Alex Sherman

Web caching with consistent hashing

Ak ey performance measure for the World Wide Web is the speed with which content is served to users. As traff ic on the Web increases, users are faced with increasing delays and failures in data delivery. Web caching is one of the key strategies that has been explored to improve performance. An important issue in many caching systems is how to decide what is cached where at any given time. Solutions have included multicast queries and directory schemes. In this paper, we offer a new Web caching strategy based on consistent hashing .C onsistent hashing provides an alternative to multicast and directory schemes, and has several other advantages in load balancing and fault tolerance. Its performance was analyzed theoretically in previous work; in this paper we describe the implementation of a consistent-hashing-based system and experiments that support our thesis that it can provide performance improvements.  1999 Published by Elsevier Science B.V. All rights reserved.

Mitigating the Effect of Free-Riders in BitTorrent using Trusted Agents

Even though Peer-to-Peer (P2P) systems present a cost-effective and scalable solution to content distribution, most entertainment, media and software, content providers continue to rely on expensive, centralized solutions such as Content Delivery Networks. One of the main reasons is that the current P2P systems cannot guarantee reasonable performance as they depend on the willingness of users to contribute bandwidth. Moreover, even systems like BitTorrent, which employ a tit-for-tat protocol to encourage fair bandwidth exchange between users, are prone to free-riding (i.e. peers that do not upload). Our experiments on PlanetLab extend previous research [14, 12, 11] demonstrating that such selfish behavior can seriously degrade the performance of regular users in many more scenarios beyond simple free-riding: we observed an overhead of upto 430% for 80% of free-riding identities easily generated by a small set of selfish users. To mitigate the effects of selfish users, we propose a new P2P architecture that classifies peers with the help of a small number of trusted nodes that we call Trusted Auditors (TAs). TAs participate in P2P download like regular clients and detect free-riding identities by observing their neighbors’ behavior. Using TAs, we can separate compliant users into a separate service pool resulting in better performance. Furthermore, we show that TAs are more effective ensuring the performance of the system than a mere increase in bandwidth capacity: for 80% of freeriding identities a single-TA system has a 6% download time overhead while without the TA and three times the bandwidth capacity we measure a 100% overhead.

Can P2P Replace Direct Download for Content Distribution

While peer-to-peer (P2P) file-sharing is a powerful and cost-effective content distribution model, most paid-for digital-content providers (CPs) rely on direct download to deliver their content. CPs such as Apple iTunes that command a large base of paying users are hesitant to use a P2P model that could easily degrade their user base into yet another free file-sharing community. We present TP2, a system that makes P2P file sharing a viable delivery mechanism for paid digital content by providing the same security properties as the currently used direct-download model. TP2 introduces the novel notion of trusted auditors (TAs) – P2P peers that are controlled by the system operator. TAs monitor the behavior of other peers and help detect and prevent formation of illegal file-sharing clusters among the CP’s user base. TAs both complement and exploit the strong authentication and authorization mechanisms that are used in TP2 to control access to content. It is important to note that TP2 does not attempt to solve the out-of-band file-sharing or DRM problems, which also exist in the direct-download systems currently in use. We analyze TP2 by modeling it as a novel game between misbehaving users who try to form unauthorized file-sharing clusters and TAs who curb the growth of such clusters. Our analysis shows that a small fraction of TAs is sufficient to protect the P2P system against unauthorized file sharing. In a system with as many as 60% of misbehaving users, even a small fraction of TAs can detect 99% of unauthorized cluster formation. We developed a simple economic model to show that even with such a large fraction of malicious nodes, TP2 can improve CP’s profits (which could translate to user savings) by 62 to 122%, even while assuming conservative estimates of content and bandwidth costs. We implemented TP2 as a layer on top of BitTorrent and demonstrated experimentally using PlanetLab that our system provides trusted P2P file sharing with negligible performance overhead.

Aequitas: A Trusted P2P System for Paid Content Delivery

P2P file-sharing has been recognized as a powerful and efficient distribution model due to its ability to leverage users’ upload bandwidth. However, companies that sell digital content on-line are hesitant to rely on P2P models for paid content distribution due to the free file-sharing inherent in P2P models. In this paper we present Aequitas, a P2P system in which users share paid content anonymously via a layer of intermediate nodes. We argue that with the extra anonymity in Aequitas, vendors could leverage P2P bandwidth while effectively maintaining the same level of trust towards their customers as in traditional models of paid content distribution. As a result, a content provider could reduce its infrastructure costs and subsequently lower the costs for the end-users. The intermediate nodes are incentivized to contribute their bandwidth via electronic micropayments. We also introduce techniques that prevent the intermediate nodes from learning the content of the files they help transmit. In this paper we present the design of our system, an analysis of its properties and an implementation and experimental evaluation. We quantify the value of the intermediate nodes, both in terms of efficiency and their effect on anonoymity. We argue in support of the economic and technological merits of the system.

Feasibility of Voice over IP on the Internet

VoIP (Voice over IP) services are using the Internet infrastructure to enable new forms of communication and collaboration. A growing number of VoIP service providers such as Skype, Vonage, Broadvoice, as well as many cable services are using the Internet to offer telephone services at much lower costs. However, VoIP services rely on the user’s Internet connection, and this can often translate into lower quality communication. Overlay networks offer a potential solution to this problem by improving the default Internet routing and overcome failures. To assess the feasibility of using overlays to improve VoIP on the Internet, we have conducted a detailed experimental study to evaluate the benefits of using an overlay on PlanetLab nodes for improving voice communication connectivity and performance around the world. Our measurements demonstrate that an overlay architecture can significantly improve VoIP communication across most regions and provide their greatest benefit for locations with poorer default Internet connectivity. We explore overlay topologies and show that a small number of well-connected intermediate nodes is sufficient to improve VoIP performance. We show that there is significant variation over time in the best overlay routing paths and argue for the need for adaptive routing to account for this variation to deliver the best performance.

A Case for P2P Delivery of Paid Content

P2P file sharing provides a powerful content distribution model by leveraging users’ computing and bandwidth resources. However, companies have been reluctant to rely on P2P systems for paid content distribution due to their inability to limit the exploitation of these systems for free file sharing. We present TP2, a system that combines the more cost-effective and scalable distribution capabilities of P2P systems with a level of trust and control over content distribution similar to direct download content delivery networks. TP2 uses two key mechanisms that can be layered on top of existing P2P systems. First, it provides strong authentication to prevent free file sharing in the system. Second, it introduces a new notion of trusted auditors to detect and limit malicious attempts to gain information about participants in the system to facilitate additional out-of-band free file sharing. We analyze TP2 by modeling it as a novel game between malicious users who try to form free file sharing clusters and trusted auditors who curb the growth of such clusters. Our analysis shows that a small fraction of trusted auditors is sufficient to protect the P2P system against unauthorized file sharing. Using a simple economic model, we further show that TP2 provides a more cost-effective content distribution solution, resulting in higher profits for a content provider even in the presence of a large percentage of malicious users. Finally, we implemented TP2 on top of BitTorrent and use PlanetLab to show that our system can provide trusted P2P file sharing with negligible performance overhead.