Showing papers presented at "International Conference on Peer-to-Peer Computing in 2007"

Globase.KOM - A P2P Overlay for Fully Retrievable Location-based Search

Abstract: Location based services are becoming increasingly popular as devices that determine geographical position become more available to end users. The main problem of existing solutions to location-based search is keeping information updated requires centralized maintenance at specific times. Therefore, retrieved results do not include all objects that exist in reality. A peer-to-peer (P2P) approach can easily overcome this issue as peers are responsible for the information users are searching for. Unfortunately, current state-of-the-art overlays cannot fulfill the requirements for efficient and fully retrievable location-based search. In this paper we present Globase.KOM, a hierarchical tree-based P2P overlay that enables fully retrievable location-based overlay operations which proved to be highly efficient and logarithmically scalable.

Reputation Systems for Fighting Pollution in Peer-to-Peer File Sharing Systems

Abstract: Content pollution is a de facto problem in several currently popular Peer-to-Peer file sharing systems. Previously proposed strategies to fight content pollution include a peer reputation system named Scrubber as well as the alternative Credence object reputation system. This paper builds on previous work into three directions. First, it evaluates the cost-effectiveness of Scrubber and Credence in reducing pollution dissemination when polluters make use of collusion and Sybil attacks. Second, it modifies the Scrubber system to increase its effectiveness, in particular under collusion. Finally, it proposes a hybrid peer and object reputation system that combines the benefits of both strategies. We performed an extensive evaluation of all three systems, for various configurations and pollution mechanisms, as well as collusion and Sybil attacks. Our results show that the new hybrid system is much more effective and robust to malicious actions than any individual strategy, even in very uncooperative and unreliable peer communities.

Attacking a Swarm with a Band of Liars: evaluating the impact of attacks on BitTorrent

Abstract: BitTorrent has become one of the most popular Internet applications, given the number of users and the fraction of the Internet traffic it consumes. Its wide adoption has exposed some potential problems, like selfish peer behavior. Related research efforts so far have focused on modeling the dynamics of swarms, as well as devising incentive mechanisms that improve fairness without sacrificing efficiency. To the best of our knowledge, this is the first paper to evaluate the impact of attacks that exploit BitTorrent vulnerabilities with the sole intention of harming a swarm. The paper sheds light on BitTorrent behavior by presenting state diagrams, describes two attacks, and then evaluates their negative impact in realistic swarm settings. To evaluate the impact of attacks, a discrete-event simulator was developed and validated against an experimental evaluation performed in a controlled environment. Our findings show the seriousness of the problem and should be the basis for the development of new mechanisms to increase BitTorrent security.

Rescuing Tit-for-Tat with Source Coding

Abstract: Tit-for-tat is widely believed to be the most effective strategy to enforce collaboration among selfish users. However, it has been shown that its usefulness for decentralized and dynamic environments such as peer-to-peer networks is marginal, as peers can rapidly end up in a deadlock situation. Many proposed solutions to this problem are either less resilient to freeloading behavior or induce a computational overhead that cannot be sustained by regular peers. In contrast, we retain tit-for-tat, but enhance the system with a novel form of source coding and an effective scheme to prevent peers from freeloading from seeding peers. We show that our system performs well without the risk of peer starvation and without sacrificing fairness. The proposed solution has a reasonably low overhead, and may hence be suitable for fully distributed content distribution applications in real networks.

Handling Network Partitions and Mergers in Structured Overlay Networks

Abstract: Structured overlay networks form a major class of peer- to-peer systems, which are touted for their abilities to scale, tolerate failures, and self-manage. Any long-lived Internet-scale distributed system is destined to face network partitions. Although the problem of network partitions and mergers is highly related to fault-tolerance and self-management in large-scale systems, it has hardly been studied in the context of structured peer-to-peer systems. These systems have mainly been studied under churn (frequent joins/failures), which as a side effect solves the problem of network partitions, as it is similar to massive node failures. Yet, the crucial aspect of network mergers has been ignored. In fact, it has been claimed that ring-based structured overlay networks, which constitute the majority of the structured overlays, are intrinsically ill-suited for merging rings. In this paper, we present an algorithm for merging multiple similar ring-based overlays when the underlying network merges. We examine the solution in dynamic conditions, showing how our solution is resilient to churn dur- ng the merger, something widely believed to be difficult or mpossible. We evaluate the algorithm for various scenar- os and show that even when falsely detecting a merger, the algorithm quickly terminates and does not clutter the network with many messages. The algorithm is flexible as the tradeoff between message complexity and time complexity can be adjusted by a parameter.

