Mark D. Corner

Capacity Enhancement using Throwboxes in DTNs

Disruption tolerant networks (DTNs) are designed to overcome limitations in connectivity due to conditions such as mobility, poor infrastructure, and short range radios. DTNs rely on the inherent mobility in the network to deliver packets around frequent and extended network partitions using a store-carry-and-forward paradigm. However, missed contact opportunities decrease throughput and increase delay in the network. We propose the use of throwboxes in mobile DTNs to create a greater number of contact opportunities, consequently improving the performance of the network. Throwboxes are wireless nodes that act as relays, creating additional contact opportunities in the DTN. We propose algorithms to deploy stationary throwboxes in the network that simultaneously consider routing as well as placement. We also present placement algorithms that use more limited knowledge about the network structure. We perform an extensive evaluation of our algorithms by varying both the underlying routing and mobility models. Our results suggest several findings to guide the design and operation of throwbox-augmented DTNs

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.

Relays, base stations, and meshes: enhancing mobile networks with infrastructure

Networks composed of mobile nodes inherently suffer from intermittent connections and high delays. Performance can be improved by adding supporting infrastructure, including base stations, meshes, and relays, but the cost-performance trade-offs of different designs is poorly understood. To examine these trade-offs, we have deployed a large-scale vehicular network and three infrastructure enhancement alternatives. The results of these deployments demonstrate some of the advantages of each kind of infrastructure; however, these conclusions can be applied only to other networks of similar characteristics, including size, wireless technologies, and mobility patterns. Thus we complement our deployment with a demonstrably accurate analytical model of large-scale networks in the presence of infrastructure. Based on our deployment and analysis, we make several fundamental observations about infrastructure-enhanced mobile networks. First, if the average packet delivery delay in a vehicular deployment can be reduced by a factor of two by adding x base stations, the same reduction requires 2x mesh nodes or 5x relays. Given the high cost of deploying base stations, relays or mesh nodes can be a more cost-effective enhancement. Second, we observe that adding small amount of infrastructure is vastly superior to even a large number of mobile nodes capable of routing to one another, obviating the need for mobile-to-mobile disruption tolerant routing schemes.

Turducken: hierarchical power management for mobile devices

Maintaining optimal consistency in a distributed system requires that nodes be always-on to synchronize information. Unfortunately, mobile devices such as laptops do not have adequate battery capacity for constant processing and communication. Even by powering off unnecessary components, such as the screen and disk, current laptops only have a lifetime of a few hours. Although PDAs and sensors are similarly limited in lifetime, a PDAs power requirement is an order-of-magnitude smaller than a laptops, and a sensors is an order-of-magnitude smaller than a PDAs. By combining these diverse platforms into a single integrated laptop, we can reduce the power cost of always-on operation. This paper presents the design, implementation, and evaluation of Turducken, a Hierarchical Power Management architecture for mobile systems. We focus on a particular instantiation of HPM, which provides high levels of consistency in a laptop by integrating two additional low power processors. We demonstrate that a Turducken system can provide battery lifetimes of up to ten times that of a standard laptop for always-on operation and three times for a system that periodically sleeps.

An Energy-Efficient Architecture for DTN Throwboxes

Disruption Tolerant Networks rely on intermittent contacts between mobile nodes to deliver packets using store-carry-and-forward paradigm. The key to improving performance in DTNs is to engineer a greater number of transfer opportunities. We earlier proposed the use of throwbox nodes, which are stationary, battery powered nodes with storage and processing, to enhance the capacity of DTNs. However, the use of throwboxes without efficient power management is minimally effective. If the nodes are too liberal with their energy consumption, they will fail prematurely. However if they are too conservative, they may miss important transfer opportunities, hence increasing lifetime without improving performance. In this paper, we present a hardware and software architecture for energy efficient throwboxes in DTNs. We propose a hardware platform that uses a multi-tiered, multi-radio, scalable, solar powered platform. The throwbox employs an approximate heuristic for solving the NP-Hard problem of meeting an average power constraint while maximizing the number of bytes forwarded by it. We built and deployed prototype throwboxes in UMassDieselNet -a bus DTN testbed. Through extensive trace-driven simulations and prototype deployment we show that a single throwbox with a 270 cm2 solar panel can run perpetually while improving packet delivery by 37% and reducing message delivery latency by at least 10% in the network.

Virtual compass: relative positioning to sense mobile social interactions

There are endless possibilities for the next generation of mobile social applications that automatically determine your social context. A key element of such applications is ubiquitous and precise sensing of the people you interact with. Existing techniques that rely on deployed infrastructure to determine proximity are limited in availability and accuracy. Virtual Compass is a peer-based relative positioning system that relies solely on the hardware and operating system support available on commodity mobile handhelds. It uses multiple radios to detect nearby mobile devices and places them in a two-dimensional plane. It uses adaptive scanning and out-of-band coordination to explore trade-offs between energy consumption and the latency in detecting movement. We have implemented Virtual Compass on mobile phones and laptops, and we evaluate it using a sample application that senses social interactions between Facebook friends.

