Structured Peer-to-Peer Overlays for NATed Churn Intensive Networks

The wide-spread coverage and ubiquitous presence of mobile networks has propelled the usage and adoption of mobile phones to an unprecedented level around the globe. The computing capabilities of these mobile phones have improved considerably, supporting a vast range of third party applications. Simultaneously, Peer-to-Peer (P2P) overlay networks have experienced a tremendous growth in terms of usage as well as popularity in recent years particularly in fixed wired networks. In particular, Distributed Hash Table (DHT) based Structured P2P overlay networks offer major advantages to users of mobile devices and networks such as scalable, fault tolerant and self-managing infrastructure which does not exhibit single points of failure. Integrating P2P overlays on the mobile network seems a logical progression; considering the popularities of both technologies. However, it imposes several challenges that need to be handled, such as the limited hardware capabilities of mobile phones and churn (i.e. the frequent join and leave of nodes within a network) intensive mobile networks offering limited yet expensive bandwidth availability. This thesis investigates the feasibility of extending P2P to mobile networks so that users can take advantage of both these technologies: P2P and mobile networks. This thesis utilises OverSim, a P2P simulator, to experiment with the performance of various P2P overlays, considering high churn and bandwidth consumption which are the two most crucial constraints of mobile networks. The experiment results show that Kademlia and EpiChord are the two most appropriate P2P overlays that could be implemented in mobile networks. Furthermore, Network Address Translation (NAT) is a major barrier to the adoption of P2P overlays in mobile networks. Integrating NAT traversal approaches with P2P overlays is a crucial step for P2P overlays to operate successfully on mobile networks. This thesis presents a general approach of NAT traversal for ring based overlays without the use of a single dedicated server which is then implemented in OverSim. Several experiments have been performed under NATs to determine the suitability of the chosen P2P overlays under NATed environments. The results show that the performance of these overlays is comparable in terms of successful lookups in both NATed and non-NATed environments; with Kademlia and EpiChord exhibiting the best performance. The presence of NATs and also the level of churn in a network influence the routing techniques used in P2P overlays. Recursive routing is more resilient to IP connectivity restrictions posed by NATs but not very robust in high churn environments, whereas iterative routing is more suitable to high churn networks, but difficult to use in NATed environments. Kademlia supports both these routing schemes whereas EpiChord only supports the iterating routing. This undermines the usefulness of EpiChord in NATed environments. In order to harness the advantages of both routing schemes, this thesis presents an adaptive routing scheme, called Churn Aware Routing Protocol (ChARP), combining recursive and iterative lookups where nodes can switch between recursive and iterative routing depending on their lifetimes. The proposed approach has been implemented in OverSim and several experiments have been carried out. The experiment results indicate an improved performance which in turn validates the applicability and suitability of ChARP in NATed environments

EGOIST: Overlay Routing Using Selfish Neighbor Selection

A foundational issue underlying many overlay network applications ranging from routing to P2P file sharing is that of connectivity management, i.e., folding new arrivals into an existing overlay, and re-wiring to cope with changing network conditions. Previous work has considered the problem from two perspectives: devising practical heuristics for specific applications designed to work well in real deployments, and providing abstractions for the underlying problem that are analytically tractable, especially via game-theoretic analysis. In this paper, we unify these two thrusts by using insights gleaned from novel, realistic theoretic models in the design of Egoist – a prototype overlay routing system that we implemented, deployed, and evaluated on PlanetLab. Using measurements on PlanetLab and trace-based simulations, we demonstrate that Egoist's neighbor selection primitives significantly outperform existing heuristics on a variety of performance metrics, including delay, available bandwidth, and node utilization. Moreover, we demonstrate that Egoist is competitive with an optimal, but unscalable full-mesh approach, remains highly effective under significant churn, is robust to cheating, and incurs minimal overhead. Finally, we discuss some of the potential benefits Egoist may offer to applications.National Science Foundation (CISE/CSR 0720604, ENG/EFRI 0735974, CISE/CNS 0524477, CNS/NeTS 0520166, CNS/ITR 0205294; CISE/EIA RI 0202067; CAREER 04446522); European Commission (RIDS-011923