Bounds on the Performance of P2P Networks Using Tit-for-Tat Strategies

Abstract: Current P2P systems employ tit-for-tat strategies, where peers only upload when they are simultaneously downloading, to avoid free riding. We derive optimal tit-for-tat strategies and obtain theoretical bounds on the performance of any P2P network employing such strategies. These are fundamental limitations that stem from peers unwillingness to cooperate without getting something in return. We show that the number of cooperating peers in a tit-for-tat strategy can, at best, grow linearly in time, as opposed to exponentially for a fully cooperative strategy. However, tit-for-tat strategies are fairer than a fully cooperative strategy. Our results show that there exists a seed capacity threshold for tit-for-tat strategies. Increasing seed capacity beyond this threshold brings significantly reduced marginal gains.

A Data Placement Scheme with Time-Related Model for P2P Storages

Abstract: Maintaining desired data availability while minimizing costs is the primary challenge in designing P2P storages. Data placement schemes and data availability calculation methods are two key design choices. Prior work has used the random placement and steady-state availability analysis model widely. We find this approach has two drawbacks: (1) it ignores the time-related property and greatly underestimates data availability in the short term and (2) it ignores differences in peers' availability and uses a mean value, which also leads to the underestimation of data availability. Both of these drawbacks influence the efficiency of storage and bandwidth resource usage. Also, this approach can not ensure all the objects achieve their expected availability due to the second drawback. This paper develops a different data placement scheme called similar-MTTF-MTTR placement, which takes into consideration differences in pees' dynamic characteristics. Under this scheme, this paper also presents a fine-gained analysis model for short-term data availability calculation. As a result, our scheme improves the efficiency of data maintaining significantly. We validate this by comparing with random placement under three different dynamic environments: a wide-area system (PlanetLab), a corporative environment (desktop personal computers at Microsoft Corporation), and a file-sharing system (Maze). The results show that our scheme can save the storage resource usage significantly under all environments.

A DHT-based Infrastructure for Content-based Publish/Subscribe Services

Abstract: Publish/subscribe model has become a prevalent paradigm for building distributed event delivering systems. Content-based publish/subscribe allows high expresses in subscriptions and thus is more appropriate for content dissemination. However, the scalability has remained a challenge in the design of distributed content-based publish/subscribe systems due to the expensive matching and delivering cost of content-based events. In this paper we propose an infrastructure built on top of distributed hash table for efficient content-based data distribution. Based on efficient subscription installation, event publication and event delivery mechanisms, the proposed infrastructure can simultaneously support any numbers of pub/sub schemas with different number of attributes. There are three key features in our design: (1) a locality-preserving hashing mechanism which partitions and maps the content space to nodes. Subscriptions and events are mapped to the corresponding node for efficiently matching; (2) an efficient event delivery algorithm which exploits the embedded trees in the underlying DHT to deliver events to the corresponding subscribers; (3) light-weighted load balancing mechanisms to adjust the load among peers and ensure that no peer is unduly loaded.

A Feasibility Study on Defending Against Ultra-Fast TopologicalWorms

Abstract: Self-propagating worms have been terrorizing the Internet for several years and they are becoming imminent threats to large-scale Peer-to-Peer (P2P) systems featuring rich host connectivity and popular data services. In this paper, we consider topological worms, which exploit P2P host vulnerabilities and topology information to spread in an ultra-fast way. We study the feasibility of leveraging the existing P2P overlay structure for distributing automated security patches to vulnerable machines. Two approaches are examined: a partition-based approach, which utilizes immunized hosts to proactively stop worm spread in the overlay graph, and a Connected Dominating Set(CDS)-based approach, which utilizes a group of dominating nodes in the overlay to achieve fast patch dissemination in a race with the worm. We demonstrate through analysis and simulations that both methods can result in effective worm containment.

