Yatin Chawathe

Making gnutella-like P2P systems scalable

Napster pioneered the idea of peer-to-peer file sharing, and supported it with a centralized file search facility. Subsequent P2P systems like Gnutella adopted decentralized search algorithms. However, Gnutellas notoriously poor scaling led some to propose distributed hash table solutions to the wide-area file search problem. Contrary to that trend, we advocate retaining Gnutellas simplicity while proposing new mechanisms that greatly improve its scalability. Building upon prior research [1, 12, 22], we propose several modifications to Gnutellas design that dynamically adapt the overlay topology and the search algorithms in order to accommodate the natural heterogeneity present in most peer-to-peer systems. We test our design through simulations and the results show three to five orders of magnitude improvement in total system capacity. We also report on a prototype implementation and its deployment on a testbed.

Accuracy characterization for metropolitan-scale Wi-Fi localization

Location systems have long been identified as an important component of emerging mobile applications. Most research on location systems has focused on precise location in indoor environments. However, many location applications (for example, location-aware web search) become interesting only when the underlying location system is available ubiquitously and is not limited to a single office environment. Unfortunately, the installation and calibration overhead involved for most of the existing research systems is too prohibitive to imagine deploying them across, say, an entire city. In this work, we evaluate the feasibility of building a wide-area 802.11 Wi-Fi-based positioning system. We compare a suite of wireless-radio-based positioning algorithms to understand how they can be adapted for such ubiquitous deployment with minimal calibration. In particular, we study the impact of this limited calibration on the accuracy of the positioning algorithms. Our experiments show that we can estimate a users position with a median positioning error of 13-40 meters (depending upon the characteristics of the environment). Although this accuracy is lower than existing positioning systems, it requires substantially lower calibration overhead and provides easy deployment and coverage across large metropolitan areas.

Towards IP geolocation using delay and topology measurements

We present Topology-based Geolocation (TBG), a novel approach to estimating the geographic location of arbitrary Internet hosts. We motivate our work by showing that 1) existing approaches, based on end-to-end delay measurements from a set of landmarks, fail to outperform much simpler techniques, and 2) the error of these approaches is strongly determined by the distance to the nearest landmark, even when triangulation is used to combine estimates from different landmarks. Our approach improves on these earlier techniques by leveraging network topology, along with measurements of network delay, to constrain host position. We convert topology and delay data into a set of constraints, then solve for router and host locations simultaneously. This approach improves the consistency of location estimates, reducing the error substantially for structured networks in our experiments on Abilene and Sprint. For networks with insufficient structural constraints, our techniques integrate external hints that are validated using measurements before being trusted. Together, these techniques lower the median estimation error for our university-based dataset to 67 km vs. 228 km for the best previous approach.

Improved access point selection

This paper presents Virgil, an automatic access point discovery and selection system. Unlike existing systems that select access points based entirely on received signal strength, Virgil scans for all available APs at a location, quickly associates to each, and runs a battery of tests to estimate the quality of each APs connection to the Internet. Virgil also probes for blocked or redirected ports, to guide AP selection in favor of preserving application services that are currently in use. Results of our evaluation across five neighborhoods in three cities show Virgil finds a usable connection from 22% to 100% more often than selecting based on signal strength alone. By caching AP test results, Virgil both improves performance and success rate. Our overhead is acceptable and is shown to be faster than manually selecting an AP with Windows XP.

Scattercast: an adaptable broadcast distribution framework

Abstract.Internet broadcasting - the simultaneous distribution of live content streams to a large audience - has a number of interesting applications ranging from real-time broadcasts of audio/video streams for online concerts or sporting events to efficient and reliable large-scale software distribution. We identify three fundamental requirements for scalable broadcasting services: an efficient infrastructure for large-scale broadcasting, an ability to adapt the infrastructure to suit the requirements of a wide range of applications, and ease of deployment of the infrastructure. Although solutions such as the network-layer IP multicast approach and a slew of overlay distribution networks exist today, none of these technologies satisfactorily addresses all of the above concerns. In this paper, we argue that an application-customizable hybrid overlay is well suited to meet these challenges. To this end, we propose an architecture called scattercast that relies on a network of strategically located agents called ScatterCast proXies or SCXs. These agents collaboratively provide the broadcast service for a session. Clients locate a nearby SCX and tap into the session via that SCX. Scattercast constructs a hybrid overlay network composed of unicast links between SCXs that interconnect locally scoped multicast regions. Rather than define a single standardized service model for transmitting data on top of the overlay, scattercast builds a customizable transport framework that provides adaptability by leveraging application-defined semantics to drive the distribution of content. We demonstrate the ability of our architecture to provide efficient distribution via a set of simulation experiments. Finally, we present our experience with the adaptability of the framework by describing two applications, a real-time Internet radio and an online slide-presentation tool, both of which we have built on top of a prototype implementation of the architecture.

Experiences with place lab: an open source toolkit for location-aware computing

Location-based computing (LBC) is becoming increasing important in both industry and academia. A key challenge is the pervasive deployment of LBC technologies; to be effective they must run on a wide variety of client platforms, including laptops, PDAs, and mobile phones, so that location data can be acquired anywhere and accessed by any application. Moreover, as a nascent area, LBC is experiencing rapid innovation in sensing technologies, the positioning algorithms themselves, and the applications they support. Lastly, as a newcomer, LBC must integrate with existing communications and application technologies, including web browsers and location data interchange standard.This paper describes our experience in developing the Place Lab architecture, a widely used first-generation open source toolkit for client-side location sensing. Using a layered, pattern-based architecture, it supports modular development in any dimension of LBC, enabling the field to move forward more rapidly as these innovations are shared with the community as pluggable components. Our experience shows the benefits of domain-specific abstractions, and how we overcame high-level language constraints to support a wide array of platforms in this emerging space. We also describe our experience in re-engineering parts of the architecture based on the needs of the user community, including insights on software licensing issues.

Broadcast federation: an application-layer broadcast internetwork

Researchers and commercial developers have proposed several protocols to enable efficient multi-point communication both at the IP layer and at the application or overlay layer. However, no single protocol has made enough headway in terms of deployment for it to span the entire Internet. In fact, we believe that none of the existing multicast or broadcast protocols will become the sole dominant Internet broadcasting technology any time in the near future. Instead, we expect islands of non-interoperable broadcast connectivity. To address this, we propose an architecture that enables the composition of different broadcast protocol, to provide an end-to-end broadcast service. We call this architecture the Broadcast Federation. In this paper, we describe the architecture along with a prototype implementation and preliminary results from the prototype.