The challenges of merging two similar structured overlays: A tale of two networks ⋆ Anwitaman Datta and Karl Aberer anwitaman.datta@epfl.ch, karl.aberer@epfl.ch Ecole Polytechnique F´ed´erale de Lausanne (EPFL) CH-1015 Lausanne, Switzerland Abstract. Structured overlay networks is an important and interesting primitive that can be used by diverse peer-to-peer applications. Multi- ple overlays can result either because of network partitioning or (more likely) because different groups of peers build such overlays separately before coming in contact with each other and wishing to coalesce the overlays together. This paper is a first look into how multiple such over- lays (all using the same protocols) can be merged - which is critical for usability and adoption of such an internet-scale distributed system. We elaborate how two networks using the same protocols can be merged, looking specifically into two different overlay design principles: (i) main- taining the ring invariant and (ii) structural replications, either of which are used in various overlay networks to guarantee functional correctness in a highly dynamic (membership changes) environment. Particularly, we show that ring based networks can not operate until the merger operation completes. In contrast, from the perspective of individual peers in structurally replicated overlays there is no disruption of service, and they can continue to discover and access resources that they could originally do before the beginning of the merger process, even though resources from the other network become visible only gradually with the progress of the merger process. 1 Introduction In the recent years there has been an increasing trend to use resources at the edge of the network - typically desktop computers interconnected across the internet provide services and run applications in a peer-to-peer manner, as an alternative to the traditional paradigm of using dedicated infrastructure and centralized coordination and control. Many peer-to-peer applications need some basic functionalities, particularly that of locating resources efficiently in a dis- tributed large-scale environment in a decentralized manner. Structured overlay ⋆ The work presented in this paper was supported (in part) by the National Com- petence Center in Research on Mobile Information and Communication Systems (NCCR-MICS), a center supported by the Swiss National Science Foundation under grant number 5005-67322 and was (partly) carried out in the framework of the EPFL Center for Global Computing and supported by the Swiss National Funding Agency OFES as part of the European project Evergrow No 001935.