Users and batteries: interactions and adaptive energy management in mobile systems

Battery lifetime has become one of the top usability concerns of mobile systems. While many endeavors have been devoted to improving battery lifetime, they have fallen short in understanding how users interact with batteries. In response, we have conducted a systematic user study on battery use and recharge behavior, an important aspect of user-battery interaction, on both laptop computers and mobile phones. Based on this study, we present three important findings: 1) most recharges happen when the battery has substantial energy left, 2) a considerable portion of the recharges are driven by context (location and time), and those driven by battery levels usually occur when the battery level is high, and 3) there is great variation among users and systems. These findings indicate that there is substantial opportunity to enhance existing energy management policies, which solely focus on extending battery lifetime and often lead to excess battery energy upon recharge, by adapting the aggressiveness of the policy to match the usage and recharge patterns of the device. We have designed, deployed, and evaluated a user- and statistics-driven energy management system, Llama, to exploit the battery energy in a user-adaptive and user-friendly fashion to better serve the user. We also conducted a user study after the deployment that shows Llama effectively harvests excess battery energy for a better user experience (brighter display) or higher quality of service (more application data) without a noticeable change in battery lifetime.

Surviving attacks on disruption-tolerant networks without authentication

Disruption-Tolerant Networks (DTNs) deliver data in network environments composed of intermittently connected nodes. Just as in traditional networks, malicious nodes within a DTN may attempt to delay or destroy data in transit to its destination. Such attacks include dropping data, flooding the network with extra messages, corrupting routing tables, and counterfeiting network acknowledgments. Many existing methods for securing routing protocols require authentication supported by mechanisms such as a public key infrastructure, which is difficult to deploy and operate in a DTN, where connectivity is sporadic. Furthermore, the complexity of such mechanisms may dissuade node participation so strongly that potential attacker impacts are dwarfed by the loss of contributing participants. In this paper, we use connectivity traces from our UMass DieselNet project and the Haggle project to quantify routing attack effectiveness on a DTN that lacks security. We introduce plausible attackers and attack modalities and provide complexity results for the strongest of attackers. We show that the same routing with packet replication used to provide robustness in the face of unpredictable mobility allows the network to gracefully survive attacks. In the case of the most effective attack, acknowledgment counterfeiting, we show a straightforward defense that uses cryptographic hashes but not a central authority. We conclude that disruption-tolerant networks are extremely robust to attack; in our trace-driven evaluations, an attacker that has compromised 30% of all nodes reduces delivery rates from 70% to 55%, and to 20% with knowledge of future events. By comparison, contemporaneously connected networks are significantly more fragile.

Ferret: RFID localization for pervasive multimedia

The pervasive nature of multimedia recording devices enables novel pervasive multimedia applications with automatic, inexpensive, and ubiquitous identification and locationing abilities. We present the design and implementation of Ferret, a scalable system for locating nomadic objects augmented with RFID tags and displaying them to a user in real-time. We present two alternative algorithms for refining a postulation of an objects location using a stream of noisy readings from an RFID reader: an online algorithm for real-time use on a mobile device, and an offline algorithm for use in post-processing applications. We also present methods for detecting when nomadic objects move and how to reset the algorithms to restarts the refinement process. An experimental evaluation of the Ferret prototype shows that (i) Ferret can refine object locations to only 1% of the readers coverage region in less than 2 minutes with small error rate (2.22%): (ii) Ferret can detect nomadic objects with 100% accuracy when the nomadic distances exceed 20cm; and (iii) Ferret works with a variety of user mobility patterns.

Design and field experimentation of an energy-efficient architecture for DTN throwboxes

Disruption-tolerant networks (DTNs) rely on intermittent contacts between mobile nodes to deliver packets using a store-carry-and-forward paradigm. We earlier proposed the use of throwbox nodes, which are stationary, battery-powered nodes with storage and processing, to enhance the capacity of DTNs. However, the use of throwboxes without efficient power management is minimally effective. If the nodes are too liberal with their energy consumption, they will fail prematurely. However, if they are too conservative, they may miss important transfer opportunities, hence increasing lifetime without improving performance. In this paper, we present a hardware and software architecture for energy-efficient throwboxes in DTNs. We propose a hardware platform that uses a multitiered, multiradio, scalable, solar-powered platform. The throwbox employs an approximate heuristic for solving the NP-hard problem of meeting an average power constraint while maximizing the number of bytes forwarded by the throwbox. We built and deployed prototype throwboxes in UMass DieselNet, a bus-based DTN testbed. Through extensive trace-driven simulations and prototype deployment, we show that a single throwbox with a 270-cm2 solar panel can run perpetually while improving packet delivery by 37% and reducing message delivery latency by at least 10% in the network.