On Routing in Distributed Hash Tables

Abstract: There have been many proposals for constructing routing tables for distributed hash tables (DHT). They can be classified into two groups: A) those that assume that the peers are uniformly randomly distributed in the identifier space, and B) those that allow order-preserving hash functions that lead to a skewed peer distribution in the identifier space. Good solutions for group A have been known for many years. However, DHTs in group A are limited to use randomized hashing and therefore, queries over whole identifier ranges thus do not scale. Group B can handle such queries easily. However, it is more difficult to connect the peers such that the resulting topology provides efficient routing, small routing tables, and balanced routing load. We present an elegant new solution to construct an efficient DHT for group B. Our main idea is to decouple the identifier space from the routing topology. In consequence, our DHT allows arbitrarily skewed peer distributions in the identifier space and does not require the overhead of sampling. Furthermore, the table construction is cheap and does not require active replacement of lost routing entries. To evaluate the performance of routing cost and table construction under high churn, we built an efficient simulator. Using the right data structures, we can easily process the state of over one million peers in RAM.

A Trust-Based Exchange Framework for Multiple Services in P2P Systems

Abstract: In this paper we propose fully distributed trust-based policies for p2p systems to regulate the exchange of different type of services and discourage misbehaviour. In a system of peers with different service valuations and capabilities, the resource allocation and server selection policies are based on local reputation vectors, whose elements are the reputations of the peers in providing each service of the system. Our studies demonstrate that proposed policies lead to the dynamic formation of coalitions (cooperation) among peers, which mutually profit by the exchange of their services, without pre-existing knowledge of one another's capabilities and values of services. In this way utilities of all peers progressively increase. Only misbehaving (non contributive) peers do not benefit by such a system, as proposed policies efficiently recognize and block misbehaviour.

Peer-to-Peer Rating

Abstract: This paper proposes to utilize algorithms from the probabilistic graphical models domain for Peer-to-Peer rating of data items and for computing "social influence" of nodes in a Peer-to-peer social network. We evaluate the practicality of our approach using large- scale simulations over a MSN Live Messenger subgraph consisting of about a million nodes. Our algorithms are general since they can be used for Peer-to-peer monitoring and for the efficient computation of other node ranking methods, such as PageRank and Information Centrality.

PISA: P2P Wi-Fi Internet Sharing Architecture

Abstract: The demand for cheap broadband Internet for nomadic users has created a market for Internet sharing. Wi- Fi communities which allow their users to share their wired Internet connections have emerged and become increasingly popular. Organizations like FON promise to provide free wireless Internet access in many places. However, user authentication is the Achilles heel of these systems. A user that allows other community members to use its access point must expect to be held responsible for other users' actions. Moreover, these Wi-Fi sharing systems are often insecure which allows eavesdroppers to gather sensitive information on the wireless link. This work provides efficient, scalable, and secure access control for large Wi-Fi sharing systems. The host identity protocol (HIP) is used as a building block for a solution which supports strong user authentication as well as mobility support for nomadic users. In our presentation, we show the feasibility and effectiveness of this approach by demonstrating the PISA authentication protocol in action.

Modelling Real P2P Networks: The Effect of Altruism

Abstract: We develop a model of the interaction of rational peers in an incentive-free peer-to-peer (P2P) network and use game theoretic analysis to derive results about peer and network behavior. We calculate and discuss Nash equilibria and predict peer behavior in terms of individual contribution. At the heart of our model is altruism, an intrinsic parameter reflecting peers inherent willingness to contribute. Two different approaches for modelling altruistic behavior and its attendant benefit are introduced and discussed. We consider the cases of P2P networks of peers that (i) have homogeneous altruism levels or (ii) have heterogeneous altruism levels, but with known probability distributions. We find that, under the effects of altruism, a substantial fraction of peers will contribute when altruism levels are within certain intervals, even though no incentive mechanism is used. Our results corroborate empirical evidence of large P2P networks surviving or even flourishing without or with barely functioning incentive mechanisms.

