Thomas Bocek

Game theoretical analysis of incentives for large-scale, fully decentralized collaboration networks

One of the key challenges in peer-to-peer networks is the design of incentives to encourage peers to share their resources. Incentives are necessary in order to exploit the full potential of these systems. The tit-for-tat incentive scheme, as used in BitTorrent for example, has proven to be a successful approach in P2P file sharing systems, where peers have direct relations and share the same kind of resources. However, in P2P systems where different kind of resources are shared between peers with non-direct relations, the design of incentives remains a challenge. In this paper, a large-scale, fully decentralized P2P collaboration network is shown, where peers share not only bandwidth and storage space, but also contribute by editing articles and voting for or against changes. A new incentive scheme is proposed which supports non-direct relations and provides incentives for sharing different kind of resources. The incentive scheme is based on a reputation system that assigns a reputation value to every peer reflecting its previous behavior in the network. Based on this value, the service level is differentiated, i.e. the higher a peers reputation the better the quality of service it can get from the network. The service differentiation has been analyzed and simulated with rational, irrational and altruistic peers based on game theory concepts.

LiveShift: Peer-to-Peer Live Streaming with Distributed Time-Shifting

The increasing assortment of devices with IP connectivity contributes to the high popularity of video sharing over the Internet. High traffic generated by such applications at the source can be better distributed using a peer-to-peer overlay, since every user forwards information to other users. Current implementations target either live or on demand video streaming. LiveShift is an application that combines both approaches. While video is transmitted through the peer-to-peer network in a live fashion, all peers participate in a distributed storage. This adds ability to replay time-shifted streams from other peers in a distributed and scalable manner. For the demonstration, a decentralized network is used, with peers running on EMANICSLab nodes and notebook computers.

Fast similarity search in peer-to-peer networks

Peer-to-peer (P2P) systems show numerous advantages over centralized systems, such as load balancing, scalability, and fault tolerance, and they require certain functionality, such as search, repair, and message and data transfer. In particular, structured P2P networks perform an exact search in logarithmic time proportional to the number of peers. However, keyword similarity search in a structured P2P network remains a challenge. Similarity search for service discovery can significantly improve service management in a distributed environment. As services are often described informally in text form, keyword similarity search can find the required services or data items more reliably. This paper presents a fast similarity search algorithm for structured P2P systems. The new algorithm, called P2P fast similarity search (P2PFastSS), finds similar keys in any distributed hash table (DHT) using the edit distance metric, and is independent of the underlying P2P routing algorithm. Performance analysis shows that P2PFastSS carries out a similarity search in time proportional to the logarithm of the number of peers. Simulations on PlanetLab confirm these results and show that a similarity search with 34,000 peers performs in less than three seconds on average. Thus, P2PFastSS is suitable for similarity search in large-scale network infrastructures, such as service description matching in service discovery or searching for similar terms in P2P storage networks.

CompactPSH: An efficient transitive TFT incentive scheme for Peer-to-Peer Networks

Incentive schemes in Peer-to-Peer (P2P) networks are necessary to discourage free-riding. One example is the Tit-for-Tat (TFT) incentive scheme, a variant of which is used in BitTorrent to encourage peers to upload. TFT uses data from local observations making it suitable for systems with direct reciprocity. This paper presents CompactPSH, an incentive scheme that works with direct and indirect reciprocity. CompactPSH allows peers to establish indirect reciprocity by finding intermediate peers, thus enabling trade with more peers and capitalizing on more resources. CompactPSH finds transitive paths while keeping the overhead of additional messages low. In a P2P file-sharing scenario based on input data from a large BitTorrent tracker, CompactPSH was found to exploit more reciprocity than TFT which enabled more chunks to be downloaded. As a consequence, peers are allowed to be stricter to fight white-washing without compromising performance.

A Cloud Storage overlay to aggregate heterogeneous Cloud services

Many Cloud services provide generic (e.g., Amazon S3 or Dropbox) or data-specific Cloud storage (e.g., Google Picasa or SoundCloud). Although both Cloud storage service types have the data storage in common, they present heterogeneous characteristics: different interfaces, accounting and charging schemes, privacy and security levels, functionality and, among the data-specific Cloud storage services, different data type restrictions. This paper proposes PiCsMu (Platform-independent Cloud Storage System for Multiple Usage), a novel approach exploiting heterogeneous data storage of different Cloud services by building a Cloud storage overlay, which aggregates multiple Cloud storage services, provides enhanced privacy, and offers a distributed file sharing system. As opposed to P2P file sharing, where data and indices are stored on peers, PiCsMu uses Cloud storage systems for data storage, while maintaining a distributed index. The main contribution of this work is to show the feasibility to store arbitrary data in different Cloud services for private use and/or for file sharing. Furthermore, the evaluation of the prototype confirms the scalability with respect to different file sizes and also shows that a moderate overhead in terms of storage and processing time is required.