Data sharing in DHT based P2P systems

International audienceThe evolution of peer-to-peer (P2P) systems triggered the building of large scale distributed applications. The main application domain is data sharing across a very large number of highly autonomous participants. Building such data sharing systems is particularly challenging because of the "extreme" characteristics of P2P infrastructures: massive distribution, high churn rate, no global control, potentially untrusted participants... This article focuses on declarative querying support, query optimization and data privacy on a major class of P2P systems, that based on Distributed Hash Table (P2P DHT). The usual approaches and the algorithms used by classic distributed systems and databases forproviding data privacy and querying services are not well suited to P2P DHT systems. A considerable amount of work was required to adapt them for the new challenges such systems present. This paper describes the most important solutions found. It also identies important future research trends in data management in P2P DHT systems

Optimizing on-demand resource deployment for peer-assisted content delivery (PhD thesis)

Increasingly, content delivery solutions leverage client resources in exchange for service in a peer-to-peer (P2P) fashion. Such peer-assisted service paradigms promise significant infrastructure cost reduction, but suffer from the unpredictability associated with client resources, which is often exhibited as an imbalance between the contribution and consumption of resources by clients. This imbalance hinders the ability to guarantee a minimum service fidelity of these services to the clients. In this thesis, we propose a novel architectural service model that enables the establishment of higher fidelity services through (1) coordinating the content delivery to optimally utilize the available resources, and (2) leasing the least additional cloud resources, available through special nodes (angels) that join the service on-demand, and only if needed, to complement the scarce resources available through clients. While the proposed service model can be deployed in many settings, this thesis focuses on peer-assisted content delivery applications, in which the scarce resource is typically the uplink capacity of clients. We target three applications that require the delivery of fresh as opposed to stale content. The first application is bulk-synchronous transfer, in which the goal of the system is to minimize the maximum distribution time -- the time it takes to deliver the content to all clients in a group. The second application is live streaming, in which the goal of the system is to maintain a given streaming quality. The third application is Tor, the anonymous onion routing network, in which the goal of the system is to boost performance (increase throughput and reduce latency) throughout the network, and especially for bandwidth-intensive applications. For each of the above applications, we develop mathematical models that optimally allocate the already available resources. They also optimally allocate additional on-demand resource to achieve a certain level of service. Our analytical models and efficient constructions depend on some simplifying, yet impractical, assumptions. Thus, inspired by our models and constructions, we develop practical techniques that we incorporate into prototypical peer-assisted angel-enabled cloud services. We evaluate those techniques through simulation and/or implementation. (Major Advisor: Azer Bestavros

Optimizing on-demand resource deployment for peer-assisted content delivery

Increasingly, content delivery solutions leverage client resources in exchange for services in a pee-to-peer (P2P) fashion. Such peer-assisted service paradigm promises significant infrastructure cost reduction, but suffers from the unpredictability associated with client resources, which is often exhibited as an imbalance between the contribution and consumption of resources by clients. This imbalance hinders the ability to guarantee a minimum service fidelity of these services to clients especially for real-time applications where content can not be cached. In this thesis, we propose a novel architectural service model that enables the establishment of higher fidelity services through (1) coordinating the content delivery to efficiently utilize the available resources, and (2) leasing the least additional cloud resources, available through special nodes (angels) that join the service on-demand, and only if needed, to complement the scarce resources available through clients. While the proposed service model can be deployed in many settings, this thesis focuses on peer-assisted content delivery applications, in which the scarce resource is typically the upstream capacity of clients. We target three applications that require the delivery of real-time as opposed to stale content. The first application is bulk-synchronous transfer, in which the goal of the system is to minimize the maximum distribution time - the time it takes to deliver the content to all clients in a group. The second application is live video streaming, in which the goal of the system is to maintain a given streaming quality. The third application is Tor, the anonymous onion routing network, in which the goal of the system is to boost performance (increase throughput and reduce latency) throughout the network, and especially for clients running bandwidth-intensive applications. For each of the above applications, we develop analytical models that efficiently allocate the already available resources. They also efficiently allocate additional on-demand resource to achieve a certain level of service. Our analytical models and efficient constructions depend on some simplifying, yet impractical, assumptions. Thus, inspired by our models and constructions, we develop practical techniques that we incorporate into prototypical peer-assisted angel-enabled cloud services. We evaluate these techniques through simulation and/or implementation

