The core motive of this research is to construct a new hierarchical non-DHT based architecture for Peer-to-Peer (P2P) networks that facilitate common interests clustering. DHT based network maintenance is on the high end and it churning management is a complex task here. Providing efficient data querying performance and ensuring minimal churn management effort has interested us to pursue non-DHT route of P2P networking. And at each level of the proposed architecture hierarchy, existing networks are all structured and each such network has the diameter of 1 overlay hop. Such low diameters have immense importance in designing very efficient data lookup algorithms. We shall use a mathematical model based on the Chinese Remainder Theorem (CRT), generally used in cryptography, to define the neighborhood relations among peers to obtain the above-mentioned diameters. To the best of our knowledge, use of CRT in P2P network design is a completely new idea; it does not exist in the literature so far. It is worth mentioning its most important advantage from the viewpoint of speed of communication, that is its diameter, which is only 3 overlay hops. The protocol is not restricted to a single data source, and it incorporates peer heterogeneity as well.

This dissertation consists of three separate essays on Peer-to-Peer (P2P) online credit markets. The first essay presents new empirical evidence of decreases in loan demand and repayment when prices in the market are determined by competing lenders in auctions as compared to the case in which a platform directly controls all prices. The paper develops an econometric model of loan demand and repayment which is then used to predict borrower choices when they are offered prices set by lenders in a market. I find that when lenders set prices, borrowers are more likely to pick loans of shorter maturity and smaller sizes, and repay less. Aggregated at the market level, demand and repayment of credit fall by 10% and 2%, respectively. In the second paper, I quantify the effects of implementation of finer credit scoring on credit demand, defaults and repayment in the context of a large P2P online credit platform. I exploit an exogenous change in the platform's credit scoring policy where the centralized price setting rules ensure that the one-to-one relationship between credit scores and prices remains intact unlike in a traditional credit market where it is broken. The results show that a 1% increase in interest rate due to the implementation of finer credit scoring results in an average decrease of 0.29% in the requested loan amount, an average increase of 0.01 in the fraction of borrowers who default and an average increase of 0.02 in the fraction of loan repaid. These findings contribute to a better understanding of how a reduction in information asymmetry affects borrower choices in a credit market. The third paper explores the main drivers behind the geographic expansion in demand for credit from P2P online platforms. It uses data from the two largest platforms in the United States to conduct an empirical analysis. By exploiting heterogeneity in local credit markets before the entry of P2P online platforms, the paper estimates the effect of local credit market conditions on demand for credit from P2P platforms. The paper uses a spatial autoregressive model for the main specification. We find that P2P consumer credit expanded more in counties with poor branch networks, lower concentration of banks, and lower leverage ratios.

Indiana University-Purdue University Indianapolis (IUPUI) / Patients and providers need to exchange medical records. Electronic Health Records and Health Information Exchanges leave a patient’s health record fragmented and controlled by the provider. This thesis proposes a Peer-to-Peer Personal Health Record network that can be extended with third-party services. This design enables patient control of health records and the tracing of exchanges. Additionally, as a demonstration of the functionality of a potential third-party, a Hypertension Predictor is developed using MEPS data and deployed as a service in the proposed framework.

No description available.

