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 ...

The X Window System -- has become widely accepted by many manufacturers. X provides network transparent access to display servers, allowing local and remote client programs to access a user's display. X is used on high performance workstation displays as well as terminals, and client programs run on everything from micro to super computers. This paper describes the tradeoffs and basic design decisions made during the design of X Version 11. We presume familiarity with the paper describing X Version 10. Keywords: X Window System, interactive human-computer interface system, distributed systems. c flDigital Equipment Corporation and Silicon Graphics Computer Systems 1990. All rights reserved. -- The X Window System is a Massachusetts Institute of Technology trademark. This paper will appear in a special issue of Software Practice and Experience. y Digital Equipment Corporation, Cambridge Research Lab, One Kendall Square, Bldg. 700, Cambridge, MA 02139, U.S.A. jg@crl.dec.com z ...

. Mobile agents are code-containing objects that may be transmitted between communicating participants in a distributed system. Compared to systems that only allow the exchange of nonexecutable data, those incorporating mobile agents can achieve significant gains in performance and functionality. Languages with first-class functions provide a good starting point for agent programming, as they make it easy to express the construction, transmission, receipt, and subsequent execution of agents. However, for developing real agent-based systems, a language implementation must handle architectural heterogeneity between communicating machines and provide sufficient performance for applications based on agents. In addition, agents need to be able to access resources on remote execution sites yet remain in a framework that provides sufficient security. In this paper we consider the uses of mobile agents and how a distributed functional language can be extended to support them. We review other a...

v Introduction 1 Development Challenges ......................................................................................................... 2 Development Options .............................................................................................................. 4 Course Objectives .................................................................................................................... 5 Relevant Collaborative Applications ....................................................................................... 6 Examples of Our Own Work ................................................................................................... 7 World-Wide Web `Shell' 12 Set of Emerging Standards ..................................................................................................... 14 Hypertext Transfer Protocol (HTTP) ............................................................................... 15 Common Gateway Interface (CGI) .......................

Network and server-centric computing paradigms are quickly returning to being the dominant methods by which we use computers. Web applications are so prevalent that the role of a PC today has been largely reduced to a terminal for running a client or viewer such as a Web browser. Implementers of network-centric applications typically rely on the limited capabilities of HTML, employing proprietary "plug ins" or transmitting the binary image of an entire application that will be executed on the client. Alternatively, implementers can develop without regard for remote use, requiring users who wish to run such applications on a remote server to rely on a system that creates a virtual frame buffer on the server, and transmits a copy of its raster image to the local client.

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A graph browsing system has been extended to support several general graph operations related to topological graph theory, such as Cartesian product, suspension, face tracing, genus distribution, crosscap distribution, and converting between graphical rotation projections and combinatorial rotation systems. Facilities have also been provided for generating topological building blocks such as bouquets, cubes, paths, cycles, and complete graphs. This system can be used to define graphs diagrammatically and explore their topological properties. The graph browser, called Bonsai, provides an interpreter for a simple graph description language and a graphical interface for modeling information in the form of graph and set diagrams. Graph structures can be created with the browser and converted into a variety of textual forms to interface with other applications. Similarly, separate applications can generate text streams and direct them into Bonsai's interpreter to display the resulting anima...

There is an increasing demand in making web user interfaces richer and more interactive. Currently, there are two approaches aiming at improving web user interfaces. First, downloaded code in form of Java Applet or ActiveX can be executed in browsers. And more recently, AJAX (Asynchronous JavaScript + XML) leverages browsers ' JavaScript engine to render user interfaces without reloading pages. Both approaches have some weaknesses. In this paper, we present an alternative approach to creating highly interactive web user interfaces. Our approach is based on the Extensible User Interface Protocol (XUP), a SOAP-based protocol for communicating events and incremental user interface updates on the web. On top of XUP, we have built a web user interface development framework, OpenXUP, consisting of a thin client and a server toolkit which offers a set of event-driven APIs. The framework allows for the rapid development of highly interactive web applications and services.

The Xremote protocol is a compressed transformation of the X Window System protocol, designed to efficiently implement X connections across relatively slow serial lines. Using an Xremote simulator and 11 traces of X sessions, we found that Xremote's overall compression ratio is 2.4:1. This figure varies widely depending on the trace. A study of compression ratio as a function of message type shows text based messages commonly achieving 3:1 compression, while geometric messages usually achieve only 1.6:1 compression. By examining bandwidth requirements and compression performance as a function of time, we see that Xremote performs adequately for some applications which are text based or which use small geometric datasets, except at application startup where more bandwidth is required. Further work is required to adequately support the initialization stage of X applications and medium to large geometric databases. 1 Introduction Window systems provide an abstraction for the display scree...

) Submitted to ACM MULTIMEDIA '94 John Danskin 3 Princeton University Computer Science Department Princeton NJ, 08540 jmd@cs.princeton.edu Abstract Network bandwidth has always been a key issue for multimedia protocols. Many potential users of networked multimedia protocols will continue to have low bandwidth network connections for some time: copper wire ISDN, infra-red, cellular modems, etc.. Compression provides potential relief for users of slow networks by increasing effective bandwidth. HBX introduces a new technique, based on arithmetic coding and statistical modeling, for compressing structured data. Applied to the X networked graphics protocol, this technique yields 4.5:1 compression across a representative set of traces, performing twice as well as the popular LZW-based Xremote compression protocol. HBX's coding techniques are generally applicable to the graphics and imaging subset of multimedia protocols. Future work will determine whether HBX's coding techniques can be appl...