We consider the problem of providing network access to hosts whose physical location changes with time. Such hosts cannot depend on traditional forms of network connectivity and routing because their location, and hence the route to reach them, cannot be deduced from their network address. In this paper, we explore the concept of providing continuous network access to mobile computers, and present a set of IP-based protocols that achieve that goal. They are primarily targeted at supporting a campus environment with mobile computers, but also extend gracefully to accommodate hosts moving between different networks. The key feature is the dependence on ancillary machines, the Mobile Support Stations (MSSs), to track the location of the Mobile Hosts. Using a combination of caching, forwarding pointers, and timeouts, a minimal amount of state is kept in each MSS. The state information is kept in a distributed fashion; the system scales well, reacts quickly to changing topologies, and does ...

Communication-based design represents a formal approach to systemon -a-chip design that considers communication between components as important as the computations they perform. Our "network-on-chip" approach partitions the communication into layers to maximize reuse and provide a programmer with an abstraction of the underlying communication framework. This layered approach is cast in the structure advocated by the OSI Reference Model and is demonstrated with a reconfigurable DSP example. The Metropolis methodology of deriving layers through a sequence of adaptation steps between incompatible behaviors is illustrated through the Intercom design example. In another approach, MESCAL provides a designer with tools for a correct-by-construction protocol stack.

research relevant to the design and application of high performance scientific computers. We test our ideas by designing, building, and using real systems. The systems we build are research prototypes; they are not intended to become products. There is a second research laboratory located in Palo Alto, the Systems Research Center (SRC). Other Digital research groups are located in Paris (PRL) and in Cambridge,

The increasing use of internetworking protocols to connect administratively heterogeneous networks has raised the question of how an organization can control the flow of information across its network boundaries. One method for doing so is the use of visas, a cryptographic technique for authenticating and authorizing a flow of datagrams. This report presents and evaluates two visa protocols ---- one that requires distributed state information in gateways and one that uses additional encryption operations instead of distributed state. Applications for such visa protocols include access control, accounting and billing for packet transit, and network resource management. This technical report is based, in large part, upon a shorter paper [8]. We have extended the discussion of design issues and added an appendix describing a visa protocol using dual-key (public key) encryption. Key Words: Computer networks, network interconnection, network security, access control, authentication, crypt...

The course “SEMA 6201 ” System Modeling and Analysis is a 5 day course. The first two days address the CAFCR model that serves as a framework for systems architecting and design. Core to the CAFCR model is the use of multiple viewpoints and multiple visualizations. The last three days of the course address quantification, modeling and analysis. Distribution This article or presentation is written as part of the Gaudí project. The Gaudí project philosophy is to improve by obtaining frequent feedback. Frequent feedback is pursued by an open creation process. This document is published as intermediate or nearly mature version to get feedback. Further distribution is allowed as long as the document remains complete and unchanged. All Gaudí documents are available at:

For natural language dialog systems, going beyond domain independence means the attempt to create a core system that can serve as a basis for interfaces to various application classes that differ not only with respect to the domain of discourse but also with respect to dialog type, user type, intended system behavior, and background system. In the design and implementation of HAM-ANS, which is presently operational as an interface to an expert system, a vision system and a data base system, we have shown that going beyond domain independence is possible. HAM-ANS is a large natural language dialog system with both considerable depth and breadth, which accepts typed input in colloquial German and produces typed German responses quickly enough to make it practical for realtime interaction. One goal of this paper is to report on the lessons learned during the realization of the system HAM-ANS. This paper introduces the overall structure of the system's processing units and knowledge sources. In addition we describe some of the innovative features concerning the strategy of semantic interpretation. I.

Loose coupling is often quoted as a desirable property of systems architectures. One of the main goals of building systems using Web technologies is to achieve loose coupling. However, given the lack of a widely accepted definition of this term, it becomes hard to use coupling as a criterion to evaluate alternative Web technology choices, as all options may exhibit, and claim to provide, some kind of “loose ” coupling effects. This paper presents a systematic study of the degree of coupling found in service-oriented systems based on a multi-faceted approach. Thanks to the metric introduced in this paper, coupling is no longer a one-dimensional concept with loose coupling found somewhere in between tight coupling and no coupling. The paper shows how the metric can be applied to real-world examples in order to support and improve the design process of service-oriented systems.

Distributed object technology is profoundly changing the ways in which software systems evolve over time. To a large extent, the focus of reengineering has been to understand legacy systems and to extract their essential functionality so that they can be rewritten as more robust and more maintainable systems over the long term. However, object technology, wrapping strategies, and the Web may be changing the focus and economics of reengineering. The question posed by this paper is the extent to which reengineering strategies ought to continue to use program understanding technology. The cost/benefit ratio of certain forms of program understanding appears to be staying roughly the same over time, while the cost/benefit ratio of wrapping legacy systems or their subsystems is dropping rapidly. As a result, new reengineering strategies that place less emphasis on deep program understanding, and more emphasis on distributed object technologies, should now be considered.

. With the increasing use of distributed hard real-time systems, the ability of computer networks to handle hard real-time message traffic is becoming more important. For traditional networks, maximizing the throughput or minimizing the average message delay is the most important performance criteria. In the hard real-time domain, however, concern focuses on satisfying the time constraints of individual messages. This paper examines recent developments in hard real-time communication in local area multiple-access networks. Two general strategies are used in hard real-time communication: the guarantee strategy and the best-effort strategy. In the former, messages are guaranteed to meet their deadlines during normal operation of the network. In the best-effort strategy, the network will attempt to send messages before their deadlines, but no guarantees are given. Real-time message traffic can be distinguished according to whether it is best suited for the guarantee strategy or the best-e...

We use an event-driven process model to specify a version of the High-Level Data Link Control (HDLC) protocol between two communicating protocol entities. The protocol is verified using the method of projections. The verification serves as a rigorous exercise to demonstrate the applicability of this method to the analysis of real-life communication protocols. The HDLC protocol has two characteristics found in most real-life communication protocols. First, the HDLC protocol operates under real-time constraints that are important not only for its performance but also for its correct logical behavior. We specify this real-time behavior using time variables and time events. Second, the HDLC protocol has three distinguishable functions: connection management, and one-way data transfers between the protocol entities. For each of these functions, we construct an image protocol using the method of projections. With each image protocol we obtain inductively complete invariant assertions that state various desirable logical safety properties. From the properties of image protocols it follows that these safety properties as proved for the image protocols are also satisfied by the HDLC protocol presented herein. We also suggest a minor modification to HDLC that will make it well-structured.