1Myconet: A Fungi-inspired Model forSuperpeer-based Peer-to-Peer Overlay TopologiesPaul L. Snyder and Rachel Greenstadt and Giuseppe ValettoDepartment of Computer ScienceDrexel University{pls29,greenie,valetto}@cs.drexel.eduAbstract—Unstructured peer-to-peer networks can be ex-tremely flexible, but, because of size, complexity, and highvariability in peers’ capacity and reliability, it is a continuingchallenge to build peer-to-peer systems that are resilient tofailure and effectively manage their available resources. Wehave drawn inspiration from the sophisticated, robust, root-likestructures of fungal hyphae to design Myconet, an approach tosuperpeer overlay construction, which models regular peers asbiomass, and superpeers as hyphae that attract and concentratebio-mass, while maintaining strong inter-connections with oneanother. Simulations of the Myconet peer-to-peer protocol showpromising results in terms of network stabilization, response tocatastrophic failure, capacity utilization, and proportion of peersto superpeers, when compared to other unstructured approaches.I. INTRODUCTIONPeer-to-peer (P2P) networks are highly-decentralized, large-scale distributed systems. These networks can be extremelylarge, scaling to millions of peers, with levels of networkmembership that can be highly unstable. Many P2P networksrequire high levels of self-management, resilience in the faceof changing conditions, and central authorization or supervi-sion is impractical or impossible. The inspiration for this workcame from the root structures of fungi, mycelia, and the re-alization that these systems display self-organizing propertiesand can be used as a guiding metaphor for improving overlaynetworks in peer-to-peer systems. Fungi-inspired models havebeen previously proposed as a paradigm for pervasive adaptivesystems [1], [2].Overlay networks impose a topology on top of these oftenchaotic networks, often acting as an enabler for other services(such as search or routing). Overlays may be either structured(as, for example, with deterministically-placed distributed hashtables) or unstructured.We have developed a biologically-inspired model, calledMyconet, for the construction of a robust overlay model withinunstructured P2P networks with superpeers. Myconet is basedon models of fungal growth, and uses the concept of hyphae(the root-like structures of some species of fungi) to guide theself-organization of the network topology.Biologically-inspired metaphors and models have beenlately the subject of active research [3], as they hold promise toenable properties, such as resilience, emergent adapation, andself-organization, that are desirable for large-scale, distributedsystems. In Myconet, the robustness and sophistication ofFig. 1. Mycelium growing [5]natural hyphal structures inspires interconnection strategiesbetween peers and superpeers, the promotion of regular peersto superpeers, and the incremental aggregation of regular peersaround superpeers.This paper describes the Myconet model for overlay net-work constuction, and reports a set of empirical resultsobtained from a simulation of that model developed usingPeerSim [4]. Our results show how Myconet exhibits a setof desirable properties: it spontaneously and quickly achieveshigh levels of capacity utilization; at the same time, itstopology converges towards a nearly optimal proportion ofsuperpeers to regular peers; finally, the network is resilientand recovers quickly in the face of catastrophic networkdisruption. The Myconet overlay is designed to dynamicallymaintain a configurable number links between superpeers tofacilitate network tasks. Myconet makes a design choice toslightly underutilize a subset of superpeers for the purposes ofmaintaining greater flexibility and robustness against failure.We discuss all of these results, and compare them to otherworks in recent literature that also aim at the construction ofrobust and efficient superpeer-based overlay topologies. Wealso report on the engineering lessons that we have learnedwhile developing Myconet and the insights we have gained onthe challenges of designing and implementing a biologically-inspired, self-organized system. We conclude by outliningfuture directions for this line of research and for Myconet.2II. BACKGROUNDSuperpeer approaches to peer-to-peer overlays attempt toexploit the heterogenous capacities of the participating peersto improve performance for the entire network [6], [7]. Su-perpeers may take on service roles for other peers, such asindexing files, routing data, or forwarding searches. Connec-tions between superpeers serve to reduce the network diameterand make these services more efficient.Designing superpeer-based overlay topologies on large-scaleP2P networks is difficult, as no global view of the networkexists. Further, such networks can be extremely dynamicas peers frequently join and leave (whether by failure ordeliberate disconnection). The number of peers needed fora particular network is unlikely to be known in advance, sodecentralized protocols are needed.Fungi, the inspiration for our overlay model, are much morethan mushrooms and yeast. Many fungi reproduce primarilyby vegetative growth; that is, by extending filamentous strands(called hyphae) through the soil (or other growth medium)as depicted in Figure 1. These hyphae search for biomass toassimilate, collecting nutrients and water. The hyphae concen-trate the biomass and also use it to fuel hyphal growth. Thesystem of hyphae is referred to as a mycelium. The myceliumconstantly adapts to changing environmental conditions byrouting nutrients and biomass to areas of need.Mycelia have a number of self-organizing properties thathave led researchers in the pervasive computing communityto propose the use of fungal networks as the inspiration for anew paradigm for decentralized systems [1], [2]. Mycelia growusing decentralized, local interactions from which organizationemerges, the are able to adjust to changing conditions ordamage by dynamically altering hyphal structures. Their manyinterconnections make the overall network both efficient attransporting nutrients and robust to environmental stresses andlocal failures.These properties match particularly well with the desideratafor unstructured peer-to-peer systems, however the fungalmetaphor has not yet been applied in this context. Our ap-proach to superpeer overlay construction, using a protocol wecall Myconet, is modeled after hyphal growth