Mapping Small Worlds

Abstract: Social networks are usually navigable small worlds: individuals are able to find short chains of acquaintances connecting pairs of unrelated nodes. This property can be explained by the fact that nodes are characterized by a series of properties, such as geographical position, work or educational background; the navigation proceeds towards the node that is "most similar" to the destination. Since nodes are likely to be linked with similar individuals, this strategy permits to quickly reach the destination. We approach the problem of creating the information that makes a network navigable. Starting from a given network, and without any other information, we show how nodes can reconstruct, with a scalable and decentralized algorithm, a "network map": a d-dimensional layout that places nodes in a way that reflects the network structure, so that navigability is achieved. Euclidean distance on the layout is used as a measure for node similarity, and efficient routing can be simply achieved by iteratively jumping towards the neighbor that is closest to the destination. The network map provides a means for implementing routing on social networks that can be used in "darknets", that is, anonymous networks where nodes establish connections only if they are mutually trusted. Moreover, the distance between nodes on the network map can be used as a measure of node affinity, and may help in various types of network analysis, for instance to help evaluate reputation in webs of trust, or in order to perform "personalized" ranking.

Cooperative Keep-Alives: An Efficient Outage Detection Algorithm for P2P Overlay Networks

Abstract: One of the challenges of today's overlay networks, especially P2P, is still scalability. A key issue in almost all of the current overlay architectures is the link count per single node. If the link count is too high, the management overhead in terms of keep-alive messages increases. If the amount of links per node is too low, the resilience of the system against network splits decreases and the system can hardly route in an optimal way. Moreover, if keep-alive messages are not sent frequently enough, outdated information could be propagated, which again could cause net splits. This paper presents a new cooperative keep-alive algorithm that strongly reduces the costs for sending keep- alive messages and, at the same time, preserves the effectiveness and reliability of standard keep-alive mechanisms in today's overlay networks. The algorithm allows to increase the number of links per node, and, thus, to improve the connectivity and routing efficiency in the network, while keeping the keep-alive overhead low. When used without increasing the link count, the algorithm reduces drastically the keep-alive traffic. The properties of the algorithm are evaluated analytically and simulatively and compared to existing keep-alive techniques.

Grid Resource Scheduling with Gossiping Protocols

Abstract: Grid resource providers can use gossiping to disseminate their available resource state to remote regions of the grid to attract application load. Pairwise gossiping protocols exchange information about limited subsets of other resources between pairs of potentially remote participants. In epidemic gossiping protocols, the provider disseminates information to multiple neighbors, who in turn forward it to their neighbors, and so on. One important metric for these protocols is their coverage, which characterizes how many and which resources receive the information. Coverage characteristics of epidemic protocols are non-uniform, concentrated within the vicinity of a disseminating node; they can exhibit bi-modal behavior where information either reaches distant nodes or dies out quickly. Pairwise gossiping protocols, on the other hand, provide a more uniform coverage, but it can take longer for the dissemination to reach desired uniformity. In this paper, we study performance characteristics of three gossiping protocols: (1) epidemic gossiping, (2) pairwise gossiping, and (3) adaptive information dissemination (which is based on a form of epidemic gossiping). We report experimental results based on our simulation framework that compare the three protocols in terms of packet overhead and query satisfaction rates. We show that pairwise gossiping protocols work best when resource distribution on the grid is uniform, but that they can be configured to perform well in support of grid scheduling.

W*-Grid: A Robust Decentralized Cross-layer Infrastructure for Routing and Multi-Dimensional Data Management in Wireless Ad-Hoc Sensor Networks

Abstract: Sensor networks are usually composed by small units able to sense and transmit to a sink elementary data which are successively processed by an external machine. However recent improvements in the memory and computational power of sensors, together with the reduction of energy consumptions, are rapidly changing the potential of such systems, moving the attention towards data-centric sensor networks. This paper presents W*-Grid, a fully decentralized and robust infrastructure for self-organizing data- centric sensor networks, where wireless communications occur through multi-hop routing among devices. The solution extends W-Grid by strongly improving the network recovery performance from link and/or device failures. In particular W*-Grid guarantees, by construction, at least two disjoint paths between each couple of nodes. This implies that the recovery in W*-Grid occurs without broadcasting transmissions and guaranteeing robustness while drastically reducing the energy consumption. An extensive number of simulations show the efficiency, robustness and traffic load of resulting networks under several scenarios of device density and of number of coordinates.

