An ad-hoc mobile network is a collection of mobile nodes that are dynamically and arbitrarily located in such a manner that the interconnections between nodes are capable of changing on a continual basis. In order to facilitate communication within the network, a routing protocol is used to discover routes between nodes. The primary goal of such an ad-hoc network routing protocol is correct and efficient route establishment between a pair of nodes so that messages may be delivered in a timely manner. Route construction should be done with a minimum of overhead and bandwidth consumption. This paper examines routing protocols for ad-hoc networks and evaluates these protocols based on a given set of parameters. The paper provides an overview of eight different protocols by presenting their characteristics and functionality, and then provides a comparison and discussion of their respective merits and drawbacks.

The difficulty of developing reliable distributed softwme is an impediment to applying distributed computing technology in many settings. Expeti _ with the Isis system suggests that a structured approach based on virtually synchronous _ groups yields systems that are substantially easier to develop, exploit sophisticated forms of cooperative computation, and achieve high reliability. This paper reviews six years of resemr,.hon Isis, describing the model, its impl_nentation challenges, and the types of applicatiom to which Isis has been appfied. 1 In oducfion One might expect the reliability of a distributed system to follow directly from the reliability of its con-stituents, but this is not always the case. The mechanisms used to structure a distributed system and to implement cooperation between components play a vital role in determining how reliable the system will be. Many contemporary distributed operating systems have placed emphasis on communication performance, overlooking the need for tools to integrate components into a reliable whole. The communication primitives supported give generally reliable behavior, but exhibit problematic semantics when transient failures or system configuration changes occur. The resulting building blocks are, therefore, unsuitable for facilitating the construction of systems where reliability is impo/tant. This paper reviews six years of research on Isis, a syg_,,m that provides tools _ support the construction of reliable distributed software. The thesis underlying l._lS is that development of reliable distributed software can be simplified using process groups and group programming too/_. This paper motivates the approach taken, surveys the system, and discusses our experience with real applications.

In this paper we evaluate the effectiveness of Random Early Detection (RED) over traffic types categorized as nonadaptive, fragile and robust, according to their responses to congestion. We point out that RED allows unfair bandwidth sharing when a mixture of the three traffic types shares a link. This unfairness is caused by the fact that at any given time RED imposes the same loss rate on all flows, regardless of their bandwidths. We propose Flow Random Early Drop (FRED), a modified version of RED. FRED uses per-active-flow accounting to impose on each flow a loss rate that depends on the flow's buffer use. We show that FRED provides better protection than RED for adaptive (fragile and robust) flows. In addition, FRED is able to isolate non-adaptive greedy traffic more effectively. Finally, we present a "two-packet-buffer" gateway mechanism to support a large number of flows without incurring additional queueing delays inside the network. These improvements are demonstrated by simulation of TCP and UDP traffic. FRED

Finding information in a peer-to-peer system currently requires either a costly and vulnerable central index, or ooding the network with queries. In this paper we introduce the concept of Routing Indices (RIs), which allow nodes to forward queries to neighbors that are more likely to have answers. If a node cannot answer a query, it forwards the query to a subset of its neighbors, based on its local RI, rather than by selecting neighbors at random or by ooding the network by forwarding the query to all neighbors. We present three RI schemes: the compound, the hop-count, and the exponential routing indices. We evaluate their performance via simulations, and nd that RIs can improve performance by one or two orders of magnitude vs. a ooding-based system, and by up to 100 % vs. a random forwarding system. We also discuss the tradeo s between the di erent RIschemes and highlight the e ects of key design variables on system performance.

We study the problem of media access control in the novel regime of sensor networks, where unique application behavior and tight constraints in computation power, storage, energy resources, and radio technology have shaped this design space to be very different from that found in traditional mobile computing regime. Media access control in sensor networks must not only be energy efficient but should also allow fair bandwidth allocation to the infrastructure for all nodes in a multihop network. We propose an adaptive rate control mechanism aiming to support these two goals and find that such a scheme is most effective in achieving our fairness goal while being energy effcient for both low and high duty cycle of network traffic.

