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Peer-to-peer (P2P) and Grid computing systems have emerged as popular models aiming at further utilizing Internet information and resources, complementing the available client-server services. However, the mechanism of peers randomly choosing logical neighbors without any knowledge about underlying physical topology can cause a serious topology mis...
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... mechanisms that a peer joins the P2P network by randomly picking some peers as its logical neighbors, and the nature of flooding search make an inefficient mismatched overlay network and cause large amount of unnecessary traffic. Figure 1 shows two examples of P2P overlay topology (A, B, and D are three participating peers) and physical topology (nodes A, B, C, and D) map- pings, where solid lines denote physical connections and dashes lines denote overlay (logical) connections. Consider the case of a message delivery from peer A to peer B. In the left figure, A and B are both P2P neighbors and physical neighbors. ...
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... only one communication is involved. In the right figure, since A and B are not P2P neighbors, A has to send the message to D before forwarding to B. This will involve 5 communications as indicated in Figure 1. Clearly, such a mapping creates much unnecessary traffic and lengthens the query response time. ...
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... each query, each peer records the neighbors that relay the query to it. Therefore, in the worst case, the same query message can be sent on each link at most twice as illustrated in Figure 1. For an overlay network with n peers, we use c n to denote the average number of neighbors, and use c e to denote the average cost of the logical links. ...
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Citations
... First, it prevents the underlying network topology to be taken into consideration by the peer sampling service. This problem is known as topology mismatch [18]: it usually leads to scenarios where many overlay links are suboptimal with regard to a given network efficiency criteria such as bandwidth or latency. Second, because the overlay structure is random, it fails to exploit the natural heterogeneity [19] of large-scale peer-to-peer systems, and does not take advantage of nodes and links that have a higher capacity. ...
Unstructured peer-to-peer overlay networks are very resilient to churn and topology changes, while requiring little maintenance
cost. Therefore, they are an infrastructure to build highly scalable large-scale services in dynamic networks. Typically,
the overlay topology is defined by a peer sampling service that aims at maintaining, in each process, a random partial view
of peers in the system. The resulting random unstructured topology is suboptimal when a specific performance metric is considered.
On the other hand, structured approaches (for instance, a spanning tree) may optimize a given target performance metric but
are highly fragile. In fact, the cost for maintaining structures with strong constraints may easily become prohibitive in
highly dynamic networks. This chapter discusses different techniques that aim at combining the advantages of unstructured
and structured networks. Namely we focus on two distinct approaches, one based on optimizing the overlay and another based
on optimizing the gossip mechanism itself.
