SoS: self-organizing substrates

Large-scale networked systems often, both by design or chance exhibit self-organizing properties. Understanding self-organization using tools from cybernetics, particularly modeling them as Markov processes is a first step towards a formal framework which can be used in (decentralized) systems research and design.Interesting aspects to look for include the time evolution of a system and to investigate if and when a system converges to some absorbing states or stabilizes into a dynamic (and stable) equilibrium and how it performs under such an equilibrium state. Such a formal framework brings in objectivity in systems research, helping discern facts from artefacts as well as providing tools for quantitative evaluation of such systems. This thesis introduces such formalism in analyzing and evaluating peer-to-peer (P2P) systems in order to better understand the dynamics of such systems which in turn helps in better designs. In particular this thesis develops and studies the fundamental building blocks for a P2P storage system. In the process the design and evaluation methodology we pursue illustrate the typical methodological approaches in studying and designing self-organizing systems, and how the analysis methodology influences the design of the algorithms themselves to meet system design goals (preferably with quantifiable guarantees). These goals include efficiency, availability and durability, load-balance, high fault-tolerance and self-maintenance even in adversarial conditions like arbitrarily skewed and dynamic load and high membership dynamics (churn), apart of-course the specific functionalities that the system is supposed to provide. The functionalities we study here are some of the fundamental building blocks for various P2P applications and systems including P2P storage systems, and hence we call them substrates or base infrastructure. These elemental functionalities include: (i) Reliable and efficient discovery of resources distributed over the network in a decentralized manner; (ii) Communication among participants in an address independent manner, i.e., even when peers change their physical addresses; (iii) Availability and persistence of stored objects in the network, irrespective of availability or departure of individual participants from the system at any time; and (iv) Freshness of the objects/resources' (up-to-date replicas). Internet-scale distributed index structures (often termed as structured overlays) are used for discovery and access of resources in a decentralized setting. We propose a rapid construction from scratch and maintenance of the P-Grid overlay network in a self-organized manner so as to provide efficient search of both individual keys as well as a whole range of keys, doing so providing good load-balancing characteristics for diverse kind of arbitrarily skewed loads - storage and replication, query forwarding and query answering loads. For fast overlay construction we employ recursive partitioning of the key-space so that the resulting partitions are balanced with respect to storage load and replication. The proper algorithmic parameters for such partitioning is derived from a transient analysis of the partitioning process which has Markov property. Preservation of ordering information in P-Grid such that queries other than exact queries, like range queries can be efficiently and rather trivially handled makes P-Grid suitable for data-oriented applications. Fast overlay construction is analogous to building an index on a new set of keys making P-Grid suitable as the underlying indexing mechanism for peer-to-peer information retrieval applications among other potential applications which may require frequent indexing of new attributes apart regular updates to an existing index. In order to deal with membership dynamics, in particular changing physical address of peers across sessions, the overlay itself is used as a (self-referential) directory service for maintaining the participating peers' physical addresses across sessions. Exploiting this self-referential directory, a family of overlay maintenance scheme has been designed with lower communication overhead than other overlay maintenance strategies. The notion of dynamic equilibrium study for overlays under continuous churn and repairs, modeled as a Markov process, was introduced in order to evaluate and compare the overlay maintenance schemes. While the self-referential directory was originally invented to realize overlay maintenance schemes with lower overheads than existing overlay maintenance schemes, the self-referential directory is generic in nature and can be used for various other purposes, e.g., as a decentralized public key infrastructure. Persistence of peer identity across sessions, in spite of changes in physical address, provides a logical independence of the overlay network from the underlying physical network. This has many other potential usages, for example, efficient maintenance mechanisms for P2P storage systems and P2P trust and reputation management. We specifically look into the dynamics of maintaining redundancy for storage systems and design a novel lazy maintenance strategy. This strategy is algorithmically a simple variant of existing maintenance strategies which adapts to the system dynamics. This randomized lazy maintenance strategy thus explores the cost-performance trade-offs of the storage maintenance operations in a self-organizing manner. We model the storage system (redundancy), under churn and maintenance, as a Markov process. We perform an equilibrium study to show that the system operates in a more stable dynamic equilibrium with our strategy than for the existing maintenance scheme for comparable overheads. Particularly, we show that our maintenance scheme provides substantial performance gains in terms of maintenance overhead and system's resilience in presence of churn and correlated failures. Finally, we propose a gossip mechanism which works with lower communication overhead than existing approaches for communication among a relatively large set of unreliable peers without assuming any specific structure for their mutual connectivity. We use such a communication primitive for propagating replica updates in P2P systems, facilitating management of mutable content in P2P systems. The peer population affected by a gossip can be modeled as a Markov process. Studying the transient spread of gossips help in choosing proper algorithm parameters to reduce communication overhead while guaranteeing coverage of online peers. Each of these substrates in themselves were developed to find practical solutions for real problems. Put together, these can be used in other applications, including a P2P storage system with support for efficient lookup and inserts, membership dynamics, content mutation and updates, persistence and availability. Many of the ideas have already been implemented in real systems and several others are in the way to be integrated into the implementations. There are two principal contributions of this dissertation. It provides design of the P2P systems which are useful for end-users as well as other application developers who can build upon these existing systems. Secondly, it adapts and introduces the methodology of analysis of a system's time-evolution (tools typically used in diverse domains including physics and cybernetics) to study the long run behavior of P2P systems, and uses this methodology to (re-)design appropriate algorithms and evaluate them. We observed that studying P2P systems from the perspective of complex systems reveals their inner dynamics and hence ways to exploit such dynamics for suitable or better algorithms. In other words, the analysis methodology in itself strongly influences and inspires the way we design such systems. We believe that such an approach of orchestrating self-organization in internet-scale systems, where the algorithms and the analysis methodology have strong mutual influence will significantly change the way future such systems are developed and evaluated. We envision that such an approach will particularly serve as an important tool for the nascent but fast moving P2P systems research and development community