The blockchain technology promises innovation by moving away from conventional centralized architectures, where trust is placed in a small number of actors, to a decentralized environment where a collection of actors must work together to maintain consensus in the overall system. Blockchain offers security and pseudo-anonymity to its adopters, through the use of various cryptographic methods. While much attention has focused on creating new applications that make use of this technology, equal importance must be given to studying naturally occurring phenomena in existing blockchain ecosystems and mitigating their effects where harmful. In this dissertation, we develop a novel open-source log-to-file system that provides the ability to record information relevant to events as they take place in live blockchain networks. Specifically, our open-source software facilitates in-situ measurements on full nodes in the live Bitcoin and Bitcoin Cash blockchain networks. This measurement framework sheds new light on many phenomena that were previously unknown or scarcely studied. First, we examine the presence and impact of churn, namely nodes joining and leaving, on the behavior of the Bitcoin network. Our data analysis over a two-month period shows that a large number of Bitcoin nodes churn at least once. We perform statistical distribution fitting to this churn and emulate it in our measurement nodes to evaluate the impact of churn on the performance of the Bitcoin protocol. From our experiments, we find that blocks received by churning nodes experience as much as five times larger propagation delay than those received by non-churning nodes. We introduce and evaluate a novel synchronization scheme to mitigate such effects on the performance of the protocol. Our empirical evaluation shows that blocks received by churning nodes that synchronize their mempools with peers have roughly half the delay in propagation experienced by those that do not synchronize their mempools. We next evaluate and compare the performance of three block relay protocols, namely the default protocol, and the more recent compact block and Graphene protocols. This evaluation is conducted over full nodes running the Bitcoin Unlimited client (which is used in conjunction with the Bitcoin Cash network). We find that in most scenarios, the Graphene block relay protocol outperforms the other two in terms of the block propagation delay and the amount of total communication associated with block relay. An exception is when nodes churn frequently and spend a significant fraction of time off the network, in which case the compact block relay protocol performs best. In-depth analyses reveal subtle inefficiencies of the protocols. Thus, in the case of frequent churns, the Graphene block relay protocol performs as many as two extra round-trips of communication to recover information necessary to reconstruct blocks. Likewise, an inspection of the compact block relay protocol indicates that the full transactions included in the initial block message are either unnecessary or insufficient for the successful reconstruction of blocks. Finally, we investigate the occurrence of orphan transactions which are those whose parental income sources are missing at the time that they are processed. These transactions typically languish in a local buffer until they are evicted or all their parents are discovered, at which point they may be propagated further. Our data reveals that slightly less than half of orphan transactions end up being included in the blockchain. Surprisingly, orphan transactions tend to have fewer parents on average than non-orphan transactions, and their missing parents have a lower fee, a larger size, and a lower transaction fee per byte than all other received transactions. Moreover, the network overhead incurred by these orphan transactions can be significant when using the default orphan memory pool size (i.e., 100 transactions), although this overhead can be made negligible if the pool size is simply increased to 1,000 transactions. In summary, this dissertation demonstrates the importance of characterizing the inner behavior of the peer-to-peer network underlying a blockchain. While our results primarily focus on the Bitcoin network and its variants, this work provides foundations that should prove useful for studying and characterizing other blockchains.

Currently, multiplayer online games use the client-server architecture which is very resource intensive, expensive, and time consuming. Peer-to-peer protocols are a less resource intensive alternative to the client-server model. We implement a peer-to-peer protocol called NEO in a multiplayer game and run experiments in a lab setting and over the Internet. These experiments show us that NEO is able to run a smooth playable game, with low unused updates and low location error. This happens as long as the arrival delay is long enough to allow updates to arrive in the given time limit and the round length is short enough to keep the location error down. However, the experiments also show that NEO has scalability problems that need to be corrected. When more than 4 clients are used the playout delay is the same length as the round which causes high location error. Also, more clients cause more updates to go unused which also causes high location error.

BitTorrent is a peer-to-peer protocol useful for distributing large files over the Internet. Many organizations use BitTorrent to distribute their software in order to reduce client download time and reduce the load on their servers. While there is a lot of legitimate content available via BitTorrent, some organizations ban BitTorrent usage due to concerns over copyright infringement and the amount of bandwidth that peers can consume. A BitTorrent proxy will allow organizations to control those risks and allow its members to use the BitTorrent protocol for approved uses. It will allow the organization to control the files that are downloaded and the total bandwidth that can be used, and it will eliminate redundant downloads by acting as a cache.

No description available.

