Brian D. Noble

Random waypoint considered harmful

This study examines the random waypoint model widely used in the simulation studies of mobile ad hoc networks. Our findings show that this model fails to provide a steady state in that the average nodal speed consistently decreases over time, and therefore should not be directly used for simulation. We show how unreliable results can be obtained by using this model. In particular, certain ad hoc routing metrics can drop by as much as 40% over the course of a 900-second simulation using the random waypoint model. We give both an intuitive and a formal explanation for this phenomenon. We also propose a simple fix of the problem and discuss a few alternatives. Our modified random waypoint model is able to reach a steady state and simulation results are presented.

Agile application-aware adaptation for mobility

In this paper we show that application-aware adaptation, a collaborative partnership between the operating system and applications, offers the most general and effective approach to mobile information access. We describe the design of Odyssey, a prototype implementing this approach, and show how it supports concurrent execution of diverse mobile applications. We identify agility as a key attribute of adaptive systems, and describe how to quantify and measure it. We present the results of our evaluation of Odyssey, indicating performance improvements up to a factor of 5 on a benchmark of three applications concurrently using remote services over a network with highly variable bandwidth.

Pastiche: making backup cheap and easy

Backup is cumbersome and expensive. Individual users almost never back up their data, and backup is a significant cost in large organizations. This paper presents Pastiche, a simple and inexpensive backup system. Pastiche exploits excess disk capacity to perform peer-to-peer backup with no administrative costs. Each node minimizes storage overhead by selecting peers that share a significant amount of data. It is easy for common installations to find suitable peers, and peers with high overlap can be identified with only hundreds of bytes. Pastiche provides mechanisms for confidentiality, integrity, and detection of failed or malicious peers. A Pastiche prototype suffers only 7.4% overhead for a modified Andrew Benchmark, and restore performance is comparable to cross-machine copy.

BreadCrumbs: forecasting mobile connectivity

Mobile devices cannot rely on a single managed network, but must exploit a wide variety of connectivity options as they travel. We argue that such systems must consider the derivative of connectivity--the changes inherent in movement between separately managed networks, with widely varying capabilities. With predictive knowledge of such changes, devices can more intelligently schedule network usage. To exploit the derivative of connectivity, we observe that people are creatures of habit; they take similar paths every day. Our system, BreadCrumbs, tracks the movement of the devices owner, and customizes a predictive mobility model for that specific user. Combined with past observations of wireless network capabilities, BreadCrumbs generates connectivity forecasts. We have built a BreadCrumbs prototype, and demonstrated its potential with several weeks of real-world usage. Our results show that these forecasts are sufficiently accurate, even with as little as one week of training, to provide improved performance with reduced power consumption for several applications.

Sound mobility models

Simulation has become an indispensable tool in the construction and evaluation of mobile systems. By using mobility models that describe constituent movement, one can explore large systems, producing repeatable results for comparison between alternatives. Unfortunately, the vast majority of mobility models---including all those in which nodal speed and distance or destination are chosen independently---suffer from decay; average speed decreases until converging to some long-term average. Such decay provides an unsound basis for simulation studies that collect results averaged over time, complicating the experimental process.This paper shows via analysis that such decay is inevitable in a wide variety of mobility models, including the most common in use today. We derive a general framework for describing this decay, and apply it to a number of practical cases. Furthermore, this framework allows us to transform any given mobility model into a stationary one: choose initial speeds from the steady-state distribution, and subsequent speeds from the original. This transformation provides sound models for simulation, eliminating variations in average nodal speed.

Samsara: honor among thieves in peer-to-peer storage

Peer-to-peer storage systems assume that their users consume resources in proportion to their contribution. Unfortunately, users are unlikely to do this without some enforcement mechanism. Prior solutions to this problem require centralized infrastructure, constraints on data placement, or ongoing administrative costs. All of these run counter to the design philosophy of peer-to-peer systems.Samsara enforces fairness in peer-to-peer storage systems without requiring trusted third parties, symmetric storage relationships, monetary payment, or certified identities. Each peer that requests storage of another must agree to hold a claim in return---a placeholder that accounts for available space. After an exchange, each partner checks the other to ensure faithfulness. Samsara punishes unresponsive nodes probabilistically. Because objects are replicated, nodes with transient failures are unlikely to suffer data loss, unlike those that are dishonest or chronically unavailable. Claim storage overhead can be reduced when necessary by forwarding among chains of nodes, and eliminated when cycles are created. Forwarding chains increase the risk of exposure to failure, but such risk is modest under reasonable assumptions of utilization and simultaneous, persistent failure.

Surface street traffic estimation

In this paper, we propose a simple yet effective method of identifying traffic conditions on surface streets given location traces collected from on-road vehicles---this requires only GPS location data, plus infrequent low-bandwidth cellular updates. Unlike other systems, which simply display vehicle speeds on the road, our system characterizes unique traffic patterns on each road segment and identifies unusual traffic states on a segment-by-segment basis. We developed and evaluated the system by applying it to two sets of location traces. Evaluation results show that higher than 90% accuracy in characterization can be achieved after ten or more traversals are collected on a given road segment. We also show that traffic patterns on a road are very consistent over time, provided that the underlying road conditions do not change. This allows us to use a longer history in identifying traffic conditions with higher accuracy.

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.

Zero-interaction authentication

Laptops are vulnerable to theft, greatly increasing the likelihood of exposing sensitive files. Unfortunately, storing data in a cryptographic file system does not fully address this problem. Such systems ask the user to imbue them with long-term authority for decryption, but that authority can be used by anyone who physically possesses the machine. Forcing the user to frequently reestablish his identity is intrusive, encouraging him to disable encryption.Our solution to this problem is Zero-Interaction Authentication, or ZIA. In ZIA, a user wears a small authentication token that communicates with a laptop over a short-range, wireless link. Whenever the laptop needs decryption authority, it acquires it from the token; authority is retained only as long as necessary. With careful key management, ZIA imposes an overhead of only 9.3% for representative workloads. The largest file cache on our hardware can be re-encrypted within five seconds of the users departure, and restored in just over six seconds after detecting the users return. This secures the machine before an attacker can gain physical access, but recovers full performance before a returning user resumes work.

The end-to-end performance effects of parallel TCP sockets on a lossy wide-area network

This paper examines the effects of using parallel TCP flows to improve end-to-end network performance for distributed data intensive applications. A series of transmission experiments were conducted over a wide-area network to assess how parallel flows improve throughput, and to understand the number of flows necessary to improve throughput while avoiding congestion. An empirical throughput expression for parallel flows based on experimental data is presented, and guidelines for the use of parallel flows are discussed.