Robert S. Gray

Experimental evaluation of wireless simulation assumptions

All analytical and simulation research on ad~hoc wireless networks must necessarily model radio propagation using simplifying assumptions. We provide a comprehensive review of six assumptions that are still part of many ad hoc network simulation studies, despite increasing awareness of the need to represent more realistic features, including hills, obstacles, link asymmetries, and unpredictable fading. We use an extensive set of measurements from a large outdoor routing experiment to demonstrate the weakness of these assumptions, and show how these assumptions cause simulation results to differ significantly from experimental results. We close with a series of recommendations for researchers, whether they develop protocols, analytic models, or simulators for ad~hoc wireless networks.

Mobile agents and the future of the internet

Use of the Internet has exploded in recent years with the appearance of the World-Wide Web. In this paper, we show how current technological trends may lead to a system based substantially on mobile code, and in many cases, mobile agents. We discuss several technical and non-technical hurdles along the path to that eventuality. It seems likely that, within a few years, nearly all major Internet sites will be capable of hosting and willing to host some form of mobile code or mobile agents.

AGENT TCL: targeting the needs of mobile computers

Mobile computers have become increasingly popular as users discover the benefits of having their electronic work available at all times. Using Internet resources from a mobile platform, however, is a major challenge. Mobile computers do not have a permanent network connection and are often disconnected for long periods. When the computer is connected, the connection is often prone to sudden failure, such as when a physical obstruction blocks the signal from a cellular modem. In addition, the network connection often performs poorly and can vary dramatically from one session to the next, since the computer might use different transmission channels at different locations. Finally, depending on the transmission channel, the computer might be assigned a different network address each time it reconnects. Mobile agents are one way to handle these unforgiving network conditions. A mobile agent is an autonomous program that can move from machine to machine in a heterogeneous network under its own control. It can suspend its execution at any point, transport itself to a new machine, and resume execution on the new machine from the point at which it left off. Agent Tcl is a mobile agent system whose agents can be written in Tcl, Java, and Scheme. Agent Tcl has extensive navigation and communication services, security mechanisms, and debugging and tracking tools. We focus on Agent Tcls architecture and security mechanisms, its RPC system, and its docking system, which lets an agent move transparently among mobile computers, regardless of when they are connected to the network.

Mobile Agents for Distributed Information Retrieval

A mobile agent is an executing program that can migrate during execution from machine to machine in a heterogeneous network. On each machine, the agent interacts with stationary service agents and other resources to accomplish its task. Mobile agents are particularly attractive in distributed information-retrieval applications. By moving to the location of an information resource, the agent can search the resource locally, eliminating the transfer of intermediate results across the network and reducing end-to-end latency. In this chapter, we first discuss the strengths of mobile agents, and argue that although none of these strengths are unique to mobile agents, no competing technique shares all of them. Next, after surveying several representative mobile-agent systems, we examine one specific information-retrieval application, searching distributed collections of technical reports, and consider how mobile agents can be used to implement this application efficiently and easily. Then we spend the bulk of the chapter describing two planning services that allow mobile agents to deal with dynamic network environments and information resources: (1) planning algorithms that let an agent choose the best migration path through the network, given its current task and the current network conditions, and (2) planning algorithms that tell an agent how to observe a changing set of documents in a way that detects changes as soon as possible while minimizing overhead. Finally, we consider the types of errors that can occur when information from multiple sources is merged and filtered, and argue that the structure of a mobile-agent application determines the extent to which these errors affect the final result.

Transportable Information Agents

Transportable agents are autonomous programs. They can movethrough a heterogeneous network of computers migrating from host tohost under their own control. They can sense the state of thenetwork, monitor software conditions, and interact with other agentsor resources. The network-sensing tools allow our agents to adapt tothe network configuration and to navigate under the control ofreactive plans. In this paper we describe the design andimplementation of a transportable-agent system and focus on navigationtools that give our agents autonomy. We also discuss the intelligentand adaptive behavior of autonomous agents in distributed information-access tasks.