A P2PSIP Demonstrator Powered by OverSim

Abstract: A fundamental problem in studying peer-to-peer networks is the evaluation of new protocols. This commonly involves both the simulation of the protocol in a large-scale network as well as the testing of the protocol in connection with real applications in networks like PlanetLab. To facilitate these tasks we developed the overlay simulation framework OverSim. It was designed to fulfill a number of requirements that have been partially neglected by existing simulation frameworks. In our demonstrator we show how OverSim can not only be used to simulate overlay protocols in large-scale networks, but also demonstrate its real-world capabilities by means of a P2PSIP testbed.

The BitCod Client: A BitTorrent Clone using Network Coding

Abstract: Network coding is an emerging field of research with sound and mature theory supporting it. Recent works shows that it has many benefits like improved fault tolerance, higher flexibility in selection of file parts to transfer and resiliency to network partitions [4, 3]. Despite those appealing properties there is no wide usage of network coding in real file sharing applications. In this work, we try to bridge the gap between theory of network coding and practice. From the one hand, we deploy one the most successful file sharing client, the BitTorrent client. We use the BitTorrent algorithm for optimizing the neighbor selections for maximizing the upload bandwidth. From the other hand, we propose several simple heuristics that improve significantly the efficiency of the network coding deployed. In a nutshell, we propose computation intensive variant of network coding that can be applied to most of the existing network coding protocols. By changing the random selection of coded parts to a selection based on feedback from the network, we significantly improve the network utilization and the efficiency of the protocol. In this paper we report our work in progress building the BitCod client. Using extensive simulations we demonstrate that our technique can compete with the performance of the state-of-the-art BitTorrent [2] file sharing client. Next, we plan to implement and test a prototype of the BitCod client over the WAN.

Self-Stabilization in Preference-Based Networks

Abstract: Participants in a decentralized system often use some local ranking information in selecting effective collaborations. We say that such systems are preference-based for most practical types of preferences, such systems converge towards a unique stable configuration. In this paper, we investigate the speed and quality of the convergence process with respect to the model parameters. Our results provide insight into the design of system parameters, such as the number of connections or the algorithm for choosing new partners.

P2P-IM: A P2P Presence System for the Internet

Abstract: Traditional instant messaging applications rely on central server infrastructure to broker user information. The cost and complexity of this infrastructure makes it difficult for developers to build and deploy lightweight presence and instant messaging systems within their own applications. In this paper, we describe P2P-IM, a peer-to-peer instant messaging client that does not rely on any hosted server infrastructure. The system provides the rich facilities available from traditional client-server systems but enables easy deployment and integration with existing applications. The solution provides simplified identity generation, connectivity, and rich per-application data publication.

An Efficient Eigenvalue-based P2P XML Routing Framework

Abstract: Many emerging applications that use XML are distributed, usually over large peer-to-peer (P2P) networks on the Internet. The deployment of an XML query shipping system over P2P networks requires a specialized synopsis to capture XML data in routing tables. In this paper, we propose a novel eigenvalue-based routing synopsis for deployment over unstructured P2P networks. Based on well-established Cauchy's interlacing theorem, our approach employs multiple eigenvalues as routing synopsis and facilitates query routing process with computationally-inexpensive range inequality checking. Through extensive experiments, we demonstrate the effectiveness of our approach.

Price of Structured Routing and Its Mitigation in P2P Systems under Churn

Abstract: We study the resilience of structured peer-to-peer (P2P) networks in the presence of churn. Using the lifetime-based failure assumptions, we first show that a realistic churn model has an equivalent uniform failure model in the steady state. We then determine via percolation analysis and simulation the gap between the size of the connected component and the reachable component of a randomly picked node for symphony and chord. This gap represents the price of structured routing: the size of the set of nodes that are reachable by any unstructured routing method (e.g. broadcast on the unstructured overlay) from a randomly picked surviving node, but are not reachable using structured routing on the structured overlay. As an illustration, for 24- minute average node lifetime with 1-minute average node-search delay, the gap is around 12 thousand nodes or 1.2% in a chord network of around 1 million nodes. We finish by discussing potential techniques to mitigate the price of structured routing.

