protocols under Churn

An overlay network is formed on top of - and generally independently from - the underlying physical computer network, by the peers (nodes) of a P2P system. The dynamics of peers is taken into account by devising appropriate overlay maintenance protocols that are able to join and leave peers from the overlay. Due to the need for scaling in the number of nodes, overlay maintenance protocols have been simulated only in environments showing a very restricted behavior with respect to the possible concurrent and interleaved execution of join/leave operations. In this paper we compare two overlay maintenance protocols well suited to unstructured P2P systems, namely SCAMP and Cyclon, in an event-based simulation setting including concurrent and interleaved join and leave operations as well as variable message transfer delay. This simulation setting allows to point out surprising results for both protocols. In particular, under a continuous and concurrent replacement of nodes, permanent partitioning of the overlay arises after a very small number of join/leave operations.