An information agent manages all or a portion of a user's information space. The electronic resources in this space are often distributed across a network and can contain tremendous quantities of data. Mobile agents provide efficient access to such resources and are a powerful tool for implementing information agents. A mobile agent is an autonomous program that can migrate from machine to machine in a heterogeneous network. By migrating to the location of a resource, the agent can access the resource efficiently even if network conditions are poor or the resource has a low-level interface. Telescript is the best-known mobile-agent system. Telescript, however, requires the programmer to learn and work with a complex object-oriented language and a complex security model. Agent Tcl, on the other hand, is a simple, flexible, and secure system that is based on the Tcl scripting language and the Safe Tcl extension. In this paper we describe the architecture of Agent Tcl and its current implementation.

. Programs that use mobility as a mechanism to adapt to resource changes have three requirements that are not shared with other mobile programs. First, they need to monitor the level and quality of resources in their operating environment. Second, they need to be able to react to changes in resource availability. Third, they need to be able to control the way in which resources are used on their behalf (by libraries and other support code). In this chapter, we describe the design and implementation of Sumatra, an extension of Java that supports resourceaware mobile programs. We also describe the design and implementation of a distributed resource monitor that provides the information required by Sumatra programs. 1 Introduction Mobile programs can move an active thread of control from one site to another during execution. This flexibility has many potential advantages. For example, a program that searches distributed data repositories can improve its performance by migrating to the re...

Mobile agents are code-containing objects that may be transmitted between communicating participants in a distributed system. As opposed to systems that only allow the exchange of nonexecutable data, systems incorporating mobile agents can achieve significant gains in performance and functionality. A programming language for mobile agents must be able to express their construction, transmission, receipt, and subsequent execution. Its implementation must handle architectural heterogeneity between communicating machines and provide sufficient performance for applications based on agents. In addition to these essential properties, an agent language may support desirable properties such as high-level abstractions for code manipulation and the ability to access resources on remote execution sites. We designed and implemented an agent programming language that satisfies the essential properties and a number of desirable ones. A key feature of our language is the use of strong static typing ...

In this paper, we investigate network-aware mobile programs, programs that can use mobility as a tool to adapt to variations in network characteristics. We present infrastructural support for mobility and network monitoring and show how adaptalk, a Java-based mobile Internet chat application can take advantage of this support to dynamically place the chat server so as to minimize response time. Our conclusion was that on-line network monitoring and adaptive placement of shared data-structures can significantly improve performance of distributed applications on the Internet. 1

Transportable agents are autonomous programs. They can move through a heterogeneous network of computers under their own control, migrating from host to host. They can sense the state of the network, monitor software conditions, and interact with other agents or resources. The network-sensing tools allow our agents to adapt to the network con guration and to navigate under the control of reactive plans. In this paper we describe the design and implementation of the navigation system that gives our agents autonomy. We also discuss the intelligent and adaptive behavior of autonomous agents in distributed information-gathering tasks.

In many execution environments software components cannot interoperate easily because of differences in their interfaces and implementations. Additional software is often required to integrate such components and implement the interfacing decisions between them. For example, a procedure call across architectures may require extensive software to relocate data and coerce parameters. Even when powerful integration facilities are available, application programs need some additional software --- often called `stubs' --- so they can access the available communication media. Interface software can be more expensive to program that other software, since its creation requires knowledge of the machine architectures and communication mechanisms. Moreover, it must be rewritten whenever components are reused in different configurations. This paper describes a way to automatically generate custom interface software for heterogeneous configurations. Whereas previous research focused on `stub generat...

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, e cient 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. In this paper, we describe the design and implementation of our mobile-agent system, Agent Tcl, and the speci c 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. 1

As network information resources grow in size, it is often most efficient to process queries and updates at the site where the data is located. This processing can be accomplished by using a traditional client-server network interface, which constrains the client to the set of queries supported by the server, or requires the server to send all data to the client for processing. The former is inflexible; the latter is inefficient. Transportable agents, which support the movement of the client computation to the location of the remote resource, have the potential to be more flexible and more efficient. Transportable agents are capable of suspending their execution, transporting themselves to another host on a network, and resuming execution from the point at which they were suspended. Transportable agents consume fewer network resources and can support systems that do not have permanent network connections, such as mobile computers and personal digital assistants. We describe a prototype transportable-agent implementation that facilitates research in this area. Agents are written in a script language that supports agent relocation, and the language is processed at each host by an agent interpreter. Electronic mail and UNIX remote shell (rsh) are the two current transport mechanisms and we plan to explore others. We present a technical-report searching agent as a demonstration of the capabilities of our prototype implementation. 1

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 informationretrieval 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-toend latency. In this chapter, we rst 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 speci c information-retrieval application, searching distributed collections of technical reports, and consider how mobile agents can be used to implement this application e ciently 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 howto observe achanging 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 ltered, and argue that the structure of a mobile-agent application determines the extent to which these errors a ect the nal result. 1

We have designed and implemented autonomous software agents. Our agents are programs that can move independently through a heterogeneous network of computers. They can sense the state of the network, monitor software conditions, and interact with other agents. The network-sensing tools allow our agents to adapt to the network con guration and to navigate under the control of reactive plans. In this paper we illustrate the intelligent and adaptive behavior of autonomous agents in distributed information-gathering tasks. 1