We propose a small number of basic concepts that can be used to explain the architecture of fault-tolerant distributed systems and we discuss a list of architectural issues that we find useful to consider when designing or examining such systems. For each issue we present known solutions and design alternatives, we discuss their relative merits and we give examples of systems which adopt one approach or the other. The aim is to introduce some order in the complex discipline of designing and understanding fault-tolerant distributed systems. 1 Introduction Computing systems consist of a multitude of hardware and software components that are bound to fail eventually. In many systems, such component failures can lead to unanticipated, potentially disruptive failure behavior and to service unavailability. Some systems are designed to be fault-tolerant: they either exhibit a well-defined failure behavior when components fail or mask component failures to users, that is, continue to provid...

We propose a scheme for congestion avoidance in networks using a connectionless protocol at the network layer. The scheme uses feedback from the network to the users of the network. The interesting challenge for the scheme is to use a minimal amount of feedback (one bit in each packet) from the network to adjust the amount of traffic allowed into the network. The servers in the network detect congestion and set a congestion indication bit on packets flowing in the forward direction. The congestion indication is commu-nicated back to the users through the transport level acknowledgement. The scheme is distributed, adapts to the dynamic state of the network, converges to the optimal operating point, is quite simple to implement, and has low overhead while operational. The scheme also addresses a very important aspect of fairness in the service provided to the various sources utilizing the network. The scheme at-tempts to maintain fairness in service provided to multiple sources. This paper presents the scheme and the analysis that went into the choice of the various decision mechanisms. We also address the performance of the scheme under transient changes in the network and for pathological conditions.

VLSI communication networks are wire limited. The cost of a network is not a function of the number of switches required, but rather a function of the wiring density required to construct the network. This paper analyzes communication networks of varying dimension under the assumption of constant wire bisection. Expressions for the latency, average case throughput, and hot-spot throughput of k-ary n- cube networks with constant bisection are derived that agree closely with experimental measurements. It is shown that low-dimensional networks (e.g., tori) have lower latency and higher hot-spot throughput than high-dimensional networks (e.g., binary n-cubes) with the same bisection width. Keywords Communication networks, interconnection networks, concurrent computing, message-passing multiprocessors, parallel processing, VLSI. 1 Introduction The critical component of a concurrent computer is its communication network. Many algorithms are communication rather than processing limited. Fi...

Abstract-One measure of usefulness of a general-purpose distrib-uted computing system is the system’s ability to provide a level of per-formance commensurate to the degree of multiplicity of resources pres-ent in the system. Many different approaches and metrics of performance have been proposed in an attempt to achieve this goal in existing systems. In addition, analogous problem formulations exist in other fields such as control theory, operations research, and produc-tion management. However, due to the wide variety of approaches to this problem, it is difficult to meaningfully compare different systems since there is no uniform means for qualitatively or quantitatively eval-uating them. It is difficult to successfully build upon existing work or identify areas worthy of additional effort without some understanding of the relationships between past efforts. In this paper, a taxonomy of approaches to the resource management problem is presented in an attempt to provide a common terminology and classification mecha-nism necessary in addressing this problem. The taxonomy, while pre-sented and discussed in terms of distributed scheduling, is also appli-cable to most types of resource management. As an illustration of the usefulness of the taxonomy an annotated bibliography is given which classifies a large number of distributed scheduling approaches accord-ing to the taxonomy. Index Terms-Distributed operating systems, distributed resource management, general-purpose distributed computing systems, sched-uling, task allocation, taxonomy. T I.

In many recent applications, data may take the form of continuous data streams, rather than finite stored data sets. Several aspects of data management need to be reconsidered in the presence of data streams, offering a new research direction for the database community. In this paper we focus primarily on the problem of query processing, specifically on how to define and evaluate continuous queries over data streams. We address semantic issues as well as efficiency concerns. Our main contributions are threefold. First, we specify a general and flexible architecture for query processing in the presence of data streams. Second, we use our basic architecture as a tool to clarify alternative semantics and processing techniques for continuous queries. The architecture also captures most previous work on continuous queries and data streams, as well as related concepts such as triggers and materialized views. Finally, we map out research topics in the area of query processing over data streams, showing where previous work is relevant and describing problems yet to be addressed.