Jussi Kangasharju

Object replication strategies in content distribution networks

Recently, the Internet has witnessed the emergence of content distribution networks (CDNs). In this paper, we study the problem of optimally replicating objects in CDN servers. In our model, each Internet autonomous system (AS) is a node with finite storage capacity for replicating objects. The optimization problem is to replicate objects so that when clients fetch objects from the nearest CDN server with the requested object, the average number of ASs traversed is minimized. We formulate this problem as a combinatorial optimization problem. We show that this optimization problem is NP complete. We develop four natural heuristics and compare them numerically using real Internet topology data. We find that the best results are obtained with heuristics that have all the CDN servers cooperating in making the replication decisions. We also develop a model for studying the benefits of cooperation between nodes, which provides insight into peer-to-peer content distribution.

Bubblestorm: resilient, probabilistic, and exhaustive peer-to-peer search

Peer-to-peer systems promise inexpensive scalability, adaptability, and robustness. Thus, they are an attractive platform for file sharing, distributed wikis, and search engines. These applications often store weakly structured data, requiring sophisticated search algorithms. To simplify the search problem, most scalable algorithms introduce structure to the network. However, churn or violent disruption may break this structure, compromising search guarantees. This paper proposes a simple probabilistic search system, BubbleStorm, built on random multigraphs. Our primary contribution is a flexible and reliable strategy for performing exhaustive search. BubbleStorm also exploits the heterogeneous bandwidth of peers. However, we sacrifice some of this bandwidth for high parallelism and low latency. The provided search guarantees are tunable, with success probability adjustable well into the realm of reliable systems. For validation, we simulate a network with one million low-end peers and show BubbleStorm handles up to 90% simultaneous peer departure and 50% simultaneous crash.

Distributing layered encoded video through caches

The efficient distribution of stored information has become a major concern in the Internet which has increasingly become a vehicle for the transport of stored video. Because of the highly heterogeneous access to the Internet, researchers and engineers have argued for layered encoded video. We investigate delivering layered encoded video using caches. Based on the stochastic knapsack theory, we develop a model for the layered video caching problem. We propose heuristics to determine which videos and which layers in the videos should be cached in order to maximize the revenue from the streaming service. We evaluate the performance of our heuristics through extensive numerical experiments. We find that, for typical scenarios, the revenue increases nearly logarithmically with the cache size and linearly with the link bandwidth that connects the cache to the origin servers. We also consider service models with request queuing and negotiations about the delivered stream quality and find that both extensions provide only small revenue increases.

Mocha: a quality adaptive multimedia proxy cache for internet streaming

Multimedia proxy caching is a client-oriented solution for large-scale delivery of high quality streams over heterogeneous networks such as the Internet. Existing solutions for multimedia proxy caching are unable to adjust quality of cached streams. Thus these solutions either can not maximize delivered quality or exhibit poor caching efficiency. This paper presents the design and implementation ofMocha, a quality adaptive multimedia proxy cache for layered encoded streams. The main contribution of Mocha is its ability to adjust quality of cached streams based on their popularity and on the available bandwidth between proxy and interested clients. Thus Mocha can significantly improve caching efficiency without compromising delivered quality. To perform quality adaptive caching, Mocha implementsfine-grained replacementandfine-grained prefetchingmechanisms. We describe our prototype implementation of Mocha on top of Squid and address various design challenges such as managing partially cached streams. Finally, we validate our implementation and present some of our preliminary results.

Performance evaluation of redirection schemes in content distribution networks

Content distribution on the Web is moving from an architecture where objects are placed on a single, designated server to an architecture where objects are replicated on geographically distributed servers and clients transparently access a nearby copy of an object. In this paper we study how the different redirection schemes used in modern content distribution networks affect the user-perceived performance in normal Web page viewing. Using both simulations and experiments with real Web servers we show that redirection schemes that require clients to retrieve different parts of a Web page from different servers yield sub-optional performance compared to schemes where a client accesses only one server for all the parts of a Web page. This implies that when replicating Web pages, we should treat the whole page (HTML and images) as a single entity.

Floating content: Information sharing in urban areas

Content sharing using personal web pages, blogs, or online social networks is a common means for people to maintain contact with their friends, colleagues, and acquaintances. While such means are essential to overcome distances, using infrastructure services for location-based services may not be desirable. In this paper, we analyze a fully distributed variant of an ephemeral content sharing service, solely dependent on the mobile devices in the vicinity using principles of opportunistic networking. The net result is a best effort service for floating content in which: 1) information dissemination is geographically limited; 2) the lifetime and spreading of information depends on interested nodes being available; 3) content can only be created and distributed locally; and 4) content can only be added, but not explicitly deleted. First we present our system design and summarize its analytical modeling. Then we perform extensive evaluation for a map-based mobility model in downtown Helsinki to assess the operational range for floating content, which, at the same time also validate the analytical results obtained for a more abstract model of the system.

MundoCore: A light-weight infrastructure for pervasive computing

MundoCore is a communication middleware specifically designed for the requirements of pervasive computing. To address the high degree of heterogeneity of platforms and networking technologies, it is based on a microkernel design, supports dynamic reconfiguration, and provides a common set of APIs for different programming languages (Java, C++, Python) on a wide range of different devices. The architectural model addresses the need for proper language bindings, different communication abstractions, peer-to-peer overlays, different transport protocols, different invocation protocols, and automatic peer discovery.

Realizing the Internet of Nano Things: Challenges, Solutions, and Applications

Embedding nanosensors in the environment would add a new dimension to the Internet of Things, but realizing the IoNT vision will require developing new communication paradigms and overcoming various technical obstacles.

Optimizing File Availability in Peer-to-Peer Content Distribution

A fundamental paradigm in peer-to-peer (P2P) content distribution is that of a large community of intermittently-connected nodes that cooperate to share files. Because nodes are intermittently connected, the P2P community must replicate and replace files as a function of their popularity to achieve satisfactory performance. In this paper, we develop an analytical optimization theory for benchmarking the performance of replication/replacement algorithms, including algorithms that employ erasure codes. We also consider a content management algorithm, the Top-K Most Frequently Requested algorithm, and show that in most cases this algorithm converges to an optimal replica profile. Finally, we present two approaches for achieving an evenly balanced load over all the peers in the community.

iClouds – Peer-to-Peer Information Sharing in Mobile Environments

The future mobile and ubiquitous computing world will need new forms of information sharing and collaboration between people. In this paper we present iClouds, an architecture for spontaneous mobile user interaction, collaboration, and transparent data exchange. iClouds relies on wireless ad hoc peer-to-peer communications. We present the iClouds architecture and different communication models, which closely resemble familiar communication forms in the real world. We also design a hierarchical information structure for storing the information in iClouds. We present our prototype implementation of iClouds which runs on wireless-enabled PDAs.