Distributed, Automatic File Description Tuning in Peer-to-Peer File-Sharing Systems

Abstract: Peers in peer-to-peer file-sharing systems cannot effectively share their files if they are poorly described. Terms one user employs to describe an instance of a file may not be those that are commonly associated with the file, making this instance difficult to locate. To alleviate this problem, a server can ask its peers for help in improving the description of files they have in common. We consider the design of a fully distributed, automatic system for the exchange of descriptive metadata. Experimental results show that the proposed techniques are effective in improving search accuracy with reasonable cost.

Fabric: Synergistic Proximity Neighbour Selection Method

Abstract: In this paper, verification of a synergistic, two-staged method for self-optimization of DHT-based overlay networks is presented. The method, named Fabric, minimizes overlay network stretch during a node insertion by using static, readily available information that does not involve costly periodic probing of many nodes. Furthermore, Fabric enables an overlay to optimize its topology at runtime in an adaptive manner.

Completeness Estimation of Range Queries in Structured Overlays

Abstract: Range queries are a very powerful tool in a wide range of data management systems and are vital to a multitude of applications. The hierarchy of structured overlay systems can be utilized in order to provide efficient techniques for processing them, resulting in the support of applications and techniques based on range queries in large-scale distributed information systems. But, due to the limited knowledge and the usually best-effort characteristics, deciding about the completeness of query results, e.g., getting an idea when a query is finished or what amount of results is still missing, is very challenging. There is not only an urgent need to provide this information to the user issuing queries, but also for implementing sophisticated and efficient processing techniques based on them. In this work, we propose a method for solving this task. We discuss the applicability and quality of our estimations, present an implementation and evaluation for the P-grid system and show how to adapt the technique to other overlays.

Running the Windows P2P Infrastructure on Mobile Phones

Abstract: The recent desktop versions of Windows (XP SP2 and Vista) include a Peer-to-Peer (P2P) infrastructure that simplifies the development and deployment of true P2P applications. We have ported the latest version of this infrastructure to Windows Embedded CE 5.0, which is the underlying OS for Windows Mobile 5.0. To our knowledge, this is the first native implementation of a P2P infrastructure for Windows Mobile. This paper provides a short overview of the infrastructure and design considerations when running P2P applications on mobile phones. For demonstration purposes, we have developed a community-based photo sharing and chat application that solely uses the P2P infrastructure for communication. We will demonstrate the ease of creating P2P communities in an ad-hoc manner, and the interoperability between Windows Mobile and Windows Vista.

The Design and Evaluation of Techniques for Route Diversity in Distributed Hash Tables

Abstract: We present a replica placement scheme for any distributed hash table that uses a prefix-matching routing scheme and evaluate the number of replicas necessary to produce a desired number of disjoint routes. We show through simulation that this placement can make a significant improvement in routing robustness over other placements. Furthermore, we consider another route diversity mechanism that we call neighbor set routing and show that, when used with our replica placement, it can successfully route messages to a correct replica even with a quarter of the nodes in the system failed at random. Finally, we demonstrate a family of replica query strategies that can trade off response time and system load. We present a hybrid query strategy that keeps response time low without producing too high a load.

Impact of Vertical Handovers on Cooperative Content Distribution Systems

Abstract: In this paper, we evaluate the performance of a P2P- based content distribution system in heterogeneous, wireless networks. The mobile users coordinate each other with cooperation strategies enabled by the multi-source download mechanism, as in eDonkey or BitTorrent. Due to the mobility, vertical handovers between different wireless access technologies are required which may result in transmission delays and IP address changes of the switching peer. Hence, connections among users have to be reestablished and downloading users are requeued at a providing peer's waiting queue. In detail, we investigate the impact of requeueing with each VHO as well as the use of mechanisms that preserve the IP address and connections beyond VHOs, like MobilelP. Another important phenomenon occurring with VHOs is the abrupt change of the available bandwidth, e.g., from a fast WLAN connection to a rather slow UMTS connection. We evaluate the download times for files by means of simulation while considering different load scenarios in today's and future network layouts of the B3G network. As a result of the performance evaluation, we derive a new time-based cooperation strategy that counters the impact of mobility. Instead of downloading individual blocks of a file, a user gets a time slot at a providing peer. We show that this leads to a significant performance gain.