Several recent proposals for an "active networks" architecture advocate the placement of user-defined computation within the network as a key mechanism to enable a wide range of new applications and protocols, including reliable multicast transports, mechanisms to foil denial of service attacks, intra-network real-time signal transcoding, and so forth. This laudable goal, however, creates a number of very difficult research problems, and although a number of pioneering research efforts in active networks have solved some of the preliminary small-scale problems, a large number of wide open problems remain. In this paper, we propose an alternative to active networks that addresses a restricted and more tractable subset of the active-networks design space. Our approach, which we (and others) call "active services", advocates the placement of user-defined computation within the network as with active networks, but unlike active networks preserves all of the routing and forwarding semantics o...

This paper proposes an approach to the design of configurable and open middleware platforms based on the concept of reflection. More specifically, the paper introduces a language-independent reflective architecture featuring a perobject meta-space, the use of meta-models to structure meta-space, and a consistent use of object graphs for composite components. This is complemented by a component framework supporting the construction of meta-spaces. The paper also reports on experiences of implementing the architecture (with emphasis on experiments with open bindings). Keywords Middleware, (re-)configuration, reflection, open implementation, open bindings. 1 INTRODUCTION Middleware has emerged as an important architectural component in supporting distributed applications. The role of middleware is to present a unified programming model to application writers and to mask out problems of heterogeneity and distribution. The importance of the topic is reflected in the increasing visibili...

Employing an emulation capability in network simulation provides the ability for real-world traffic to interact with a simulation. The benefits of emulation include the ability to expose experimental algorithms and protocols to live traffic loads, and to test real-world protocol implementations against repeatable interference generated in simulation. This paper describes the design and implementation of the emulation facility in the NS simulator, a commonly-used publicly available network research simulator. 1. Introduction Simulation and testbed construction represent the two most important methodologies available to network protocol developers for design and evaluation of both novel and existing network protocols. Simulation provides for repeatable, controlled experimentation with a modest overhead required to construct and carry out a simulation. Unfortunately, simulations often make simplifying assumptions which may obscure understanding of behavior seen in real-world situations....

Although IP Multicast is an effective network primitive for best-effort, large-scale, multi-point communication, many multicast applications such as shared whiteboards, multi-player games and software distribution require reliable data delivery. Building services like reliable sequenced delivery on top of IP Multicast has proven to be a hard problem. The enormous extent of network and end-system heterogeneity in multipoint communication exacerbates the design of scalable end-to-end reliable multicast protocols. In this paper, we propose a radical departure from the traditional end-to-end model for reliable multicast and instead propose a hybrid approach that leverages the successes of unicast reliability protocols such as TCP while retaining the efficiency of IP multicast for multi-point data delivery. Our approach splits a large heterogeneous reliable multicast session into a number of multicast data groups of co-located homogeneous participants. A collection of application-aware agents--Reliable Multicast proxies (RMXs)--organizes these data groups into a spanning tree using an overlay network of TCP connections. Sources transmit data to their local group, and the RNLX in that group forwards the data towards the rest of the data groups. RMXs use detailed knowledge of application semantics to adapt to the effects of heterogeneity in the environment. To demonstrate the efficacy of our architecture, we have built a prototype implementation that can be customized for different kinds of applications.

The development of context-aware applications will require tools that are based on clearly defined models of context and system software architecture. This essay introduces models for each of these, examines the tradeoffs among the different alternatives, and describes a blackboard-based context architecture that is being used in the construction of interactive workspaces. 1.

Coordination policies vary from collaboration to collaboration and are even subject to evolution in different phases of the same collaboration. It is vital for collaborative systems to be flexible enough to accommodate changes to the coordination policies during development and the lifetime of the collaboration. Motivated by

IP Multicast has proven to be an effective communication primitive for best effort, large-scale, multi-point audio/video conferencing applications. While the best-effort transport of real-time digital audio/video is a relatively straightforward and well understood problem, many other applications like multicast-based shared whiteboards and shared text editors are more challenging to design because their underlying media require reliable transport, i.e., a "reliable multicast" protocol. The design of scalable end-to-end reliable multicast protocols has unfortunately proven to be an especially hard problem, exacerbated by the enormous degree of network and system heterogeneity present in the Internet. In this paper, we propose to tackle the heterogeneity problem with a hybrid model for reliable multicast that relies in part on end-to-end loss recovery mechanisms and in part on intelligent and application-aware adaptation carried out within the network. In our framework, application-aware agents -- or proxies -- use detailed knowledge of application semantics to hide the effects of heterogeneity from the rest of the system. We present a general architecture for proxy-based reliable multicast called the Reliable Multicast proXy (RMX) model and describe a prototype implementation of an RMX for a shared whiteboard application for hand-held PDAs.

The Application Level Framing (ALF) protocol architecture [2] encourages application control over mechanisms that traditionally fall within the "transport layer", e.g., loss detection and recovery. Traditional ARQ-based reliable protocols for unicast (e.g., TCP) as well as multicast (e.g., Horus [30], RMTP [15], etc.) number data units sequentially to detect losses. Unfortunately, these transport-level sequence numbers do not permit receivers to flexibly tailor their reliability semantics. Achieving receiver-driven reliability is cumbersome in the existing "layered" architecture of the network protocol stack where the receiving application has no knowledge of how application-level objects map onto transport level sequence numbers. In this paper, we propose a new data naming scheme that exposes the structure of application data to the transport layer, thereby enhancing the expressibility of an applications' reliability and ordering semantics. We apply this data naming scheme to a reliab...

Abstract. This paper describes videoSpace, a software toolkit designed to facilitate the integration of image streams into existing or new documents and applications to support new forms of human-computer interaction and collaborative activities. In this perspective, videoSpace is not focused on performance or reliability issues, but rather on the ability to support rapid prototyping and incremental development of video applications. The toolkit is described in extensive details, by showing the architecture and functionalities of its class library and basic tools. Several projects developed with videoSpace are also presented, illustrating its potential and the new uses of video it will allow in the future. 1

In this thesis, we describe our comprehensive design and implementation of a multicast based shared drawing tool called MediaBoard. Like its predecessor, the LBL whiteboard application, wb, its design follows the principles of application level framing and lightweight sessions. Both applications use the Scalable, Reliable Multicast protocol (SRM) to distribute data over the network. But unlike wb, which embeds the networking code in the application, MediaBoard uses a separate SRM framework library and customizes it to achieve application level semantics and thus performance gains. Based on actual deployment experiences --- MediaBoard was used in an "online classroom" at U.C. Berkeley --- we extended the design to improve its usability, provide increased awareness of remote participants, and integrate support for interactive browsing of the drawing space history. The application is also extremely flexible.