Using sensor networks and data fusion for early detection of active worms

Identification of an Internet worm is a manual process where security analysts must observe and analyze unusual activity on multiple firewalls, intrusion-detection systems or hosts. A worm might not be positively identified until it already has spread to most of the Internet, eliminating many defensive options. In this paper, we present an automated system that can identify active worms seconds or minutes after they first begin to spread, a necessary precursor to halting the spread of a worm, rather than simply cleaning up afterward. Our implemented system collects ICMP Unreachable messages from instrumented network routers, identifies those patterns of unreachable messages that indicate malicious scanning activity, and then searches for patterns of scanning activity that indicate a propagating worm. In this paper, we examine the problem of active worms, describe our ICMP-based detection system, and present simulation results that illustrate the speed with which it can detect a worm.

Experimental Evaluation of Wireless Simulation Assumptions

All analytical and simulation research on ad hoc wireless networks must necessarily model radio propagation using simplifying assumptions. A growing body of research, however, indicates that the behavior of the protocol stack may depend significantly on these underlying assumptions. The standard response to this problem is a call for more realism in designing radio models. But how much realism is enough? This study is the first to approach this question by validating simulator performance (both at the physical and application layers) with the results of real-world data. Referencing an eXtensive set of measurements from a large outdoor routing eXperiment, we start by evaluating the relative realism of common assumptions made in radio model design, identifying those which provide a reasonable approXimation of reality. Although several such investigations have been made for static sensor networks, radio behavior in mobile network deployments is a much less-studied topic. We then reproduce our eXperimental setup in our simulator, and generate the same application-layer metrics under progressively smaller sets of these assumptions. By comparing the simulated outcome to the outcome of our eXperiment, we are able to discern at what point our balance of simplification and realism captures the real behavior of our target environment.

Mobile-Agent versus Client/Server Performance: Scalability in an Information-Retrieval Task

Building applications with mobile agents often reduces the bandwidth required for the application, and improves performance. The cost is increased server workload. There are, however, few studies of the scalability of mobile-agent systems. We present scalability experiments that compare four mobile-agent platforms with a traditional client/server approach. The four mobile-agent platforms have similar behavior, but their absolute performance varies with underlying implementation choices. Our experiments demonstrate the complex interaction between environmental, application, and system parameters.

Simulation validation using direct execution of wireless Ad-Hoc routing protocols

Computer simulation is the most common approach to studying wireless ad-hoc routing algorithms. The results, however, are only as good as the models the simulation uses. One should not underestimate the importance of validation, as inaccurate models can lead to wrong conclusions. In this paper, we use direct-execution simulation to validate radio models used by ad-hoc routing protocols, against real-world experiments. This paper documents a common testbed that supports direct execution of a set of ad-hoc routing protocol implementations in a wireless network simulator. The testbed reads traces generated from real experiments, and uses them to drive direct-execution implementations of the routing protocols. Doing so we reproduce the same network conditions as in real experiments. By comparing routing behavior measured in real experiments with behavior computed by the simulation, we are able to validate the models of radio behavior upon which protocol behavior depends. We conclude that it is possible to have fairly accurate results using a simple radio model, but the routing behavior is quite sensitive to one of this models parameters. The implication is that one should: i) use a more complex radio model that explicitly models point-to-point path loss; or ii) use measurements from an environment typical of the one of interest; or iii) study behavior over a range of environments to identify sensitivities.

Mobile agents: the next generation in distributed computing

Mobile agents are programs that can move through a network under their own control, migrating from host to host and interacting with other agents and resources on each. We argue that these mobile, autonomous agents have the potential to provide a convenient, efficient and robust programming paradigm for distributed applications, particularly when partially connected computers are involved. Partially connected computers include mobile computers such as laptops and personal digital assistants as well as modem connected home computers, all of which are often disconnected from the network. We describe the design and implementation of our mobile agent system, Agent Tcl, and the specific features that support mobile computers and disconnected operation. These features include network sensing tools and a docking system that allows an agent to transparently move between mobile computers, regardless of when the computers connect to the network.