Building and maintaining overlay networks for bandwidth-demanding applications
The demands of Internet applications have grown significantly in terms of required resources and types of services. Overlay networks have emerged to accommodate such applications by implementing more services on top of IP (Internet Protocol). However, while overlay networks are successful in circumventing limitations of IP, the task of building and maintaining an overlay network is still challenging. In an overlay network, participating hosts are virtually fully-connected through the underlying Internet. However, since the quality of overlay connections varies, the performance of the overlay network is dependent on which connections are chosen to be utilized. Therefore, maintaining a “good” overlay network topology is crucial in achieving high performance. To demonstrate how much performance gain can be achieved through topology changes, a distributed algorithm to build an overlay multicast tree is proposed for streaming media distribution. The algorithm finds an optimal tree such that the average bandwidth of receivers is maximized under an abstract network model. However, increasing bandwidth does not necessarily lead to a better overlay topology; in overlay networks, interference between overlay connections should be taken into account. Since such interference occurs when different overlay connections pass through a congested link simultaneously, detecting congestion shared by multiple overlay connections is necessary to avoid bottlenecks. For shared congestion detection, a novel technique called DCW (Delay Correlation with Wavelet denoising) is proposed. Previous techniques to detect shared congestion have limitations in applying to overlay networks; they assume a common source or destination node, drop-tail queueing, or a single point of congestion. However, DCW is applicable to any pair of paths on the Internet without such limitations. It employs a signal processing method, wavelet denoising, to separate queueing delay caused by network congestion from various other delay variations. The proposed technique is evaluated through both simulations and Internet experiments. They show that for paths with a common synchronization point, DCW provides faster convergence and higher accuracy while using fewer packets than previous techniques. Furthermore, DCW is robust and accurate without a synchronization point; more specifically, it can tolerate a synchronization offset of up to one second between two packet flows. Because DCW is designed to detect shared congestion between a pair of paths, there is a concern about scalability when it is used in a large-scale overlay network. To cluster N paths, a straightforward approach of using pairwise tests would require O(N2 ) time complexity. To address this issue, a scalable approach to cluster Internet paths using multidimensional indexing is presented. By storing per-path data in a multidimensional space indexed using a tree-like structure, the computational complexity of clustering is reducible to O(N log N). The indexing overhead can be further improved by reducing dimensionality of the space through the wavelet transform. Computation cost is kept low by using the same wavelet transform for both denoising in DCW and dimensionality reduction. The proposed approach is evaluated using simulations and found to be effective for large N. The tradeoff between indexing overhead and clustering accuracy is shown empirically. As a case study, an algorithm that improves overlay multicast topology is designed. Because overlay multicast forwards data without support from routers, data may be delivered multiple times over the same physical link, causing a bottleneck. This problem is more serious for applications demanding high bandwidth such as multimedia distribution. Although such bottlenecks can be removed by overlay topology changes, a na¨ıve approach may create bottlenecks in other parts of the network. The proposed algorithm removes all bottlenecks caused by the redundant data delivery of overlay multicast, detecting such bottlenecks using DCW. In a case where the source rate is constant and the available bandwidth of each link is not less than the rate, the algorithm guarantees that every node receives at the full source rate. Simulation results show that even in a network with a dense receiver population, the algorithm finds a tree that satisfies all the receiving nodes while other heuristic-based approaches often fail. A similar approach to finding bottlenecks and removing them through topology changes is applicable to other types of overlay networks. This research will enable bandwidth-demanding applications to build more efficient overlay networks to achieve higher throughput.