Walkabout : an asynchronous messaging architecture for mobile devices

Modern mobile devices are prolific producers and consumers of digital data, and wireless networking capabilities enable them to transfer their data over the Internet while moving. Applications running on these devices may perform transfers to upload data for backup or distribution, or to download new content on demand. Unfortunately, the limited connectivity that mobile devices experience can make these transfers slow and impractical as the amount of data increases. This thesis argues that asynchronous messaging supported by local proxies can improve the transfer capabilities of mobile devices, making it practical for them to participate in large Internet transfers. The design of the Walkabout architecture follows this approach: proxies form store-and-forward overlay networks to deliver messages asynchronously across the Internet on behalf of devices. A mobile device uploads a message to a local proxy at rapid speed, and the overlay delivers it to one or more destination devices, caching the message until each one is able to retrieve it from a local proxy. A device is able to partially upload or download a message whenever it has network connectivity, and can resume this transfer at any proxy if interrupted through disconnection. Simulation results show that Walkabout provides better throughput for mobile devices than is possible under existing methods, for a range of movement patterns. Upload and end-to-end to transfer speeds are always high when the device sending the message is mobile. In the basic Walkabout model, a message sent to a mobile device that is repeatedly moving between a small selection of connection points experiences high download and end-to-end transfer speeds, but these speeds fall as the number of connection points grows. Pre-emptive message delivery extensions improve this situation, making fast end-to-end transfers and device downloads possible under any pattern of movement. This thesis describes the design and evaluation of Walkabout, with both practical implementation and extensive simulation under real-world scenarios

A machine-checked proof of correctness of Pastry

A distributed hash table (DHT) is a peer-to-peer network that offers the function of a classic hash table, but where different key-value pairs are stored at different nodes on the network. Like a classic hash table, the main function provided by a DHT is key lookup, which retrieves the value stored at a given key. Examples of DHT protocols include Chord, Pastry, Kademlia and Tapestry. Such DHT protocols certain correctness and performance guarantees, but formal verification typically discovers border cases that violate those guarantees. In his PhD thesis, Tianxiang Lu reported correctness problems in published versions of Pastry and developed a model called LuPastry, for which he provided a partial proof of correct delivery of lookup messages assuming no node failure, mechanized in the TLA+ Proof System. In analyzing Lu's proof, I discovered that it contained unproven assumptions, and found counterexamples to several of these assumptions. The contribution of this thesis is threefold. First, I present LuPastry+, a revised TLA+ specification of LuPastry. Aside from needed bug fixes, LuPastry+ contains new definitions that make the specification more modular and significantly improve proof automation. Second, I present a complete TLA+ proof of correct delivery for LuPastry+. Third, I prove that the final step of the node join process of LuPastry/LuPastry+ is not necessary to achieve consistency. In particular, I develop a new specification with a simpler node join process, which I denote by Simplified LuPastry+, and prove correct delivery of lookup messages for this new specification. The proof of correctness of Simplified LuPastry+ is written by reusing the proof for LuPastry+, which represents a success story in proof reuse, especially for proofs of this size. Each of the two proofs amounts to over 32,000 proof steps; to my knowledge, they are currently the largest proofs written in the TLA+ language, and---together with Lu's proof---the only examples of applying full theorem proving for the verification of DHT protocols.Eine verteilte Hashtabelle (DHT) ist ein P2P Netzwerk, das die gleiche Funktion wie eine klassische Hashtabelle anbietet, wo aber verschiedene Schlüssel-Inhalt Paare an verschiedenen Knoten im Netzwerk gespeichert werden. Chord, Pastry, Kademlia und Tapestry sind einige bekannte Implementierungen von DHT. Solche Protokolle versprechen bestimmte Eigenschaften bezüglich Korrektheit und Leistung. Die formale Verifikation von diesen Protokollen führt jedoch normalerweise zu Widersprüchen dieser Eigenschaften. In seiner Doktorarbeit entdeckt Tianxiang Lu Gegenbeispiele zu veröffentlichten Versionen von Pastry und entwickelt LuPastry, ein Pastry Model ausschließlich des Knotenausfalles. Zusätzlich bietet Lu einen Teilbeweis für korrekte Lieferung von Suchnachrichten in LuPastry in der Sprache TLA+ an. Lus Beweis basiert auf unbewiesenen Annahmen. Beim Untersuchen des Beweises habe ich Gegenbeispiele zu mehreren dieser Annahmen entdeckt. Diese Doktorarbeit deckt drei Hauptthemen ab. Erstens, es wird LuPastry+ entwickelt: eine revidierte TLA+ Spezifikation zu LuPastry. Neben den benötigten Fehlerkorrekturen, bietet LuPastry+ zusätzlich neue Definitionen an, welche die Spezifikation modularer machen, und die Automatisierung des Beweises signifikant verbessern. Zweitens, biete ich einen vollständigen TLA+ korrektheitsbeweis für LuPastry+ an. Drittens, zeige ich, dass der letzte Schritt des Beitrittsprotokolles in LuPastry/LuPastry+ nicht notwendig für Korrektheit ist. Insbesondere, biete ich eine neue Spezifikation mit einem einfacheren Beitrittsprotokoll an, und einen Korrektheitsbeweis dafür.Nach bestem Wissen sind diese Beweise (2 Beweise je von über 32.000 Schritten) bis dato die größten in TLA+ geschriebenen Beweise