LiveShift: Mesh-pull live and time-shifted P2P video streaming

The popularity of video sharing over the Internet has increased significantly. High traffic generated by such applications at the source can be better distributed using a peer-to-peer (P2P) overlay. Unlike most P2P systems, LiveShift combines both live and on-demand video streaming — while video is transmitted through the peer-to-peer network in a live fashion, all peers participate in distributed storage. This adds the ability to replay time-shifted streams from other peers in a distributed and scalable manner. This paper describes an adaptive fully-distributed mesh-pull protocol that supports the envisioned use case and a set of policies that enable efficient usage of resources, discussing interesting trade-offs encountered. User-focused evaluation results, including both channel switching and time shifting behavior, show that the proposed system provides good quality of experience for most users, in terms of infrequent stalling, low playback lag, and a small proportion of skipped blocks in all the scenarios studied, even in presence of churn.

Blockchains everywhere - a use-case of blockchains in the pharma supply-chain

Blockchains are on the top of the Gartner Hype Cycle 2016 and many start-ups are integrating blockchains into their technology portfolio. While blockchains have emerged in the context of financial applications, non-financial application areas are of interest as well. In this paper, modum.io is presented, a start-up that uses IoT (Internet of Things) sensor devices leveraging blockchain technology to assert data immutability and public accessibility of temperature records, while reducing operational costs in the pharmaceutical supply-chain. The medical industry has many complex and strict environmental control process (e.g., temperature and humidity) to ensure quality control and regulatory compliance over the transport of medical products. The sensor devices monitor the temperature of each parcel during the shipment to fully ensure GDP regulations. All data is transferred to the blockchain where a smart contract assesses against the product attributes. As modum.io is not the only non-financial start-up working with blockchains, a list of areas and other start-ups is provided that aim to reduce bureaucracy, distribute the infrastructure, and saving costs using blockchains.

B-Tracker: Improving load balancing and efficiency in distributed P2P trackers

Trackers are used in peer-to-peer (P2P) networks for provider discovery, that is, mapping resources to potential providers. Centralized trackers, e.g., as in the original BitTorrent protocol, do not benefit from P2P properties, such as no single point of failure, scalability, and load balancing. Decentralized mechanisms have thus been proposed, based on distributed hash tables (DHTs) and gossiping, such as BitTorrents Peer Exchange (PEX). While DHT-based trackers suffer from load balancing problems, gossip-based ones cannot deliver new mappings quickly. This paper presents B-Tracker, a fully-distributed, pull-based tracker. B-Tracker extends DHT functionality by distributing the tracker load among all providers in a swarm. Bloom filters are used to avoid redundant mappings to be transmitted. This results in the important properties of load balancing and scalability, while adding the ability for peers to fetch new mappings instantly. B-Tracker shows, through simulations, improved load balancing and better efficiency when compared to pure DHTs and PEX.

The Design and Evaluation of a Distributed Reliable File System

Peer-to-peer (P2P) systems are, in contrast to clientserver (C/S) systems, fault-tolerant, robust, and scalable. While C/S distributed file systems, such as NFS (Network File System) or SMB (Server Message Block), do not scale with respect to the number of clients and exhibit a single point of failure, P2P file systems have the potential to cope with an increasing number of participants. Thus, this paper presents DRFS (Distributed Reliable File System), a P2P file system for cooperative environments. DRFS uses random, content-independent identifiers for data storage, while maintaining high performance and low overhead with many concurrent reads and writes. A dynamic replication mechanism ensures data availability, even under high churn. The application scenario considers an office environment, where DRFS is installed on employees’ machines, who store and request files. DRFS has been implemented using the Filesystem in Userspace (FUSE) interface, in order to provide users with transparent read and write operations. Experiments show the benefits of such a peer-to-peer architecture, when a small number of peers reads or writes in parallel: DRFS performs better than NFS, as soon as 6 peers read or write in parallel a 32 MB file. For unpopular files, it is also more reliable than IgorFS.

PeerVote: A Decentralized Voting Mechanism for P2P Collaboration Systems

Peer-to-peer (P2P) systems achieve scalability, fault tolerance, and load balancing with a low-cost infrastructure, characteristics from which collaboration systems, such as Wikipedia, can benefit. A major challenge in P2P collaboration systems is to maintain article quality after each modification in the presence of malicious peers. A way of achieving this goal is to allow modifications to take effect only if a majority of previous editors approve the changes through voting. The absence of a central authority makes voting a challenge in P2P systems. This paper proposes the fully decentralized voting mechanism PeerVote, which enables users to vote on modifications in articles in a P2P collaboration system. Simulations and experiments show the scalability and robustness of PeerVote, even in the presence of malicious peers.