Die Erfindung von Bitcoin hat ein großes Interesse an dezentralen Systemen geweckt. Eine häufige Zuschreibung an dezentrale Systeme ist dabei, dass eine Dezentralisierung automatisch zu einer höheren Sicherheit und Widerstandsfähigkeit gegenüber Angriffen führt. Diese Dissertation widmet sich dieser Zuschreibung, indem untersucht wird, ob dezentralisierte Anwendungen tatsächlich so robust sind. Dafür werden exemplarisch drei Systeme untersucht, die häufig als Komponenten in komplexen Blockchain-Anwendungen benutzt werden: Ethereum als Infrastruktur, IPFS zur verteilten Datenspeicherung und schließlich "Stablecoins" als Tokens mit Wertstabilität. Die Sicherheit und Robustheit dieser einzelnen Komponenten bestimmt maßgeblich die Sicherheit des Gesamtsystems in dem sie verwendet werden; darüber hinaus erlaubt der Fokus auf Komponenten Schlussfolgerungen über individuelle Anwendungen hinaus. Für die entsprechende Analyse bedient sich diese Arbeit einer empirisch motivierten, meist Netzwerklayer-basierten Perspektive -- angereichert mit einer ökonomischen im Kontext von Wertstabilen Tokens. Dieses empirische Verständnis ermöglicht es Aussagen über die inhärenten Eigenschaften der studierten Systeme zu treffen. Ein zentrales Ergebnis dieser Arbeit ist die Entdeckung und Demonstration einer "Eclipse-Attack" auf das Ethereum Overlay. Mittels eines solchen Angriffs kann ein Angreifer die Verbreitung von Transaktionen und Blöcken behindern und Netzwerkteilnehmer aus dem Overlay ausschließen. Des weiteren wird das IPFS-Netzwerk umfassend analysiert und kartografiert mithilfe (1) systematischer Crawls der DHT sowie (2) des Mitschneidens von Anfragenachrichten für Daten. Erkenntlich wird hierbei, dass die hybride Overlay-Struktur von IPFS Segen und Fluch zugleich ist, da das Gesamtsystem zwar robust gegen Angriffe ist, gleichzeitig aber eine umfassende Überwachung der Netzwerkteilnehmer ermöglicht wird. Im Rahmen der wertstabilen Kryptowährungen wird ein Klassifikations-Framework vorgestellt und auf aktuelle Entwicklungen im Gebiet der "Stablecoins" angewandt. Mit diesem Framework wird somit (1) der aktuelle Zustand der Stablecoin-Landschaft sortiert und (2) ein Mittel zur Verfügung gestellt, um auch zukünftige Designs einzuordnen und zu verstehen. / The inception of Bitcoin has sparked a large interest in decentralized systems. In particular, popular narratives imply that decentralization automatically leads to a high security and resilience against attacks, even against powerful adversaries. In this thesis, we investigate whether these ascriptions are appropriate and if decentralized applications are as robust as they are made out to be. To this end, we exemplarily analyze three widely-used systems that function as building blocks for blockchain applications: Ethereum as basic infrastructure, IPFS for distributed storage and lastly "stablecoins" as tokens with a stable value. As reoccurring building blocks for decentralized applications these examples significantly determine the security and resilience of the overall application. Furthermore, focusing on these building blocks allows us to look past individual applications and focus on inherent systemic properties. The analysis is driven by a strong empirical, mostly network-layer based perspective; enriched with an economic point of view in the context of monetary stabilization. The resulting practical understanding allows us to delve into the systems' inherent properties. The fundamental results of this thesis include the demonstration of a network-layer Eclipse attack on the Ethereum overlay which can be leveraged to impede the delivery of transaction and blocks with dire consequences for applications built on top of Ethereum. Furthermore, we extensively map the IPFS network through (1) systematic crawling of its DHT, as well as (2) monitoring content requests. We show that while IPFS' hybrid overlay structure renders it quite robust against attacks, this virtue of the overlay is simultaneously a curse, as it allows for extensive monitoring of participating peers and the data they request. Lastly, we exchange the network-layer perspective for a mostly economic one in the context of monetary stabilization. We present a classification framework to (1) map out the stablecoin landscape and (2) provide means to pigeon-hole future system designs. With our work we not only scrutinize ascriptions attributed to decentral technologies; we also reached out to IPFS and Ethereum developers to discuss results and remedy potential attack vectors.

No description available.