A machine-checked proof of correctness of Pastry

A distributed hash table (DHT) is a peer-to-peer network that offers the function of a classic hash table, but where different key-value pairs are stored at different nodes on the network. Like a classic hash table, the main function provided by a DHT is key lookup, which retrieves the value stored at a given key. Examples of DHT protocols include Chord, Pastry, Kademlia and Tapestry. Such DHT protocols certain correctness and performance guarantees, but formal verification typically discovers border cases that violate those guarantees. In his PhD thesis, Tianxiang Lu reported correctness problems in published versions of Pastry and developed a model called LuPastry, for which he provided a partial proof of correct delivery of lookup messages assuming no node failure, mechanized in the TLA+ Proof System. In analyzing Lu's proof, I discovered that it contained unproven assumptions, and found counterexamples to several of these assumptions. The contribution of this thesis is threefold. First, I present LuPastry+, a revised TLA+ specification of LuPastry. Aside from needed bug fixes, LuPastry+ contains new definitions that make the specification more modular and significantly improve proof automation. Second, I present a complete TLA+ proof of correct delivery for LuPastry+. Third, I prove that the final step of the node join process of LuPastry/LuPastry+ is not necessary to achieve consistency. In particular, I develop a new specification with a simpler node join process, which I denote by Simplified LuPastry+, and prove correct delivery of lookup messages for this new specification. The proof of correctness of Simplified LuPastry+ is written by reusing the proof for LuPastry+, which represents a success story in proof reuse, especially for proofs of this size. Each of the two proofs amounts to over 32,000 proof steps; to my knowledge, they are currently the largest proofs written in the TLA+ language, and---together with Lu's proof---the only examples of applying full theorem proving for the verification of DHT protocols.Eine verteilte Hashtabelle (DHT) ist ein P2P Netzwerk, das die gleiche Funktion wie eine klassische Hashtabelle anbietet, wo aber verschiedene Schlüssel-Inhalt Paare an verschiedenen Knoten im Netzwerk gespeichert werden. Chord, Pastry, Kademlia und Tapestry sind einige bekannte Implementierungen von DHT. Solche Protokolle versprechen bestimmte Eigenschaften bezüglich Korrektheit und Leistung. Die formale Verifikation von diesen Protokollen führt jedoch normalerweise zu Widersprüchen dieser Eigenschaften. In seiner Doktorarbeit entdeckt Tianxiang Lu Gegenbeispiele zu veröffentlichten Versionen von Pastry und entwickelt LuPastry, ein Pastry Model ausschließlich des Knotenausfalles. Zusätzlich bietet Lu einen Teilbeweis für korrekte Lieferung von Suchnachrichten in LuPastry in der Sprache TLA+ an. Lus Beweis basiert auf unbewiesenen Annahmen. Beim Untersuchen des Beweises habe ich Gegenbeispiele zu mehreren dieser Annahmen entdeckt. Diese Doktorarbeit deckt drei Hauptthemen ab. Erstens, es wird LuPastry+ entwickelt: eine revidierte TLA+ Spezifikation zu LuPastry. Neben den benötigten Fehlerkorrekturen, bietet LuPastry+ zusätzlich neue Definitionen an, welche die Spezifikation modularer machen, und die Automatisierung des Beweises signifikant verbessern. Zweitens, biete ich einen vollständigen TLA+ korrektheitsbeweis für LuPastry+ an. Drittens, zeige ich, dass der letzte Schritt des Beitrittsprotokolles in LuPastry/LuPastry+ nicht notwendig für Korrektheit ist. Insbesondere, biete ich eine neue Spezifikation mit einem einfacheren Beitrittsprotokoll an, und einen Korrektheitsbeweis dafür.Nach bestem Wissen sind diese Beweise (2 Beweise je von über 32.000 Schritten) bis dato die größten in TLA+ geschriebenen Beweise