Charles Reis

Measurement-based models of delivery and interference in static wireless networks

We present practical models for the physical layer behaviors of packet reception and carrier sense with interference in static wireless networks. These models use measurements of a real network rather than abstract RF propagation models as the basis for accuracy in complex environments. Seeding our models requires N trials in an N node network, in which each sender transmits in turn and receivers measure RSSI values and packet counts, both of which are easily obtainable. The models then predict packet delivery and throughput in the same network for different sets of transmitters with the same node placements. We evaluate our models for the base case of two senders that broadcast packets simultaneously. We find that they are effective at predicting when there will be significant interference effects. Across many predictions, we obtain an RMS error for 802.11a and 802.11b of a half and a third, respectively, of a measurement-based model that ignores interference.

Measurement-based characterization of 802.11 in a hotspot setting

We analyze wireless measurements taken during the SIGCOMM 2004 conference to understand how well 802.11 operates in real deployments. We find that the overhead of 802.11 is high, with only 40% of the transmission time spent in sending original data. Most of the remaining time is consumed by retransmissions due to packet losses that are caused by both contention and transmission errors. Our analysis also shows that wireless nodes adapt their transmission rates with an extremely high frequency. We comment on the difficulties and opportunities of working with wireless traces, rather than the wired traces of wireless activity that are presently more common.

BrowserShield: Vulnerability-driven filtering of dynamic HTML

Vulnerability-driven filtering of network data can offer a fast and easy-to-deploy alternative or intermediary to software patching, as exemplified in Shield [43]. In this paper, we take Shields vision to a new domain, inspecting and cleansing not just static content, but also dynamic content. The dynamic content we target is the dynamic HTML in web pages, which have become a popular vector for attacks. The key challenge in filtering dynamic HTML is that it is undecidable to statically determine whether an embedded script will exploit the browser at run-time. We avoid this undecidability problem by rewriting web pages and any embedded scripts into safe equivalents, inserting checks so that the filtering is done at run-time. The rewritten pages contain logic for recursively applying run-time checks to dynamically generated or modified web content, based on known vulnerabilities. We have built and evaluated BrowserShield, a system that performs this dynamic instrumentation of embedded scripts, and that admits policies for customized run-time actions like vulnerability-driven filtering.

Isolating web programs in modern browser architectures

Many of todays web sites contain substantial amounts of client-side code, and consequently, they act more like programs than simple documents. This creates robustness and performance challenges for web browsers. To give users a robust and responsive platform, the browser must identify program boundaries and provide isolation between them. We provide three contributions in this paper. First, we present abstractions of web programs and program instances, and we show that these abstractions clarify how browser components interact and how appropriate program boundaries can be identified. Second, we identify backwards compatibility tradeoffs that constrain how web content can be divided into programs without disrupting existing web sites. Third, we present a multi-process browser architecture that isolates these web program instances from each other, improving fault tolerance, resource management, and performance. We discuss how this architecture is implemented in Google Chrome, and we provide a quantitative performance evaluation examining its benefits and costs.

A friendly face for Eclipse

Eclipse is a powerful integrated development environment (IDE) for Java1 targeted at professional software developers. However, Eclipse is poorly suited for use in introductory computing education because the complexity of its interface and the associated computing environment can overwhelm beginners. In contrast, DrJava is a friendly, highly interactive IDE targeted at teaching Java to beginners. DrJava has a simple interface consisting of a Definitions pane for entering and editing program text and an Interactions pane for evaluating arbitrary Java statements and expressions given the program in the Definitions pane. This interface frees students from the complication of defining main methods for their programs and encourages them to explore the Java language by conducting simple experiments.We have developed a plug-in for Eclipse, based largely on the existing DrJava code base, that provides an Interactions pane to Eclipse with precisely the same capabilities as the Interactions pane in DrJava, along with a simplified user interface. With this plug-in, Eclipse becomes a suitable vehicle for teaching introductory programming, enabling instructional programs to use the same IDE for all levels of the programming curriculum. In addition, it provides professional developers with a convenient mechanism for interactively evaluating arbitrary program text during program development --- a common feature of programming interfaces for functional languages like Scheme and ML.