Development of an adaptable multicast overlay network

Dissertação de mestrado em Informatics EngineeringMulticast is a group communication paradigm created in order to reduce, as much as possible, the amount of data generated to the network. However, limited deployment of IP Multicast protocols has motivated an interest in alternative approaches which implement a similar process of Multicast at an application-level (using solely end-systems and not the routers). In this context, different methodologies are presented, entitled Application-Layer Multicast or Overlay Multicast, which may vary in the way they operate. This dissertation’s objective is to develop and experiment a prototype of an overlay multicast system. This system should be easily configurable and adaptable in order to assume different strategies when establishing the multicast distribution tree. It is also expected to explore and integrate collaborative mechanisms between the overlay network and the Internet Service Providers (ISP). With the presented context, the first step to take is an investigation on the state of the art, where technologies relevant to this work will be presented. After this initial step, the developed system’s architecture will be described, one which enables different ways of building and maintaining the multicast distribution tree. The envisioned system can operate independently, integrating mechanisms where the distribution tree relies solely on peer decisions, which will be firstly addressed. Then, this work will move on to collaborative mechanisms between the overlay’s management (the central node) and the Internet Service Providers. Based on the proposed system architecture, several mechanisms are explored, not only focusing on alternative ways to build distribution trees, but also mechanisms allowing for some traffic engineering objectives involving the Internet Service Providers. Using the CORE network emulator, all the proposed mechanisms are tested, and results are analyzed to corroborate the system’s correct operation.O multicast é um paradigma de comunicação em grupo que tem como objetivo reduzir, tanto quanto possível, a quantidade de tráfego gerada para a rede. No entanto, a implantação limitada de protocolos IP Multicast tem motivado o interesse em abordagens alternativas que implementam processos de distribuição Multicast na camada aplicacional (ou seja, usando apenas os sistemas/aplicações finais e não os routers). Neste contexto, surgem as soluções denominadas por Application-Layer Multicast ou Overlay Multicast, podendo estas apresentar algumas variantes na sua operação. Nesta dissertação, tem-se como objetivo o desenvolvimento e experimentação de um protótipo de um sistema de Overlay Multicast. Este sistema deverá ser capaz de ser facilmente (re)configurado para assumir diferentes estratégias no estabelecimento da árvore de distribuição Multicast, e integrar mecanismos de colaboração entre a rede Overlay e os Internet Service Providers. No contexto apresentado, o primeiro passo consiste na investigação do estado da arte, onde tecnologias relevantes ao atual trabalho serão apresentadas. Após este passo inicial, a arquitectura do sistema será apresentada, uma arquitectura que considera diferentes maneiras de construir e manter a árvore de distribuição multicast. O sistema proposto pode operar de forma independente, contemplando mecanismos onde a árvore de distribuição depende apenas das decisões dos vários peers, sendo que estes serão os primeiros mecanismos a serem apresentados. De seguida, o sistema direcciona-se para mecanismos colaborativos entre a gestão da rede overlay e o ISP, de maneira a incluir conhecimento acerca da topologia da rede que nenhuma outra entidade seria capaz de providenciar. Com base na arquitectura do sistema proposto, vários mecanismos são explorados, não só mecanismos que se concentram em formas alternativas de construir a árvore de distribuição, mas também mecanismos que permitem cumprir os objetivos de engenharia de tráfico dos ISPs. Por fim, utilizando o emulador de redes CORE, todas as soluções serão testadas, e os seus resultados analisados por forma a validar a correta operação de todo o sistema

Enabling Large-Scale Peer-to-Peer Stored Video Streaming Service with QoS Support

This research aims to enable a large-scale, high-volume, peer-to-peer, stored-video streaming service over the Internet, such as on-line DVD rentals. P2P allows a group of dynamically organized users to cooperatively support content discovery and distribution services without needing to employ a central server. P2P has the potential to overcome the scalability issue associated with client-server based video distribution networks; however, it brings a new set of challenges. This research addresses the following five technical challenges associated with the distribution of streaming video over the P2P network: 1) allow users with limited transmit bandwidth capacity to become contributing sources, 2) support the advertisement and discovery of time-changing and time-bounded video frame availability, 3) Minimize the impact of distribution source losses during video playback, 4) incorporate user mobility information in the selection of distribution sources, and 5) design a streaming network architecture that enables above functionalities.To meet the above requirements, we propose a video distribution network model based on a hybrid architecture between client-server and P2P. In this model, a video is divided into a sequence of small segments and each user executes a scheduling algorithm to determine the order, the timing, and the rate of segment retrievals from other users. The model also employs an advertisement and discovery scheme which incorporates parameters of the scheduling algorithm to allow users to share their life-time of video segment availability information in one advertisement and one query. An accompanying QoS scheme allows reduction in the number of video playback interruptions while one or more distribution sources depart from the service prematurely.The simulation study shows that the proposed model and associated schemes greatly alleviate the bandwidth requirement of the video distribution server, especially when the number of participating users grows large. As much as 90% of load reduction was observed in some experiments when compared to a traditional client-server based video distribution service. A significant reduction is also observed in the number of video presentation interruptions when the proposed QoS scheme is incorporated in the distribution process while certain percentages of distribution sources depart from the service unexpectedly